arXiv subjects

Mengjia He

Publications and source records attributed to Mengjia He.

4 recordsLinked to original sources

arXiv2025

Finite elements and moving asymptotes accelerate quantum optimal control -- FEMMA

Quantum optimal control is central to designing spin manipulation pulses. Gradient-based pulse optimization can be facilitated by either accelerating gradient evaluation or enhancing the convergence rate. In this work, we accelerated single-spin optimal control by combining the finite element method with the method of moving asymptotes. By treating discretized time as spatial coordinates, the Liouville - von Neumann equation was reformulated as a linear system, efficiently yielding a joint solution of the spin trajectory and control gradient. The method of moving asymptotes, relying on the ensemble fidelities and gradients, achieves rapid convergence for a target fidelity of 0.995.

physics.chem-ph↗

arXiv2019

Valley kink states and topological channel intersections in substrate-integrated photonic circuitry

Valley degrees of freedom, providing a novel way to increase capacity and efficiency of information processing, have become an important instrument for photonics. Experimental studies on photonic topological valley kink states at interfaces between regions with opposite valley-Chern numbers have attracted much attention, however, were restricted to zigzag-type interfaces, largely limiting their applications such as geometry-dependent topological channel intersections. Here, we experimentally demonstrate and manipulate valley kink states at generic interfaces in subwavelength substrate-integrated photonic circuitry. We verify the robustness of the kink states by measuring transmissions of the kink states through twisted interfaces and interfaces with disorders. Based on the valley kink states at generic interfaces, we realize several topological channel intersections where photonic transport paths are related to geometries of the intersections. In comparison to those in previous works, the present valley photonic crystals have subwavelength thicknesses and excellent self-consistent electrical shielding, which are perfectly compatible with conventional substrate-integrated photonic circuitry. Our work opens a door to manipulate photonic valley pseudo-spins in substrate-integrated circuitry with robustness, easy access, and lightweight.

physics.app-ph↗

arXiv2019

Realization of a three-dimensional photonic topological insulator

Confining photons in a finite volume is in high demand in modern photonic devices. This motivated decades ago the invention of photonic crystals, featured with a photonic bandgap forbidding light propagation in all directions. Recently, inspired by the discoveries of topological insulators (TIs), the confinement of photons with topological protection has been demonstrated in two-dimensional (2D) photonic structures known as photonic TIs, with promising applications in topological lasers and robust optical delay lines. However, a fully three-dimensional (3D) topological photonic bandgap has never before been achieved. Here, we experimentally demonstrate a 3D photonic TI with an extremely wide (> 25% bandwidth) 3D topological bandgap. The sample consists of split-ring resonators (SRRs) with strong magneto-electric coupling and behaves as a 'weak TI', or a stack of 2D quantum spin Hall insulators. Using direct field measurements, we map out both the gapped bulk bandstructure and the Dirac-like dispersion of the photonic surface states, and demonstrate robust photonic propagation along a non-planar surface. Our work extends the family of 3D TIs from fermions to bosons and paves the way for applications in topological photonic cavities, circuits, and lasers in 3D geometries.

physics.class-ph↗

arXiv2018

Valley-Hall photonic topological insulators with dual-band kink states

Extensive researches have revealed that valley, a binary degree of freedom (DOF), can be an excellent candidate of information carrier. Recently, valley DOF has been introduced into photonic systems, and several valley-Hall photonic topological insulators (PTIs) have been experimentally demonstrated. However, in the previous valley-Hall PTIs, topological kink states only work at a single frequency band, which limits potential applications in multiband waveguides, filters, communications, and so on. To overcome this challenge, here we experimentally demonstrate a valley-Hall PTI, where the topological kink states exist at two separated frequency bands, in a microwave substrate-integrated circuitry. Both the simulated and experimental results demonstrate the dual-band valley-Hall topological kink states are robust against the sharp bends of the internal domain wall with negligible inter-valley scattering. Our work may pave the way for multi-channel substrate-integrated photonic devices with high efficiency and high capacity for information communications and processing.

physics.app-ph↗