Spectral Imaging of Plasmonic Nano-StructuresCustomer Stories
Prof. Adi Salomon
Bar-Ilan University, Department of Chemistry
Background
Prof. Adi Salomon’s lab is primarily interested in understanding the interactions of molecules with light at the nanoscale and build devices for sensing molecules using light. The group designs and fabricates metallic nanostructures and uses them to influence light on the nanoscale due to interactions with surface plasmons. These plasmonic devices enhance and focus the light into a deep sub-wavelength volume, depending on their size, shape and arrangement of nano particles as well as the light’s energy. Molecules located in close vicinity to these hotspots can experience strong interactions that changes their physical properties and increases signal response, for example in surface enhanced Raman scattering.
One project in the lab investigates the color and intensity of light transmission through triangular, nano-cavities in metallic surfaces. The color of light can be precisely controlled by changing the distance between just two cavities or by controlling the polarization state of the incoming field (light). The Salomon lab uses different sets of cavities milled in thin metallic films to investigate hybrid modes between molecular transition states and plasmonic modes, to change the interaction strength.
Using plasmonic structures the Salomon lab also performs experiments using plasmonic structures to control second harmonic generation (SHG) of light. SHG is ideal for probing thin layers of molecules on surfaces and as well as molecular events occurring on surfaces. Since inversion symmetry has to be broken for SHG to occur, the observed signal is from the surface of a metal without interference from the bulk of the substrate.
Challenge
The Salomon lab uses various spectroscopic methods for the characterization of nanostructures such as cathodoluminescence, transmission and reflection spectroscopy. Spectroscopy provides information about the location of electric field hotspots, the position of resonances as well as changes due to changing light polarization and device design. However, it is important to collect this data as a function of position with sufficient spatial resolution. Spectral signals could be obtained by scanning every point on the surface which would be rather slow and inefficient. For this reason, the Salomon lab is using a spectral imaging approach for transmission spectroscopy, where a region of the sample across the plasmonic structures is projected on the spectrometer entrance slit. All spectra from points along the entrance slit will then be measured simultaneously on different rows of the spectrometer camera.
Spectral resolution is also an important consideration particularly when considering molecular spectra. While plasmonic transmission peaks have broad width due to the short lifetime of the surface plasmon excitations, when looking at interactions with molecules fine structure in the molecular spectra contain information about the spectral coupling to the electromagnetic fields.
IsoPlane and PIXIS camera are part of one of the key setups in our lab for characterization of plasmonic structures. The IsoPlane allows us to do spectral imaging without aberrations so hotspots on plasmonic surfaces can be more easily identified.
Solution
The Salomon lab implemented an aberration corrected spectrograph (IsoPlane SCT-320 and PIXIS 1024 camera) in their spectral imaging microscopy setups. This system is designed to minimize or eliminate optical aberrations that are limiting for more conventional Czerny-Turner spectrograph designs. Reducing aberration increases spectral resolution, sensitivity and reduces distortions of sharp spectral lines as the signal is focused on a smaller area in the focal plane. At the same time the aberration free design increases the spatial resolution as spectra coming from different points on the sample are better resolved on the detector. The spatial location and structure of electromagnetic field hotspots in plasmonic structures can be more precisely and rapidly characterized using the spectral imaging approach.
Control of Ultrafast Non-Linear Interactions in Materials and Plasmonic NanostructuresCustomer Stories
Prof. Adi Salomon
Bar-Ilan University, Department of Chemistry
Background
Prof. Adi Salomon’s lab is primarily interested in understanding the interactions of molecules with light at the nanoscale and build devices for sensing molecules using light. The group designs and fabricates metallic nanostructures and uses them to influence light on the nanoscale due to interactions with surface plasmons. These plasmonic devices enhance and focus the light into a deep sub-wavelength volume, depending on their size, shape and arrangement of nano particles as well as the light’s energy. Molecules located in close vicinity to these hotspots can experience strong interactions that changes their physical properties and increases signal response, for example in surface enhanced Raman scattering.
One project in the lab investigates the color and intensity of light transmission through triangular, nano-cavities in metallic surfaces. The color of light can be precisely controlled by changing the distance between just two cavities or by controlling the polarization state of the incoming field (light). The Salomon lab uses different sets of cavities milled in thin metallic films to investigate hybrid modes between molecular transition states and plasmonic modes, to change the interaction strength.
Using plasmonic structures the Salomon lab also performs experiments using plasmonic structures to control second harmonic generation (SHG) of light. SHG is ideal for probing thin layers of molecules on surfaces and as well as molecular events occurring on surfaces. Since inversion symmetry has to be broken for SHG to occur, the observed signal is from the surface of a metal without interference from the bulk of the substrate.
Figure 1: Plasmonic-mediated enhancement of second harmonic generation. Enabled by the enhanced sensitivity and precision of the IsoPlane and Pixis, coherent control of nonlinear plasmonic dynamics reveal an ultrafast, femtosecond, dynamics in the formation of nonlinear plasmonic excitation, capturing the temporal structure in the extreme ultrafast collective electronic response in nanostructures.
Figure 2: Enabled by the precision and sensitivity of the PIXIS and combined with the intuitive LightField software enabling effortless operation of online data analysis, Eyal Bahar observes unique spatio-temporal effectes in nanostructures where the nanostructured resonance couples with a surface lattice resonance to produce Lattice-induced transparency (LIT) modes that indicate the formation of slow-light propagation centered around the 96% transparent wavelength.
Figure 3: Eyal Bahar next to the homebuilt microspectroscopy setup and IsoPlane-SCT320 with PIXIS camera.
Challenge
The Salomon lab uses various spectroscopic methods for the characterization of nanostructures such as cathodoluminescence, transmission and reflection spectroscopy. Spectroscopy provides information about the location of electric field hotspots, the position of resonances as well as changes due to changing light polarization and device design. However, it is important to collect this data as a function of position with sufficient spatial resolution. Spectral signals could be obtained by scanning every point on the surface which would be rather slow and inefficient. For this reason, the Salomon lab is using a spectral imaging approach for transmission spectroscopy, where a region of the sample across the plasmonic structures is projected on the spectrometer entrance slit. All spectra from points along the entrance slit will then be measured simultaneously on different rows of the spectrometer camera.
