Stefano Longhi
It is theoretically shown that an end-pumped microchip laser formed by a thin laser crystal with plane-plane but slightly tilted facets can emit, under appropriate pumping conditions and near a crystal edge, a truncated self-accelerating Airy output beam.
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By cryogenically cooling the Yb:YAG/Cr:YAG medium, one can break through the damage limit of Yb:YAG/Cr:YAG passively Q-switched microchip lasers at room temperature and thus achieve a shorter minimum pulse duration. In the proof of principle experiment we carried out, a 160.6 ps pulse duration was obtained. To the best of our knowledge, this is the first realization of sub-200 ps pulse operation for an Yb:YAG/Cr:YAG microchip laser
The spatially periodic 2D patterns at output mirror of solid state microchip laser with high Fresnel number (100-1000) are discussed in view of numerical modeling with split-step FFT code comprising nonlinear gain, relaxation of inversion and paraxial diffraction.
Using a Nd:YVO4 microchip laser with a relaxation frequency in the megahertz range, we have experimentally compared a heterodyne interferometer based on a Michelson configuration with an autodyne interferometer based on the laser optical feedback imaging (LOFI) method regarding their signal to noise ratios. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity while in the autodyne configuration, the wave beating takes place inside the laser cavity and the relaxation oscillations of the laser intensity then play an impo…
Microchip semiconductor membrane external-cavity surface-emitting laser
该研究首次展示微芯片薄膜外腔表面发射激光器，通过薄增益膜与散热片构成紧凑腔体，实现超1瓦连续输出、51.4%斜率效率及优异光束质量，为高功率可调波长激光提供紧凑方案。
the realization of high repetition rate passively Q-switched monolithic microlaser is a challenge since a decade. To achieve this goal, we report here on the first passively Q-switched diode-pumped microchip laser based on the association of a Nd:GdVO4 crystal and a Cr4+:YAG saturable absorber. The monolithic design consists of 1 mm long 1% doped Nd:GdVO4 optically contacted on a 0.4 mm long Cr4+:YAG leading to a plano-plano cavity. A repetition rate as high as 85 kHz is achieved. The average output power is approximately 400 mW for 2.2 W of absorbed pump power and the pulse length is 1.1 ns.
Square vortex lattices predicted theoretically and experimentally observed in diode-pumped solid state microchip lasers are shown to have a remarkable symmetry. These lattices are formed by counter-rotating vortices. The interaction of vortices with nonlinear gain medium leads to precession of vortices and collective excitations with dynamics identical to acoustical and optical vibrations of atomic lattices.
The spatiotemporal vortex lattices generated in high Fresnel number solid-state microchip lasers are studied in connection with Talbot phenomenon generic to spatially periodic electromagnetic fields. The spatial layout of light field is obtained via dynamical model based on Maxwell-Bloch equations for class-B laser, discrete Fox-Lee map with relaxation of inversion and static model based on superposition of copropagating Gaussian beams. The spatial patterns observed experimentally and obtained numerically are interpreted as nonlinear superposition of vortices with helicoidal phase dislocations…
We analytically calculate the transverse energy fluxes that would be respectively induced in high-power Airy beams by the Kerr self-focusing and the Airy profile itself if they were the only active process. In experimental condition representative of laser filamentation experiments of high-power ultrashort laser pulses in air and condensed media, the Kerr lens induces transverse energy fluxes much larger than the Airy "prism" at the main peak. As a consequence, the curved plasma channels in Airy beams are not only a plasma spark on a curved focus, but indeed self-guided filaments, and their cu…
Spatiotemporal coupled Airy-Airy wavepacket and its propagation dynamics
本文首次生成并控制时空耦合 Airy-Airy 波包，在保持垂直分布的同时实现旋转，并验证其自加速与自愈特性，为超快激光管理、光学微操控和时域编码通信提供新策略。
Spontaneous transverse mode locking to generate optical vortex arrays is already widely studied for the frequency degenerated transverse modes. While, the mode locking between transverse modes in different orders (non-frequency degenerated families) to achieve spatial stationary beam patterns is rarely reported. The theory of transverse mode locking is discussed. Concepts of general transverse mode locking (G-TML) and restricted transverse mode locking (R-TML) are mentioned. It's experimentally shown that R-TML is possible in microchip cavities with high nonlinearity. More interestingly, some …
Detailed polarization properties have been examined in laser-diode-pumped (LD-pumped) micro-grained ceramic Nd:YAG lasers in different microchip cavity configurations. Stable linearly-polarized single-frequency oscillations, whose polarization direction coincide with that of an LD pump light, were observed in an external cavity scheme. While, in the case of a thin-slice laser scheme with coated reflective ends, elliptically-polarized single-frequency operations took place in the low pump-power regime and dynamic instabilities appeared, featuring self-induced antiphase modulations among counter…
