Over the past decade, the proliferation of pulsed laser sources with high repetition rates has facilitated a merger of ultrafast time-resolved spectroscopy with imaging microscopy. In transient absorption microscopy (TAM), the excited-state dynamics of a system are tracked by measuring changes in the transmission of a focused probe pulse following photoexcitation of a sample. Typically, these experiments are done using a photodiode detector and lock-in amplifier synchronized with the laser and images highlighting spatial heterogeneity in the TAM signal are constructed by scanning the probe across a sample. Performing TAM by instead imaging a spatially defocused widefield probe with a multipixel camera could dramatically accelerate the acquisition of spatially resolved dynamics, yet approaches for such widefield imaging generally suffer from reduced signal-to-noise due to an incompatibility of multipixel cameras with high-frequency lock-in detection. Herein, we describe implementation of a camera capable of high-frequency lock-in detection, thereby enabling widefield TAM imaging at rates matching those of high repetition rate lasers. Transient images using a widefield probe and two separate pump pulse configurations are highlighted. In the first, a widefield probe was used to image changes in the spatial distribution of photoexcited molecules prepared by a tightly focused pump pulse, while in the second, a widefield probe detected spatial variations in photoexcited dynamics within a heterogeneous organic crystal excited by a defocused pump pulse. These results highlight the ability of high-sensitivity lock-in detection to enable widefield TAM imaging, which can be leveraged to further our understanding of excited-state dynamics and excitation transport within spatially heterogeneous systems.

pubs.acs.org/doi/abs/10.1021/acs.jpcc.4c02984

The quantum diamond microscope (QDM) is a recently developed technology for near-field imaging of magnetic fields with micron-scale spatial resolution. In the present work, we integrate a QDM with a narrowband measurement protocol and a lock-in camera; and demonstrate imaging of radiofrequency (RF) magnetic field patterns produced by microcoils, with spectral resolution ≈1\,Hz. This RF-QDM provides multi-frequency imaging with a central detection frequency that is easily tunable over the MHz-scale, allowing spatial discrimination of both crowded spectral peaks and spectrally well-separated signals. The present instrument has spatial resolution ≈2μm, field-of-view ≈300×300μm², and per-pixel sensitivity to narrowband fields ∼1nT⋅Hz^−1/2. Spatial noise can be reduced to the picotesla scale by signal averaging and/or spatial binning. The RF-QDM enables simultaneous imaging of the amplitude, frequency, and phase of narrowband magnetic field patterns at the micron-scale, with potential applications in real-space NMR imaging, AC susceptibility mapping, impedance tomography, analysis of electronic circuits, and spatial eddy-current-based inspection.

arXiv:2406.15450

Time-resolved spectroscopy of plasmonic nanoparticles is a vital technique for probing their ultrafast electron dynamics and subsequent acoustic and photothermal properties. Traditionally, these experiments are performed with spectrally broad probe beams on the ensemble level to achieve high signal amplitudes. However, the relaxation dynamics of plasmonic nanoparticles is highly dependent on their size, shape, and crystallinity. As such, the inherent heterogeneity of most nanoparticle samples can complicate efforts to build microscopic models for these dynamics solely on the basis of ensemble measurements. Although approaches for collecting time-resolved microscopy signals from individual nanoparticles at selected probe wavelengths have been demonstrated, acquiring time-resolved spectra from single objects remains challenging. Here, we demonstrate an alternate method that efficiently yields the time-resolved spectra of a single gold nanodisk in one measurement. By modulating the frequency-doubled output of a 96 MHz Ti:sapphire oscillator at 8 kHz, we are able to use a lock-in pixel-array camera to detect photoinduced changes in the transmission of a white light continuum probe derived from a photonic crystal fiber to produce broadband femtosecond transmission spectra of a single gold nanodisk. We also compare the performance of the lock-in camera for the same single nanoparticle to measurements with a single-element photodiode and find comparable sensitivities. The lock-in camera thus provides a major advantage due to its ability to multiplex spectral detection, which we utilize here to capture both the electronic dynamics and acoustic vibrations of a single gold nanodisk following ultrafast laser excitation.

pubs.acs.org/doi/abs/10.1021/acs.jpcc.4c01814

Nitrogen-vacancy (NV–) centers in nanodiamonds have emerged as a versatile platform for a wide range of applications, including bioimaging, photonics, and quantum sensing. However, the widespread adoption of nanodiamonds in practical applications has been hindered by the challenges associated with patterning them into high-resolution features with sufficient throughput. In this work, we overcome these limitations by introducing a direct laser-writing bubble printing technique that enables the precise fabrication of two-dimensional nanodiamond patterns. The printed nanodiamonds exhibit a high packing density and strong photoluminescence emission, as well as robust optically detected magnetic resonance (ODMR) signals. We further harness the spatially resolved ODMR of the nanodiamond patterns to demonstrate the mapping of two-dimensional temperature gradients using high frame rate widefield lock-in fluorescence imaging. This capability paves the way for integrating nanodiamond-based quantum sensors into practical devices and systems, opening new possibilities for applications involving high-resolution thermal imaging and biosensing.