Spectral resolution is also an important consideration particularly when considering molecular spectra. While plasmonic transmission peaks have broad width due to the short lifetime of the surface plasmon excitations, when looking at interactions with molecules fine structure in the molecular spectra contain information about the spectral coupling to the electromagnetic fields.
IsoPlane and PIXIS camera are part of one of the key setups in our lab for characterization of plasmonic structures. The IsoPlane allows us to do spectral imaging without aberrations so hotspots on plasmonic surfaces can be more easily identified.
Solution
The Salomon lab implemented an aberration corrected spectrograph (IsoPlane SCT-320 and PIXIS 1024 camera) in their spectral imaging microscopy setups. This system is designed to minimize or eliminate optical aberrations that are limiting for more conventional Czerny-Turner spectrograph designs. Reducing aberration increases spectral resolution, sensitivity and reduces distortions of sharp spectral lines as the signal is focused on a smaller area in the focal plane. At the same time the aberration free design increases the spatial resolution as spectra coming from different points on the sample are better resolved on the detector. The spatial location and structure of electromagnetic field hotspots in plasmonic structures can be more precisely and rapidly characterized using the spectral imaging approach.
Ultra-High-Sensitivity emICCD Cameras Enable Diamond
Quantum Dynamics Research
Introduction
Interest in the various crystal defects found within diamonds is growing quickly amongst physicists and biologists worldwide. This increasing attention is attributable in no small part to the ability of such defects, when embedded in nanocrystals, to function as single-photon sources or as highly photostable, low-cytotoxicity fluorescent biomarkers.1
One of these defects, the nitrogen-vacancy (NV) center, can be utilized to detect and measure local magnetic2 and electric fields,3 a capability based on the quantum mechanical interactions of the defect’s spin state. The ongoing study of NV centers holds fantastic promise for a broad range of advanced applications, including ion concentration measurements,4 membrane potential measurements,5 nanoscale thermometry,6 and single-spin nuclear magnetic resonance.7
Recently, researchers at theUniversité de Sherbrooke in Québec, Canada, led by Ph.D. student David Roy-Guay and supervised by professors Denis Morris and Michel Pioro- Ladrière, employed a scientific emICCD camera from Princeton Instruments to image the quantum state of NV centers in diamond over a wide surface (100 μm x 100 μm) on a pixel basis. Their work on diamond quantum dynamics is the focus of this application note.
Nitrogen-Vacancy Basics
Nitrogen-vacancy centers in diamond show exceptional light-to-spin properties that can be exploited in quantum information,8 magnetic resonance imaging, and entangled photon sources.9 Embedded in the rigid structure of diamond, the radiative defect is composed of a single substitutional nitrogen atom adjacent to a carbon vacancy (see Figure 1a) freeing two electrons with a spin, a quantum property. Combination of their spin forms a triplet state,10 sensitive to the external magnetic field applied. Usually, even the smallest change in the magnetic field induced by other spins in a material affects the spin properties of a defect. In the case of the NV centers, such modulations are limited by the low spin-phonon coupling and the low concentration (~1%) of other spin species, resulting in the preservation of the spin triplet coherence even at room temperature.11 Combined with the ability to read out the spin state optically, NV centers make outstanding magnetic field sensors with nanoscale sensitivity.12
Figure 1: (a) Lattice structure of diamond, including the NV center. (b) Energy-level structure of the NV center. (c) Optically detected magnetic resonance (ODMR) of the NV center for a 2 mT external magnetic field. Courtesy of David Roy-Guay, Université de Sherbrooke.
A closer look (see Figure 1b) at the energy levels of the NV center shows the close relationship between light and spin property. The ground triplet state of the NV center is linked to the excited state by a radiative transition at 637 nm. Its spin projection is either 0 (symmetric state, |0>) or ±1 (both spins up or down, |±1>), split by 2.87 GHz. Following excitation with laser pulses at 532 nm, the NV center initially in state 0 will re-emit red photons at 637 nm. On the other hand, if the initial state is ±1, it will de-excite via a non-radiative transition to the 0 state on a timescale of approximately 300 nsec. Consequently, the intensity of the red light collected depends on the state, and the NV center can be initialized by sending a short (µsec) laser pulse. Upon application of an external magnetic field along the NV quantization axis, the ±1 state is split by quantity β (28 MHz/mT), proportional to the magnetic field.
By sending microwaves in an optically detected magnetic resonance (ODMR) experiment, the spin state can be manipulated (see Figure 1c). Recording the photoluminescence while scanning the frequency of the microwaves, the amplitude and orientation of an unknown magnetic field can be extracted via measurement of the distance between the two spin states, which appear as dips in Figure 1c. Consequently, one sees that narrow resonance lines improve the magnetic field sensitivity.
Coherent Control of the NV Center State
The experimental setup used by David Roy-Guay to measure the ODMR is shown in Figure 2. The green laser goes through a double-pass acousto-optical modulator (AOM) to generate laser pulses for initialization and readout of the NV centers. A 60X plano-convex microscope lens focuses the laser on the surface of the NV-containing CVD diamond and the emitted light is collected by a PI-MAX4:512EM emICCD camera.
Figure 2: Experimental setup for NV center imaging. Laser pulses are generated by the AOM in a double-pass configuration and sent to the sample with a polarizing beam splitter (PBS). Courtesy of David Roy-Guay, Université de Sherbrooke.
In the spin-manipulation experiment shown in Figure 3a, an external field (10 mT) is applied to split the levels ±1 into the four possible orientations of the NV in the lattice structure. The resonant microwave pulses for the orientation at 2751 MHz are amplified and sent through a photolithographically defined wire on the surface of the diamond — these pulses’ widths are stepped in-between the acquisitions of the camera.
Synchronization of the microwave generator, the AOM, and the emICCD camera is achieved by operating the camera in its external trigger mode and counting the number of frames out-put by the ‘logic out’ output.
