Measuring Large Scale Interactions Between Surfaces with nm Precision to Better Understand Geological FormationsCustomer Stories

Dr. Joanna Dziadkowiec, Dr. Anja Røyne

University of Oslo

Background

Although we perceive geological processes on macroscopic length scales, the mechanical behavior of geological structures can be significantly influenced by the microscopic mineral structure of rocks as well as the micro-scale interactions at the contacting mineral surfaces. Microscopic and nanoscale spaces between mineral grains often contain fluids and water that can reactively erode or deposit material, e.g. by promoting crystallization processes. Overall, these reactive dissolution and growth processes may have consolidating or destabilizing influences. These can further express in macroscopic, observable changes over time. A better understanding of these processes is important for predicting the behavior of geological structures both by natural development, but also due to human impact, for example, due to large-scale oil drilling or potential capture of carbon dioxide in geological structures.

Joanna Dziadkowiec and Anja Røyne performed research at the Njord Center at the University of Oslo trying to understand what happens at contacting mineral surfaces when they are pushed together and exposed to influences of fluids and water. Over time, the solution- and pressure-induced changes in the surface structure alter how the reactive, contacting surfaces interact with each other. This, in turn, affects the overall mechanical behaviour of granular rocks. For their experiments, the researchers use a surface force apparatus (SFA), a measurement device, which determines the interaction force between 2 solid surfaces. Unlike atomic force microscopy, where surfaces are probed point-wise on a nm scale, the SFA technique samples a much larger contact area of up to 100 μm diameter. This scale is more suitable and relevant to relate their observations to geologically relevant processes. For complete characterization of the interaction dynamics, the researchers also measure the distance between the surfaces (imagine a spring where the spring constant is characterized by the applied force and the displacement from the relaxed position). The nm-sized space between the surfaces is measured using multi-beam interferometry where the sample surfaces sit between 2 highly reflective substrates which are illuminated by a white light source. The transmitted signal is imaged onto the entrance slit of a spectrograph and dispersed to visualize fringes of equal chromatic order (FECO fringes). For example, Figure 1 shows the FECO fringes of 2 surfaces in contact with van der Waals forces acting between them. Fringes detected at different horizontal positions in the image correspond to a different wavelength, whereas different vertical positions correspond to different points on the sample surface.

The FECO interference technique allows the researchers to measure the distance profile between the 2 mineral surfaces and detect reactive changes that alter the nano- and micron-scale surface forces and separation distances. By observing the behavior of the interference patterns over time, the researchers are able to investigate reactive changes in the presence of fluids. Figure 2 shows how the interference pattern is modified by the presence of a capillary water bridge between two surfaces placed in close proximity: the capillary bridge is indicated by the discontinuity in the pattern and disappears once water is injected.

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 spectroscopy system used for multibeam FECO interference in the SFA requires high spectral accuracy and resolution as well as good imaging capability. The changes in the surface profiles of the mineral sample can be in the nm range so a spectral resolution in a nm or sub-nm range is needed to accurately determine fringe positions.

The interference patterns are observed by dispersing the signal along the spectrometer’s entrance slit into a hyperspectral interference image on the system camera, where the horizontal positions, which correspond to wavelength and the vertical position, can be mapped onto different points within the relatively large sample region of interest. The good imaging quality of the spectrograph ensures that the signal is undistorted at all points on the camera. Spectrographs that are optimized for the detection of single spectral channels often show severe image aberrations outside of the focal plane center, leading to distortions, signal reduction, and loss of measurement precision.

“The low aberrations of the IsoPlane system allow us to make very precise measurements of 2D spectral interference patterns. In combination with LightField and Intellical, this system is very efficient to setup for challenging scientific measurements, such as imaging of reactive mineral surfaces in confinement”

Solution

The multi-beam interference setup at the Njord center uses an IsoPlane-SCT320 spectrograph coupled with a PIXIS-2KBUV camera for hyperspectral detection of FECO fringes.

The IsoPlane is an advanced imaging spectrograph with aberration-corrected optics, significantly reducing image aberrations such as astigmatism that could distort the spectral image. The good image quality at any point of the focal plane also increases resolution due to better focusing of the signal light onto the camera. The IsoPlane is ideally suited for multi-channel and hyperspectral measurements requiring this high spectral and spatial resolution.

