Optical microscopy provides sub-cellular and high-speed imaging to capture neuron dynamics in a living brain, but its penetration depth is limited by tissue scattering. Multiphoton excitation improves the depth to over 1 mm, while combining with a gradient refractive index (GRIN) lens enables centimeter penetration with minimal invasiveness. However, the system performance is compromised due to the intrinsic optical aberrations of GRIN lenses, which severely reduce the contrast, spatial resolution, and effective field of view (FoV). To address this issue, we developed a 3D aberration correction approach for GRIN lenses by combining spiral scanning with cylindrical deconvolution. This method leverages the cylindrical symmetry of GRIN-induced aberrations and incorporates the spatially varying point-spread function (PSF) across the imaging volume. Radially adaptive excitation implemented through spiral scanning expanded the usable FoV diameter by nearly 2-fold and achieved 30- and 10-fold improvement, respectively, in peripheral signal intensity and signal-to-noise ratio (SNR) compared to conventional raster scanning with uniform excitation, while cylindrical deconvolution improved spatial resolution by up to 3.5-fold. We further validated this method through 3D imaging of neuronal structures, demonstrating enhanced effective volume size and a 2-fold improvement in neuronal SNR. These results indicate that the spiral scanning and algorithm-augmented GRIN 2PF system is promising toward resolving structure/functional connectomics in deep brain regions.

Gradient refractive index (GRIN) lenses are widely used in multiphoton microscopy to access deep brain regions with minimal invasiveness. However, intrinsic aberrations, particularly field-dependent astigmatism, severely degrade spatial resolution, contrast, and usable field of view (FOV), limiting their effectiveness and throughput for volumetric imaging. Here, we present an aberration correction framework that combines spiral scanning with a cylindrical deconvolution algorithm without requiring additional optical components. This method leverages the cylindrical symmetry of GRIN-induced aberrations and the spatially varying point spread functions (PSFs) across the volume, resulting in an expanded effective FOV and up to a 3.5-fold improvement in spatial resolution. Our method provides a simple and efficient solution for high-contrast deep-brain imaging using GRIN-based microendoscopy.

In this work, we present a novel indolo[2,3-a]carbazole-based compound for potential bioimaging applications. The introduction of a planar electron-accepting naphthyridine moiety allows to facilitate charge separation, reduce the energy gap, and to improve intersystem crossing. Notably, the singlet-triplet energy splitting of 0.49 eV, as shown from theoretical and experimental data, could promote not only efficient triplet state formation via conventional ISC to T1 -> S1 but also suggests the potential for the involvement of higher-lying triplet states like T2 -> S1 in the emission pathway. This large splitting may allow for efficient population of upper triplet states that can then undergo efficient high-lying reverse intersystem crossing to emissive singlet state. The near-infrared excitation spectra and square dependency of fluorescence intensity over excitation power confirm the potential of the indolo[2,3-a]carbazole-based probe for advanced two-photon imaging applications.

In this work, we present two very sensitive new heavy metal-free thianthrene-based oxygen analytes. Experimental results in combination with theoretical calculations, confirm that both the compounds display roomtemperature phosphorescence, with a significant difference in intensity observed in air and oxygen-free environments. The compounds show long emission lifetimes reaching up to 27.08 ms. This indicates the involvement of oxygen-sensitive triplet emission which occur due to intersystem crossing. The compounds show record-high Stern-Volmer constants, reaching 1.57 x 10- 2 ppm- 1. These properties suggest that 2-(pentan-3-yl)-6-(anthracen-1-yl)-1H-benzo[de]isoquinolin-1,3(2 H)-dione and 4-(thianthren-1-yl)benzonitrile are perfect candidates for metal-free oxygen sensors with one of the highest sensitivity detection at low oxygen concentrations reported so far.

Vanadium dioxide (VO 2 ), a representative phase‐transition material, exhibits nonlinear optical responses during its transition from an insulating to a metallic state. By investigating the transient photoresponses of a VO 2 thin film at different temperatures, additional slow‐growth and hybrid phases are discovered between the insulating phase at low‐temperature and metallic phase at high‐temperature. The amplitude of the VO 2 photoresponse at 68 °C in the slow‐growth phase is observed to be 5.5 times greater than that at room temperature. The controllability of relaxation time is also demonstrated from a few tens of picoseconds to a few nanoseconds. The optimized conditions for all‐optical modulation applications are explored.