Because of the fast repolarization of the spin state to 0, the gate must be precisely aligned with the laser readout pulse. This critical alignment is easy to perform via the sequential gating function of the PI-MAX4:512EM camera, which moves the gate pulse relative to the trigger pulse. Once the gate pulse is set at the beginning of the readout laser pulse, microwaves are applied with a variable time τ for coherent control of the NV spin state (see inset Figure 3a). The quantum dynamics of an NV ensemble caught on a single pixel (blue curve Figure 3a) or averaged over a 10×10 pixel region (red curve) show excellent signal-to-noise ratio.
Figure 3: (a) Rabi oscillations of an NV ensemble on an individual pixel (blue) and 10×10 pixel region (red). Inset: pulse sequence for the laser initialization (I) and readout (R) of the NV centers. Microwave (MW) pulses are applied between the laser pulses.
(b) Photoluminescence readout of the NV center state. Maximum contrast is obtained by integrating the signal over the first µsec of the laser pulse. Courtesy of David Roy-Guay, Université de Sherbrooke.
The oscillations show that the spin state can be flipped from 0 to -1 by applying a 150 nsec pulse, corresponding to a π-pulse. Curve decay is due to relaxation: over 1 µsec, the ensemble of NV centers probed interacts with other spins in the diamond lattice so that their quantum state can no longer be manipulated coherently with high fidelity. Once the π pulse duration has been determined, the spin repolarization dynamics can be measured by preparing the NV in the 0 or -1 state and scanning the gate across the readout pulse (see Figure 3b).
Figure 4: The PI-MAX4:512EM from Princeton Instruments gives users the benefits of both an intensified CCD (ICCD) camera and an electron-multiplying CCD (EMCCD) camera.
This fiber-optically bonded Princeton Instruments PI-MAX®4 camera system provides <500 psec gate widths using standard fast-gate intensifiers while preserving quantum efficiency. Its integrated SuperSynchro timing generator allows camera users to set gate pulse widths and delays under GUI software control, and significantly reduces the inherent insertion delay (~27 nsec).
Complete control over all PI-MAX4:512EM hardware features is simple with the latest version of Princeton Instruments’LightField® data acquisition software (available as an option). Precision intensifier gating control and gate delays, as well as a host of novel functions for easy capture and export of image data, are provided via the exceptionally intuitive LightField user interface. The PI-MAX4:512EM uses a high-bandwidth (125 MB/sec or 1000 Mbps) GigE data interface to afford camera users real-time image transmission. This interface supports remote operation from more than 50 meters away.
Future Directions
The use of a scientific emICCD camera from Princeton Instruments has allowed researchers at the Université de Sherbrooke to image the quantum state of nitrogen-vacancy centers in diamond over a wide surface (100 µm x 100 µm) on a pixel basis. Observing and studying the effects of various magnetic systems on the optically detected magnetic resonance, ranging from functionalized biological samples to magnetic domains in materials, will be possible by tracking the position of the ODMR peaks down to 1 Gauss variations.
Furthermore, higher-sensitivity magnetometry can be achieved by applying spin-echo pulse techniques enabled by the emICCD camera’s precision gating features. Such a capability is key to the development of a new scientific tool based on diamond technology, one which will provide unique insights for biology, medicine, chemistry, physics, and geology.
Acknowledgments
Princeton Instruments thanks David Roy-Guay (Université de Sherbrooke) for his invaluable contributions to this application note. The research team at the university would like to acknowledge funding from FRQNT and CIFAR.
Schirhagl R. et al. Nitrogen-vacancy centers in diamond: nanoscale sensors for physics and biology. Annu. Rev. Phys. Chem. 65, 83–105 (2014).
Maze J.R. et al. Nanoscale magnetic sensing with an individual electronic spin in diamond. Nature 455, 644–647 (2008).
Dolde F. et al. Electric-field sensing using single diamond spins. Nat. Phys. 7, 459–463 (2011).
Hall L.T. et al. Monitoring ion-channel function in real time through quantum decoherence. Proc. Natl. Acad. Sci. U.S.A. 107, 18777–18782 (2010).
Hall L.T. et al. High spatial and temporal resolution wide-field imaging of neuron activity using quantum NV-diamond. Sci. Rep. 2, 401 (2012).
Toyli D.M. et al. Fluorescence thermometry enhanced by the quantum coherence of single spins in diamond. Proc. Natl. Acad. Sci. U.S.A. 110, 8417–8421 (2013).
Mamin H.J. et al. Nanoscale nuclear magnetic resonance with a nitrogen-vacancy spin sensor. Science (80-) 339, 557–560 (2013).
Bernien H. et al. Heralded entanglement between solid-state qubits separated by three metres. Nature 497, 86–90 (2013).
Togan E. et al. Quantum entanglement between an optical photon and a solid-state spin qubit. Nature 466, 730–734 (2010).
Gali A. et al. Theory of spin-conserving excitation of the N-V-center in diamond. Phys. Rev. Lett. 103, 1–4 (2009).
Balasubramanian G. et al. Ultralong spin coherence time in isotopically engineered diamond. Nat. Mater. 8, 383–387 (2009).
Balasubramanian G. et al. Nanoscale imaging magnetometry with diamond spins under ambient conditions. Nature 455, 648–651 (2008).
Further Reading
Ultra-Low-Light Imaging in Quantum Research
Find out about the different types of camera technologies that can be used for quantum research, their advantages and their limitations.
Measuring Large Scale Interactions Between Surfaces with nm Precision to Better Understand Geological FormationsCustomer Stories
Prof. Adi Salomon
Bar-Ilan University, Department of Chemistry
Background
Prof. Adi Salomon’s lab is primarily interested in understanding the interactions of molecules with light at the nanoscale and build devices for sensing molecules using light. The group designs and fabricates metallic nanostructures and uses them to influence light on the nanoscale due to interactions with surface plasmons. These plasmonic devices enhance and focus the light into a deep sub-wavelength volume, depending on their size, shape and arrangement of nano particles as well as the light’s energy. Molecules located in close vicinity to these hotspots can experience strong interactions that changes their physical properties and increases signal response, for example in surface enhanced Raman scattering.
