Optics (physics.optics)

Abstract: We examine properties and propagation of the energy-density and photon-probability centroids of electromagnetic wavepackets in free space. In the second-order paraxial approximation, both of these centroids propagate with the same subluminal velocity because of the transverse confinement of the wavepacket and its diffraction. The tiny difference between the energy and probability centroid velocities appears only in the forth order. We consider three types of wavepackets: Gaussian, Bessel, and non-diffracting Bessel. In all these cases, the subluminal propagation is clearly visible in the intensity distributions and can be measured experimentally in both classical-light and single-photon regimes. For Gaussian wavepackets, the half-wavelength delay is accumulated after propagation over about 12 Rayleigh lengths.

Abstract: Bound states in the continuum (BICs) are localized electromagnetic modes within the continuous spectrum of radiating waves. Due to their infinite lifetimes without radiation losses, BICs are driving research directions in lasing, non-linear optical processes, and sensing. However, conventional methods for converting BICs into leaky resonances, or quasi-BICs, with high-quality factors typically rely on breaking the in-plane inversion symmetry of the metasurface and often result in resonances that are strongly dependent on the angle of the incident light, making them unsuitable for many practical applications. Here, we numerically analyze and experimentally demonstrate an emerging class of BIC-driven metasurfaces, where the coupling to the far field is controlled by the displacement of individual resonators. In particular, we investigate both all-dielectric and metallic as well as positive and inverse displacement-mediated metasurfaces sustaining angular-robust quasi-BICs in the mid-infrared spectral region. We explore their behavior with changes in the incidence angle of illumination and experimentally show their superior performance compared to two conventional alternatives: silicon-based tilted ellipses and cylindrical nanoholes in gold. We anticipate our findings to open exciting perspectives for bio-sensing, conformal optical devices, and photonic devices using focused light.

Abstract: Fiber photometry is a significantly less invasive method compared to other deep brain imaging microendoscopy approaches due to the use of thin multimode fibers (MMF diameter $<$ 500 $\mu$m). Nevertheless, the transmitted signals get scrambled upon propagation within the MMF, thus limiting the technique's potential in resolving temporal readouts with cellular resolution. Here, we demonstrate how to separate the time trace signals of several fluorescent sources probed by a thin ($\approx$ 200 $\mu$m) MMF with typical implantable length in a mouse brain. We disentangled several spatio-temporal fluorescence signals by using a general unconstrained non-negative matrix factorization (NMF) algorithm directly on the raw video data. Furthermore, we show that commercial and low-cost open-source miniscopes display enough sensitivity to image the same fluorescence patterns seen in our proof of principle experiment, suggesting that a whole new avenue for novel minimally invasive deep brain studies with multimode fibers in freely-behaving mice is possible.

Abstract: Generating widely tunable, continuous wave light at long wavelengths via difference frequency generation (DFG) remains challenging due to high absorption and dispersion. The relatively new platform of thin film lithium niobate enables high-confinement nonlinear waveguides, which could improve efficiency. We simulated DFG in thin film lithium niobate waveguides that are periodically poled for surface emission at 30 THz. Maximum efficiency for a 1 cm device is 9.16 $\times$ 10$^{-6}$ W$^{-1}$ assuming $d_{33}$ = 30 pm/V. The tuning range within 50$\%$ of efficiency at 30 THz is as wide as 25 THz, from 25 THz to 50 THz.

Abstract: Optical imaging with mechanical contrast is critical for material and biological discovery since it allows contactless light-radiation force-excitation within the sample, as opposed to traditional mechanical imaging. Whilst optical microscopy based on stimulated Brillouin scattering (SBS) enables mechanical imaging of materials and living biological systems with high spectrospatial resolution, its temporal resolution remains limited. Here, we develop Brillouin gain microscopy (BGM) with a 200-fold higher temporal resolution by detecting the Brillouin gain at a mechanically contrasting frequency in the sample with high sensitivity. Using BGM, we demonstrate mechanical imaging of materials, living organisms and cells at high spectro-spatiotemporal resolution.

Abstract: Brillouin microscopy is an emerging technique for all-optical biomechanical imaging without the need for physical contact with the sample or for an external mechanical stimulus. However, Brillouin microscopy often retrieves a single, averaged Brillouin frequency shift of all the materials in the sampling volume, introducing significant spectral artifacts in the Brillouin shift images produced. To enable the identification between single- and multi-peak Brillouin signatures in the sample voxels, we developed here a new physics-driven model selection framework based on information theory and an overfit Brillouin water peak threshold. The model selection framework was applied to Brillouin data of NIH/3T3 cells measured by stimulated Brillouin scattering microscopy, facilitating the improved quantification of the Brillouin shift of different regions in the cells, and substantially minimizing spectral artifacts in their Brillouin shift images.