acs.nanolett.4c02519

This thesis documents research done in the development and the exploration of feasibility for a high-throughput method to measure local thermal properties. The present capabilities in the measurement of local thermophysical properties such as thermal conductivity, thermal diffusivity, and Kapitza resistance are very inefficient and impractical to fully understand and characterize heat transport through certain materials and features. This work follows up on past work in local thermal property measurement via the spatial domain thermoreflectance (SDTR) method, and explores the possibility of parallelizing the process. The parallelized SDTR (P-SDTR) method involves using laser projector sources to periodically heat and measure the changes of reflectivity of a sample surface at multiple locations simultaneously. These measurements are made possible by the development of a lock-in camera that can measure the characteristics of modulated light using lock-in amplification at several spots across an area with an advanced camera sensor. This method allows for the measurement of local thermal properties across features such as grain boundaries, or directional properties in anisotropic materials. An experimental setup is developed to determine at which heating and probing parameters a thermoreflectance signal can be measured. A finite element model is also made to simulate the P-SDTR process, and validate that the assumptions made in SDTR can be made in P-SDTR measurements. It is shown that at an appropriate separation of heating/measurement locations, the solutions from the simulation approach that of a single measurement spot. An initial device design is proposed and tested. Future work in the development of the P-SDTR device is also laid out.

hdl.lib.byu.edu/1877/etd13238

AC susceptometry, unlike static susceptometry, offers a deeper insight into magnetic materials. By employing AC susceptibility measurements, one can glean into crucial details regarding magnetic dynamics. Nevertheless, traditional AC susceptometers are constrained to measuring changes in magnetic moments within the range of a few nano-joules per tesla. Additionally, their spatial resolution is severely limited, confining their application to bulk samples only. In this study, we introduce the utilization of a Nitrogen Vacancy (NV) center-based quantum diamond microscope for mapping the AC susceptibility of micron-scale ferromagnetic specimens. By employing coherent pulse sequences, we extract both magnitude and the phase of the field from samples within a field of view spanning 70 micro-meters while achieving a resolution of 1 micro-meter. Furthermore, we quantify changes in dipole moment on the order of a femto-joules per tesla induced by excitations at frequencies reaching several hundred kilohertz.

arxiv.org/abs/2309.06587

Wide-field imaging of magnetic signals using ensembles of nitrogen-vacancy (NV) centers in diamond has garnered increasing interest due to its combination of micron-scale resolution, millimeter-scale field of view, and compatibility with diverse samples from across the physical and life sciences. Recently, wide-field NV magnetic imaging based on the Ramsey protocol has achieved uniform and enhanced sensitivity compared to conventional measurements. Here, we integrate the Ramsey-based protocol with spin-bath driving to extend the NV spin dephasing time and improve magnetic sensitivity. We also employ a high-speed camera to enable dynamic wide-field magnetic imaging. We benchmark the utility of this quantum diamond microscope (QDM) by imaging magnetic fields produced from a fabricated wire phantom. Over a 270×270μm2 field of view, a median per-pixel magnetic sensitivity of 4.1(1)nT/√ Hz is realized with a spatial resolution ≲10μm and sub-millisecond temporal resolution. Importantly, the spatial magnetic noise floor can be reduced to the picotesla scale by time-averaging and signal modulation, which enables imaging of a magnetic-field pattern with a peak-to-peak amplitude difference of about 300pT. Finally, we discuss potential new applications of this dynamic QDM in studying biomineralization and electrically-active cells.

arxiv.org/abs/2309.06587

Multiple scattering poses a fundamental limitation in deep imaging, especially for high-resolution optical imaging methods. The amalgamation of reflection matrix measurements and optical coherence tomography (OCT) has afforded significant advantages for deep imaging through highly scattering media. To empirically exhibit the superior performance of reflection matrix OCT (RMOCT), this study proposes a unique method to ascertain the actual resolutions at each imaging point. In contrast to conventional theoretical lateral resolutions, these resolutions are derived by applying time-reversal decomposition to the time-gated reflection matrix. Moreover, the concept of contribution rate, which quantifies the imaging contributions for each point, is introduced by considering the local imaging point itself and its neighboring points. The contribution rate provides a quantitative evaluation of the imaging quality afforded by a system. To the best of our knowledge, this study represents the comprehensive assessment of the practical performance of RMOCT in terms of actual resolving power and imaging quality.

doi.org/10.1063/5.0154739

Quantum diamond magnetometers using lock-in detection have successfully detected weak bio-magnetic fields from neurons, a live mammalian muscle, and a live mouse heart. This opens up the possibility of quantum diamond magnetometers visualizing microscopic distributions of the bio-magnetic fields. Here, we demonstrate a lock-in-based wide-field quantum diamond microscopy, achieving a mean volume-normalized per pixel sensitivity of 43.9 nT⋅μm^1.5/Hz^0.5. We obtain the sensitivity by implementing a double resonance with hyperfine driving and magnetic field alignment along the <001> orientation of the diamond. Additionally, we have demonstrated that sub-ms temporal resolution (∼ 0.4 ms) can be achieved at a micrometer scale with tens of nanotesla per-pixel sensitivity using quantum diamond microscopy. This lock-in-based diamond quantum microscopy could be a step forward in mapping functional activity in neuronal networks in micrometer spatial resolution.

doi.org/10.1063/5.0142448

The interest in quantum technology has increased over the last few decades. Quantum processes have enabled measurements of electric and magnetic fields, temperature etc. with previously unheard-of precision and spatial resolution. Negatively charged Nitrogen vacancy centers (NV– centers) in diamond are an emerging example of such a quantum sensor system. In this article, I will discuss the developments and uses of sensing using “NV– centers” for applications in biology, investigating quantum materials, and developing a platform for teaching quantum technologies in university laboratories using these room-temperature operable systems.

