This PDF file contains the front matter associated with SPIE Proceedings Volume 13139, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Ultrashort energetic terahertz (THz) pulses have opened up exciting new avenues of research in the field of light-matter interactions. For material studies in small laboratories, researchers often require widely tunable femtosecond THz pulses with a peak field strength close to MV/cm. Currently, these pulses can be generated through optical rectification and difference frequency generation in crystals without inversion symmetry. We present in this talk a novel approach for generating THz pulses with no frequency tuning gap. Our method is based on Raman-resonance-enhanced four-wave mixing in centrosymmetric media, specifically diamond. We demonstrate that this technique enables the generation of highly stable, few-cycle pulses with near-Gaussian spatial and temporal profiles. Using a 0.5-mm-thick diamond, we were able to generate THz pulses with a stable and controllable carrier-envelope phase. These pulses carried approximately 15 nJ of energy per pulse at 10 THz, with a peak field strength of about 1 MV/cm at the focus. Experimental measurements of the THz pulse characteristics were in good agreement with theoretical predictions. We also discuss the way to improve output energy.

We show two of our work on multi-mode nonlinear optical process. In a Ta₂O₅ waveguide of 100 nm in thickness, 8 μm in width and 4.3 mm in length, all-optical signal switching and power limiting operation were demonstrated by nonlinear multi-mode interference while the modes were properly excited by a near infrared pulse at the wavelength of 1064 nm, peak power of 100 W and the pulse duration of 1 ps. In supercontinuum process, a waveguide of 760 nm in thickness and 1.1 μm in width was designed where different guided modes were anomalously dispersive at different wavelengths. Supercontinuum spectrum spanning over an octave with high degree of spectral flatness was excited by a 100-fs laser pulse at the wavelength of 1030 nm. Spectral synthesis with a diffraction gratings mapped the spectral components attributed to specific waveguide modes. It inferred the possibility to engineer supercontinuum generation through waveguide-mode arrangement.

We present pulse compression by double-pass spectral broadening in ZnS, KGW and YAG crystals pumped with 16 W average power amplified Yb:KGW oscillator pulses at a 76 MHz repetition rate. We demonstrate nearly transform-limited compressed laser pulses with excellent spatio-spectral homogeneity of the beam. This relatively simple and cost-effective method for pulse compression is applicable to almost any high-average power and low pulse energy laser system. It has the potential to open the opportunities for applications in ultrafast spectroscopy systems operating at pulse repetition rates in the tens or hundreds of MHz.

Single shot SPIFI operating in the Fourier Domain is demonstrated for the first time to our knowledge. We present initial results capturing Fourier Domain single shot images with both one and two-dimensional detectors and demonstrate that the same enhanced frequency support that is characteristic of classical SPIFI translates directly into single shot SPIFI architectures as well. Linear Fourier Domain single shot SPIFI is systematically analyzed, for both types of detectors. Finally, we show that despite the complex pulse structure imposed on the illumination beam by SPIFI, nonlinear single shot SPIFI can be realized, and third harmonic generation imaging is demonstrated. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-860152.

High speed optical imaging is a critical tool for the observation of transient, nonrepeatable phenomena. In this talk, we discuss our recent progress on a spatiotemporally encoded ultrafast imaging system. Our approach involves recording of ultrafast events encoded using nano – scribed spatiotemporal masks on a slow camera. The captured data is then reconstructed into a sequence of ultrafast frames via a U – net based deep learning model. We will present both simulation and experimental results.

For antibiotics that target Gram-positive bacterial cell structures, optimizing their interaction with the cytoplasmic membrane is of paramount importance. Recent time-resolved second harmonic scattering (trSHS) experiments with living bacterial cells have shown that some amphiphilic small molecules display signals consistent with organization within the membrane environment. Such organization could arise, for example, from aggregation, solvent interactions, and/or environmental rigidity. To expand our study of this system, we turn to polarization-resolved SHS (pSHS). PSHS has previously been used with model membranes to extract information about the angular distribution of integrated small molecules. Here we apply pSHS, for the first time, to cells, specifically living Staphylococcus aureus. In doing so, we aim to address contributions ascribed to the organization of amphiphilic molecules in bacterial membranes.