Vanadium dioxide (VO2), a representative phase-transition material, exhibits nonlinear optical responses during its transition from an insulating to a metallic state. By investigating the transient photoresponses of a VO2 thin film at different temperatures, additional slow-growth and hybrid phases are discovered between the insulating phase at low-temperature and metallic phase at high-temperature. The amplitude of the VO2 photoresponse at 68 degrees C in the slow-growth phase is observed to be 5.5 times greater than that at room temperature. The controllability of relaxation time is also demonstrated from a few tens of picoseconds to a few nanoseconds. The optimized conditions for all-optical modulation applications are explored.

The biological clock synchronizes with the environmental light-dark cycle through circadian photoentrainment. While intracellular pathways regulating clock gene expression after light exposure in the suprachiasmatic nucleus are well studied in mammals, the neuronal circuits driving phase shifts remain unclear. Here, using a mouse model, we show that chemogenetic activation of early-night light-responsive neurons induces phase delays at any circadian time, potentially breaking the photoentrainment dead zone. In contrast, activating late-night light-responsive neurons mimics light-induced phase shifts. Using in vivo two-photon microscopy, we found that most neurons in the suprachiasmatic nucleus exhibit stochastic light responses, while a small subset is consistently activated in the early subjective night and another is inhibited in the late subjective night. Our findings suggest a dynamic bi-stable network model for circadian photoentrainment, where phase shifts arise from a functional circuit integrating signals to groups of outcome neurons, rather than a labeled-line principle seen in sensory systems.

A novel double differential photothermal technique substantially enhances the sensitivity of Raman signal detection by 50 times compared to the conventional Stimulated Raman scattering method, offering great potential for ultra-sensitive label-free molecular imaging.

Artificial nanostructures with ultrafine and deep-subwavelength features have emerged as a paradigm-shifting platform to advanced light-field management, becoming key building blocks for high-performance integrated optoelectronics and flat optics. However, direct optical inspection of integrated chips remains a missing metrology gap that hinders quick feedback between design and fabrications. Here, we demonstrate that photothermal nonlinear scattering microscopy can be used for direct imaging and resolving of integrated optoelectronic chips beyond the diffraction limit. We reveal that the inherent coupling among deep-subwavelength nanostructures supporting leaky resonances allows for the pronounced heating effect to access reversible nonlinear modulations of the confocal reflection intensity, yielding optical resolving power down to 80 nm (~λ/7). The versatility of this approach has been exemplified by imaging silicon grating couplers and metalens with minimum critical dimensions of 100 nm, as well as central processing unit chip with 45-nm technology, unfolding the long-sought possibility of in situ, nondestructive, high-throughput optical inspection of integrated optoelectronic and nanophotonic chips.

We demonstrate an all-optical method for transmissivity control in silicon metasurfaces, overcoming the tradeoff between absorption for photothermal heating and scattering intensity. By leveraging interference between dipole and quadrupole modes, we achieve efficient wavelength separation, enabling high absorption at the control wavelength without sacrificing signal transmission. This approach results in a 381% intensity modulation at λ S = 473 nm under control light irradiation (λ C = 405 nm) at 3.67 mW/μm 2 . Our method significantly enhances modulation efficiency compared to conventional techniques, paving the way for ultrathin all-optical switches and active wavefront control devices.

A novel double differential photothermal technique substantially enhances the sensitivity of Raman signal detection by 50 times compared to the conventional Stimulated Raman scattering method, offering great potential for ultra-sensitive label-free molecular imaging.

We demonstrated a far-field super-resolution optical imaging for mapping the resonance mode within semiconductor nanowires, where periodic distributions are found with good agreement between simulation and experiment. The pronounced absorption at the antinodes leads to localized photothermal heating, as well as consequent scattering nonlinearity via the thermo-optic effect. To break the diffraction limit, we combine the scattering nonlinearity with tightly focused laser scanning. Based on the principle of saturated excitation (SAX) microscopy, the nonlinear scattering signals are extracted to significantly improve the spatial resolution (1.7 fold), enabling visualization of the resonant modes that are not visible with conventional far-field optical imaging. Our results pave the way for optical inspection of semiconductor photonic integrated circuits with subdiffraction-limit spatial resolution.