PHYSICS NEWS

We present a technique for determining the micro-scale AC susceptibility of magnetic materials. We use the magnetic field sensing properties of nitrogen-vacancy (NV⁻) centers in diamond to gather quantitative data about the magnetic state of the magnetic material under investigation. A quantum diamond microscope with an integrated lock-in camera is used to perform pixel-by-pixel, lock-in detection of NV⁻ photo-luminescence for high-speed magnetic field imaging. In addition, a secondary sensor is employed to isolate the effect of the excitation field from fields arising from magnetic structures on NV⁻ centers. We demonstrate our experimental technique by measuring the AC susceptibility of soft permalloy micro-magnets at excitation frequencies of up to 20 Hz with a spatial resolution of 1.2 µm and a field of view of 100 µm. Our work paves the way for microscopic measurement of AC susceptibilities of magnetic materials relevant to physical, biological, and material sciences.

doi.org/10.1063/5.0138301 or PDF (preprint)

Shifted Excitation Raman Difference Spectroscopy (SERDS) is a non-destructive chemical analysis method capable of removing the fluorescence background and other disturbances from the Raman spectrum, thanks to the independence of the fluorescence with respect to the small difference in excitation wavelength. The spectrum difference is computed in a post-processing step. Here, we demonstrate the use of a lock-in camera to obtain an on-line analog SERDS spectra allowing longer exposure times and no saturation, leading to an improved Signal-to-Noise Ratio (SNR) and reduced data storage. Two configurations are presented: the first one uses a single laser and can remove excitation-independent disturbances, such as ambient light; the second employs two-wavelength shifted sources and removes fluorescence background similarly to SERDS. In both cases, we experimentally extrapolate the expected SNR improvement.

doi.org

Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in it’s current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This experimental technique allows magnetic field imaging of sub-second varying microscale currents in planar microcoils with imaging frame rates in the range of 50–200 frames per s (fps). Our work demonstrates that widefield per-pixel lock-in detection of frequency modulated NV ODMR enables dynamic magnetic field microscopy.

Multiple scattering inside the random medium limits the imaging depth of optical coherence tomography (OCT) to 1–2 mm, as well as the degree of focus at the deep imaging depth. In this paper, by combining the concept of matrix measurement with a wide-field optical coherence tomography, we have done two aspects of work. The first one is for deeper imaging depth. By reconstructing the huge reflection matrix of the sample and then applying a time-reversal operation to it, we successfully filter out the single scattered light for imaging at the depth of 15 times of the scattering mean free path (SMFP). Since the imaging depth of conventional OCT is 6–7 times of the SMFP, our proposed reflection matrix optical coherence tomography (RMOCT) is about one time deeper than the conventional OCT. The second part of the work is a high-speed wavefront shaping (WFS) method based on a one-time in-and-out complex light field analysis. With the help of a phase-only spatial light modulator, we realize the light focusing through a random medium is ~113 ms. It is about three times faster than the iterative feedback wavefront shaping method. We believe that our work might pave the way to apply WFS to optical imaging methods and open new methods toward deeper imaging through a scattering medium.

doi.org/10.1007/978-981-16-7258-3_32

PDF

In a prior research work, published in Communication Physics 2020 28, we have proposed a novel algorithm to reconstruct 3D magnetic field activity produced by action potentials AP of mammalian neurons located in a cortical volume. We have simulated the expected 2D microscale magnetic field patterns measured by diamond NV microscopy. In fundamental work by Roth et. al 29, it has been shown that 3D current source reconstruction is a non-unique inverse problem, unless additionally constrained with prior source information. Our analyses have shown that the axon hillock segment in the neuron provides a unique dominant signature to resolve neuronal activity in 3D with sufficient accuracy. Since AP magnetic fields are millisecond scale phenomena, our dynamic magnetic microscopy setup may enable us to probe novel AP associated magnetic fields at microscope resolution. In Ref. 30, we demonstrated for the first time a wide-field magnetic field microscope capable of probing dynamically varying microscale magnetic field features at tunable imaging frame rates of 50-200 frames per second.

Defense Technical Information Center

Microscopic visualisation of optically transparent samples has been a topic of interest for several decades. Features such as density or chemical composition can influence the optical phase of transmitted light, and phase contrast can reveal these structures. Several methods of phase contrast have been developed, which can be categorised as either interferometric or non-interferometric, based on the type of coherence properties of the light used. In this work, I focus on incoherent based phase contrast, in particular Differential Phase Contrast (DPC). The choice of incoherent light brings benefits such as the absence of distortions like speckle patterns and ringing patterns, increased spatial resolution, and a simpler setup that can be used for in-vivo applications. Moreover, DPC allows reconstruction of quantitative phase maps of the samples. On the other hand, coherent-based techniques demonstrate superior performance in terms of phase sensitivity. The first part of this thesis offers a quantitative analysis of the phase sensitivity of DPC and investigates the influence of optical parameters and sample characteristics. With simulations and experiments, a relation between numerical aperture and phase sensitivity is demonstrated, and the concept of spectral matching is introduced to enhance the contrast. The methods can be generalised to any DPC setup, and allow a-priori investigation of the sensitivity of a DPC microscope at the design stage rather than through testing. Comparison between the best sensitivity that can be achieved in DPC and state-of-the-art interferometric techniques, shows that it is not possible to reach comparable single-shot performances. In this thesis, it is shown that the reason for this limitation is the strong background in DPC images, which degrades the dynamic range. DPC images are obtained with mirrored illuminations, for which the background is identical and the phase term switches in sign. The difference between these pairs of images is computed digitally, which does not improve the limited dynamic range. Lock-in DPC is proposed as a solution: instead of sampling different illumination states, lock-in DPC demodulates the phase signal when the illumination is switched, and the background is never encoded. This is enabled by periodical switching of sources coupled with a smart pixel detector, the so-called “lock-in camera”. Part of this work is dedicated to the theoretical description of this method, and the analysis of the expected benefit. Experiments are also described, that demonstrate a factor of 8 improvement in single-shot sensitivity compared to standard DPC. DPC is not the only imaging technique to suffer from high intensity background: it is easy to see how the use of a lock-in camera for differential imaging can be generalised to any situation where weak modulations can be induced over a strong background. Here, an example is presented with lock-in Shifted Excitation Raman Difference Spectroscopy (SERDS). SERDS is an established Raman spectroscopy technique that takes advantage of the Raman emission spectrum being relative to the excitation wavelength to remove unwanted fluorescence emission. Two spectra with shifted excitation wavelengths are measured, their difference is computed, and the fluorescence is thus digitally removed. The parallel with DPC is immediately apparent. Both simulations and experiments are used to demonstrate the advantage of analog demodulation.