The PIXIS-2KBUV has a large 2048×512 pixel sensor, covering a wide area in the focal plane, so a wider profile of sample positions can be measured simultaneously. The PIXIS is a deep-cooled scientific camera with very low noise that is optimized for precise, quantitative signal detection.

As the researchers work frequently at different spectral ranges they also use an automated system for accurate wavelength calibration called Intellical. This system, which is controlled by Princeton Instruments LightField software, makes sure that spectral calibration can be done quickly, with very high precision (up to 10x higher than other common calibration methods).

The researchers also have to monitor the interactions between the sample surfaces for long times to be able to detect the reactive changes. Using a time-lapse feature in LightField, the experiments can be easily setup to automatically acquire images for long times with arbitrary time intervals between them, so the acquisition can be optimized considering the speed of the surface evolution.

TERS Imaging Resolves Individual Molecular Vibrational ModesCustomer Stories

Dr. Joanna Dziadkowiec, Dr. Anja Røyne

University of Oslo

Background

Although we perceive geological processes on macroscopic length scales, the mechanical behavior of geological structures can be significantly influenced by the microscopic mineral structure of rocks as well as the micro-scale interactions at the contacting mineral surfaces. Microscopic and nanoscale spaces between mineral grains often contain fluids and water that can reactively erode or deposit material, e.g. by promoting crystallization processes. Overall, these reactive dissolution and growth processes may have consolidating or destabilizing influences. These can further express in macroscopic, observable changes over time. A better understanding of these processes is important for predicting the behavior of geological structures both by natural development, but also due to human impact, for example, due to large-scale oil drilling or potential capture of carbon dioxide in geological structures.

Joanna Dziadkowiec and Anja Røyne performed research at the Njord Center at the University of Oslo trying to understand what happens at contacting mineral surfaces when they are pushed together and exposed to influences of fluids and water. Over time, the solution- and pressure-induced changes in the surface structure alter how the reactive, contacting surfaces interact with each other. This, in turn, affects the overall mechanical behaviour of granular rocks. For their experiments, the researchers use a surface force apparatus (SFA), a measurement device, which determines the interaction force between 2 solid surfaces. Unlike atomic force microscopy, where surfaces are probed point-wise on a nm scale, the SFA technique samples a much larger contact area of up to 100 μm diameter. This scale is more suitable and relevant to relate their observations to geologically relevant processes. For complete characterization of the interaction dynamics, the researchers also measure the distance between the surfaces (imagine a spring where the spring constant is characterized by the applied force and the displacement from the relaxed position). The nm-sized space between the surfaces is measured using multi-beam interferometry where the sample surfaces sit between 2 highly reflective substrates which are illuminated by a white light source. The transmitted signal is imaged onto the entrance slit of a spectrograph and dispersed to visualize fringes of equal chromatic order (FECO fringes). For example, Figure 1 shows the FECO fringes of 2 surfaces in contact with van der Waals forces acting between them. Fringes detected at different horizontal positions in the image correspond to a different wavelength, whereas different vertical positions correspond to different points on the sample surface.

The FECO interference technique allows the researchers to measure the distance profile between the 2 mineral surfaces and detect reactive changes that alter the nano- and micron-scale surface forces and separation distances. By observing the behavior of the interference patterns over time, the researchers are able to investigate reactive changes in the presence of fluids. Figure 2 shows how the interference pattern is modified by the presence of a capillary water bridge between two surfaces placed in close proximity: the capillary bridge is indicated by the discontinuity in the pattern and disappears once water is injected.

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 spectroscopy system used for multibeam FECO interference in the SFA requires high spectral accuracy and resolution as well as good imaging capability. The changes in the surface profiles of the mineral sample can be in the nm range so a spectral resolution in a nm or sub-nm range is needed to accurately determine fringe positions.

The interference patterns are observed by dispersing the signal along the spectrometer’s entrance slit into a hyperspectral interference image on the system camera, where the horizontal positions, which correspond to wavelength and the vertical position, can be mapped onto different points within the relatively large sample region of interest. The good imaging quality of the spectrograph ensures that the signal is undistorted at all points on the camera. Spectrographs that are optimized for the detection of single spectral channels often show severe image aberrations outside of the focal plane center, leading to distortions, signal reduction, and loss of measurement precision.

