Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams or beams with helical phase structure. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 1936, when he reported a tiny torque developed in a quartz waveplate due to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating. We have observed rotation rates in excess of 350 Hz.

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Optical alignment and spinning of laser-trapped microscopic particles

Thermal performance of a thermosyphon heat pipe evacuated tube solar collector using silver-water nanofluid for commercial applications

Generating electron coherence in quantum materials is essential in optimal control of many-body interactions and correlations. In a multidomain system this signifies nonlocal coherence and emergence of collective phenomena, particularly in layered 2D quantum materials possessing novel electronic structures and high carrier mobilities. Here we report nonlocal ac electron coherence induced in dispersed MoS2 flake domains, using coherent spatial self-phase modulation (SSPM). The gap-dependent nonlinear dielectric susceptibility χ(3) measured is surprisingly large, where direct interband transition and two-photon SSPM are responsible for excitations above and below the bandgap, respectively. A wind-chime model is proposed to account for the emergence of the ac electron coherence. Furthermore, all-optical switching is achieved based on SSPM, especially with two-color intraband coherence, demonstrating that electron coherence generation is a ubiquitous property of layered quantum materials.

https://www.pnas.org/content/pnas/112/38/11800.full.pdf

Emergence of electron coherence and two-color all-optical switching in MoS2 based on spatial self-phase modulation

Femtosecond laser ablation in solution (FLAS) is a unique and simple technique addressing the drawbacks of the conventional methods giving access to preparation of an ultra-broad spectrum of nanomaterials (NMs), since the NMs can be produced from nearly any solid materials and variable precursors in different solutions. After the energy injection in ultra-short time, extreme conditions with high temperature, high pressure and high cooling rate are created, under which many interesting phenomena occur, including nanomaterial formation. Characteristic advantages of this method are the applications in preparing a huge variety of NMs, the high purity of the products, and the in situ dispersion and functionalization of the as-prepared NMs in various solutions. This review is devoted to give a comprehensive understanding of the fundamental mechanisms of FLAS. A map of variable NMs prepared by FLAS and the promising applications are built up to pave the way for more future work in this field.

Preparation of functional nanomaterials with femtosecond laser ablation in solution

Synthesis of TiO2 nanoparticles by electrochemical method and their antibacterial application

Femtosecond laser ablation of silver plate placed in water is used to produce nanoparticle suspension. The method is easy to operate and the suspension is relatively stable. The optical properties and the size distribution of the suspension are studied with UV–vis absorption spectroscopy and dynamic light scattering, respectively. The shape of the nanoparticles is investigated by an atomic force microscope, which is near spherical. There are two kinds of nanoparticles, small particles with diameter about 35 nm, and large particles with diameter about 120nm.

https://iopscience.iop.org/article/10.1088/0256-307X/25/12/078/pdf

Preparation and Size Characterization of Silver Nanoparticles Produced by Femtosecond Laser Ablation in Water

We show that the refractive index change induced by a focused incident beam with an additional incoherent uniform illumination in photovoltaic-photorefractive crystals under open-circuit condition has a saturable nonlinearity form. The incoherent uniform background illumination can be used to increase the effective dark irradiance. The formation time of the photovoltaic soliton can be decreased by keeping the intensity of the soliton at a higher value without over-saturation by use of the background illumination.

Saturable Nonlinearity in Photovoltaic-Photorefractive Crystals Under Open-circuit Condition

Normalized Mandel Parameter (Q) is introduced as a new measurement of photon statistics. Dependences of Mandel parameter Q and corresponding normalized Mandel parameter (Q) on photon-counting time interval are experimentally investigated for pseudo-thermal light. We demonstrate that (Q) is more appropriate than Q to measure the statistical deviation from Poisson distribution, because (Q) presents clearly both the intrinsic statistical properties and measuring effects. The advantages of (Q) in charactering nonclassical emissions are also discussed.

https://iopscience.iop.org/article/10.1088/0256-307X/25/11/038/pdf

Characterization of Photon Statistical Properties with Normalized Mandel Parameter

We report our laser-driven method used to make large quantities of straight thin silver nanowires, and experimentally demonstrate that femtosecond laser pulse polarization has a prominent effect on formation of non-spherical shapes of nanoscale particles. Further, our experiment directly reveals that the underlying mechanism is plasmon-plasmon interaction, which can be controlled by polarization and plays a decisive role in this non-synthetic method for metal nanowire formation.

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Laser-Driven Silver Nanowire Formation: Effect of Femtosecond Laser Pulse Polarization

Forward degenerate four-wave mixing (DFWM) processes are investigated with a femtosecond pulsed laser in lithium niobate crystal doubly-doped with magnesium and iron (LiNbO3:Fe,Mg). The pulse energy dependence reveals a pure third-order nonlinear response, and the third-order nonlinear susceptibility χ(3) in the material is evaluated to be 4.96×10−13 esu. The time-resolved DFWM process shows a response time of χ(3) shorter than 100 fs, which is due to the nonresonant electronic nonlinearities. Our results indicate that LiNbO3 crystals have potentials for ultrafast real-time optical processing systems, which require a large and fast χ(3) optical nonlinearity.

Zhang Xin-Zheng

Papers

Preparation and Size Characterization of Silver Nanoparticles Produced by Femtosecond Laser Ablation in Water

Saturable Nonlinearity in Photovoltaic-Photorefractive Crystals Under Open-circuit Condition

Characterization of Photon Statistical Properties with Normalized Mandel Parameter

Laser-Driven Silver Nanowire Formation: Effect of Femtosecond Laser Pulse Polarization

Time-Resolved Femtosecond Degenerate Four-Wave Mixing in LiNbO 3 :Fe,Mg Crystal