Crystal-strain variation imposes significant limitations on many quantum sensing and information applications for solid-state defect qubits in diamond. Thus, the precision measurement and control of diamond crystal strain is a key challenge. Here, we report diamond strain measurements with a unique set of capabilities, including micron-scale spatial resolution, a millimeter-scale field of view, and a 2-order-of-magnitude improvement in volume-normalized sensitivity over previous work, reaching 5(2)×10⁻⁸(Hz μm^-3)^1/2 (with spin-strain coupling coefficients representing the dominant systematic uncertainty). We use strain-sensitive spin-state interferometry on ensembles of nitrogen-vacancy (N-V) color centers in single-crystal bulk diamond with low strain gradients. This quantum interferometry technique provides insensitivity to magnetic-field inhomogeneity from the electronic and nuclear spin bath, thereby enabling long N-V–ensemble electronic spin dephasing times and enhanced strain sensitivity, as well as broadening the potential applications of the technique beyond isotopically enriched or high-purity diamond. We demonstrate the strain-sensitive measurement protocol first on a confocal scanning laser microscope, providing quantitative measurement of sensitivity as well as three-dimensional strain mapping; and second on a wide-field-imaging quantum diamond microscope. Our strain-microscopy technique enables fast, sensitive characterization for diamond material engineering and nanofabrication; as well as diamond-based sensing of strains applied externally, as in diamond anvil cells or embedded diamond stress sensors, or internally, as by crystal damage due to particle-induced nuclear recoils.

The ability to measure the passage of electrical current with high spatial and temporal resolution is vital for applications ranging from inspection of microscopic electronic circuits to biosensing. The ability to image such signals passively and remotely is of great importance, in order to measure without invasive disruption of the system under study or the signal itself. A recent approach to achieving this utilizes point defects in solid-state materials; in particular, nitrogen-vacancy centers in diamond. Acting as a high-density array of independent sensors, addressable opto-electronically and highly sensitive to factors including temperature and magnetic field, these are ideally suited to microscopic wide-field imaging. In this work, we demonstrate simultaneous spatially and temporally resolved recovery signals from a microscopic lithographically patterned circuit. Through application of a lock-in amplifier camera, we demonstrate micrometer-scale imaging resolution with a millimeter-scale field of view with simultaneous spatially resolved submillisecond (up to 3500 frames s⁻¹) recovery of dc to kilohertz alternating and broadband pulsed-current electrical signals, without aliasing or undersampling. We demonstrate as examples of our method the recovery of synthetic signals replicating digital pulses in integrated circuits and signals that would be observed in a biological neuronal network in the brain.

doi.org

Supplementing Material

We introduce a lock-in method to increase the phase contrast in incoherent differential phase contrast (DPC)
Imaging. This method improves the phase sensitivity by the analog removal of the background. The use of a
smart pixel detector with in-pixel signal demodulation, paired with synchronized switching illumination, provides
the basis of a bit-efficient approach to emulate a lock-in DPC. The experiments show an increased sensitivity by a
factor of up to 8, as expected from theory, and a reduction of collected data by a factor of 70, for equivalent standard
DPC measurements; single-shot sensitivity of 0.7 mrad at a frame rate of 1400 frames per second is demonstrated.
This new approach may open the way for the use of incoherent phase microscopy in biological applications where
extreme phase sensitivity and millisecond response time are required.

doi.org

Multiple light scattering is considered as the major limitation for deep imaging and focusing in turbid media. In this paper, we present an innovative method to overcome this limitation and enhance the delivery of light energy ultra-deep into turbid media with significant improvement in focusing. Our method is based on a wide-field reflection matrix optical coherence tomography (RM-OCT). The time-reversal decomposition of the RM is calibrated with the Tikhonov regularization parameter in order to get more accurate reversal results deep inside the scattering sample. We propose a concept named model energy matrix, which provides a direct mapping of light energy distribution inside the scattering sample. To the best of our knowledge, it is the first time that a method to measure and quantify the distribution of beam intensity inside a scattering sample is demonstrated. By employing the inversion of RM to find the matched wavefront and shaping with a phase-only spatial light modulator, we succeeded in both focusing a beam deep (~9.6 times of scattering mean free path, SMFP) inside the sample and increasing the delivery of light energy by an order of magnitude at an ultra-deep (~14.4 SMFP) position. This technique provides a powerful tool to understand the propagation of photon in a scattering medium and opens a new way to focus light inside biological tissues.