One project in the lab investigates the color and intensity of light transmission through triangular, nano-cavities in metallic surfaces. The color of light can be precisely controlled by changing the distance between just two cavities or by controlling the polarization state of the incoming field (light). The Salomon lab uses different sets of cavities milled in thin metallic films to investigate hybrid modes between molecular transition states and plasmonic modes, to change the interaction strength.
Using plasmonic structures the Salomon lab also performs experiments using plasmonic structures to control second harmonic generation (SHG) of light. SHG is ideal for probing thin layers of molecules on surfaces and as well as molecular events occurring on surfaces. Since inversion symmetry has to be broken for SHG to occur, the observed signal is from the surface of a metal without interference from the bulk of the substrate.
Figure 1:Movie showing FECO of the attractive van der Waals force acting between two mica surfaces in the Surface Forces Apparatus. Surfaces are slowly brought into contact. The sudden jump into a flat contact corresponds to the attraction between the surfaces.
Figure 2:Formation of a capillary bridge between hydrophilic surface and Au in humid air. Flattened region is the contact region between the surfaces. The discontinuity between the flattened region and curved part of the FECO indicates a difference in the refractive index in the contact region and outside of it (there is a capillary water bridge between the surfaces, outside the contact region there is air.) Upon injection of water between the surfaces, the capillary bridge disappears discontinuity in the FECO fringes is gone.
Challenge
The Salomon lab uses various spectroscopic methods for the characterization of nanostructures such as cathodoluminescence, transmission and reflection spectroscopy. Spectroscopy provides information about the location of electric field hotspots, the position of resonances as well as changes due to changing light polarization and device design. However, it is important to collect this data as a function of position with sufficient spatial resolution. Spectral signals could be obtained by scanning every point on the surface which would be rather slow and inefficient. For this reason, the Salomon lab is using a spectral imaging approach for transmission spectroscopy, where a region of the sample across the plasmonic structures is projected on the spectrometer entrance slit. All spectra from points along the entrance slit will then be measured simultaneously on different rows of the spectrometer camera.
Spectral resolution is also an important consideration particularly when considering molecular spectra. While plasmonic transmission peaks have broad width due to the short lifetime of the surface plasmon excitations, when looking at interactions with molecules fine structure in the molecular spectra contain information about the spectral coupling to the electromagnetic fields.
IsoPlane and PIXIS camera are part of one of the key setups in our lab for characterization of plasmonic structures. The IsoPlane allows us to do spectral imaging without aberrations so hotspots on plasmonic surfaces can be more easily identified.
Solution
The Salomon lab implemented an aberration corrected spectrograph (IsoPlane SCT-320 and PIXIS 1024 camera) in their spectral imaging microscopy setups. This system is designed to minimize or eliminate optical aberrations that are limiting for more conventional Czerny-Turner spectrograph designs. Reducing aberration increases spectral resolution, sensitivity and reduces distortions of sharp spectral lines as the signal is focused on a smaller area in the focal plane. At the same time the aberration free design increases the spatial resolution as spectra coming from different points on the sample are better resolved on the detector. The spatial location and structure of electromagnetic field hotspots in plasmonic structures can be more precisely and rapidly characterized using the spectral imaging approach.
TERS Imaging Resolves Individual Molecular Vibrational ModesCustomer Stories
Prof. Adi Salomon
Bar-Ilan University, Department of Chemistry
Background
Prof. Adi Salomon’s lab is primarily interested in understanding the interactions of molecules with light at the nanoscale and build devices for sensing molecules using light. The group designs and fabricates metallic nanostructures and uses them to influence light on the nanoscale due to interactions with surface plasmons. These plasmonic devices enhance and focus the light into a deep sub-wavelength volume, depending on their size, shape and arrangement of nano particles as well as the light’s energy. Molecules located in close vicinity to these hotspots can experience strong interactions that changes their physical properties and increases signal response, for example in surface enhanced Raman scattering.
One project in the lab investigates the color and intensity of light transmission through triangular, nano-cavities in metallic surfaces. The color of light can be precisely controlled by changing the distance between just two cavities or by controlling the polarization state of the incoming field (light). The Salomon lab uses different sets of cavities milled in thin metallic films to investigate hybrid modes between molecular transition states and plasmonic modes, to change the interaction strength.
Using plasmonic structures the Salomon lab also performs experiments using plasmonic structures to control second harmonic generation (SHG) of light. SHG is ideal for probing thin layers of molecules on surfaces and as well as molecular events occurring on surfaces. Since inversion symmetry has to be broken for SHG to occur, the observed signal is from the surface of a metal without interference from the bulk of the substrate.
Figure 1 : STM-TERS spectra of a single CuNc molecule comparison with a spectrum several nm away shows that the origin of the Raman signal is localized to the molecule. The images of TERS mapping of the molecule reveal the structure of different vibrational modes.
Challenge
The Salomon lab uses various spectroscopic methods for the characterization of nanostructures such as cathodoluminescence, transmission and reflection spectroscopy. Spectroscopy provides information about the location of electric field hotspots, the position of resonances as well as changes due to changing light polarization and device design. However, it is important to collect this data as a function of position with sufficient spatial resolution. Spectral signals could be obtained by scanning every point on the surface which would be rather slow and inefficient. For this reason, the Salomon lab is using a spectral imaging approach for transmission spectroscopy, where a region of the sample across the plasmonic structures is projected on the spectrometer entrance slit. All spectra from points along the entrance slit will then be measured simultaneously on different rows of the spectrometer camera.
Spectral resolution is also an important consideration particularly when considering molecular spectra. While plasmonic transmission peaks have broad width due to the short lifetime of the surface plasmon excitations, when looking at interactions with molecules fine structure in the molecular spectra contain information about the spectral coupling to the electromagnetic fields.
IsoPlane and PIXIS camera are part of one of the key setups in our lab for characterization of plasmonic structures. The IsoPlane allows us to do spectral imaging without aberrations so hotspots on plasmonic surfaces can be more easily identified.
