Chiral sum-frequency generation (SFG) has proven to be a versatile spectroscopic and imaging tool for probing chirality. However, due to polarization restriction, the conventional chiral SFG microscopes have mostly adopted noncollinear beam configurations, which only partially cover the aperture of microscope and strongly spoil the spatial resolution. In this study, we report the first experimental demonstration of collinear chiral SFG microscopy, which fundamentally supports diffraction-limited resolution. This advancement is attributed to the collinear focus of a radially polarized vectorial beam and a linearly polarized (LP) beam. The tightly focused vectorial beam has a very strong longitudinal component, which interacts with the LP beam and produces the chiral SFG. The collinear configuration can utilize the full aperture and thus push the spatial resolution close to the diffraction limit. This technique can potentially boost the understanding of chiral systems.

The article comments on a recent advance for wavelength tuning of laser particles, through a new photoelectrochemical etching method.

Chaotic dynamics in optical microcavities, governed dominantly by manifolds, is of great importance for both fundamental studies and photonic applications. Here, we report the experimental observation of a stable manifold characterized by energy and momentum evolution in the nearly chaotic phase space of an asymmetric optical microcavity. By controlling the radius of a fiber coupler and the coupling azimuth of the cavity, corresponding to the momentum and position of the input light, the injected light can in principle excite the system from a desired position in phase space. It is found that once the input light approaches the stable manifold, the angular momentum of the light experiences a rapid increase, and the energy is confined in the cavity for a long time. Consequently, the distribution of the stable manifold is visualized by the output power and the coupling depth to high-Q modes extracted from the transmission spectra, which is consistent with theoretical predictions by the ray model. This work opens a new path to understand the chaotic dynamics and reconstruct the complex structure in phase space, providing a new paradigm of manipulating photons in wave chaos.

The ability to sense dynamic biochemical reactions and material processes is particularly crucial for a wide range of applications, such as early-stage disease diagnosis and biomedicine development. Optical microcavities-based label-free biosensors are renowned for ultrahigh sensitivities, and the detection limit has reached a single nanoparticle/molecule level. In particular, a microbubble resonator combined with an ultrahigh quality factor (Q) and inherent microfluidic channel is an intriguing platform for optical biosensing in an aqueous environment. In this work, an ultrahigh Q microbubble resonator-based sensor is used to characterize dynamic phase transition of a thermosensitive hydrogel. Experimentally, by monitoring resonance wavelength shift and linewidth broadening, we (for the first time to our knowledge) reveal that the refractive index is increased and light scattering is enhanced simultaneously during the hydrogel hydrophobic transition process. The platform demonstrated here paves the way to microfluidical biochemical dynamic detection and can be further adapted to investigating single-molecule kinetics.

Synchronization is of importance in both fundamental and applied physics, but its demonstration at the micro/nanoscale is mainly limited to low-frequency oscillations such as mechanical resonators. We report the synchronization of two coupled optical microresonators, in which the high-frequency resonances in the optical domain are aligned with reduced noise. It is found that two types of synchronization regimes emerge with either the first- or second-order transition, both presenting a process of spontaneous symmetry breaking. In the second-order regime, the synchronization happens with an invariant topological character number and a larger detuning than that of the first-order case. Furthermore, an unconventional hysteresis behavior is revealed for a time-dependent coupling strength, breaking the static limitation and the temporal reciprocity. The synchronization of optical microresonators offers great potential in reconfigurable simulations of many-body physics and scalable photonic devices on a chip.

Optical trapping techniques are of great interest since they have the advantage of enabling the direct handling of nanoparticles. Among various optical trapping systems, photonic crystal nanobeam cavities have attracted great attention for integrated on-chip trapping and manipulation. However, optical trapping with high efficiency and low input power is still a big challenge in nanobeam cavities because most of the light energy is confined within the solid dielectric region. To this end, by incorporating a nanoslotted structure into an ultracompact one-dimensional photonic crystal nanobeam cavity structure, we design a promising on-chip device with ultralarge trapping potential depth to enhance the optical trapping characteristic of the cavity. In this work, we first provide a systematic analysis of the optical trapping force for an airborne polystyrene (PS) nanoparticle trapped in a cavity model. Then, to validate the theoretical analysis, the numerical simulation proof is demonstrated in detail by using the three-dimensional finite element method. For trapping a PS nanoparticle of 10 nm radius within the air-slot, a maximum trapping force as high as 8.28 nN/mW and a depth of trapping potential as large as 1.15×105 kBT mW 1 are obtained, where kB is the Boltzmann constant and T is the system temperature. We estimate a lateral trapping stiffness of 167.17 pN·nm 1· mW 1 for a 10 nm radius PS nanoparticle along the cavity x-axis, more than two orders of magnitude higher than previously demonstrated on-chip, near field traps. Moreover, the threshold power for stable trapping as low as 0.087 μW is achieved. In addition, trapping of a single 25 nm radius PS nanoparticle causes a 0.6 nm redshift in peak wavelength. Thus, the proposed cavity device can be used to detect single nanoparticle trapping by monitoring the resonant peak wavelength shift. We believe that the architecture with features of an ultracompact footprint, high integrability with optical waveguides/circuits, and efficient trapping demonstrated here will provide a promising candidate for developing a lab-on-a-chip device with versatile functionalities.