www.nature.com/articles/s41377-022-00795-8

State-of-the-art time-of-flight (ToF) based 3D sensors suffer from poor lateral and depth resolutions. In this work, we introduce a novel sensor concept that provides ToF-based 3D measurements of real world objects with depth precisions up to 35μm and point cloud densities
at the native sensor-resolutions of state-of-the-art CMOS/CCD cameras (up to several megapixels). Unlike other continuous-wave amplitude-modulated ToF principles, our approach exploits
wavelength diversity for an interferometric surface measurement of macroscopic objects with
rough or specular surfaces. Based on this principle, we introduce three different embodiments
of prototype sensors, exploiting three different sensor architectures.

arxiv.org

We develop a new type of high-speed wavefront determination method with single feedback measurement to focus light through a 15.2 scattering mean free path in ∼113 ms. Our method is based on a heterodyne-detection phase sensitivity interferometer. First, the matrix which describes the light propagation process in the sample is measured by single input and output optical fields’ analysis. Then, by using a spatial light modulator to reshape the incident light with a matched wavefront, a focused beam is observed behind the sample. The proposed high-speed light focusing method will open new spot scanning mode toward deeper imaging through highly scattering biological tissues.

doi.org

State-of-the-art time-of-flight (ToF) based 3D sensors suffer from poor lateral and depth resolutions. In this work, we introduce a novel sensor concept that provides ToF-based 3D measurements of real world objects with depth precisions up to 35µm and point cloud densities at the native sensor-resolutions of state-of-the-art CMOS/CCD cameras (up to several megapixels). Unlike other continuous-wave amplitude-modulated ToF principles, our approach exploits wavelength diversity for an interferometric surface measurement of macroscopic objects with rough or specular surfaces. Based on this principle, we introduce three different embodiments of prototype sensors, exploiting three different sensor architectures.

arxiv.org

We introduce a double quantum (DQ) 4-Ramsey measurement protocol that enables wide-field magnetic imaging using nitrogen-vacancy (N-V) centers in diamond, with enhanced homogeneity of the magnetic sensitivity relative to conventional single quantum (SQ) techniques. The DQ 4-Ramsey protocol employs microwave-phase alternation across four consecutive Ramsey (4-Ramsey) measurements to isolate the desired DQ magnetic signal from any residual SQ signal induced by microwave pulse errors. In a demonstration experiment employing a 1-μm-thick N-V layer in a macroscopic diamond chip, the DQ 4-Ramsey protocol provides a volume-normalized dc magnetic sensitivity of η^V = 34 nT Hz ^−1/2 μm ^3/2 across a 125 μm × 125 μm
field of view, with about 5 × less spatial variation in sensitivity across the field of view compared to a SQ measurement. The improved robustness and magnetic sensitivity homogeneity of the DQ 4-Ramsey protocol enables imaging of dynamic broadband magnetic sources such as integrated circuits and electrically active cells.

aps.org

We develop a new type of high-speed wavefront determination method with single feedback measurement to focus light through a 15.2 scattering mean free path in ∼113 ms. Our method is based on a heterodyne-detection phase sensitivity interferometer. First, the matrix which describes the light propagation process in the sample is measured by single input and output optical fields’ analysis. Then, by using a spatial light modulator to reshape the incident light with a matched wavefront, a focused beam is observed behind the sample. The proposed high-speed light focusing method will open new spot scanning mode toward deeper imaging through highly scattering biological tissues.

APL Photonics: Vol 6, No 3 (scitation.org)

The ability to measure the passage of electrical current with high spatial and temporal resolution is vital for applications ranging from inspection of microscopic electronic circuits to biosensing. Being able to image such signals passively and remotely at the same time is of high importance, to measure without invasive disruption of the system under study or the signal itself. A new approach to achieve this utilises point defects in solid state materials, in particular nitrogen vacancy (NV) centres in diamond. Acting as a high density array of independent sensors, addressable opto-electronically and highly sensitive to factors including temperature and magnetic field, these are ideally suited to microscopic widefield imaging. In this work we demonstrate such imaging of signals from a microscopic lithographically patterned circuit at the micrometer scale. Using a new type of lock-in amplifier camera, we demonstrate sub-millisecond (up to 3500 frames-per-second) spatially resolved recovery of AC and pulsed electrical current signals, without aliasing or undersampling. Finally, we demonstrate as a proof of principle the recovery of synthetic signals replicating the exact form of signals in a biological neural network: the hippocampus of a mouse.

arxiv.org

Wide field-of-view magnetic field microscopy has been realized by probing shifts in optically detected magnetic resonance (ODMR) spectrum of nitrogen-vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require a few to several minutes of acquisition to get a single magnetic field image, rendering the technique as temporally static in its current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency-modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This modified diamond NV imaging allows up to ten thousand pixels to simultaneously track an applied magnetic field waveform of sub-second temporal variation at imaging frame rates of 50-200 Hz. Our work demonstrates that widefield per-pixel lock-in detection, in combination with frequency-modulated NV ODMR, enables millisecond scale dynamic magnetic field microscopy.

