Abstract: Mid-infrared (MIR) ultra-short pulses with multiple spectral-band coverage and good freedom in spectral and temporal shaping are desired by broad applications such as steering strong-field ionization, investigating bound-electron dynamics, and minimally invasive tissue ablation. However, the existing methods of light transient generation lack freedom in spectral tuning and require sophisticated apparatus for complicated phase and noise control. Here,with both numerical analysis and experimental demonstration, we report the first attempt, to the best our knowledge, at generating MIR pulses with dualwavelength spectral shaping and exceptional freedom of tunability in both the lasing wavelength and relative spectral amplitudes, based on a relatively simple and compact apparatus compared to traditional pulse synthesizers. The proof-of-concept demonstration in steering the high-harmonic generation in a polycrystalline ZnSe plate is facilitated by dual-wavelength MIR pulses shaped in both spectral and temporal domains, spanning from 5.6 to 11.4 μm, with multi-microjoule pulse energy and hundred- milliwatt average power. Multisets of harmonics corresponding to different fundamental wavelengths are simultaneously generated in the deep ultraviolet region, and both the relative strength of individual harmonics sets and the spectral shapes of harmonics are harnessed with remarkable freedom and flexibility. This work would open new possibilities in exploring femtosecond control of electron dynamics and light–matter interaction in composite molecular systems.
Abstract: As a new energy source, hydrogen (H2) detection is a hot topic in recent years. Because of the weak absorption characteristic, laser spectroscopy-based H2 detection is challenging. In this paper, a highly sensitive H2 sensor based on light-induced thermoelastic spectroscopy (LITES) technique is demonstrated for the first time. A continuous-wave, distributed feedback diode laser with emission in the 2.1 μm region was adopted as the excitation source to target the strongest H2 absorption line of 4,712.90 cm−1. A Herriott multipass cell with an optical length of 10.1 m was chosen to further
improve the H2 absorption. With the feature of processing the raw input data without data preprocessing and extracting the desired features automatically, the robust shallow neural network (SNN) fitting algorithm was brought in to denoise the sensor. For the LITES-based H2 sensor, the concentration response was tested, and an excellent linear response to H2 concentration levels was achieved. A minimum detection limit (MDL) of ~80 ppm was obtained. On the basis of implementation of the H2-LITES sensor, a heterodyne H2-LITES sensor was further constructed to realize a fast measurement of resonance frequency of quartz tuning fork and H2 concentration simultaneously. The resonance frequency can be retrieved in several hundred milliseconds with the measurement accuracy of ±0.2 Hz, and the result of 30,713.76Hz is exactly same as the experimentally determined value of 30,713.69 Hz. After the SNN algorithm was applied, an MDL of ~45 ppm was achieved for this heterodyne H2-LITES sensor.
Abstract: Optical beams carrying orbital angular momentum (OAM) play an important role in micro-/nanoparticle manipulation and information multiplexing in optical communications.Conventional OAM generation setups require bulky optical elements and are unsuitable for on-chip integration. OAM generators based on metasurfaces can achieve ultracompact designs. However, they generally have limited working bandwidth and require complex designs and multistep time-consuming fabrication processes.In comparison, graphene metalenses based on the diffraction principle have simple designs and can be fabricated by laser nanoprinting in a single step. Here, we demonstrate that a single ultrathin (200 nm) graphene OAM metalens can integrate OAM generation and high-resolution focusing functions in a broad bandwidth, covering the entire visible wavelength region. Broadband graphene OAM metalenses with flexibly controlled topological charges are analytically designed using the detour phase method considering the dispersionless feature of the graphene material and fabricated using ultrafast laser nanoprinting. The experimental results agree well with the theoretical predictions, which demonstrate the accuracy of the design method. The broadband graphene OAM metalenses can find broad applications in miniaturized and integrated photonic devices enabled by OAM beams.
Abstract: In the recent decade, single-shot ultrafast optical imaging by active detection,called single-shot active ultrafast optical imaging (SS-AUOI) here, has made great progress, e.g., with a temporal resolution of 50 fs and a frame rate beyond 10 trillion frames per second. Now, it has become indispensable for charactering the nonrepeatable and difficult-to-reproduce events and revealing the underlying physical, chemical, and biological mechanisms. On the basis of this delightful status, we would like to make a review of SS-AUOI. On the basis of a brief introduction of SS-AUOI, our review starts with discussing its characteristics and then focuses on the survey and prospect of SS-AUOI technology.The art that appears on the front cover of this issue was created by the Ultrafast Science Editorial Office. The art is derived from Jia et al., “Broadband Diffractive Graphene Orbital Angular Momentum Metalens by Laser Nanoprinting,” featured in this issue. Figures incorporated on the cover are used with permission from the authors.