Solution
The Salomon lab implemented an aberration corrected spectrograph (IsoPlane SCT-320 and PIXIS 1024 camera) in their spectral imaging microscopy setups. This system is designed to minimize or eliminate optical aberrations that are limiting for more conventional Czerny-Turner spectrograph designs. Reducing aberration increases spectral resolution, sensitivity and reduces distortions of sharp spectral lines as the signal is focused on a smaller area in the focal plane. At the same time the aberration free design increases the spatial resolution as spectra coming from different points on the sample are better resolved on the detector. The spatial location and structure of electromagnetic field hotspots in plasmonic structures can be more precisely and rapidly characterized using the spectral imaging approach.
COSMOS for the Detection and Characterization of Exoplanets
Introduction
Exoplanets are planets that orbit a star other than the Sun, and their detection and characterization has been at the forefront within astronomy for multiple years. To date there has been greater than 4000 exoplanets confirmed across a range of different methods, with more than 5000 additional exoplanet candidates. Over the next decade, characterizing habitable exoplanets will be one of the key research areas within astronomy, with the goal to find Earth-like planets. Although there are multiple detection methods, transit photometry and radial velocity are the most common techniques for exoplanet detection/characterization, with each method discovering 71.3% and 22.9% of exoplanets respectively [1].
Transit Photometry
Transit photometry is an indirect method that measures the decrease in flux of a star caused by an orbiting planet passing in front of the star [2]. Measurement of the star’s flux produces a light curve, and when a planet passes in front of the star some of its light will be blocked, indicated by a slight dip in the light curve. Figure 1 shows a schematic of the transit of an exoplanet and the typical light curve produced (figure is only illustrative, and the curve is exaggerated).
Figure 1: Schematic showing an orbiting planet passing in front of a star with a typical lightcurve, at an orbital inclination denoted by i. The light curve measures the change in flux over time, with the diminishing flux (ΔF) indicating the presence of a planet.
By assuming that the planet’s flux is negligible, and that the star and planet are both spherical, the light curve can be used to determine the relative size of the planet, the inclination of the orbit and the orbital period. Transit photometry can also be combined with the orbital elements whenever an orbital solution, based on radial velocity measurements is available (see below). In this case the inclination of the orbit can be used to determine the mass of the planet. Combining the mass with the planet radius we get mean density and surface gravity. These parameters represent the first step in planet characterization, helping to distinguish between gas giants, icy and rocky planets[2].
Atmospheric characterization can also be achieved via transit photometry. By measuring the transit of the planet using different filters, the variations in the light curve can indicate the presence of certain molecules. For example, water molecules absorb near-infrared light so if they are present within the atmosphere of a planet, the atmosphere will be opaque to near-infrared wavelengths. When a near-infrared filter is used, the planet will block more light overall, due to the less transparent atmosphere. This means the eclipse will start earlier and end later in comparison to optical wavelengths, producing a light curve with a deeper and wider minimum.
However, this method is limited by geometry, as only planets that pass in front of the star, relative to Earth, can be studied in this way. Transit photometry also suffers from false positives, although a light curve characteristic of a transiting planet is measured, the flux may instead come from a multiple stellar system. An example of a false positive is an eclipsing binary star that is on the same line of sight with a bright single star. The flux of the single star reduces the apparent depth of the light curve dip caused by eclipsing binary. The resulting light curve is similar in shape to a transiting planet [2]. In order to confirm that a transit planet candidate is not a false positive, radial velocity measurements of the system are required.
Radial Velocity
Radial velocity is another indirect technique that measures the spectra of a star to look for a variable Doppler shift, indicative of an orbiting planet. Star-planet interaction is governed by gravity, so as the star pulls on the planet the planet pulls on the star, resulting in the star moving in a periodic motion, often referred to as a “wobble”. By measuring the spectra of the star, any movement towards and away from Earth will be observed as blueshift or redshift of the spectrum respectively, indicative of an orbiting planet.
In this way the Doppler effect can be used to determine the planet’s orbital period, the size of the orbit and orbital velocity. The latter can provide a minimum mass of the orbiting planet. The more massive the planet, the larger the velocity amplitude. However, to determine the true mass of the planet, transit photometry is required as radial velocity by itself cannot account for the inclination of the orbit [3]. Figure 2 shows a schematic of a star-planet interaction and how this influences the spectra of the star.
Figure 2: Schematic illustrating how the gravitational pull from an exoplanet causes a star to move in a circular motion, causing blueshift and redshift as the star moves towards and away from Earth respectively. Image not to scale.
As radial velocity measures the relative masses of the orbiting bodies, it can be used to rule out any false positives from exoplanets detected via transit photometry. In case of an eclipsing binary, the expected Doppler shift amplitude is large as the objects are of comparable masses. Therefore, radial velocity variations in a ball park of a few hundred meters per second or more will robustly distinguish a binary star from a transiting planet, confirming any false positive.
Both transit photometry and radial velocity measure the slightest of variations in a star’s flux as function of time or wavelength. Therefore, these techniques rely heavily on advanced detector technology to successfully detect and characterize exoplanets.
Camera requirements
Detection and characterization of exoplanets via transit photometry relies on the detection of a small variation in the fluxof a star. These variations, or dips, in flux are typically less than 1% of the true flux of the star. Therefore, a camera with high precision is essential to measure any slight decrease in flux. In addition, a camera with large dynamic range and low noise further improves the likelihood of identifying an exoplanet, as any dip, or spectral feature, can be differentiated from the noise level.
Transit photometry typically requires dense observations once the planet traverses in front of the star. These high frame rate observations are essential to capture the maximum amount of data for each transit and to counteract for the inhomogeneity of the star, in particular the edge of the star or active regions (stellar spots) that a planet may cover during its passage. It also allows for parameters such as planet size and inclination to be determined. Therefore, a camera with a high duty cycle (i.e. minimal readout with respect to exposure) is vital to allow for this high cadence imaging. The combination of high precision, low noise, large dynamic range and high duty cycle allows for dense observations while maintaining a high signal to noise ratio.
Radial velocity, although a spectroscopic technique, still requires advanced camera parameters such as those stated above. Fringing (interference of photons within the camera sensor) can be extremely problematic for radial velocity as it causes a sinusoidal modulation on the top of any measured spectrum. Radial velocity relies on the cross correlation of spectra, so any fringing artifacts will affect the accuracy of cross correlation and therefore reduce the accuracy of exoplanet characterization.