arxiv.org

Crystal strain variation imposes significant limitations on many quantum sensing and information applications for solid-state defect qubits in diamond. Thus, precision measurement and control of diamond crystal strain is a key challenge. Here, we report diamond strain measurements with a unique set of capabilities, including micron-scale spatial resolution, millimeter-scale field-of-view, and a two order-of-magnitude improvement in volume-normalized sensitivity over previous work [1], reaching 5(2)×10−8/sprt(Hz⋅μm3) (with spin-strain coupling coefficients representing the dominant systematic uncertainty). We use strain-sensitive spin-state interferometry on ensembles of nitrogen vacancy (NV) color centers in single-crystal CVD bulk diamond with low strain gradients. This quantum interferometry technique provides insensitivity to magnetic-field inhomogeneity from the electronic and nuclear spin bath, thereby enabling long NV ensemble electronic spin dephasing times and enhanced strain sensitivity. We demonstrate the strain-sensitive measurement protocol first on a scanning confocal laser microscope, providing quantitative measurement of sensitivity as well as three-dimensional strain mapping; and second on a wide-field imaging quantum diamond microscope (QDM). Our strain microscopy technique enables fast, sensitive characterization for diamond material engineering and nanofabrication; as well as diamond-based sensing of strains applied externally, as in diamond anvil cells or embedded diamond stress sensors, or internally, as by crystal damage due to particle-induced nuclear recoils.

arxiv.org

Multiple light scattering in biomedical tissue limits the penetration depth of optical imaging systems such as optical coherence tomography. To increase the imaging depth in scattering media, a computational method based on coherent reflection matrix measurement has been developed using low coherence interferometry. The complex reflection matrix is obtained via point-by-point scanning followed by a phase-shifting method; then singular value decomposition is used to retrieve the singly back-scattered light. However, the in vivo application of the current reported method is limited due to the slow acquisition speed of the matrix. In this Letter, a wide-field heterodyne-detection method is adopted to speed up the complex matrix measurement at a deep tissue layer. Compared to the phase-shifting method, the heterodyne-detection scheme retrieves depth-resolved complex amplitudes faster and is more stable without mechanical movement of the reference mirror. As a result, the matrix measurement speed is increased by more than one order of magnitude.

osapublishing.org

The presence of a scattering medium in the imaging path between an object and an observer is known to severely limit the visual acuity of the imaging system. We present an approach to circumvent the deleterious effects of scattering, by exploiting spectral correlations in scattered wavefronts. Our Synthetic Wavelength Holography (SWH) method is able to re cover a holographic representation of hidden targets with high resolution over a wide field of view. The complete object field is recorded in a snapshot-fashion, by monitoring the scattered light return in a small probe area. This unique combination of attributes opens up a plethora of new Non-Line-of-Sight imaging applications ranging from medical imaging and forensics, to
13 early-warning navigation systems and reconnaissance. Adapting the findings of this work to other wave phenomena will help unlock a wider gamut of applications beyond those envisioned in this paper.

researchsquare.com

We present a novel technique for Non-Line-of-Sight imaging that borrows ideas from Multi-Wavelength Interferometry and Remote Digital Holography. Our method reaches a resolution of a few mm, which by far surpasses the resolution of conventional methods.

osapublishing.org

Conventional line-of-sight imaging techniques rely on detecting light paths bouncing from the object and reaching directly to the detector. Absence of any such direct light paths from object to detector results in a failure to recover any useful information using conventional techniques. The absence of direct light paths from object to detector can be observed in several real-world scenarios such as looking around a corner, imaging through turbid media, imaging through tissue etc.

The focus of this thesis is pertaining to the problem of looking around corners (or) imaging object hidden from line of sight at macroscopic scales. This thesis focuses on adapting heterodyne interferometry to circumvent the radiometry losses due to scattering and thereby enabling its use in more challenging practical scenarios. Objects hidden around a corner were reconstructed with 500 µm resolution at 0.8 meters standoff. Using heterodyne interferometry and lock-in detection techniques, the hologram of the hidden object could be obtained even under significant radiometry losses without any power matching. Also discussed is the estimation of rapidly varying and slowly varying motion of objects around a corner using doppler shifts and speckle correlations respectively.

smu.edu

Dynamic interferometry enables snapshot recovery of phase images by using polarization phase shifting. However, the phase estimate is susceptible to influence from sources of ambient light having uncontrolled polarization. We present a novel method, dynamic heterodyne interferometry (DHI), as a means to mitigate phase bias from ambient light sources, while retaining dynamic potential.

IEEE Conference Publication | IEEE Xplore

The OMNISCIENT research effort seeks to tackle the challenge of indirect imaging through the use of active
illumination. The basic framework leverages the concept of virtual sources of illumination and virtual
detectors, which represent scattering surfaces that are in direct view of both the hidden object and the indirect
Imaging System. The approach exploits the intrinsic roughness of scattering surfaces to facilitate the indirect
illumination of objects hidden from view and intercept the light reflected by the hidden objects. The effort is
focused on the design, development and integration of two distinct pathways to recovering latent scene
information.

researchgate.net

Sensitive, real-time optical magnetometry with nitrogen-vacancy centers in diamond relies on accurate imaging of small (≪10−2), fractional fluorescence changes across the diamond sample. We discuss the limitations on magnetic field sensitivity resulting from the limited number of photoelectrons that a camera can record in a given time. Several types of camera sensors are analyzed, and the smallest measurable magnetic field change is estimated for each type. We show that most common sensors are of a limited use in such applications, while certain highly specific cameras allow achieving nanotesla-level sensitivity in 1 s of a combined exposure. Finally, we demonstrate the results obtained with a lock-in camera that paves the way for real-time, wide-field magnetometry at the nanotesla level and with a micrometer resolution.

scitation.org

Structured illumination has been utilized to super-resolve microscopic objects and provide topographic information in computer vision applications. Motivated by the achievements in these fields and leveraging techniques found in astronomical sparse aperture systems, an approach is developed to super-resolve macroscopic objects in typical real world scenarios. The challenges of super-resolving uncontrolled 3D environments are addressed. An approach is presented which enables the collection of 3D topographic information while super-resolving. These techniques use incoherent illumination to resolve spatial detail in an intensity image. For indirect imaging scenarios, this approach is adapted with structured coherent illumination to super-resolve phase at a distance.

