High-field nonlinear optical response and phase control in a dielectric laser accelerator

Coherent EUV light can be generated using the process of high‐harmonic upconversion of pulses from a high‐intensity femtosecond laser. Recent advances in ultrafast laser technology, combined with the development of efficient techniques for high‐harmonic generation (HHG), now make routine small‐scale, table‐top experiments using coherent EUV light. New advances, such as the use of pulse shaping and quasi‐phase matched geometries, give new flexibility in the design of coherent EUV sources.

Recent advances in ultrafast laser technology have spurred investigations of microheterogeneous solutions. In particular, researchers have explored details of reverse micelles (RMs), which present isolated droplets of polar solvent sequestered from a continuous nonpolar phase by a surfactant layer. This review explores recent studies utilizing a variety of ultrafast laser techniques to uncover details about structure and dynamics in various RMs. Using ultrafast vibrational spectroscopy, researchers have probed hydrogen-bond dynamics and vibrational energy relaxation in RMs. These studies have developed our understanding of reverse micellar structure, identifying varying water environments in the RMs. In a plethora of experiments employing probe molecules, researchers have explored the confined environment presented by RMs and their impact on a range of chemical reactions. These studies have shown that confinement, rather than the specific interactions with surfactants, is an important factor determining the impact of the reverse micellar environment on the chemistry.

With recent advances in ultrafast laser technology, ablation using femtosecond laser pulses has found a wide range of applications. It is widely believed that ablation using femtosecond lasers has many unique advantages compared to longer-pulse ablation. One of the most significant features is believed to be that a negligible amount of energy should remain in the sample following ablation because the deposited heat does not have enough time to travel into the bulk sample during the femtosecond laser pulse. In this paper, we discuss our new findings on thermal effects in femtosecond laser ablation of metals. In contrast to the previous common belief, we find that a significant amount of residual thermal energy is deposited in metal samples following multi-shot femtosecond laser ablation. We further discuss how the absorptance of metals depends on the structural modifications. Lastly, we discuss the formation of periodic surface structures on metals induced by femtosecond laser pulses.

In this study, we aimed to develop a novel femtosecond laser-based technique for individual embryo tagging. We have demonstrated for the first time the possibility to create individual alphanumeric codes be means of femtosecond laser microsurgery directly on embryo’s outer shell. Femtosecond laser pulses (wavelength of 514 nm, pulse duration of 280 fs, repetition rate of 2.5 kHz, pulse energy of 20 nJ) were applied for precise alphanumeric code engraving on the zona pellucida of mouse embryos at the zygote stage. The code consisted of 4-5 characters (numerical and/or alphabetical) typically. Embryo quality assessment was performed every 24 hours post laser-assisted marking by light microscopy and compared with that of non-treated control embryos. The codes engraved could be clearly recognized until the thinning of the zona pellucida prior to hatching. No differences in morphology and developmental rates of laser-treated embryos and control embryos were found. Our results demonstrate the suitability of femtosecond laser as a novel tool for noninvasive embryo tagging, enabling embryo identification from day 0.5 post coitum to at least early blastocyst stage. The technique proposed is relatively fast, reliable and simple, and can be fully automated in the future. It can be performed in a contactless mode under sterile conditions and does not require any additional equipment (except microscope) to visualize the code and to identify the embryo. As far as femtosecond lasers offer several important advantages (such as high precision, minimal invasiveness, versatility etc.) over conventional milli/microsecond lasers we believe that further advances in ultrafast laser technology aimed at reducing complexity, size and high price of femtosecond lasers will help them gain popularity in the field of assisted reproduction.

