Optics (physics.optics)

Abstract: The interaction between a quantum particle's spin angular momentum and its orbital angular momentum is ubiquitous in nature. In optics, the spin-orbit optical phenomenon is closely related with the light-matter interaction and has been of great interest. With the development of laser technology, the high-power and ultrafast light sources now serve as a crucial tool in revealing the behaviour of matters under extreme conditions. The comprehensive knowledge of the spin-orbit interaction for the intense light is of utmost importance. Here, we achieve the in-situ modulation and visualization of the optical orbital-to-spin conversion in strong-field regime. We show that, through manipulating the morphology of femtosecond cylindrical vector vortex pulses by a slit, the photons' orbital angular momentum can be controllably transformed into spin after focusing. By employing strong-field ionization experiment, the orbital-to-spin conversion can be imaged and measured through the photoelectron momentum distributions. Such detection and consequent control of spin-orbit dynamics of intense laser fields have implications on controlling the photoelectron holography and coherent extreme ultraviolet radiation.

Abstract: High harmonic generation (HHG) with longitudinal optical orbital angular momentum has attracted much attention over the past decade. Here, we present the first study on the HHG with transverse orbital angular momentum driven by the spatiotemporal optical vortex (STOV) pulses. We show that the produced spatial resolved harmonic spectra reveal unique structures, such as the spatially spectral tilt and the fine interference patterns. We show these spatio-spectral structures originate from both the macroscopic and microscopic effect of spatiotemporal optical singularity in HHG. Employing two-color counter-spin and counter-vorticity STOV pulses, we further discuss a robust method to control the spatiotemporal topological charge and spectral structure of high-order harmonics. The conservation rule of photon transverse orbital angular momentum in HHG process is also discussed when mixing with photon spin angular momenta.

Abstract: The control of light through all-optical means is a fundamental challenge in nanophotonics and a key effect in optical switching and logic. The optical bistability effect enables this control and can be observed in various planar photonic systems such as microdisk and photonic crystal cavities and waveguides. However, the recent advancements in flat optics with wavelength-thin optical elements require nonlinear elements based on metastructures and metasurfaces. The performance of these systems can be enhanced with high-Q bound states in the continuum (BIC), which leads to intense harmonic generation, improved light-matter coupling, and pushes forward sensing limits. In this study, we report on the enhanced thermo-optical nonlinearity and the observation of optical bistability in an all-dielectric metasurface membrane with BICs. Unlike many other nanophotonic platforms, metasurfaces allow for fine control of the quality factor of the BIC resonance by managing the radiative losses. This provides an opportunity to control the parameters of the observed hysteresis loop and even switch from bistability to optical discrimination by varying the angle of incidence. Additionally, we propose a mechanism of nonlinear critical coupling that establishes the conditions for maximal hysteresis width and minimal switching power, which has not been reported before. Our work suggests that all-dielectric metasurfaces supporting BICs can serve as a flat-optics platform for optical switching and modulation based on strong thermo-optical nonlinearity.

Abstract: Optical skyrmion serves as a crucial interface between optics and topology. Recently, it has attracted great interest in linear optics. Here, we theoretically introduce a framework for the all-optical generation and control of freespace optical skyrmions in extreme ultraviolet regions via high harmonic generation. We show that by employing full Poincare beams, the created extreme ultraviolet fields manifest as skyrmionic structures in Stokes vector fields, whose skyrmion number is relevant to harmonic orders. We reveal that the generation of skyrmionics structure is attributed to spatial-resolved spin constraint of high harmonic generation. Through qualifying the geometrical parameters of full Poincar\'e beams, the topological texture of extreme ultraviolet fields can be completely manipulated, generating the Bloch-type, Neel-type, anti-type, and higher-order skyrmions, etc. This work promotes the investigation of topological optics in optical highly nonlinear processes, with potential applications towards ultrafast spintronics with structured light fields.

Abstract: We report on the highly nonlinear behavior observed in the central nervous system tissue of zebrafish (Danio rerio) when exposed to femtosecond pulses at 1030 nm. At this irradiation wavelength, photo damage becomes detectable only after exceeding a specific peak intensity threshold, which is independent of the photon flux and irradiation time, distinguishing it from irradiation at shorter wavelengths. Furthermore, we investigate and quantify the role of excessive heat in reducing the damage threshold, particularly during high-repetition-rate operations, which are desirable for label-free and multi-dimensional microscopy techniques. To verify our findings, we examined cellular responses to tissue damage, including apoptosis and the recruitment of macrophages and fibroblasts at different time points post-irradiation. These findings substantially contribute to advancing the emerging nonlinear optical microscopy techniques and provide a strategy for inducing deep-tissue, precise and localized injuries using near-infrared femtosecond laser pulses.

Abstract: The ability to detect quantum superpositions lies at the heart of fundamental and applied aspects of quantum mechanics. The time-frequency degree of freedom of light enables encoding and transmitting quantum information in a multi-dimensional fashion compatible with fiber and integrated platforms. However, the ability to efficiently detect time-frequency superpositions is not yet available. Here we show, that multidimensional time-bin superpositions can be detected using a single time-resolved photon detector. Our approach uses off-the shelf components and is based on the temporal Talbot effect -- a time-frequency counterpart of the well-know near field diffraction effect. We provide experimental results and discuss the possible applications in quantum communication, quantum information processing, and time-frequency quantum state tomography.

Abstract: A low-cost single frequency laser emitting in the mid-infrared spectral region and dissipating minimal electrical power is a key ingredient for the next generation of portable gas sensors for high-volume applications involving chemical sensing of important greenhouse and pollutant gases. We propose here a Quantum Cascade Surface Emitting Laser (QCSEL), which we implement as a short linear cavity with high reflectivity coated end-mirrors to suppress any edge emission and use a buried semiconductor diffraction grating to extract the light from the surface. By wafer-level testing we investigate the cavity length scaling, extract mirror reflectivities larger than 0.9, and achieve a pulsed threshold power dissipation of 237 mW for an emission wavelength near 7.5 $\mu$m. Finally, we demonstrate single mode emission with a side-mode suppression ratio larger than 33 dB of a 248 $\mu$m short cavity mounted with the epitaxial layer up and operated in continuous wave at 20 $^\circ$C.