spiedigitallibrary.org

Sensitive, real-time optical magnetometry with nitrogen-vacancy centers in diamond relies on accurate imaging of small (≪10−2) fractional fluorescence changes across the diamond sample. We discuss the limitations on magnetic-field sensitivity resulting from the limited number of photoelectrons that a camera can record in a given time. Several types of camera sensors are analyzed and the smallest measurable magnetic-field change is estimated for each type. We show that most common sensors are of a limited use in such applications, while certain highly specific cameras allow to achieve nanotesla-level sensitivity in 1~s of a combined exposure. Finally, we demonstrate the results obtained with a lock-in camera that pave the way for real-time, wide-field magnetometry at the nanotesla level and with micrometer resolution.

arxiv.org

The paper describes an adaptation of heterodyne interferometry for recovering holograms of objects hidden behind scattering surfaces, in a single shot. A lock-in camera featuring pixel level synchronous demodulation aids in the process.

osapublishing.org

A number of extensions to the Standard Model of particle physics predict a permanent electric dipole moment of the electron (eEDM) in the range of the current experimental limits. Trapped ThF⁺ will be used in a forthcoming generation of the JILA eEDM experiment. Here, we present extensive survey spectroscopy of ThF⁺ in the 700–1000 nm spectral region, with the 700–900 nm range fully covered using frequency comb velocity modulation spectroscopy. We have determined that the ThF⁺ electronic ground state is X 3Δ1, which is the eEDM-sensitive state. In addition, we report high-precision rotational and vibrational constants for 14 ThF⁺ electronic states, including excited states that can be used to transfer and readout population in the eEDM experiment.

doi.org/10.1016/j.jms.2015.11.001

Time-reversed ultrasonically encoded optical focusing measures the wavefront of ultrasonically tagged light, and then phase conjugates the tagged light back to the ultrasonic focus, thus focusing light deep inside the scattering media. In previous works, the speed of wavefront measurement was limited by the low frame rates of conventional cameras. In addition, these cameras used most of their bits to represent an informationless background when the signal-to-background ratio was low, resulting in extremely low efficiencies in the use of bits. Here, using a lock-in camera, we increase the bit efficiency and reduce the data transfer load by digitizing only the signal after rejecting the background. With this camera, we obtained the wavefront of ultrasonically tagged light after a single frame of measurement taken within 0.3 ms, and focused light in between two diffusers. The phase sensitivity has reached 0.51 rad even when the SBR is 6×10⁻⁴.

osapublishing.org

Ultrasound-modulated optical tomography (UOT) images optical contrast deep inside scattering media. Heterodyne holography based UOT is a promising technique that uses a camera for parallel speckle detection. In previous works, the speed of data acquisition was limited by the low frame rates of conventional cameras. In addition, when the signal-to-background ratio was low, these cameras wasted most of their bits representing an informationless background, resulting in extremely low efficiencies in the use of bits. Here, using a lock-in camera, we increase the bit efficiency and reduce the data transfer load by digitizing only the signal after rejecting the background. Moreover, compared with the conventional four-frame based amplitude measurement method, our single-frame method is more immune to speckle decorrelation. Using lock-in camera based UOT with an integration time of 286 μs, we imaged an absorptive object buried inside a dynamic scattering medium exhibiting a speckle correlation time (τc) as short as 26 μs. Since our method can tolerate speckle decorrelation faster than that found in living biological tissue (τc ∼ 100–1000 μs), it is promising for in vivo deep tissue non-invasive imaging.
This work was sponsored in part by National Institutes of Health Grant Nos. DP1 EB016986 and R01 CA186567.

Applied Physics Letters: Vol 108, No 23 (scitation.org)

For super-resolution microscopy methods based on single molecule stochastic switching and localization, to simultaneously improve the spatial–temporal resolution, it is necessary to maximize the number of photons that can be collected from single molecules per unit time. Here, we describe a novel approach to enhance the signal intensity (collected photons per second) from fluorescence probes by introducing a stimulated emission (SE) optical process. This process is based on the following two properties: first, with reasonable parameters, the photon emission rate can be significantly increased with SE; and second, the SE photons, which are spatially coherent with the stimulation beam, are more favorable for collection than fluorescence. Theoretical results have shown that signal intensity from a single fluorescent molecule can be greatly improved with SE. We therefore showed, using SE in combination with single molecule localization methodology, that fast imaging at a rate of 0.05 s per reconstructed image with lateral resolutions of ∼30  nm∼30 nm can be obtained.

doi.org/10.1364/AO.54.006919

doi.org/10.1117/12.2040216

White-light interferometry is a highly accurate technology for 3D measurements. The principle is widely utilized in surface metrology instruments but rarely adopted for in-line inspection systems. The main challenges for rolling out inspection systems based on white-light interferometry to the production floor are its sensitivity to environmental vibrations and relatively long measurement times: a large quantity of data needs to be acquired and processed in order to obtain a single topographic measurement. Heliotis developed a smart-pixel CMOS camera (lock-in camera) which is specially suited for white-light interferometry. The demodulation of the interference signal is treated at the level of the pixel which typically reduces the acquisition data by one orders of magnitude. Along with the high bandwidth of the dedicated lock-in camera, vertical scan-speeds of more than 40mm/s are reachable. The high scan speed allows for the realization of inspection systems that are rugged against external vibrations as present on the production floor. For many industrial applications such as the inspection of wafer-bumps, surface of mechanical parts and solar-panel, large areas need to be measured. In this case either the instrument or the sample are displaced laterally and several measurements are stitched together. The cycle time of such a system is mostly limited by the stepping time for multiple lateral displacements. A line-scanner based on white light interferometry would eliminate most of the stepping time while maintaining robustness and accuracy. A. Olszak proposed a simple geometry to realize such a lateral scanning interferometer. We demonstrate that such inclined interferometers can benefit significantly from the fast in-pixel demodulation capabilities of the lock-in camera. One drawback of an inclined observation perspective is that its application is limited to objects with scattering surfaces. We therefore propose an alternate geometry where the incident light is normal to the object surface and where an inclined grating is used as reference mirror.

