Burst mode enabled ultrafast laser inscription inside gallium arsenide

Ultrafast laser inscription(ULI)inside semiconductors offers new perspectives for 3D monolithic structures to be fabricated and new functionalities to be added in electronic and photonic microdevices.However,important challenges remain because of nonlinear effects such as strong plasma generation that distort the energy delivery at the focal point when exposing these materials to intense infrared light.Up to now,the successful technological demonstrations have primarily concentrated on silicon(Si).In this paper,we target at another important semiconductor:gallium arsenide(GaAs).With nonlinearities higher than those of Si,3D-machining of GaAs with femtosecond pulses becomes even harder.However,we show that the difficulty can be circumvented by burst-mode irradiation.We generate and apply trains of pulses at terahertz repetition rates for efficient pulse-to-pulse accumulation of laser-induced free carriers in the focal region,while avoiding an overdose of prefocal excitations.The superior performance of burst-mode irradiation is confirmed by a comparative study conducted with infrared luminescence microscopy.The results indicate a successful reduction of the plasma density in the prefocal region so that higher pulse energy reaches the focal spot.The same method is applied to identify optimum irradiation conditions considering particular cases such as asymmetric pulse trains and aberrated beams.With 64-pulse trains,we successfully manage to cross the writing threshold providing a solution for ULI inside GaAs.The application potential is finally illustrated with a stealth dicing demonstration by taking benefit of the burst mode.The irradiation method opens wide possibilities for 3D structuring inside GaAs by ULI.

Two-dimensional materials for ultrafast lasers

As the fundamental optical properties and novel photophysics of graphene and related two-dimensional(2D) crystals are being extensively investigated and revealed, a range of potential applications in optical and optoelectronic devices have been proposed and demonstrated. Of the many possibilities, the use of 2D materials as broadband, cost-effective and versatile ultrafast optical switches(or saturable absorbers) for short-pulsed lasers constitutes a rapidly developing field with not only a good number of publications, but also a promising prospect for commercial exploitation. This review primarily focuses on the recent development of pulsed lasers based on several representative 2D materials. The comparative advantages of these materials are discussed, and challenges to practical exploitation, which represent good future directions of research, are laid out.

Ultrafast laser dynamics of metal organic frameworks/TiO_2 nano-arrays hybrid composites for energy conversion applications

Herein,we report the victorious synthesis of metal-organic frameworks(MOFs) on TiO_2 nanotubes(NTs)using a layer-by-layer(LbL) approach.Highly crystalline and homogenous thin films of MOFs were grown and characterized using XRD,SEM,FT-IR and UV/Vis spectroscopy.Moreover,the utilization of the MOF films as sensitizers was probed in bespoke Graetzel type liquid junction solar cells.The constructed cell performance revealed an I_(sc) of 1.16 mA cm^(–2),Vocof 0.63 V,FF of 0.33,and E_(ff) of 0.42%.Further,pumpprobe transient laser spectroscopy was performed to investigate the energy and charge transfer dynamics of the MOFs/TiO_2 NTs interface.The results indicated 86% injection efficiency.The ultrafast pump-probe spectroscopy allows the investigation of this process and the differences between MOFs.It also showed that the relaxation of the MOF chromophores is in competition with electron injection in the Ti O2 motif.Thus this study provides a new insight into electron transfer from photoexcited metal-organic frameworks(MOFs) into titanium dioxide.

Ultrafast laser-chemical modification hybrid fabrication of hydrostatic bearings with a superhydrophobicity solid-liquid interface

Oil film vortex severely reduces the stability of hydrostatic bearings. A solid-liquid interface with drag and slip properties can weaken the oil film vortex of the bearing. Here, a combined picosecond laser ablation and chemical modification method is proposed to prepare surfaces with microbulge array structure on 6061 aluminum alloy substrates. Because of the low surface energy of the perfluorododecyltriethoxysilane modification and the bulge geometry of the microbulge array structure, the surface shows excellent superhydrophobicity. The optimum contact angle in air for water is 164°, and that for oil is 139°. Two surfaces with “lotus-leaf effect” and “rose-petal effect” were obtained by controlling the processing parameters. The drag reduction properties of superhydrophobic surfaces were systematically investigated with slip lengths of 22.26 and 36.25 μm for deionized water and VG5 lubricant, respectively. In addition, the superhydrophobic surface exhibits excellent mechanical durability and thermal stability. The proposed method provides a new idea for vortex suppression in hydrostatic bearings and improves the stability of bearings in high-speed operation.