“The low aberrations of the IsoPlane system allow us to make very precise measurements of 2D spectral interference patterns. In combination with LightField and Intellical, this system is very efficient to setup for challenging scientific measurements, such as imaging of reactive mineral surfaces in confinement”

Solution

The multi-beam interference setup at the Njord center uses an IsoPlane-SCT320 spectrograph coupled with a PIXIS-2KBUV camera for hyperspectral detection of FECO fringes.

The IsoPlane is an advanced imaging spectrograph with aberration-corrected optics, significantly reducing image aberrations such as astigmatism that could distort the spectral image. The good image quality at any point of the focal plane also increases resolution due to better focusing of the signal light onto the camera. The IsoPlane is ideally suited for multi-channel and hyperspectral measurements requiring this high spectral and spatial resolution.

The PIXIS-2KBUV has a large 2048×512 pixel sensor, covering a wide area in the focal plane, so a wider profile of sample positions can be measured simultaneously. The PIXIS is a deep-cooled scientific camera with very low noise that is optimized for precise, quantitative signal detection.

As the researchers work frequently at different spectral ranges they also use an automated system for accurate wavelength calibration called Intellical. This system, which is controlled by Princeton Instruments LightField software, makes sure that spectral calibration can be done quickly, with very high precision (up to 10x higher than other common calibration methods).

The researchers also have to monitor the interactions between the sample surfaces for long times to be able to detect the reactive changes. Using a time-lapse feature in LightField, the experiments can be easily setup to automatically acquire images for long times with arbitrary time intervals between them, so the acquisition can be optimized considering the speed of the surface evolution.

High Dynamic Range, Hyperspectral and Multidimensional CARS Measurements of Combustion GasesCustomer Stories

Dr. Joanna Dziadkowiec, Dr. Anja Røyne

University of Oslo

Background

Although we perceive geological processes on macroscopic length scales, the mechanical behavior of geological structures can be significantly influenced by the microscopic mineral structure of rocks as well as the micro-scale interactions at the contacting mineral surfaces. Microscopic and nanoscale spaces between mineral grains often contain fluids and water that can reactively erode or deposit material, e.g. by promoting crystallization processes. Overall, these reactive dissolution and growth processes may have consolidating or destabilizing influences. These can further express in macroscopic, observable changes over time. A better understanding of these processes is important for predicting the behavior of geological structures both by natural development, but also due to human impact, for example, due to large-scale oil drilling or potential capture of carbon dioxide in geological structures.

Joanna Dziadkowiec and Anja Røyne performed research at the Njord Center at the University of Oslo trying to understand what happens at contacting mineral surfaces when they are pushed together and exposed to influences of fluids and water. Over time, the solution- and pressure-induced changes in the surface structure alter how the reactive, contacting surfaces interact with each other. This, in turn, affects the overall mechanical behaviour of granular rocks. For their experiments, the researchers use a surface force apparatus (SFA), a measurement device, which determines the interaction force between 2 solid surfaces. Unlike atomic force microscopy, where surfaces are probed point-wise on a nm scale, the SFA technique samples a much larger contact area of up to 100 μm diameter. This scale is more suitable and relevant to relate their observations to geologically relevant processes. For complete characterization of the interaction dynamics, the researchers also measure the distance between the surfaces (imagine a spring where the spring constant is characterized by the applied force and the displacement from the relaxed position). The nm-sized space between the surfaces is measured using multi-beam interferometry where the sample surfaces sit between 2 highly reflective substrates which are illuminated by a white light source. The transmitted signal is imaged onto the entrance slit of a spectrograph and dispersed to visualize fringes of equal chromatic order (FECO fringes). For example, Figure 1 shows the FECO fringes of 2 surfaces in contact with van der Waals forces acting between them. Fringes detected at different horizontal positions in the image correspond to a different wavelength, whereas different vertical positions correspond to different points on the sample surface.

The FECO interference technique allows the researchers to measure the distance profile between the 2 mineral surfaces and detect reactive changes that alter the nano- and micron-scale surface forces and separation distances. By observing the behavior of the interference patterns over time, the researchers are able to investigate reactive changes in the presence of fluids. Figure 2 shows how the interference pattern is modified by the presence of a capillary water bridge between two surfaces placed in close proximity: the capillary bridge is indicated by the discontinuity in the pattern and disappears once water is injected.