In this paper, we theoretically demonstrate enhanced supercontinuum generation in a trench-assisted silicon–germanium (Si–Ge) rib waveguide on a sapphire substrate. The computational analysis of the proposed design is done by solving the nonlinear Schrödinger equation using the Split-Step Fourier Method (SSFM) in MATLAB and finite element method (FEM) simulations. By incorporating sidewall trenches, the waveguide exhibits a 30.07% reduction in the effective mode area of the fundamental mode at a pump wavelength of 3.5[Formula: see text][Formula: see text]m, leading to a 42.98% increase in the nonlinear coefficient. These enhancements intensify nonlinear interactions such as self-phase modulation and soliton dynamics, resulting in a broader and more intense supercontinuum spectrum. Using the SSFM with secant hyperbolic pump pulses centered at 3.5[Formula: see text][Formula: see text]m having a pulse width of 45[Formula: see text]fs and a peak power of 5[Formula: see text]kW, we simulate supercontinuum generation and compare the results with those of a conventional rib waveguide. While the standard design yields a coherent spectrum spanning 1.7–6.2[Formula: see text][Formula: see text]m, the trench-assisted configuration extends the bandwidth to 1.6–7.4[Formula: see text][Formula: see text]m. The suggested waveguide design can serve as a potential candidate for mid-infrared supercontinuum sources, with strong potential for applications in spectroscopy, environmental sensing, biomedical diagnostics, and free-space optical communication.

Optical phased arrays are promising candidates for both RF signal processing and optical beam forming and steering. These platforms not only enable accurate electrically controlled beam steering at high frequencies but also have the potential to significantly improve the performance of future free-space optical communications systems. In this work we exploit recent advancements in both nano-scale hybrid silicon-slot waveguides and electro-optic (EO) polymers to demonstrate an integrated optical phased-array antenna. Specifically, we create a hybrid integrated "photonic circuit" that connects an array of optical phase modulators, fed by a common optical signal and a 1x4 splitter, to a compact optical waveguide diffraction array for optical beam steering applications. The fundamental characteristics of the resulting integrated optical beam former, including the optical insertion loss, driving voltage, and phase control from the waveguide aperture are summarized in this letter.

Control of optical phase by a liquid crystal device using micro-patterned alignment film

There is considerable interest worldwide in reconfigurable, transparent switch fabrics with low loss for both telecom and computing applications. A number of architectures employing liquid crystal devices have been suggested for switches where reconfiguration times of µsec to 1 sec are acceptable [1]. These-are either multistage, with control and latency overheads, or single stage, such as the matrix-vector multiplier approach with 1/N2 loss (N being the number on input and output channels). An alternative single stage scheme, where a computer generated hologram is written on the SLM [2,3], charges the latter with the task of control and beam shaping. The use of multi-focus diffractive optical elements to direct the beam, which would relieve the charge on the LCTV and improve the throughput efficiency, has been proposed recently [4]. However, the results on the basic system show low throughput efficiency and unacceptable crosstalk, A high throughput efficiency is required for an acceptable bit error rate on a high speed link. The principle of a low-loss link based on free space optics is described in this paper.