Localized in‑situ deposition:a new dimension to control in fabricating surface micro/nano structures via ultrafast laser ablation

Controllable fabrication of surface micro/nano structures is the key to realizing surface functionalization for various applications.As a versatile approach,ultrafast laser ablation has been widely studied for surface micro/nano structuring.Increasing research eforts in this feld have been devoted to gaining more control over the fabrication processes to meet the increasing need for creation of complex structures.In this paper,we focus on the in-situ deposition process following the plasma formation under ultrafast laser ablation.From an overview perspective,we frstly summarize the diferent roles that plasma plumes,from pulsed laser ablation of solids,play in diferent laser processing approaches.Then,the distinctive in-situ deposition process within surface micro/nano structuring is highlighted.Our experimental work demonstrated that the in-situ deposition during ultrafast laser surface structuring can be controlled as a localized micro-additive process to pile up secondary ordered structures,through which a unique kind of hierarchical structure with fort-like bodies sitting on top of micro cone arrays were fabricated as a showcase.The revealed laser-matter interaction mechanism can be inspiring for the development of new ultrafast laser fabrication approaches,adding a new dimension and more fexibility in controlling the fabrication of functional surface micro/nano structures.

Ultrafast lasers—reliable tools for advanced materials processing

The unique characteristics of ultrafast lasers,such as picosecond and femtosecond lasers,have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities.Thus,ultrafast lasers are currently used widely for both fundamental research and practical applications.This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing.Surface processing includes micromachining,microand nanostructuring,and nanoablation,while volume processing includes two-photon polymerization and three-dimensional(3D)processing within transparent materials.Commercial and industrial applications of ultrafast laser processing are also introduced,and a summary of the technology with future outlooks are also given.

Recent development of saturable absorbers for ultrafast lasers [Invited]

As one of the greatest inventions in the 20 th century, ultrafast lasers have offered new opportunities in the areas of basic scientific research and industrial manufacturing. Optical modulators are of great importance in ultrafast lasers, which directly affect the output laser performances. Over the past decades, significant efforts have been made in the development of compact, controllable, repeatable, as well as integratable optical modulators(i.e., saturable absorbers). In this paper, we review the fundamentals of the most widely studied saturable absorbers, including semiconductor saturable absorber mirrors and low-dimensional nanomaterials. Then, different fabrication technologies for saturable absorbers and their ultrafast laser applications in a wide wavelength range are illustrated. Furthermore, challenges and perspectives for the future development of saturable absorbers are discussed and presented. The development of ultrafast lasers together with the continuous exploration of reliable saturable absorbers will open up new directions for the mass production of the nextgeneration optoelectronic devices.

Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses

We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses(fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons,for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes.

Ultrafast laser-scanning time-stretch imaging at visible wavelengths

Optical time-stretch imaging enables the continuous capture of non-repetitive events in real time at a line-scan rate of tens of MHz—a distinct advantage for the ultrafast dynamics monitoring and high-throughput screening that are widely needed in biological microscopy.However,its potential is limited by the technical challenge of achieving significant pulse stretching(that is,high temporal dispersion)and low optical loss,which are the critical factors influencing imaging quality,in the visible spectrum demanded in many of these applications.We present a new pulse-stretching technique,termed free-space angular-chirpenhanced delay(FACED),with three distinguishing features absent in the prevailing dispersive-fiber-based implementations:(1)it generates substantial,reconfigurable temporal dispersion in free space(41 ns nm^(−1))with low intrinsic loss(o6 dB)at visible wavelengths;(2)its wavelength-invariant pulse-stretching operation introduces a new paradigm in time-stretch imaging,which can now be implemented both with and without spectral encoding;and(3)pulse stretching in FACED inherently provides an ultrafast all-optical laser-beam scanning mechanism at a line-scan rate of tens of MHz.Using FACED,we demonstrate not only ultrafast laser-scanning time-stretch imaging with superior bright-field image quality compared with previous work but also,for the first time,MHz fluorescence and colorized time-stretch microscopy.Our results show that this technique could enable a wider scope of applications in high-speed and high-throughput biological microscopy that were once out of reach.

Ultrafast laser ablation size and recast adjustment in dielectrics based on electron dynamics control by pulse train shaping

The manipulation of the subpulse number, pulse delay, and pulse energy distribution of an ultrafast laser enables electron dynamics control by changing absorptions, excitations, ionizations, and recombinations of electrons, which can result in smaller, cleaner, and more controllable structures. This letter experimentally reveals that ablation sizes and recasts can be controlled by shaping femtosecond pulse trains to adjust transient localized electron dynamics, material properties, and corresponding phase change mechanisms.