Figure 1: Single-shot experimental spectrogram of a flame-wall interaction event. Image courtesy of Peterson Lab (http://peterson-labs.com)

Challenge

The spectroscopy system used for multibeam FECO interference in the SFA requires high spectral accuracy and resolution as well as good imaging capability. The changes in the surface profiles of the mineral sample can be in the nm range so a spectral resolution in a nm or sub-nm range is needed to accurately determine fringe positions.

The interference patterns are observed by dispersing the signal along the spectrometer’s entrance slit into a hyperspectral interference image on the system camera, where the horizontal positions, which correspond to wavelength and the vertical position, can be mapped onto different points within the relatively large sample region of interest. The good imaging quality of the spectrograph ensures that the signal is undistorted at all points on the camera. Spectrographs that are optimized for the detection of single spectral channels often show severe image aberrations outside of the focal plane center, leading to distortions, signal reduction, and loss of measurement precision.

“The low aberrations of the IsoPlane system allow us to make very precise measurements of 2D spectral interference patterns. In combination with LightField and Intellical, this system is very efficient to setup for challenging scientific measurements, such as imaging of reactive mineral surfaces in confinement”

Solution

The multi-beam interference setup at the Njord center uses an IsoPlane-SCT320 spectrograph coupled with a PIXIS-2KBUV camera for hyperspectral detection of FECO fringes.

The IsoPlane is an advanced imaging spectrograph with aberration-corrected optics, significantly reducing image aberrations such as astigmatism that could distort the spectral image. The good image quality at any point of the focal plane also increases resolution due to better focusing of the signal light onto the camera. The IsoPlane is ideally suited for multi-channel and hyperspectral measurements requiring this high spectral and spatial resolution.

The PIXIS-2KBUV has a large 2048×512 pixel sensor, covering a wide area in the focal plane, so a wider profile of sample positions can be measured simultaneously. The PIXIS is a deep-cooled scientific camera with very low noise that is optimized for precise, quantitative signal detection.

As the researchers work frequently at different spectral ranges they also use an automated system for accurate wavelength calibration called Intellical. This system, which is controlled by Princeton Instruments LightField software, makes sure that spectral calibration can be done quickly, with very high precision (up to 10x higher than other common calibration methods).

The researchers also have to monitor the interactions between the sample surfaces for long times to be able to detect the reactive changes. Using a time-lapse feature in LightField, the experiments can be easily setup to automatically acquire images for long times with arbitrary time intervals between them, so the acquisition can be optimized considering the speed of the surface evolution.

Reducing stray light with spatial modulation and imaging spectroscopyResearch Stories

Andreas Ehn

OSA Publishing

Fiber-based stray light suppression in spectroscopy using periodic shadowing

Introduction

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”.

Further Information

Raman Characterization of Synthesized 2D Material HeterostructuresResearch Stories

Venkataraman Dharuman

ChemRxiv

Single Stage Liquid Phase Exfoliated MoS2/Graphene Hetero-Structure With Enhanced Brightness for Live Cell Imaging

Introduction

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.

Further Information

Resonance Raman Spectroscopy
of 2D Material LayersResearch Stories

David C. Smith

arXiv

Superposition of intra- and inter-layer excitons in twistronic MoSe2/WSe2 bilayers probed by resonant Raman scattering

Introduction

Raman spectroscopy has always been one of the most important measurement techniques for the characterization of 2D materials such as graphene, hexagonal boron nitride or transition metal dichalcogenides (TMDs). Analyzing their Raman spectrum reveals information on the number of layers, charge doping or stress and strain states. 2D materials can also be readily stacked into more complex structures. These so-called van der Waals heterostructures possess new physical properties and are fundamental for building new optoelectronic devices from 2D materials. It has been shown that not only the order of materials in the heterostructures is important to consider, but also their relative crystal orientation and the physical properties of so called twisted materials where the lattice of one layer is rotated relative to the layer below can change significantly.

