Optics (physics.optics)

Abstract: Spectroscopic ellipsometry is a potent method that is widely adopted for the measurement of thin film thickness and refractive index. However, a conventional ellipsometer, which utilizes a mechanically rotating polarizer and grating-based spectrometer for spectropolarimetric detection, is bulky, complex, and does not allow real-time measurements. Here, we demonstrated a compact metasurface array-based spectroscopic ellipsometry system that allows single-shot spectropolarimetric detection and accurate determination of thin film properties without any mechanical movement. The silicon-based metasurface array with a highly anisotropic and diverse spectral response is combined with iterative optimization to reconstruct the full Stokes polarization spectrum of the light reflected by the thin film with high fidelity. Subsequently, the film thickness and refractive index can be determined by fitting the measurement results to a proper material model with high accuracy. Our approach opens up a new pathway towards a compact and robust spectroscopic ellipsometry system for the high throughput measurement of thin film properties.

Abstract: Thin-film lithium niobate (TFLN) based frequency doublers have been widely recognized as essential components for both classical and quantum optical communications. Nonetheless, the efficiency of these devices is hindered by imperfections present in the quasi-phase matching (QPM) spectrum. In this study, we present a thorough analysis of the spectral imperfections in TFLN frequency doublers with varying lengths, ranging from 5 mm to 15 mm. Employing a non-destructive diagnostic method based on scattered light imaging, we identify the sources and waveguide sections that contribute to the imperfections in the QPM spectrum. Furthermore, by mapping the TFLN film thickness across the entire waveguiding regions, we successfully reproduce the QPM spectra numerically, thus confirming the prominent influence of film thickness variations on the observed spectral imperfections. This comprehensive investigation provides valuable insights into the identification and mitigation of spectral imperfections in TFLN-based frequency doublers, paving the way toward the realization of nonlinear optical devices with enhanced efficiency and improved spectral fidelity.

Abstract: It is shown that reciprocity of the optical path can be violated through asymmetric strength coupling via near-field from resonant Mie scatterers to total internal reflection modes in a dielectric slab. Numerical simulations for silicon nanospheres separated by a nanogap from the glass substrate reveal that at least two orders of magnitude rectification ratio can be realized for such an optical diode configuration. Implementation to a solar light harvesting device, a scattering solar concentrator, is discussed, indicating a similar efficiency is achievable as for the state-of-the-art devices based on luminescence.

Abstract: Optomechanical devices are being harnessed as sensors of ultraweak forces for applications ranging from inertial sensing to the search for the elusive dark matter; For the latter, the focus is on detection of either higher energy single recoils or ultralight, narrowband sources; a directional signal is expected. However, the possibility of searching for a directional broadband signal need not be excluded; with this and other applications in mind, we apply a stochastic signal with a well defined direction, $\Psi$, to a trapped and cooled levitated nanosphere. We find that cross-correlation power spectra offer a calibration-free distinctive signature of the presence of a directional force, and its orientation quadrant, unlike normal power spectral densities (PSDs). With calibration we are able to accurately measure the angle $\Psi$, akin to a force compass in a plane.

Abstract: Optical neural networks (ONNs) herald a new era in information and communication technologies and have implemented various intelligent applications. In an ONN, the activation function (AF) is a crucial component determining the network performances and on-chip AF devices are still in development. Here, we first demonstrate on-chip reconfigurable AF devices with phase activation fulfilled by dual-functional graphene/silicon (Gra/Si) heterojunctions. With optical modulation and detection in one device, time delays are shorter, energy consumption is lower, reconfigurability is higher and the device footprint is smaller than other on-chip AF strategies. The experimental modulation voltage (power) of our Gra/Si heterojunction achieves as low as 1 V (0.5 mW), superior to many pure silicon counterparts. In the photodetection aspect, a high responsivity of over 200 mA/W is realized. Special nonlinear functions generated are fed into a complex-valued ONN to challenge handwritten letters and image recognition tasks, showing improved accuracy and potential of high-efficient, all-component-integration on-chip ONN. Our results offer new insights for on-chip ONN devices and pave the way to high-performance integrated optoelectronic computing circuits.

Abstract: We discuss a fundamental question regarding the Abraham-Minkowski debate about the momentum of light in a medium: If an atom in a gas absorbs a photon, what is the momentum transferred to it? We consider a classical model for the internal degrees of freedom of the absorbing atom, computing the absorbed energy and momentum using the Lorentz force law due to the microscopic electromagnetic fields. Each non-absorbing atom from the gas is treated as a dielectric sphere, with the set of atoms forming a linear, dielectric, non-magnetic, and non-absorbing medium with a refractive index $n$ close to one. Our numerical results indicate that if the atoms are classically localized, the average absorbed momentum increases with $n$, but is smaller than Minkowski's momentum $np_0$, $p_0$ being the photon momentum in vacuum. However, experiments performed with Bose-Einstein condensates [Phys. Rev. Lett. $\mathbf{94}$, 170403 (2005)] are consistent with the atom absorbing Minkowski's momentum. We argue that there is no contradiction between these results since, in a Bose-Einstein condensate, the atoms are in a quantum state spatially superposed in a relatively large volume, forming a ``continuous'' medium. In this sense, the experimental verification of an atomic momentum recoil compatible with Minkowski's momentum would be a quantum signature of the medium state.