We report on a simple experimental scheme based on a pair of cylindrical lenses (convex and concave) of same focal length and common optical elements producing high power optical beams in 1D and/or 2D Airy intensity profiles with laser polarization as control parameter. Using an ultrafast Yb-fiber laser at 1064 nm of average power of 5 W in Gaussian spatial profile and pulse-width of ~180 fs, we have generated 1D and 2D Airy beams at an efficiency of 80% and 70%, respectively, and pulse width of ~188 fs and ~190 fs, respectively. We have measured the transverse deflection rate of 1D and 2D bea…
A transverse mode selective laser system with gain regulation by a digital micromirror device (DMD) is presented in this letter. The gain regulation in laser medium is adjusted by the switch of the patterns loaded on DMD. Structured pump beam patterns can be obtained after the reflection of the loaded patterns on DMD, and then it's defocused into a microchip laser medium by a short focal lens, so that the pump patterns can be transferred to the gain medium to regulate the gain distribution. Corresponding structured laser beams can be generated by this laser system. The laser beam pattern can b…
Airy Resonances in Photonic Crystal Superpotentials
本文在二维硅基光子晶体中观测到源于晶格常数线性变化的艾里共振，通过映射为含线性势的非厄米薛定谔方程，揭示了面外辐射导致的复数有效质量对集体光学响应的根本影响。
Microcantilever-integrated photonic circuits for broadband laser beam scanning
本文提出一种微悬臂集成光子电路激光束扫描器，在约0.1 mm²面积和31-46 mW功耗下实现410-700 nm宽带一维和二维光束转向，兼具紧凑、宽带和低功耗优势。
We show that light spiraling like a tornado can be generated by superimposing abruptly auto-focusing ring-Airy beams that carry orbital angular momentum of opposite handedness. With different parabolic propagation trajectories, the superimposing ring-Airy beams are tailored to abruptly auto-focus at overlapping focal regions. This results to a complex wave with intense lobes that twist and shrink in an accelerating fashion along propagation. By achieving angular acceleration values that exceed 295 rad\mm^2, these Tornado waves can find numerous applications in laser trapping, direct laser writ…
High-power ultrafast radially and azimuthally polarized accelerating Airy beams and their particle-like lattice topologies
本研究理论结合实验探究径向与角向偏振的高功率超快矢量Airy光束，给出空间谱解析表达，用飞秒刻写纳米光栅制备几何相位元件实现光束生成，还发现其类粒子拓扑晶格结构。
In this paper, we experimentally demonstrate reversible wavefront shaping through mimicking gravitational field. A gradient-index micro-structured optical waveguide with special refractive index profile was constructed whose effective index satisfying a gravitational field profile. Inside the waveguide, an incident broad Gaussian beam is firstly transformed into an accelerating beam, and the generated accelerating beam is gradually changed back to a Gaussian beam afterwards. To validate our experiment, we performed full-wave continuum simulations that agree with the experimental results. Furth…
Since the first experimental observation of optical Airy beams, various applications ranging from particle and cell micromanipulation to laser micromachining have exploited their non-diffracting and accelerating properties. The later discovery that Airy beams can self-heal after being blocked by an obstacle further proved their robustness to propagate under scattering and disordered environment. Here, we report the generation of Airy beams on an integrated silicon photonic chip and demonstrate that the on-chip 1D Airy beams preserve the same properties as the 2D beams. The 1D meta-optics used …
We demonstrate curved modifications with lengths of up to 2 mm within borosilicate glass produced by single 1030 nm picosecond laser shots with an Airy beam profile. Plasma ignition in the side lobes of the beam as well as surface damage prove to be the crucial limitations for confined bulk energy deposition on a curved trajectory. A combined experimental and numerical analysis reveals optimum laser parameters for confined bulk energy deposition. This way we achieved single pass cutting of a 525 $μ$m thick glass sheet with a well defined convex edge down to a bending radius of 774 $μ$m.
Nonlinear dynamics and self-healing properties of elliptical Airy beams in Kerr media
本文通过数值求解非线性薛定谔方程，研究了Kerr介质中椭圆艾里光束的非线性传播与自愈特性，提出了最佳输入功率以延长传播距离并增强稳定性，适用于远距离激光传输。
Dielectric laser acceleration (DLA) achieves remarkable gradients from the optical near fields of a grating structure. Tilting the dielectric grating with respect to the electron beam leads to deflection forces and the DLA structure can be utilized as a microchip undulator. We investigate the beam dynamics in such structures analytically and by numerical simulations. A crucial challenge is to keep the beam focused, especially in direction of the narrow channel. An alternating phase focusing scheme is optimized for this purpose and matched lattice functions are obtained. We distinguish synchron…
Pentagonal Photonic Crystal Mirrors: Scalable Lightsails with Enhanced Acceleration via Neural Topology Optimization
针对突破摄星计划对高性能低成本光帆的需求，研究采用神经拓扑优化得到新型五边形晶格光子晶体反射器，可缩短加速时间、大幅降低制造成本，还制备出高长径比纳米光子反射元件，为低成本太空探索提供了可行方案。
Light beams with extraordinary spatial structures, such as the Airy beam (AB), the Bessel-Gaussian beam (BGB) and the Laguerre-Gaussian beam (LGB), are widely studied and applied in many optical scenarios. We report on preparation of light beams with controllable spatial structures through sum frequency generation (SFG) using two Gaussian pump beams in a quasi-phase matching (QPM) crystal. The spatial structures, including multi-ring-like BGB, donut-like LGB, and super-Gaussian-like beams, can be controlled periodically via crystal phase mismatching by tuning the pump frequency or crystal temp…
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