Liam McDonnell and David Smith from the University of Southampton, UK and their coworkers are interested the physics of twisted van der Waals heterostructures. They use resonance Raman scattering to reveal interesting physics in TMD layers of MoSe2 and WSe2 and to characterize interactions between layers. In resonance Raman spectroscopy the excitation energy of the laser is brought in or close to resonance with the transition energy between electronic energy levels. The resonance effect typically leads to a massive increase in the Raman scattering signal which is often selective so only some Raman lines are enhanced. In TMDs resonance Raman has already been shown to be a probe of dark exciton states that are invisible in PL spectra, but couple to vibrational phonon modes.

In their recent article the researchers show that resonance Raman is a good tool to measure interactions between the TMD layers. For excitation of Raman spectra they use a tunable Ti:Sapph (1.24eV -1.77eV) and dye laser (1.74eV – 2.27eV) while controlling the light polarization which helps to eliminate unwanted luminescence signal from the sample. The Raman signal is analyzed in a TriVista triple stage spectrograph. The multi-stage system works particularly well for resonance Raman experiments as they can be adjusted for changing excitation laser wavelength without use of external filters that would be designed for use with a single laser excitation wavelength. They also allow observation of signals at energies <100cm-1 which are important for studying interlayer interactions of TMDs, but can’t be observe with standard Raman filters.

When scanning the excitation energy across resonance of an intra layer exciton the researcher observe spectral lines that can only be explained by hybridization of intra and interlayer exciton states. They can show that resonance Raman is a good tool to characterize the interactions and quantify the hybridization properties. Moreover, the interaction is dependent on the twisting angle between the layers. As a result, the researchers hope that their results will lead to new ways of designing optoelectronic devices of 2D materials. They also see potential applications in quantum technology and quantum information processing where hybridized excitons could be basis for q-bit states.

Further Information

Measuring Spectra of Transition Metal Dichalcogenides in Stable CavitiesResearch Stories

Alexander Högele

arXiv

Open-cavity in closed-cycle cryostat as a quantum optics platform

Introduction

Scientists around Prof. Alexander Högele from the Center for NanoScience and Center for Quantum Science and Technology at the Ludwig-Maximilians-University in Munich, Germany published an article about a new experimental setup for very sensitive cavity quantum electrodynamics measurements in closed cycle cryostats. They demonstrate the effectiveness of their passive and active vibration control by measuring strong coupling of a single layer of the transition metal dichalcogenide (TMD) WSe2 to an optical cavity.

Solid state quantum materials such as quantum dots, TMDs and rare earth ions are promising platforms for applications of quantum physics for computation, sensing and information processing. The researchers describe that these materials have to be cooled to cryogenic temperatures to reduce environment noise, however Helium that has traditionally been used is expensive and limited. The challenge of closed cycle systems are vibrations that are of the order of 10 micro-meter but need to be closer to 40 pico-meter to be useful for cavity QED experiments. The new experimental system at the University of Munich has a series of passive vibration control mechanisms, but also allows for active control receiving feedback through light reflection and interference in the optical cavity over a 100kHz bandwidth.

The setup is tested on a TMD monolayer of WSe2, encapsulated in 2 sheets of hexagonal boron nitride. The cavity is built from a concave fiber mirror and the substrate of the sample. For spectroscopic measurements of the sample the researchers use a high resolution SpectraPro spectrograph with 750mm focal length and detection using a liquid nitrogen cooled camera. The spectra show the exciton resonance around 1.723eV. The cavity is brought in resonance with the exciton (moving the fiber from 50μm distance to 2.9μm distance) while measuring the transmission spectra of a super continuum light source. Strong coupling between the photon field and the TMD is visible as an anti-crossing of the transmission signal which is a basic signature of coupled quantum oscillators.

After this successful demonstration the researchers plan to apply their experimental setup to further measurements on a wide range of quantum materials, single photon sources and qubit materials. They think that the already excellent vibration isolation can be improved even further in the future.

Further Information

Sensitive CoV-SARS-2 Detection with SERSResearch Stories

Deborah Crittenden, Mark Waterland

ACS Publications

Optical Detection of CoV-SARS-2 Viral Proteins to Sub-Picomolar Concentrations

Introduction

As the COVID-19 pandemic of 2020 and 2021 has shown, reliable, fast and cost effective detection of viral components and diagnostics is an important factor in management and mitigation of disease effects and spread. While PCR tests are the gold standard for testing they are relatively slow to process and resource intensive according to Deborah Crittenden, Mark Waterland and their team of researchers from University of Canterbury and Massey University (New Zealand). In a recent publication in ACS Omega they write that “it has been widely recognized that optical and/or electronic sensing technologies may hold the key to the development of rapid, high throughput, easy-to-use, point-of-care diagnostics.”