<sec>Ultrafast fiber laser sources with mJ-level pulse energy and kilo-watt average power are of particular importance for various science fields such as attosecond lasers. Currently, several large-scale facilities for attosecond lasers, including ELI-ALPS in Europe, SECUF in China, NeXUS in America and ALFA in Japan are under construction. High-performance femtosecond driven lasers are crucial for attosecond lasers and various ultrafast laser facilities. Fiber lasers have a large surface-to-volume ratio, which enables efficient cooling and is suitable for high average power amplification. However, due to small mode area of optical fibers, detrimental nonlinear optical effects such as self-phase modulation, four-wave mixing, and stimulated Raman scattering limit the peak power of pulse to hundreds of MW, corresponding to pulse energy of hundreds of μJ for femtosecond pulses in large mode area rod-type fibers. In addition, the average power of fiber lasers is limited by transverse mode instability, which reduces the stability and quality of beams above a certain threshold. In rod-type fibers, the threshold is about 250 W. Neither average power nor pulse energy emitted by single fiber meets the requirement for attosecond laser generation.</sec><sec>The average power and pulse energy can be further scaled by coherent beam combination, which involves splitting pulses caused by an frontend laser and recombining them after amplification. It is essential for coherent beam combination to maintain the coherence of pulse replicas, which usually involves high speed photodiode detectors, piezo-driven mirrors, and other electronics forming a feedback system to actively control the phase of all replicas. We present a high-energy high-power ultrafast fiber laser system by using filled-aperture coherent combination of four ytterbium-doped rod-type fiber amplifiers. The phase control is achieved by using stochastic parallel gradient descent method. The frontend includes a passively mode-locked Yb-fiber oscillator, a stretcher, a pulse picker, and three fiber pre-amplifiers, which delivers 1 MHz stretched pulses centered at 1032 nm with 700 ps duration and 20 W average power. The pulse is split into four replicas by polarization beam-splitter and half-wave plate pairs, and the replicas pass through delay lines formed by piezo-driven mirrors before amplification. The pulse replicas are equally split and amplified to ensure the same accumulated nonlinear phase, and are combined by thin film polarizer and half-wave plate pairs. A small portion of the combined pulse is split and collected by a photodiode detector after being filtered spectrally and spatially, serving as a signal for controlling phase. The combined pulse is compressed by a compressor using a double-pass diffraction grating pair consisting of two 1739 l/mm gratings.</sec><sec>At a repetition rate of 1 MHz, our four-channel Yb-fiber coherent beam combination system generates a combined average power value of 753 W and a combination efficiency of 87%. By utilizing an adjustable pulse stretcher and compressor, a 0.67 mJ, 242 fs near transform-limited pulse can be generated with a compressing efficiency of 89%. The compressed pulse is centered at 1032 nm, and the spectrum width is 8.8-nm. In the 30 min measurement, the root-mean-square of average power is less than 1% , while the residual phase error is less than <i>λ</i>/23, indicating excellent stability on different time scales. The beam quality factor of the 0.67 mJ compressed pulses is 1.17×1.11. At 500 kHz, we obtain pulses of 1.07 mJ and 247 fs with average power of 534 W, exhibiting similar efficiency, long-term stability, and beam quality. The residual phase error decreases below <i>λ</i>/29, indicating better short-term stability. Further scaling power and energy can be achieved by increasing the number of channels. By adding the delay stabilization system and pointing stabilization system, which are currently under development, an eight-channel CBC system can be used to generate 1 kW, 2 mJ pulses.</sec><sec>In this work, we implement a four-channel coherent beam combining system based on the SPGD method, and obtain compressed pulses of 673 W, 673 µJ, and 242 fs at 1 MHz and 534 W, 1.07 mJ, and 247 fs at 500 kHz. Both power and energy can be further improved by increasing the channel number, and adding the delay stabilization system and pointing stabilization system which are under construction. By adding coherent pulse stacking amplification technology, the coherent beam combining system ought to generate pulse energy as high as 100 mJ, which constitutes the energy source for applications such as laser wake-field acceleration.</sec>

Phase control of random lasing processes has been a challenge both in physics and in the device/materials design. Although conventional saturable absorbers can be integrated with random lasers to conceive mode-locking scheme, low intensity and random directions of the lasing radiation reduce largely the possibility. In such considerations, we put forth a new mode-locking mechanism, which is defined as cascaded absorption and stimulated emission (CASE), and have it achieved in multicrystalline microdisk structures of a hybrid perovskite. This scheme applies only to lasing materials with strong overlap between the absorption and emission spectra. In this work, we employed 2-photon pumping at 800 nm with a pulse duration of about 150 fs to realize phase-locked random lasing in MAPbBr 3 microdisks in donut shapes, which are produced by micro-imprinting using a flexibly transferred template of tricyclo[5.2.1.0 2,6 ] decanedimethanol diacrylate. The phase-locking performance is identified by the narrow-band lasing lines with equal separations. The constant phase shift for initializing phase locking is determined by the internal conversion lifetime in the MAPbBr 3 molecules. Two-photon pumping enables large penetration depth into the microdisks and consequently large numbers of phase-locked lasing modes, producing much narrowed and high-contrasted spectral lines. Lasing lines with a bandwidth as narrow as 0.26 to 0.3 nm and an equal separation ranging from 1.7 to 4.8 nm have been achieved for different microdisk schemes. These results imply marked progress in new random lasing physics and potential applications in ultrafast laser technology.

