Yi Dong Chong

Performant on-chip spectrometers are important for advancing sensing technologies, from environmental monitoring to biomedical diagnostics. As device footprints approach the scale of the operating wavelength, previous strategies, including those relying on multiple scattering in diffusive media, face fundamental accuracy constraints due to limited optical path lengths and noise-sensitive random transmission matrices. Here, we demonstrate a wavelength-scale, CMOS-compatible on-chip spectrometer that overcomes the challenges by exploiting inverse-designed quasinormal modes in a complex photonic resonator. These modes extend the effective optical path length beyond the physical device dimensions, producing highly decorrelated spectral responses. We show that this strategy is theoretically optimal for minimizing spectral reconstruction error in the presence of noise. The fabricated spectrometer occupies a lateral footprint of only 3.5 times the free-space operating wavelength, with a spectral resolution of 10 nm across the 3590-3760 nm mid-infrared band, which is suitable for various molecular sensing. The design of this miniaturized noise-resistant spectrometer is readily extensible to other portions of the electromagnetic spectrum, paving the way for lab-on-a-chip devices, chemical sensors, and other applications.

Non‐Hermiticity has introduced new physical mechanisms into sensing, with approaches based on exceptional points and non‐Hermitian skin effects demonstrating potential sensitivity enhancements over conventional sensing technologies. By monitoring the frequency shifts of specific eigenmodes, previous studies on non‐Hermitian sensors have revealed extraordinary sensitivity to local perturbations. In contrast, the influence of global perturbations such as noise and disorder, which generally involve complex spectra and may even suppress these eigenmodes, seems largely incompatible with the current non‐Hermitian sensing framework and has received far less attention. Here, motivated by recent theoretical advances on pseudospectra theory, we investigate the possibility of employing maximum transient growth to probe the level of global perturbations in non‐Hermitian skin‐effect sensors. Using discrete‐time light walks in synthetic photonic lattices, we experimentally evaluate the performance of a non‐Hermitian photonic lattice under static global phase noise. Remarkably, we demonstrate that the sensitivity grows exponentially with lattice size, manifesting in the maximum transient growth rather than the spectral shifts of previous non‐Hermitian skin‐effect sensors. Furthermore, numerical simulations reveal that this exponential sensitivity is preserved under dynamical perturbations. Our results highlight the limits as well as the potential of non‐Hermitian systems to tackle a wide range of sensing requirements for next‐generation ultrasensitive sensors.

Non‐Hermiticity has introduced new physical mechanisms into sensing, with approaches based on exceptional points and non‐Hermitian skin effects demonstrating potential sensitivity enhancements over conventional sensing technologies. By monitoring the frequency shifts of specific eigenmodes, previous studies on non‐Hermitian sensors have revealed extraordinary sensitivity to local perturbations. In contrast, the influence of global perturbations such as noise and disorder, which generally involve complex spectra and may even suppress these eigenmodes, seems largely incompatible with the current non‐Hermitian sensing framework and has received far less attention. Here, motivated by recent theoretical advances on pseudospectra theory, we investigate the possibility of employing maximum transient growth to probe the level of global perturbations in non‐Hermitian skin‐effect sensors. Using discrete‐time light walks in synthetic photonic lattices, we experimentally evaluate the performance of a non‐Hermitian photonic lattice under static global phase noise. Remarkably, we demonstrate that the sensitivity grows exponentially with lattice size, manifesting in the maximum transient growth rather than the spectral shifts of previous non‐Hermitian skin‐effect sensors. Furthermore, numerical simulations reveal that this exponential sensitivity is preserved under dynamical perturbations. Our results highlight the limits as well as the potential of non‐Hermitian systems to tackle a wide range of sensing requirements for next‐generation ultrasensitive sensors.

Coherent perfect absorption (CPA)-the time-reversed operation of lasing at threshold-relies on finely tuned interference and is intrinsically fragile to disorder and structural imperfections. Whether absorption can be endowed with topological protection, by analogy to topological lasing, has remained an open question. Here, we experimentally demonstrate a topological antilaser: the time-reversed counterpart of a topological laser, in which chiral edge modes of a photonic lattice enable perfect light absorption protected by topology. Using a nonreciprocal microwave network with low intrinsic loss, we show that the topological antilaser preserves near-unity absorption under strong disorder, and, unlike conventional antilasers, remains functional for arbitrary placements of dissipation and input ports, even when the lattice is strongly perturbed. This robustness arises from the disorder-immune propagation and stable spatial profile of the topological edge modes. Our results establish topologically protected absorption as the missing counterpart of topological lasing, opening new directions for studying robust energy dissipation, wave control, and coherent-absorption-based detection technologies.

Photonic crystal fibers (PCFs) provide a versatile platform for various applications, thanks to the flexibility with which light guiding can be customized by modifying the fiber geometry. We realize a PCF with guided modes produced by photonic band structure topology rather than conventional mode-trapping mechanisms. The design, which is compatible with the stack-and-draw fabrication process, consists of a cross-sectional photonic topological crystalline insulator with a disclination. A bulk-defect correspondence produces degenerate topological modes, lying below the cladding light line. We use various theoretical methods to confirm their topological origins, including a spectral localizer that makes minimal assumptions about the band structure. Our experiments on the fabricated fiber show it transmitting visible to near-infrared light with low losses of 10 to 20 decibels per kilometer, which do not increase substantially when the fiber is bent. A comparable solid-core PCF of conventional design exhibits substantially higher bending losses. Optical fibers based on topological modes hold promise for improved performance and versatile functionalities.