It is a basic principle that the effect cannot come before the cause. Dispersive relations that follow from this fundamental fact have proven to be an indispensable tool in physics and engineering. They are most powerful in the domain of linear response where they are known as Kramers-Kronig relations. However when it comes to nonlinear phenomena the implications of causality are much less explored, apart from several notable exceptions. Here in this work we demonstrate how to apply the dispersive formalism to analyse the ultrafast nonlinear response in the context of the paradigmatic nonlinear Kerr effect. We find that the requirement of causality introduces a noticeable effect even under assumption that Kerr effect is mediated by quasi-instantaneous off-resonant electronic hyperpolarizability. We confirm this by experimentally measuring the time resolved Kerr dynamics in GaAs by means of a hybrid pump-probe Mach- Zehnder interferometer and demonstrate the presence of an intrinsic lagging between amplitude and phase responses as predicted by dispersive analysis. Our results describe a general property of the time-resolved nonlinear processes thereby highlighting the importance of accounting for dispersive effects in the nonlinear optical processes involving ultrashort pulses.

This conference presentation was prepared for SPIE Optical Engineering + Applications, 2023.

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 the development of tunable stimulated Brillouin interactions with A) low-frequency guided acoustic waves and B) 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.

Optical interrogation of cellular electrical activity is a crucial tool for understanding how cells function and communicate in complex networks. Scientists often use voltage-sensitive dyes to measure the excitability of cells, but these dyes can interfere with cellular function. Label-free techniques offer a way to measure electrical activity without using external probes. In this study, researchers found that second-harmonic generation from live cells is highly sensitive to changes in transmembrane potential by electrode control, making it a promising label-free approach to measure electrical activity in more complex cellular networks. This research provides a promising framework for a non-invasive label-free tool to measure electrical activity in cells.

Traditional cameras are limited to capturing only the intensity distribution of light, lacking other important information. We present a novel approach to overcome this limitation by incorporating metasurfaces - thin layers of nanostructures - into standard cameras. These metasurfaces can encode light information of different dimensions, including polarization, frequency, and time, through the unique optical responses of their constituent meta-atoms. As a result, a single snapshot of the light field can capture all its properties. We also use deep-learning-based recovery algorithms that enable real-time retrieval of multi-dimensional images. This breakthrough has enormous potential to revolutionize the way we collect and analyze light information, leading to new applications and insights in various fields.

Time-resolved mid-infrared photothermal imaging is discussed to study heat diffusion dynamics in various complex environments including water interfaces and nanoparticle clusters. Boxcar detection enables the extraction of hyper-temporal image stacks along with local temporal traces of the photothermal signal with high a signal-to-noise ratio and no complex post-processing. Environments with multiple absorbers in close proximity to each other or features in highly absorbing media can be analyzed with characteristic thermal decay profiles.

Spectral imaging in the mid-infrared (MIR) range provides simultaneous morphological and chemical information of a wide variety of samples. However, current MIR technologies struggle to produce high-definition images over a broad spectral range at acquisition rates that are compatible with real-time processes. We present a novel spectral imaging technique based on non-degenerate two-photon absorption of temporally chirped optical MIR pulses. This new approach avoids complex image processing or reconstruction and enables high-speed acquisition of spectral data cubes at high pixel density in under a second.

This conference presentation was prepared for SPIE Optical Engineering + Applications, 2023.

We report on laser trapping and unidirectional angular acceleration of CO2 molecules to energies well beyond the ground state C-O bond dissociation energy. While a centrifugal dissociation barrier prevents prompt dissociation of these molecules, coherent Raman probing, and virtually imaged phased array (VIPA) detection of the scattering signals, provides direct evidence of coherence transfer and rotation-to-vibration energy transfer. The record-breaking acceleration of CO2 to J=364, corresponding to a rotational temperature of over 71,000 K, and high-resolution detection method permitted the determination of improved centrifugal distortion constants.

ZIF-8 is a widely studied metal-organic framework, but questions remain about its crystallization mechanism, especially at the initial stages. In this study, we used in-situ nonlinear optical (NLO) light scattering techniques to probe nucleation and crystal growth of ZIF-8 nanocrystals. The NLO measurements provided unique insights into the crystallization mechanism of ZIF-8. The formation of crystalline ZIF-8 nanoparticles was probed through depolarization measurements. By comparing second and third harmonic scattering the formation of defects was revealed in the earliest stages. These measurements highlight the potential of NLO techniques to probe nucleation and crystal growth in situ, providing information missed by traditional X ray based techniques.

We present a compact, spatiotemporally encoded, deep learning-enabled, single-shot ultrafast imaging system. We simulate the forward light-transport process of an ultrafast event encoded by a random spatiotemporal encoding mask and construct a U-net-based deep learning model to reconstruct the ultrafast event sequences. Trained on simulated ultrafast events consisting of various geometric shapes and handwritten digits with random locations and speeds, the deep learning model can reconstruct multi-frame ultrafast event sequences from simulated single-shot measurements by a normal camera with high reconstruction accuracy and noise tolerance. We also present preliminary fabrication results of spatiotemporal masks. This work provides a simple, cost-effective, single-shot method for studying nonrepetitive ultrafast transient processes.