ABSTRACT Exploration of neural activity at high spatiotemporal resolution in live animals is essential for advancing the understanding of brain function. Multiphoton microscopy has emerged over the past three decades as a powerful tool for in vivo neuroimaging, providing 3D subcellular spatial resolution and sub‐second temporal resolution. However, its imaging depth is fundamentally limited to approximately 2 mm due to light scattering, leaving most subcortical brain regions inaccessible in mammals. Gradient refractive index (GRIN) lens‐based multiphoton microendoscopy offers a minimally invasive approach that extends imaging depth up to 10 cm while maintaining 3D µm resolution. The technique, however, remains constrained by intrinsic optical aberrations of GRIN lenses, which degrade image quality and limit both the field of view and the imaging volume. Recent advances, including adaptive optics, aspheric correctors, and geometric transformation techniques, provide state‐of‐the‐art aberration correction and expand the volume of view to the cubic millimeter scale. Applications of GRIN multiphoton microendoscopy in functional neuroimaging demonstrate its potential for high‐throughput volumetric imaging with enhanced spatiotemporal resolution. These innovations enable longitudinal studies of large‐scale neural dynamics and support the development of next‐generation photonic systems for deep brain connectome mapping.

Nonlinear optical properties of the silicon nanostructures have attracted attention to extend the application of silicon photonics toward all-optical switching and super-resolution optical microscopy. In particular, our group discovered that the use of iterative photothermal and thermo-optic effects on Mie-resonant silicon nanostructure is the key to inducing the strong nonlinear relationships between scattering intensity and excitation intensity, with an effective nonlinear coefficient at a few orders larger scale than the Kerr nonlinearity. In this talk, we present our recent research effort to further enhance the nonlinearity via exploiting several different physical mechanisms: quasi-perfect absorbing Huygens' metasurface, displaced excitation by tightly focused laser, and optical bistability. In addition, we applied the nonlinear scattering responses of silicon nanostructures to improve the spatial resolution in laser scanning scattering microscopy. By using our home-built super-resolution imaging technique, we clearly observed densely located silicon nanostructures, which were not possible to resolve in conventional optical microscopy, with the non-label and non-contact advantages.

The circadian clock, an evolutionarily conserved mechanism regulating the majority of physiological functions in many organisms, is synchronized with the environmental light-dark cycle through circadian photoentrainment. This process is mediated by light exposure at specific times, leading to a discrete phase shift including phase delays during early subjective night, phase advances during late subjective night, and no shift at midday, known as the dead zone. In mammals, such as mice, the intrinsically photosensitive retinal ganglion cells (ipRGCs) are crucial for conveying light information to the suprachiasmatic nucleus (SCN), the central clock consisting of approximately 20,000 neurons. While the intracellular signaling pathways that modulate clock gene expression post-light exposure are well-studied, the functional neuronal circuits responsible for the three discrete light responses are not well understood. Utilizing in vivo two-photon microscopy with gradient-index (GRIN) endoscopes, we have identified seven distinct light responses from SCN neurons. Our findings indicate that light responses from individual SCN neurons are mostly stochastic from trial to trial. However, at the population level, light response composition remains similar across trials, with only minor variations between circadian times, suggesting a dynamic populational coding for light input. Additionally, only a small subset of SCN neurons shows consistent light responses. Furthermore, by utilizing the targeted recombination in active populations (TRAP) system to label neurons that respond to light during early subjective night, we demonstrate that their activation can induce phase delays at any circadian time, effectively breaking the gate that produce photoentrainment dead zone typically observed at midday. Our results suggest the existence of at least two separate time-dependent functional circuits within the SCN. We propose a dynamic bi-stable network model for circadian photoentrainment in the mammalian central clock, where a shifting clock is driven by a dynamic functional circuit utilizing population coding to integrate information flow similar to proposed cortical computational network, rather than a simplistic, consistent linear circuit. ### Competing Interest Statement The authors have declared no competing interest.