Bo-Han Chen

We present a two-dimensional electronic spectroscopy (2DES) platform driven by a Coherent Loop-based Integrated Modulating and Beamsplitting System (CLIMBS). Combined with an octave-spanning multiple-plate continuum (MPC) source, the system enables ultrabroadband, phase-coherent measurements with attosecond-level delay precision. Its Sagnac-inspired, nearly common-path geometry provides excellent long-term phase stability without active feedback while eliminating beam walk-off during delay scans. Delay calibration using spectrally resolved interferometric fringes confirmed precise, linear coherence-time control. The MPC source generates 550-980 nm excitation pulses compressed to 3.7 fs, enabling simultaneous excitation of widely separated electronic and vibronic transitions with high temporal and spectral resolution, and supporting distortion-free measurements of ground-state bleaching, stimulated emission, excited-state absorption, and vibronic cross peaks. The system's performance was evaluated using chlorophyll a in methanol, where the excitation bandwidth fully covers the and bands, enabling distortion-free 2DES measurements across the electronic manifold. This simple and robust platform provides high-fidelity ultrabroadband 2DES for multidimensional spectroscopy.

The exploration of deactivation mechanisms for near-infrared(NIR)-emissive organic molecules has been a key issue in chemistry, materials science and molecular biology. In this study, based on transient absorption spectroscopy and transient grating photoluminescence spectroscopy, we demonstrate that the aggregated Pt ^II complex 4H (efficient NIR emitter) exhibits collective out-of-plane motions with a frequency of 32 cm ⁻¹ (0.96 THz) in the excited states. Importantly, similar THz characteristics were also observed in analogous Pt ^II complexes with prominent NIR emission efficiency. The conservation of THz motions enables excited-state deactivation to proceed along low-frequency vibrational coordinates, contributing to the suppression of nonradiative decay and remarkable NIR emission. These novel results highlight the significance of excited-state vibrations in nonradiative processes, which serve as a benchmark for improving device performance.

The vortex beam (Laguerre–Gaussian, LG ₁₀ mode) is employed to alleviate crystal damage in multiple-plate continuum generation. We successfully compressed 190-fs, 1030-nm pulses to 42 fs with 590 µJ input pulse energy, which is 5.5 times higher than that obtained by a Gaussian beam setup of the same footprint. High throughput (86%) and high intensity-weighted beam homogeneity (>98%) have also been achieved. This experiment confirms the great potential of beam shaping in energy up-scaling of nonlinear pulse compression.

The vortex beam (Laguerre–Gaussian, LG₁₀ mode) is employed to alleviate crystal damage in multiple-plate continuum generation. We successfully compressed 190-fs, 1030-nm pulses to 42 fs with 590 μJ input pulse energy, which is 5.5 times higher than that obtained by a Gaussian beam setup of the same footprint. High throughput (86%) and high intensity-weighted beam homogeneity (>98%) have also been achieved. This experiment confirms the great potential of beam shaping in energy up-scaling of nonlinear pulse compression.

Near-single-cycle mid-infrared pulses with a spectrum covering 5.4–11 μm are efficiently frequency-doubled in different GaSe crystals. The second-harmonic spectrum spans 3–4.3 μm at a power conversion efficiency of >20%>20%. We measure an effective nonlinear coefficient of ?eff≈35  pm/Vdeff≈35  pm/V. We also report on self-phase modulation and spectral broadening of the mid-infrared pulses in various bulk materials and find an increase of 45% of spectral width for 5 mm of Ge. These results demonstrate that nonlinear optical conversions can efficiently be driven by few-cycle mid-infrared radiation.

We report the generation of extremely broadband and inherently phase-locked mid-infrared pulses covering the 5 to 11 µm region. The concept is based on two stages of optical parametric amplification starting from a 270-fs Yb:KGW laser source. A continuum seeded, second harmonic pumped pre-amplifier in β-BaB₂O₄ (BBO) produces tailored broadband near-infrared pulses that are subsequently mixed with the fundamental pump pulses in LiGaS₂ (LGS) for mid-infrared generation and amplification. The pulse bandwidth and chirp is managed entirely by selected optical filters and bulk material. We find an overall quantum efficiency of 1% and a mid-infrared spectrum smoothly covering 5-11 µm with a pulse energy of 220 nJ at 50 kHz repetition rate. Electro-optic sampling with 12-fs long white-light pulses directly from self-compression in a YAG crystal reveals near-single-cycle mid-infrared pulses (32 fs) with passively stable carrier-envelope phase. Such pulses will be ideal for producing attosecond electron pulses or for advancing molecular fingerprint spectroscopy.

We report a surprisingly broadband and efficient midinfrared pulse generation in LiGaS2LiGaS2 (Langasite, LGS) by invoking a simultaneous interplay of intrapulse difference-frequency generation, self-phase modulation, and dispersion. This cascaded mechanism expands the output bandwidth and output power at the same time. With 30-fs driving pulses centered at 1030-nm wavelength we obtain a broadband middle-infrared spectrum of 8–11 μm with an LGS crystal as thick as 4 mm, which is eight times longer than the walk-off length.

We investigate two approaches for the spectral broadening and compression of 1-ps long pulses of a thin-disk laser amplifier running at 50 kHz repetition rate at 1030 nm wavelength. We find that with a single, 2.66-m long stretched flexible hollow fiber filled with xenon gas, Fourier transform limited output pulse duration of 66 fs can be directly reached. For larger pulse shortening, we applied a hybrid cascaded approach involving a BBO-based pre-compressor and a long hollow fiber. We could achieve 33-times temporal shortening of 1-ps pulses down to a duration of 30 fs at an overall efficiency of ~29% with an output power level of 3.7 W. These results demonstrate the potential of stretched flexible fibers with their free length scalability for shortening laser pulses of moderate peak power.