A challenge for these measurement techniques is that they need to achieve sensitivity to detect concentration in the fM range which represents a biologically and practically relevant value. The researchers implement 3 photonics based measurements systems for measurement of Covid-19 genetic signatures including biolayer interferometry and surface plasmon resonance, but the most sensitive technique studied was surface enhanced Raman spectroscopy (SERS). SERS enhances the vibrational Raman spectra of molecules in the presence of metallic nanoparticles.

To achieve high sensitivity the team functionalizes the surface of Ag nano particles with DNA aptamers that bind specifically to the CoV-SARS-2 spike proteins. Using Raman spectroscopy they observe changes in the aptamer Raman spectrum before and after binding of the viral components.

The SERS spectra are recorded by excitation with a 532nm laser and measuring the spectra (after filtering out laser light using a longpass edge filter) with an Isoplane-81 spectrograph, which allows for data collection with high resolution and sensitivity besides its small form factor.

The researchers then identify specific Raman bands that are attributed to N-H and C-H stretching vibrations and analysis including principal component analysis reveals a characteristic reduction of intensity in these bands in the presence of the spike protein. The results show that the applied SERS technique allows detection down to subfemtomolar concentrations. Moreover this method has high speed and sensitivity that make it a good building block for the development of rapid diagnostic instruments. Future public health challenge might see techniques like SERS applied on a large scale allowing for faster and better informed decisions to prevent large scale viral outbreaks.

Further Information

Real-Space and Fourier Imaging and Spectroscopy of NIR Emission from SWCNTsCustomer Stories

Dr. Joanna Dziadkowiec, Dr. Anja Røyne

University of Oslo

Background

Although we perceive geological processes on macroscopic length scales, the mechanical behavior of geological structures can be significantly influenced by the microscopic mineral structure of rocks as well as the micro-scale interactions at the contacting mineral surfaces. Microscopic and nanoscale spaces between mineral grains often contain fluids and water that can reactively erode or deposit material, e.g. by promoting crystallization processes. Overall, these reactive dissolution and growth processes may have consolidating or destabilizing influences. These can further express in macroscopic, observable changes over time. A better understanding of these processes is important for predicting the behavior of geological structures both by natural development, but also due to human impact, for example, due to large-scale oil drilling or potential capture of carbon dioxide in geological structures.

Joanna Dziadkowiec and Anja Røyne performed research at the Njord Center at the University of Oslo trying to understand what happens at contacting mineral surfaces when they are pushed together and exposed to influences of fluids and water. Over time, the solution- and pressure-induced changes in the surface structure alter how the reactive, contacting surfaces interact with each other. This, in turn, affects the overall mechanical behaviour of granular rocks. For their experiments, the researchers use a surface force apparatus (SFA), a measurement device, which determines the interaction force between 2 solid surfaces. Unlike atomic force microscopy, where surfaces are probed point-wise on a nm scale, the SFA technique samples a much larger contact area of up to 100 μm diameter. This scale is more suitable and relevant to relate their observations to geologically relevant processes. For complete characterization of the interaction dynamics, the researchers also measure the distance between the surfaces (imagine a spring where the spring constant is characterized by the applied force and the displacement from the relaxed position). The nm-sized space between the surfaces is measured using multi-beam interferometry where the sample surfaces sit between 2 highly reflective substrates which are illuminated by a white light source. The transmitted signal is imaged onto the entrance slit of a spectrograph and dispersed to visualize fringes of equal chromatic order (FECO fringes). For example, Figure 1 shows the FECO fringes of 2 surfaces in contact with van der Waals forces acting between them. Fringes detected at different horizontal positions in the image correspond to a different wavelength, whereas different vertical positions correspond to different points on the sample surface.

The FECO interference technique allows the researchers to measure the distance profile between the 2 mineral surfaces and detect reactive changes that alter the nano- and micron-scale surface forces and separation distances. By observing the behavior of the interference patterns over time, the researchers are able to investigate reactive changes in the presence of fluids. Figure 2 shows how the interference pattern is modified by the presence of a capillary water bridge between two surfaces placed in close proximity: the capillary bridge is indicated by the discontinuity in the pattern and disappears once water is injected.