We present the binary supergrating (BSG), a digital approach to spectral engineering that permits the near-arbitrary control of optical amplitude and phase in a wavelength-dependent manner. The BSG is a guided-wave technology that consists of an aperiodic sequence of binary elements, leading to a simple, robust and practical form. This sequence, determined through the process of BSG synthesis, encodes an optical program that defines device functionality. Our approach to synthesis builds on existing knowledge in the design of "analog" gratings through a two-step process: first, exploit the best analog-domain methods, then transform the resulting structure into binary form. Accordingly, we explore the notion of diffractive structure transformation and introduce the principle of "key information". We assemble such key information for Bragg-regime structures, and employ it in the design of grating quantizers based on an atypical form of Delta-Sigma modulation. We illustrate this approach through the synthesis of a complex dense-wavelength division-multiplexed telecom filter featuring 50-GHz channel spacing, -40-dB stopbands, and 25-GHz-wide passbands that are flat to within 0.2 dB.

A novel coherent beam combination system based on double piezoelectric ceramics transducer (PZT) phase modulators is presented and demonstrated for the first time. In this system, two different PZT phase modulators are used for high frequency phase modulation and low frequency phase control, respectively, while in previous demonstrated system, LiNbO3 phase modulators are often employed. The inherent low insert loss and high laser-induced damage threshold of the PZT phase modulator makes the new proposed system more compact and stable. By the way, the experiment of coherent beam combination of two 5-W fiber laser beams based on double PZT phase modulators is done. In the experiment, the PZT phase modulator with 500-kHz frequency response point made in home is used for high frequency phase modulating and another one with 0∼30-kHz linear frequency response range for phase controlling. When the phase control system is in the closed loop, the fringe contrast of far-field intensity pattern is improved to be more than 90 % from 5 % in open loop.

: During the period of this program we made significant progress in several areas, in the development of new femtosecond lasers and techniques, in the application of ultrafast optics to fiber devices and to studies of novel materials and material structures. Some of these key advances are summarized in sections entitled: (1) Ultrashort pulse femtosecond lasers; (2) Optical phase and waveform control of sub-two-cycle pulses; (3) Dispersion managed propagation and modelocking; (4) Noise in ultrafast optical devices; (5) Generation of high intensity ultrashort pulses using novel laser design; (6) Nonlinear semiconductor mirrors for femtosecond pulse generation; (7) Erbium glass waveguide lasers and short pulse generation; (8) Nonepitaxial saturable absorber materials for nonlinear optic; (9) Ultrashort pulse fiber lasers; and (10) Photonic device fabrication using nonlinear materials processing.

Large refractive index changes during phase transition in vanadium dioxide are exploited to control the phase of a light at 1300 nm over distances of less than 100 nm without altering other properties of light.

The glass laser fusion driver, GEKKO 12 beam system, at ILE, Osaka, has operated since 1983 to deliver 20 kJ output in 1 ns pulse with the second and third harmonic frequency converters and the pulse tailoring optics. The uniform irradiation of the spherical target in the implosion experiments is a key issue to get the higher density of final compressed core. The pulse power to drive 2000 flash lamps exciting the rod and disk amplifiers is a moderate system with the total energy of 21 MJ in 300 microsecond(s) duration. High reliability and reproducibility are required to keep the laser energy balance of 12 beams within 3%, and no misfiring of 88 high-voltage switches of the laser amplifiers and 14 Pockels optical switches with the total probability of less than 0.1% per shot. Power balance technology involves not only the power conditioning of the flash power supply but also the gain control of the amplifiers and the alignment of a phase angle of the frequency converters, both working very nonlinearly. The precise control is sustained by the precise measurements of the beam energies, power waveforms, and the optical components characteristics such as reflectivity and transmittance. This report includes the optical beam control to realize the uniform illumination with an incoherent light source and the optical phase control in a focusing optics.