In addition, having a good charge transport efficiency on the camera sensor is important. If either some charge is left behind, or there is some correlated noise between pixels, this will influence cross correlation negatively affecting detection or characterization of exoplanets. However, this is only something that needs to be taken into consideration when using a CCD or EMCCD sensor.
COSMOS for transit detection and radial velocity measurements
COSMOS, the large format, back-illuminated, advanced CMOS camera from Teledyne Princeton Instruments, has many of the camera qualities required for transit photometry and radial velocity measurements. Due to its back-illuminated sensor, COSMOS has a >90% peak quantum efficiency in the visible range, as shown in Figure 3. Therefore, even with high cadence imaging, COSMOS will be able to convert a high percentage of photons into photoelectrons. COSMOS can also achieve down to 0.7 e- read noise, making COSMOS suitable for detecting the slightest dips in a light curve. In combination with high quantum efficiency, this low read noise provides a high signal to noise ratio for the ultimate large format CMOS sensitivity.
Figure 3: Quantum efficiency curve of COSMOS camera, showing high quantum efficiency over the visible range and a peak quantum efficiency of >90%.
Large dynamic range is important within transit photometry and radial velocity as it allows for slight fluctuations to be determined. It also allows for the measurement of fainter signals without saturation of the detector from surrounding brighter objects. Dynamic range is a parameter that depends on camera linearity, analog-to-digital converter bit-depth, and gain. To extend dynamic range, CMOS cameras typically sample the signal multiple times at both high and low gain. However, the cross-over point between these high and low gain readouts can produce artifacts which limit measurement precision [4].
Advanced CMOS designs, such as those within COSMOS, ensure precise cross-over between both ADCs for operation with low noise and high linearity. By combining with higher bit-depth ADCs, the technology provides unsurpassed dynamic range. More details about the large dynamic range of LACera™ technology, used within COSMOS, can be found in our article New Era in High Dynamic Range CMOS.
Traditional full frame CCD sensors rely on a mechanical shutter to block any incident light during readout. Opening and closing mechanical shutters is a relatively slow process, introducing quantitative errors for high cadence imaging. Mechanical shutters also have finite lifetimes and often need to be replaced frequently when the camera is in heavy use. As COSMOS is a CMOS detector, it utilizes a fast, electronic shutter where exposure is stopped by shifting detected photoelectrons into a frame storage area before readout. Not only is an electronic shutter more precise than a mechanical shutter, but also leads to less dead time of the detector, during which the camera is not exposing to light. This means that subsequent exposures can already start as signal is readout from the storage area, providing a 100% duty cycle. This allows dense, high cadence imaging essential for measuring characteristic parameters of an exoplanet.
Conclusion
Exoplanet detection and characterization will be at the forefront of astronomy over the next decade. Transit photometry and radial velocity, two complementary techniques, have been used to discover a cumulative 94.2% of all exoplanets. Transit photometry is an indirect method that looks for a dip in a stars brightness due to a planet traversing in front of the star, whereas radial velocity looks at the Doppler shift of the star due to the gravitational pull from an orbiting planet.
Both techniques require high precision, and a camera with low noise, large dynamic range and high sensitivity. Transit photometry also requires a camera with a high duty cycle, as a dense number of frames taken at a high cadence is typically used for imaging a potential planet as it crosses the star’s surface. Radial velocity, a spectroscopic technique, can be greatly hindered by fringing, so requires a camera with as minimal fringing as possible.
COSMOS, with >90% peak quantum efficiency, low read noise of 0.7 e-, and large dynamic range meets the minimum camera requirements for both transit photometry and radial velocity. In addition, the 100% duty cycle, made possible by the CMOS sensor architecture and electronic shutter, allows for high cadence imaging typical of exoplanet detection and characterization. This makes COSMOS optimal for transit photometry and radial velocity methods.
Acknowledgements
Teledyne Princeton Instruments would like to thank Professor Nikolai Piskunov, Uppsala University, for his invaluable contributions to this application note.
Handbook of Exoplanets, ISBN 978-3-319-55332-0. Springer International Publishing AG, part of Springer Nature, 2018, id.117, DOI: 1007/978-3-319-55333-7_117
Fischer D. A., Howard A. W., Laughlin G. P., Macintosh B., Mahadevan S., Sahlmann J., and Yee J. C. (2014) Exoplanet detection techniques. In Protostars and Planets VI (H. Beuther et al., eds.), pp. 715–737. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816531240-ch031.
High Dynamic Range, Hyperspectral and Multidimensional CARS Measurements of Combustion GasesCustomer Stories
Prof. Adi Salomon
Bar-Ilan University, Department of Chemistry
Background
Prof. Adi Salomon’s lab is primarily interested in understanding the interactions of molecules with light at the nanoscale and build devices for sensing molecules using light. The group designs and fabricates metallic nanostructures and uses them to influence light on the nanoscale due to interactions with surface plasmons. These plasmonic devices enhance and focus the light into a deep sub-wavelength volume, depending on their size, shape and arrangement of nano particles as well as the light’s energy. Molecules located in close vicinity to these hotspots can experience strong interactions that changes their physical properties and increases signal response, for example in surface enhanced Raman scattering.
One project in the lab investigates the color and intensity of light transmission through triangular, nano-cavities in metallic surfaces. The color of light can be precisely controlled by changing the distance between just two cavities or by controlling the polarization state of the incoming field (light). The Salomon lab uses different sets of cavities milled in thin metallic films to investigate hybrid modes between molecular transition states and plasmonic modes, to change the interaction strength.
Using plasmonic structures the Salomon lab also performs experiments using plasmonic structures to control second harmonic generation (SHG) of light. SHG is ideal for probing thin layers of molecules on surfaces and as well as molecular events occurring on surfaces. Since inversion symmetry has to be broken for SHG to occur, the observed signal is from the surface of a metal without interference from the bulk of the substrate.