Investigating metabolic reactions within cells is crucial for unraveling the numerous biological functions. Established imaging modalities, including MRI, PET, Fluorescence, and Mass Spectrometry, present various drawbacks. To address these issues, we have developed a nonlinear multimodal imaging system. This system combines stimulated Raman scattering, multiphoton fluorescence, and second harmonic generation. It was designed to probe the spatial distribution of metabolic activities within cells and tissues by measuring multiple molecular signals. To support the analysis scheme, we have also pioneered cutting-edge algorithms, notably the Adam-based Pointillism Deconvolution (A-PoD) and Correlation Coefficient Mapping (CoCoMap). They allow us to analyze correlations between super-resolution images of nanoscale Regions of Interest. Additionally, our research has introduced a novel clustering algorithm known as Multi-SRS reference matching (Multi-SRM). This algorithm is particularly tailored to isolate signals exclusively from specific subcellular organelles. The application of this innovation offers significant potential to study aging and disease related metabolic changes.

Second harmonic generation (SHG) microscopy, involving the doubling of the frequency of light by a material, is a valuable characterization technique that is highly sensitive to local material symmetry at lengths scales close to the diffraction limit. The use of high-NA microscope objectives introduces an additional layer of complexity when performing quantitative analysis of SHG polarimetry data due to the appreciable effects that strong focusing has on the polarization of the probe beam. A systematic investigation of this problem is presented, producing analytical and numerical solutions of SHG polarimetry generated under high-NA lenses in a microscope setup. Modeling of a variety of standard samples, from single crystals to thin films, is performed and compared against experimental data.

Understanding brain lipid metabolism is vital for unraveling brain aging and neurodegeneration mechanisms. Traditional imaging methods lack chemical specificity and resolution. Stimulated Raman scattering (SRS) imaging provides high specificity, resolution, and deep penetration. We utilize a multimodal imaging platform integrating deuterium isotope probed SRS microscopy (DO-SRS), multiphoton fluorescence (MPF), and second harmonic generation (SHG) to visualize lipid dynamics in animal brains. Deuterium incorporation generates detectable carbon-deuterium bonds in lipids, revealing turnover. We find decreased lipid activity during aging and neurodegeneration in Drosophila brains, with interventions like dietary restriction and pathway modulation enhancing turnover. Our advanced imaging and analysis methods offer high-resolution insights applicable to diverse biomedical studies.

Our work proposes the use of a collinear femtosecond Optical Kerr Gate for the first time to study the temporal behavior of the molecular interactions of carbon disulfide (CS2) vapor at 51kPa at room temperature in comparison to its liquid state. A faster molecular relaxation time for the vapor sample (800fs) is shown as it has less neighboring molecule interactions from collisions, while the liquids state has more collisions and interactions giving a 1.7ps relaxation time. This study also presents the OKE as a new optical biopsy method to differentiate different types of tissues. The main biomarker observed in our study is the doubling in the tissue’s conductivity from the dielectric response time, associated with the conductivity and permittivity observed in different grades of breast cancer tissues. Our finding suggests conductivity can be used as a new major biomarker for the classification or detection of diseases.

Raman microscopy is a valuable approach to label-free chemical imaging. Raman spectra provide a rich and powerful label-free probe of biological systems. While the narrow spectral features make Raman spectroscopy extremely attractive, the weak strength of Raman scattering makes it difficult to image deeply in many situations. In the case of spontaneous scattering, where inelastically scattered light is detected from a sample illuminated by a narrow-linewidth laser, the persistent challenge with Raman spectroscopic methods is low signals that are difficult to separate from background autofluorescence. In addition, low frequency vibrational modes are very difficult to detect with spontaneous Raman scattering. We will discuss imaging with impulsive stimulated Raman scattering (ISRS). In ISRS, the pump pulse produces a vibrational coherence that leads to a time variation of the effective linear refractive index that drives spectral scattering in the time-delayed probe pulse. ISRS microscopy allows for high quality hyperspectral imaging of low and fingerprint Raman vibrational frequencies when using optical interferometry. We will present methods that allow for high-quality low-frequenc

Silicon is a ubiquitous material in electronics, yet its implementation in opto-electronic devices is significantly less prominent due to its underwhelming optical properties compared to other semiconducting materials. In this presentation, we discuss novel strategies for improving light absorption in silicon at the nanoscale. First, we show that the principle of photon confinement on the nanometer scale enables new transitions in silicon that are otherwise momentum-forbidden, providing a mechanism for absorption enhancement by several orders of magnitude. Second, we offer new approaches for enhancing two-photon absorption in silicon and show that such strategies can be used for rapid mid-infrared imaging with Si-based cameras.

Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical signal, has traditionally been used to study molecules at interfaces as a spectroscopy technique with a spatial resolution of ~100 µm. However, the spectroscopy is not sensitive to the heterogeneity of a sample. To study mesoscopically heterogeneous samples, we, along with others, pushed the resolution limit of VSFG spectroscopy down to ~ 1 µm level, and constructed the VSFG microscope. This imaging technique not only can resolve sample morphologies through imaging, but also record a broadband VSFG spectrum at every pixel of the images. In this study, we demonstrate the capability of VSFG microscopy to discern chemically specific domain details of collagen in both mouse lung tumor and control tissues. We introduce two methods for identifying the tumor domain using chemical-specific VSFG imaging. These findings underscore the potency of VSFG microscopy as a transformative tool in the realm of bioimaging for medical research in both revealing fundamental structural of collagens and as a diagnostic tool in clinical setting.

In this study, a ResNet approach based on multipolarization SHG imaging is proposed for the categorization and regression of collagen type I and II blend hydrogels at 0%, 25%, 50%, 75%, and 100% type II, without the need for prior time-consuming model fitting. A ResNet model, pretrained on 18 progressive polarization SHG images at 10° intervals for each percentage, categorizes the five blended collagen hydrogels with a mean absolute error (MAE) of 0.021, while the model pretrained on nonpolarization images exhibited 0.083 MAE. Moreover, the pretrained models can also generally regress the blend hydrogels at 20%, 40%, 60%, and 80% type II. In conclusion, the multipolarization SHG image-based ResNet analysis demonstrates the potential for an automated approach using deep learning to extract valuable information from the collagen matrix.

Exciton-polaritons are bosonic quasi-particles generated through the strong coupling of optical excitations in a semiconductor with a resonant mode of a microcavity. While the exciton-polaritons enable manifestation of collective quantum phenomena, the emergence of coherent dynamics of polaritons essential to drive such processes is dependent on their nonlinear many-body interactions. Here we will discuss the dynamics of such interactions measured through nonlinear optical probes based on ultrashort optical pulses in strongly-coupled microcavities made of organic molecules and those that are based on two-dimensional metal halides. In addition, we present an alternative methodology reliant on spectrally entangled biphoton states as a probe of such many-body dynamics.

Macroscopic quantum phase transitions in solid-state systems hold promise for advancing high-temperature quantum technologies. However, the practical implementation of such technologies is hindered by rapid thermal dephasing, confining macroscopic quantum phenomena to cryogenic conditions. This limitation emphasizes the need for understanding the mechanisms governing phase transitions, including the properties of materials determining critical temperature and the process leading to macroscopic coherence. In this study, we delve into the superradiant phase transition in perovskites, focusing on critical temperatures and densities influencing the emergence of macroscopically coherent quantum states within electronic excitations in crystalline matter. Our analysis of the phase diagram of PEA:CsPb(Br/Cl) from 78K to 285K reveals a distinctive dome-shaped pattern, akin to quantum phenomena such as superconductivity or superfluidity. This intriguing similarity holds the potential to provide insights into the unknown mechanisms of high-temperature quantum phenomena, potentially paving the way for practical advancements in quantum technologies designed to operate at elevated temperatures.

The spin-orbit interaction in two-dimensional electron gases is responsible for a broad range of phenomena, including spin Hall effects and spin textures such as the persistent spin helix (PSH). A PSH occurs when parameters associated with the bulk (Dresselhaus) and structural (Rashba) inversion asymmetries are roughly equal in strength. This situation results in a momentum-dependent effective magnetic field providing the SU(2) symmetry in which the two-dimensional electron gas (2DEG) features a unidirectional spin grating (or helical spin-density wave). Here I will focus on several new aspects of PSH formation and manipulation obtained from temporally and spatially resolved Kerr microscopy.

In this presentation, I will describe recent advances in using ultrafast coherent multi-dimensional nonlinear optical microscopy to identify resonant electronic excitations in 2D metals. I will demonstrate that resonance matching at harmonic wavelengths results in a population inversion, which in turn saturates SHG transduction. Numerical modeling shows that internal energy transfer on sub-cycle timescales (< 2fs) mediates the population inversion. These effects are determined by atomic-level structure of the 2D metal.