Ultrafast laser surface wettability modification on alumina surface

As a new research area, laser surface wettability modification brings new applications for both laser and materials for industry. The picoseconds (10 ps) pulse laser surface micro-processing over alumina covered aluminium is researched. In the experiment, 10-ps laser pulse is employed and the energy threshold for different laser wavelength and repetition rate is measured. At the repetition rate of 5 kHz, the energy thresholds are: 1 064 nm: 4.0 μJ/pulse, 532 nm: 2.8 μJ/pulse, and 355 nm: 4.2 μJ/pulse. Picoseconds pulse laser is demonstrated feasible in surface scanning at industrial level.

Ultrafast laser system based on noncollinear optical parametric amplification for laser spectroscopy

We report the experimental demonstration of transform-limited sub-6 fs pulses at an optimal central wavelength by a tunable noncollinear optical parametric amplification(NOPA) source. Meanwhile, a white light continuum in the near-infrared(NIR) range from 900 to 1100 nm is also successfully generated by focusing the unconverted800 nm beam during NOPA generation on a sapphire rod. Both visible-pump/visible-probe and visible-pump/NIR-probe experiments are realized using the same laser system. As examples, ultrafast photo-induced exciton dynamics inside two kinds of materials are investigated by the visible-pump/visible-probe and visible-pump/NIR-probe spectroscopy, respectively.

Probing the dynamic structural changes of DNA using ultrafast laser pulse in graphene-based optofluidic device

The ultrafast monitoring of deoxyribonucleic acid(DNA)dynamic structural changes is an emerging and rapidly growing research topic in biotechnology.The existing optical spectroscopy used to identify different dynamical DNA structures lacks quick response while requiring large consumption of samples and bulky instrumental facilities.It is highly demanded to develop an ultrafast technique that monitors DNA structural changes with the external stimulus or cancer-related disease scenarios.Here,we demonstrate a novel photonic integrated graphene-optofluidic device to monitor DNA structural changes with the ultrafast response time.Our approach is featured with an effective and straightforward design of decoding the electronic structure change of graphene induced by its interactions with DNAs in different conformations using ultrafast nanosecond pulse laser and achieving refractive index sensitivity of~3×10^(−5) RIU.This innovative technique for the first time allows us to perform ultrafast monitoring of the conformational changes of special DNA molecules structures,including G-quadruplex formation by K+ions and i-motif formation by the low pH stimulus.The graphene-optofluidic device as presented here provides a new class of label-free,ultrafast,ultrasensitive,compact,and cost-effective optical biosensors for medical and healthcare applications.

Ultrafast Modulation of the Molten Metal Surface Tension under Femtosecond Laser Irradiation

We predict ultrafast modulation of the pure molten metal surface stress fields under the irradiation of the single femtosecond laser pulse through the two-temperature model molecular-dynamics simulations. High-resolution and precision calculations are used to resolve the ultrafast laser-induced anisotropic relaxations of the pressure components on the time-scale comparable to the intrinsic liquid density relaxation time. The magnitudes of the dynamic surface tensions are found being modulated sharply within picoseconds after the irradiation, due to the development of the nanometer scale non-hydrostatic regime behind the exterior atomic layer of the liquid surfaces.The reported novel regulation mechanism of the liquid surface stress field and the dynamic surface tension hints at levitating the manipulation of liquid surfaces, such as ultrafast steering the surface directional transport and patterning.

透明硬脆材料激光剥离关键问题研究(特邀)

透明硬脆材料由于其优异的力学性能、热稳定性、耐腐蚀性以及光电性能,广泛应用于半导体与电子领域。传统透明硬脆材料切片方法效率低、材料损耗大,制约了硬脆材料的推广应用。激光剥离技术是近年来新兴的一种透明硬脆材料切片新方法,较传统金刚线切割方法大幅提升硬脆材料的切片效率和材料利用率,目前已发展成为硬脆材料激光加工领域学术研究与产业应用的焦点。文中深入分析透明硬脆材料激光剥离物理过程,归纳激光剥离过程关键科学问题:透明硬脆材料对激光的非线性吸收、激光作用下材料内部微观结构演化与缺陷扩展规律,以及激光光场调控对材料改质影响机制等。基于这些科学问题,综述了近年来激光剥离不同类型透明硬脆材料的研究进展,目前用于激光剥离的材料已涵盖了SiC、Si、GaN、金刚石等半导体材料,蓝宝石、多晶Al_(2)O_(3)、氧化锆等陶瓷材料,激光剥离技术已发展出超快激光双脉冲诱导剥离、超快激光-化学辅助剥离、多激光复合剥离等。激光剥离物理过程是一个典型的激光-材料-热学-力学多学科交叉问题,尽管在实验结果方面获得了显著突破和迅猛发展,但目前对于工艺机理仍缺乏深入的理论与数值建模研究。未来透明硬脆材料激光剥离技术将会朝着百微米以下超薄厚度剥离、改质层低损伤、工艺自适应等方向发展,将为半导体与电子等领域快速发展提供更大的技术支撑。