Figure 1: Single-shot experimental spectrogram of a flame-wall interaction event. Image courtesy of Peterson Lab (http://peterson-labs.com)
Challenge
The Salomon lab uses various spectroscopic methods for the characterization of nanostructures such as cathodoluminescence, transmission and reflection spectroscopy. Spectroscopy provides information about the location of electric field hotspots, the position of resonances as well as changes due to changing light polarization and device design. However, it is important to collect this data as a function of position with sufficient spatial resolution. Spectral signals could be obtained by scanning every point on the surface which would be rather slow and inefficient. For this reason, the Salomon lab is using a spectral imaging approach for transmission spectroscopy, where a region of the sample across the plasmonic structures is projected on the spectrometer entrance slit. All spectra from points along the entrance slit will then be measured simultaneously on different rows of the spectrometer camera.
Spectral resolution is also an important consideration particularly when considering molecular spectra. While plasmonic transmission peaks have broad width due to the short lifetime of the surface plasmon excitations, when looking at interactions with molecules fine structure in the molecular spectra contain information about the spectral coupling to the electromagnetic fields.
IsoPlane and PIXIS camera are part of one of the key setups in our lab for characterization of plasmonic structures. The IsoPlane allows us to do spectral imaging without aberrations so hotspots on plasmonic surfaces can be more easily identified.
Solution
The Salomon lab implemented an aberration corrected spectrograph (IsoPlane SCT-320 and PIXIS 1024 camera) in their spectral imaging microscopy setups. This system is designed to minimize or eliminate optical aberrations that are limiting for more conventional Czerny-Turner spectrograph designs. Reducing aberration increases spectral resolution, sensitivity and reduces distortions of sharp spectral lines as the signal is focused on a smaller area in the focal plane. At the same time the aberration free design increases the spatial resolution as spectra coming from different points on the sample are better resolved on the detector. The spatial location and structure of electromagnetic field hotspots in plasmonic structures can be more precisely and rapidly characterized using the spectral imaging approach.
Time domain astronomy is the study of how astronomical objects and unique events vary over time. It provides an alternative method to understanding the extreme phases of galaxy and stellar evolution, through the investigation of events such as supernovae, and gamma-ray bursts, indicative of the collapse of massive stars and the creation of neutron stars or black holes, and objects such as variable pulsars and stars, to name a few. Characteristics of these events can be periodic, random, and bursting in nature, with variation across all objects and events. This makes time domain astronomy a challenging key field within astronomy and astrophysics.
Figure 1: An artist’s rendition of a supernovae, an example of an astronomical object of interest for time domain astronomy
Time domain astronomy covers much of the electromagnetic spectrum. It utilizes both dedicated satellites, looking at regions of the electromagnetic spectrum that are absorbed by the atmosphere, and dedicated ground-based, fully robotic telescopes. These systems work together to capture the optical, infrared, and radio waves, describing the full electromagnetic profile associated with each astronomical object or event. Typically, the dedicated satellite will detect a new event, triggering the remote, robotic telescope to point at the event (or object) and begin acquiring.
There are different methodologies used for capturing temporal information of these variable objects. Imaging is a useful technique for visualizing these temporal changes, with spectroscopy typically utilized for looking at the chemical compositions of each object or event. Polarimetry is another common technique used within time domain astronomy to understand the associated variability in magnetic field. This is particularly useful for gamma-ray bursts, with the polarized light highlighting variations within the magnetic field of the burst.
These events can vary drastically in timescale, with some events lasting only a few milliseconds, and others lasting several years. As these events are unique, it is essential that the devices used to capture any variation on the very small timescale are optimized.
Camera/Sensor requirements
As there is a lot of variance between these astronomical events, with each being truly unique, the light intensity of the source is often unknown. To avoid potentially saturating the sensor, typically a large number of frames are taken at short exposure times. Although this method reduces the likelihood of saturation, the shorter exposure time limits the number of photons that can be detected. Therefore, a high quantum efficiency is essential to ensure that a high proportion of photons detected are converted into photoelectrons and therefore signal.
A high quantum efficiency is also important as integration over longer periods of time is not feasible within time domain astronomy. Once astronomical events are over, they typically fade, sometimes within a matter of minutes. A higher quantum efficiency allows for these fainter signals to still be detected even though the event has finished (i.e. measurement of the residual signal from the event).
To gain the most out of the information gathered during the fading of an event, the images acquired are typically stacked. To maintain a high signal-to-noise ratio of these stacked images, a low read noise is required, so that fainter stars or aspects of the event/object can still be detected. When frames are stacked, not only the signal is summed, butalso the read noise, so low read noise is essential. Computational stacking also allows for atmospheric turbulence to be partially corrected for, making a low read noise as integral as a high quantum efficiency.
As these events/objects are unique and imaged on such short timescales, the true brightness is unknown. To determine the true brightness, reference stars, which have known brightness, are also imaged within the frames. By measuring how the shorter exposure times have influenced the brightness of any reference stars and comparing to the known brightness of the reference star, the true brightness of the event/object can be obtained. The more reference stars in each image, the better the image calibration can be. Therefore, the larger the physical area of the sensor, the better for image and brightness comparison.
COSMOS for Time Domain Astronomy
COSMOS, the large format, back-illuminated CMOS camera from Teledyne Princeton Instruments, has many of the camera qualities required for time domain astronomy. As it is back-illuminated, COSMOS has a high quantum efficiency over the visible range, with >90% peak quantum efficiency as shown in Figure 2. This means that it will be able to convert a high percentage of photons into photoelectrons, even with shorter exposure times. COSMOS also has a low read noise, with the ability to achieve down to 0.7 e- read noise. This make the COSMOS suitable for not only imaging the early stages of an event, or object, but also detect any event afterglow. This low read noise is also advantageous for stacking of frames, maintaining a high signal to noise ratio even with the summation of frame read noise.
Figure 2: Quantum efficiency curve of the COSMOS camera, showing high quantum efficiency over the visible range and a peak quantum efficiency of >90%.
COSMOS has a large imaging area, with 3k x 3k, 6k x 6k, and 8k x 8k formats, all with 10 μm pixels. COSMOS has a sensor diagonal of 43 mm, 92 mm, and 115 mm respectively, as depicted in Figure 3. These large imaging areas allow for the capture of multiple reference stars within each frame, ensuring a high level of image comparison and calibration.