Challenge

The spectroscopy system used for multibeam FECO interference in the SFA requires high spectral accuracy and resolution as well as good imaging capability. The changes in the surface profiles of the mineral sample can be in the nm range so a spectral resolution in a nm or sub-nm range is needed to accurately determine fringe positions.

The interference patterns are observed by dispersing the signal along the spectrometer’s entrance slit into a hyperspectral interference image on the system camera, where the horizontal positions, which correspond to wavelength and the vertical position, can be mapped onto different points within the relatively large sample region of interest. The good imaging quality of the spectrograph ensures that the signal is undistorted at all points on the camera. Spectrographs that are optimized for the detection of single spectral channels often show severe image aberrations outside of the focal plane center, leading to distortions, signal reduction, and loss of measurement precision.

“The low aberrations of the IsoPlane system allow us to make very precise measurements of 2D spectral interference patterns. In combination with LightField and Intellical, this system is very efficient to setup for challenging scientific measurements, such as imaging of reactive mineral surfaces in confinement”

Solution

The multi-beam interference setup at the Njord center uses an IsoPlane-SCT320 spectrograph coupled with a PIXIS-2KBUV camera for hyperspectral detection of FECO fringes.

The IsoPlane is an advanced imaging spectrograph with aberration-corrected optics, significantly reducing image aberrations such as astigmatism that could distort the spectral image. The good image quality at any point of the focal plane also increases resolution due to better focusing of the signal light onto the camera. The IsoPlane is ideally suited for multi-channel and hyperspectral measurements requiring this high spectral and spatial resolution.

The PIXIS-2KBUV has a large 2048×512 pixel sensor, covering a wide area in the focal plane, so a wider profile of sample positions can be measured simultaneously. The PIXIS is a deep-cooled scientific camera with very low noise that is optimized for precise, quantitative signal detection.

As the researchers work frequently at different spectral ranges they also use an automated system for accurate wavelength calibration called Intellical. This system, which is controlled by Princeton Instruments LightField software, makes sure that spectral calibration can be done quickly, with very high precision (up to 10x higher than other common calibration methods).

The researchers also have to monitor the interactions between the sample surfaces for long times to be able to detect the reactive changes. Using a time-lapse feature in LightField, the experiments can be easily setup to automatically acquire images for long times with arbitrary time intervals between them, so the acquisition can be optimized considering the speed of the surface evolution.

Transition Metal Dichalcogenides for Energy Momentum MeasurementsResearch Stories

Tian Jiang

Laser & Photonics Reviews

Ultrafast Response of a Hybrid Device Based on Strongly Coupled Monolayer WS2 and Photonic Crystals: The Effect of Photoinduced Coulombic Screening

Introduction

Transition Metal Dichalcogenides are semiconductor materials that build single atom thin layers. They are heavily researched two dimensional materials and have been discovered to be useful for nanophotonic applications either on their own or combined in stacks with other 2D materials (so called van-der-Waals heterostructures). Many experiments investigate their behavior in combination with plasmonic and cavity nanostructures. Such devices allow investigation of interesting physics and are building blocks for components of photonic and quantum devices.

A new publication by a research collaboration around Prof. Tian Jiang and Prof. Lei Shi in China is reporting on their experiments combining monolayers of Tungsten diselenide with photonic crystal structures. Such nanostructures have a strong influence on energy as well as direction of light emission

Energy-momentum spectroscopy is a perfect tool to observe the properties of these devices. This technique is equivalent to Fourier plane imaging in microscopy where instead of the direct image of the sample, the backfocal plane of the objective is observed revealing the direction of light emission. If this image is projected on the entrance slit plane of a spectrograph the relationship between light wavelength and its momentum can be directly observed. In the reported experiment the energy-momentum spectroscopy system is built around an Isoplane-320 spectrograph combined with a PIXIS 400 camera. Aberration corrected systems are perfectly suited for these measurements as important information is observed across the whole image sensor.

The researchers report that their energy momentum measurements reveal strong coupling in their device which allows them to study fundamental physics as well as show the potential of this material for applications in nanophotonic devices in the future.

Further Information