A rotating organic cation and a dynamically disordered soft inorganic cage are the hallmark features of organic-inorganic lead-halide perovskites. Understanding the interplay between these two subsystems is a challenging problem, but it is this coupling that is widely conjectured to be responsible for the unique behavior of photocarriers in these materials. In this work, we use the fact that the polarizability of the organic cation strongly depends on the ambient electrostatic environment to put the molecule forward as a sensitive probe of the local crystal fields inside the lattice cell. We measure the average polarizability of the C/N–H bond stretching mode by means of infrared spectroscopy, which allows us to deduce the character of the motion of the cation molecule, find the magnitude of the local crystal field, and place an estimate on the strength of the hydrogen bond between the hydrogen and halide atoms. Our results pave the way for understanding electric fields in lead-halide perovskites using infrared bond spectroscopy.

Nonlocal PL emission near monolayer to bilayer transitions in the two-dimensional material WS2 is an important indicator of dynamics of the system. For example, we find excitons excited in a bilayer can emit microns away at such a transition. We find that spectral shift of the emission is also important, as it indicates the bandgap in the emission region, and reduced bandgap regions can trap the excitons. We use nonlocal fluorescence measurements in conjunction with position-correlated 2nd harmonic microscopy, which is always local, AFM and Raman spectroscopy to understand the dynamic processes of the carriers. Two complementary nonlocal measurement approaches are applied to detect the Photo Luminescent (PL) emission region around the excitation spot. Quantitative analysis of the spectral and spatial dynamics is discussed.

Photothermal imaging has proven a powerful label-free chemical imaging technique. With time-resolved mid-infrared photothermal imaging heat transfer dynamics across aqueous interfaces can be studied. However, liquid water has been a limiting factor in mid-infrared imaging and spectroscopy due to its high absorption spanning across the molecular fingerprint region so that cellular imaging is often performed in less absorbing heavy water instead. Time-resolved measurements via boxcar detection enable the separation of water background and reveal how heat transfer dynamics across aqueous interfaces strongly depend on hydration and the surrounding environment. Mid-infrared photothermal imaging of extracted axon-bundles from crayfish is presented in a saline solution where the water background can be separated based on its inherently different transient response.

This presentation focuses on the use of common path, birefringent, time-delayed interferometry to detect ultrafast pump-probe effects via phase, rather than absorption of the probe pulse, in a laser-scanning microscope. The method uses balanced detection to cancel relative intensity noise inherent to fiber laser sources. We compare absorption and phase measurements in graphene, hemoglobin, and red blood cells, and present a preliminary model for determining whether, for a given wavelength, absorption or phase will yield a stronger signal.

Living cells are complex, crowded, and dynamic with heterogeneous ionic strength, which influences biological processes that are essential to cellular function and survival. Recently, we have investigated a family of newly developed donor-linker-acceptor constructs for environmental sensing of macromolecular crowding and ionic strength using integrated, ultrafast time-resolved fluorescence spectroscopy methodologies. In this contribution, we highlight a novel single-molecule approach to investigate the sensitivity of these sensors to environmental variables using fluorescence fluctuation analysis and molecular brightness spectroscopy. These single-molecule studies complement the traditional, ensemble methods for protein-protein interactions. In addition, our findings represent a stop forward towards the development of a systematic, rational design strategy for environmental sensors.

We report preliminary results on subsurface spectroscopy inside heterogeneous materials using four wave mixing phase conjugation. For our test material we use an inert plastic bonded explosive simulant that contains fluorescent guidestar particles. Using phase conjugation, we are able to obtain spectral enhancements approximately equal to the phase conjugate reflectivity. Additionally, we discuss future directions to improve this technique to obtain greater signal enhancements.

In this talk, I will talk about our recent development of a high-speed 3D microscopy technique, squeezed light field microscopy (SLIM), and its application in kilohertz volumetric imaging of transient biological events.

Third harmonic generation (THG) microscopy offers label-free imaging and enables three-dimensional imaging with inherently good optical depth-sectioning. Its utility is important for imaging and characterization of laser-induced changes in transparent materials. Here we develop a digital twin for THG microscopy that allows accurate simulation of experiments and provides insight into desired information such as the third-order nonlinear susceptibility tensor. It also enables accurate simulation of the instrument's point spread function (PSF), which is essential for relating THG measurements to the desired hidden quantities. Finally, we improve microscope throughput with adaptive optics, demonstrating how PSF engineering improves spatial probing, temporal efficiency, and information content, and comparing results between the real setup and its digital twin.

Advances in computational imaging over the last decade have sparked a revolution in metrologies aimed at understanding nano-material, magnetic, acoustic, and most recently—systems with non-repeatable and nonlinear dynamics. The most recent advances are being led by state-of-the-art single-shot, high-dimensional multiplexed imaging systems. In this talk, I will review progress on single-shot multiplexed coherent computational imaging and provide some perspective on how the technology is poised to usher in a new revolution in high-dimensional pulse-beam metrology. Uniquely, these new metrologies will enable optimizing the spatiotemporal profile of ultrashort pulses to reach the highest intensities. The talk will conclude with progress on a computational microscope capable of recording nonlinear dynamics with femtosecond frame-periods.