spiedigitallibrary.org

Imaging laser Doppler velocimetry (ILDV) is a novel flow measurement technique, which enables the measurement of the velocity in an imaging plane. It is an evolution of heterodyne Doppler global velocimetry (HDGV) and may be regarded as the planar extension of the classical dual-beam laser Doppler velocimetry (LDV) by crossing light sheets in the flow instead of focused laser beams. Seeding particles within the flow are illuminated from two different directions, and the light scattered from the moving particles exhibits a frequency shift due to the Doppler effect. The frequency shift depends on the direction of the illumination and the velocity of the particle. The superposition of the two different frequency-shifted signals on the detector creates interference and leads to an amplitude modulated signal wherein the modulation frequency depends on the velocity of the particle. This signal is detected using either a high-speed camera or alternatively a smart pixel imaging array. This detector array performs a quadrature detection on each pixel with a maximum demodulation frequency of 250 kHz. To demonstrate the feasibility of the technique, two experiments are presented: The first experiment compares the measured velocity distribution of a free jet using ILDV performed with the smart pixel detector array and a high-speed camera with a reference measurement using PIV. The second experiment shows an advanced setup using two smart pixel detector arrays to measure the velocity distribution on a rotating disk, demonstrating the potential of the technique for high-velocity flow measurements.

doi.org/10.1007/s00348-011-1192-1

Precision spectroscopy of trapped HfF⁺ will be used to search for the permanent electric dipole moment of the electron (eEDM). Prior to this study, spectroscopic information necessary for state preparation, readout, and analysis of systematic errors was not available. We have developed a powerful technique for broadband, high-resolution survey spectroscopy of molecular ions that combines cavity-enhanced direct frequency-comb spectroscopy with velocity-modulation spectroscopy (vms) and used this to measure four bands in HfF⁺ over a 1000 cm⁻¹ bandwidth near 800 nm. Additionally, we performed targeted scans with cw-laser vms to find 15 additional bands from 9950 to 14600 cm⁻¹. We present a detailed analysis of these bands to obtain high-precision rovibrational constants, Λ-doublings, and isotope splittings for eight electronic states. We also use our results to improve theoretical predictions and discuss implications of our measurements to the eEDM experiments. These results demonstrate the application of frequency-comb and cw-vms for broadband, high-resolution spectroscopy of molecular ions.

doi.org/10.1016/j.cplett.2012.06.037 PDF

In the paper the new type of mobile sensor based on optical coherence tomography is presented. For increasing the measurement range the special dynamic focusing system which moves imaging plane during axial scanning process is used. Therefore developed system allows focusing on measured layer. Additionally, for image analysis the special type of CMOS matrix (called smart-pixel camera), synchronized with a reference mirror transducer, is applied. Due to hardware realization of a fringe contrast analysis simultaneously in each pixel with high frequency, the time of measurement decreases significantly. These advantages together with a compact design allow the sensor to be used as the mobile device for measurements of surface topography, thickness of surface layers and subsurface defects detection in laboratory, workshop and out-door conditions. Calibration of the designed sensor and its application to the technological measurements of the sticker label layers are presented and discussed.

Scientific.Net

We have demonstrated a new technique that provides massively parallel comb spectroscopy sensitive specifically to ions through the combination of cavity-enhanced direct frequency comb spectroscopy with velocity-modulation spectroscopy. Using this novel system, we have measured electronic transitions of HfF+ and achieved a fractional absorption sensitivity of 3×10−7 recorded over 1500 simultaneous channels spanning 150 cm−1 around 800 nm with an absolute frequency accuracy of 30 MHz (0.001 cm−1). A fully sampled spectrum consisting of interleaved measurements is acquired in 30 min.

doi.org/10.1103/PhysRevLett.107.093002

The new mobile system based on optical coherence tomography (OCT) for measurement of surface layers structure is presented. Due to application of special type detection matrix (called smart-pixel camera) it can be used for very fast measurements also in outdoor conditions. Additionally, the dynamic focusing mechanism causes that the surface of zero optical path difference is always sustained within depth of focus of imaging system. The concept and design of the mobile system is described and exemplary results of its application are presented.

SpringerLink

Although low coherence interferometers are commercially available (e.g., white light interferometers), they are generally quite bulky, expensive, and offer limited flexibility. In the paper the new portable profilometer based on low coherence interferometry is presented. In the device the white light diode with controlled spectrum shape is used in order to increase the zero order fringe contrast, what allows for its better and quicker localization. For image analysis the special type of CMOS matrix (called smart pixel camera), synchronized with reference mirror transducer, is applied. Due to hardware realization of the fringe contrast analysis, independently in each pixel, the time of measurement decreases significantly. High speed processing together with compact design allows that profilometer to be used as the portable device for both in and out door measurements. The capabilities of the designed profilometer are well illustrated by a few application examples.

spiedigitallibrary.org