Figure 3: Representative sensor sizes for each of the three COSMOS sensor models, with the 3k x 3k having a sensor diagonal of 43 mm, the 6k x 6k having a sensor diagonal of 92 mm, and the 8k x 8k having a sensor diagonal of 115 mm.
Due to the low dark current and large sensor areas, full frame CCDs have been traditionally used within time domain astronomy. In order to completely block any incident light during readout, full frame CCDs utilize a mechanical shutter. Mechanical shutters have finite lifetimes and often need to be replaced frequently when the camera is in heavy use. This can be problematic for telescopes used within time domain astronomy, as they are typically fully robotic, remote telescopes, meaning that maintenance can be challenging.
Additionally, opening and closing a mechanical shutter is relatively slow, introducing quantitative errors for the shorter exposure times essential for time domain astronomy. As COSMOS is a CMOS detector, it utilizes a fast, electronic shutter where exposure is stopped by shifting detected photoelectrons into a frame storage area before readout. Not only is an electronic shutter more precise than a mechanical shutter, but also leads to less dead time of the detector, during which the camera is not exposing to light. This means that subsequent exposures can already start as signal is readout from the storage area, allowing image capture of the entire event, rather than periodic snippets.
As time domain astronomy relies on the synergy between dedicated satellites and ground-based telescopes, it is common for the telescopes to be robotic and remotely controlled. Therefore, it is key that any camera or device can easily integrate into the existing software. COSMOS will be completely controlled by PICam, a software development kit from Teledyne Princeton Instruments. PICam is available for both Linux and 64-bit Windows, allowing complete control of COSMOS in either of these operating systems. PICam offers direct control of the camera, with flexible configuration and integration with other languages such as Python. In this way, COSMOS can easily integrate into the software controlling any remote, robotic telescope.
Conclusion
Time domain astronomy is the study of how astronomical objects and events vary and change with time. It is a research area that utilizes a lot of different techniques to understand the most about each unique event.
Typically, time domain astronomy takes multiple images with short exposure times to prevent saturation of the sensor, requiring a camera/device with high quantum efficiency and low read noise (to maintain high signal to noise ratio when stacking frames). In addition, as each event is unique, the true brightness is unknown, so multiple reference stars are required to ensure calibration of the object of interest. To measure multiple reference stars within an image, a large physical sensor area is required.
The COSMOS, with peak quantum efficiency >90%, low read noise down to 0.7 e-, and sensor sizes up to a diagonal of 115 mm at 8k x 8k pixels, meets the essential parameters for time domain astronomy. In addition, the reduced error from its electronic shutter, and software development kit for full control with integration into operating systems such as Linux, makes the COSMOS optimal for time domain astronomy.
Reducing stray light is one of the major challenges of combustion experiments probed with laser beams such as Raman spectroscopy. Bright, constant flame background can be effectively removed using temporal filters by using a combination of ultrafast laser pulses and gated ICCD or emICCD cameras that can open an electronic shutter for ps to ns time scales exactly when the signal arrives at the detector. However, stray light from other sources still interferes with the usable signal in particular for low frequency measurements.
Researchers around Andreas Ehn from Lund University implemented a creative approach for reducing stray light based on a spatial lock-in technique called periodic shadowing. While the concept for this measurement technique was shown in previously, the researchers from Lund describe in their recent paper that they “increase its feasibility, strength, and robustness” by implementing the method using a fiber probe.
The experiment uses a bundle of 19 fibers that are arranged in a densely packed, circular pattern for signal collection side of the fiber, but in a linear pattern on the spectrograph side. Each signal carrying fiber is followed by a dark fiber to create proper spacing of the fibers along the entrance slit of an Isoplane-SCT 320 spectrograph. The fiber pattern approximates a square wave function. At the exit of the spectrograph a spectral image is collected using a fast-gated PI-MAX4, ICCD camera. The Isoplane spectrograph is an aberration corrected system that maintains high signal accuracy without distortions even across a large sensor area which helps in the reconstruction of the signal from the collected images. The periodic structure of the signal is computationally analyzed on a by column basis by Fourier transform, multiplication with reference signals and band-pass filters which results in reduction of the DC like offsets like the stray light component on the detector.
The researchers further show the application of this technique to Raman spectroscopy of gases and premixed flames. They not only show that the reconstructed signal gives quantitative correct measures of species mole fraction and the temperature in the flame, but also that the stray light is suppressed by almost 2 orders of magnitude (factor of 80). The conclude that their “concept is highly valuable for accurate spectroscopic measurements in experiments with limited optical access”.
Raman spectroscopy plays a fundamental role for the characterization and application of 2D materials such as graphene and transition metal dichalcogenides. Promising applications of 2D materials are investigated in a diverse range of fields from quantum technology to life sciences. Diverse range of these materials is available by combining the in 2D layers, so called van der Vaals heterostructures and 2D material research tries to understand the physics of these structures and their benefits for different applications.
While 2D materials for the lab are still often produced by the mechanical exfoliation process, larger scale industrial production requires techniques based on material synthesis to achieve higher throughput in production. Researchers around Venkataraman Dharuman from Alagappa University in India report liquid phase synthesis of heterostructures built from graphene and MoS2 layers and show that incubating cells in these heterostructures lead to higher contrast in fluorescence imaging.
Various optical techniques are used for the characterization of the material, a crucial one being micro Raman spectroscopy providing lots of information on structure and interaction of the 2D heterostructures. The researchers use a micro Raman system with a 532nm excitation laser excite and collect signal through a 60x objective. The signal is coupled to a SpectraPro SP2500 spectrograph with a BLAZE-100HR camera for signal detection providing the required high resolution and sensitivity for characterization.
Using the setup the researchers monitor the Raman features of both graphene and MoS2 including positions, linewidth and comparisons to exfoliated materials. The Raman analysis indicates the presence and thickness of the heterostructures after synthesis as peak positions are shifted compared to graphene and MoS2 themselves. The spectra allow the quantification of defects and show further spectral line shifts that characterize the interlayer and material-solvent interaction.
Raman characterization as performed by the researchers from India will be important for ensuring stable and reproducible production processes of 2D material structures in future applications.