Full temporal characterization of optical pulses is critical in understanding ultrafast phenomena including electronic transitions, laser physics, and others. We demonstrate a machine learning-based reconstruction technique to recover the complex field from frequency resolved optically gated (FROG) interferometric data collected in the collinear acquisition geometry. FROG, or the collinear version CFROG data are time-consuming to acquire, so training a high-performance ML network with measured data is challenging. The use of simulated data for training requires careful dataset construction, as we show here. We find that the combination of using the Fourier transform instead of the raw data as an input and accurate noise modeling for the synthetic-data based training are required for a robust and accurate deep-learning-based quantification. The result is a significantly faster computation than traditional methods for inverting the CFROG experimental data, potentially enabling CFROG imaging.

Stimulated Brillouin scattering couples optical and acoustic waves with applications for signal processing, narrow-linewidth lasers, and environmental sensors. While traditional interactions were fixed in acoustic-wave character and frequency, more recent techniques with engineered optical waves enable new flexibility. In this talk I will discuss recent developments of tunable stimulated Brillouin interactions with 1) low-frequency guided acoustic waves and 2) surface-acoustic waves. Interactions with fundamental guided acoustic waves enable record coupling strengths and linewidths ideal for sensors and signal processing, and interactions with surface-acoustic waves enable versatile contact-free optical control and spectroscopy of state-of-the-art saw cavities for classical and quantum applications.

We demonstrate a high-NA, inch-scale, dual-wavelength metalens for long-working-distance two-photon fluorescence imaging.

Superfluorescence (SF) is a unique optical phenomenon that consists of an ensemble of emitters coupling collectively to produce a short but extremely intense burst of light. Despite our recently published works showing that room temperature anti-Stokes shifted SF were achieved in a few randomly assembled or even single lanthanide-doped upconversion nanoparticle (UCNP), the coupling required to produce and optimize Burnham-Chiao ringing (echoing of pulses) is not understood. Such ringing could be particularly useful to provide timing and multiplexing in potential applications as an alternative light source device. We previously found a lack of Burnham Chiao ringing in single nanocrystals, but strong ringing in a random cluster. The ordered assembly of these crystals will not only create a SF superburst, but also enable understanding of the periodicity of the Burnham Chiao ringing. This work explores SF microrod (MR) and microplate (Mplate), with enhanced SF performance and the closely spaced assembly of MR/Mplate result in a greater active volume, which gives rise to greater Burnham-Chiao ringing.

Direct laser writing (DLW) has been an emerging technique for creating complex nanostructures due to its high flexibility, high precision, and digital-control capabilities. However, the Abbe diffraction limit of laser writing system often restricts its resolution. Combining stimulated emission depletion (STED) with DLW offers promising prospects for improvement. Yet conventional STED systems using Laguerre-Gauss depletion beams suffer from severe spherical aberration due to the index mismatching between samples and optical system, leading to resolution degradation at deeper sample depths. This work proposes a Gauss-Bessel STED (GB-STED) system employing a first-order Bessel beam for depletion. Due to its self-healing properties and minimal spherical aberration susceptibility, the GB-STED system achieves deep super-resolution DLW with linewidths nearly identical to the surface. Debye vector diffraction integral simulations has been performed to compare the optical field distributions and linewidths of STED and GB-STED systems. Experimentally, a two-photon super-resolution DLW system based on the Bessel beam demonstrated constant resolution throughout the sample depth. These results showcase the superior super-resolution DLW capability of the GBSTED system at depth, opening new avenues for high-resolution laser nanofabrication.

Optical diffractive tomography (ODT) microscopy is a normal wide-field, non-invasive and label-free three-dimensional imaging technology for cells and tissues. The traditional ODT microscopy has a little field-of-view (FOV) of about 80um, which needs to be reconstructed by regions and then spliced for large complex biological samples with sub-millimeter scale. However, there is interference of ringing effect during splicing, which limits its application in sub-millimeter biological samples. In this paper, an ODT microscopy with a wider FOV is proposed. The FOV is 196um, more than three times that of conventional technology, and the photon flux is higher. The results show that the wider-field ODT microscopy has better imaging performance, higher signal-to-noise ratio on the sub-millimeter samples without splicing.