基于超快激光定域去除的Cu/Ag复合金属网格透明电极性能优化研究

[目的]Ag网格透明电极方块电阻低,但反射率高。[方法]在钠钙玻璃基底表面利用等离子射频磁控溅射沉积Cu/Ag复合金属薄膜,然后采用飞秒脉冲激光定域去除金属层,获得Cu/Ag复合金属网格透明电极。分析了激光能量密度、激光扫描速率和激光扫描线重叠率对电极材料去除的影响机制。[结果]在激光能量密度1.4 J/cm^(2)、激光扫描速率300 mm/s和激光扫描线重叠率70%的工艺参数下获得了综合光电性能最为出色的Cu/Ag复合金属网格透明电极,其平均透光率为87.58%,方块电阻为2.15Ω,品质因子为1.235×10^(-1)Ω^(-1)。[结论]该电极的综合性能与传统商用ITO(掺锡氧化铟)透明电极相比有巨大提升。

1064nm超短激光脉冲的锁模获取与特性分析

为了获得1064 nm皮秒级的超短激光脉冲,探究其波形和功率特征以及泵浦功率和腔长对锁模特性的影响,使用SESAM(半导体可饱和吸收镜)设计相应的W型折叠腔实验系统进行分析,获得了重复频率为92.25 MHz的稳定锁模脉冲。实验结果表明,在腔长不变时,若泵浦功率过低,则锁模不会启动,若泵浦功率过高,则会观察到调Q不稳定及多脉冲现象,且在产生锁模效应的泵浦功率范围内,锁模脉冲的平均功率大小和泵浦功率大小是呈正相关的;在泵浦功率大小一定时,锁模脉冲的波形对折叠腔腔长的变化极为敏感,不足1 mm的腔长变化都会使波形出现较大的抖动,只有在一个极小的范围内才会出现稳定的梳状波。

碳化硅陶瓷超快激光双光束精密抛光技术研究

为了有效提高碳化硅陶瓷材料抛光精度,采用了一种红外脉冲和超快紫外皮秒激光双光束抛光碳化硅陶瓷技术。通过红外纳秒激光与超快紫外皮秒激光抛光碳化硅陶瓷材料激光双光束抛光方法,实验研究了激光功率、重复频率、扫描速率、离焦量等抛光工艺参数对抛光SiC表面质量的影响。结果表明,在激光功率24 W、扫描速率200 mm/s、激光重复频率500 kHz、离焦1 mm时,陶瓷表面粗糙度达到最优,激光双光束抛光和单光束抛光原始粗糙度碳化硅2.87μm分别下降至0.42μm和0.53μm;激光双光束抛光SiC陶瓷致密化,陶瓷表面呈现有规律的整齐排列,且碳化硅抛光表面的晶粒平均大小为1.48μm,表面力学性能得到改善。该研究为陶瓷等硬脆材料激光精密抛光提供了参考。

X射线超快成像原位表征激光增材制造过程研究进展

在增材制造过程中,熔池经历快速熔化、冷却、凝固的过程,其物理、化学、结构和应力等处于极端非平衡状态,容易导致孔隙、裂纹等缺陷产生,影响制件力学性能。同步辐射X射线具有高空间分辨率和时间分辨率,可以实现增材制造过程中熔池动力学和缺陷演化的原位表征。本文综述了近年来激光增材制造原位实验平台的搭建,并从熔池结构演化、缺陷形成机理和成形工艺优化三个方面介绍了同步辐射X射线原位表征激光增材制造过程的研究进展。最后基于目前增材制造原位观察的局限性,从表征系统改进、原料多样化和计算修正等方面对未来研究进行了展望。

玻璃材料超短脉冲激光焊接机理及研究进展

玻璃材料因具有一系列优异的物理化学性能而广泛应用于航空航天、汽车和集成电路等领域中,但其典型的高硬脆性特性会导致玻璃受热膨胀脆断,难以形成可靠的焊接接头。由于超快激光具有热效应小、形成非线性吸收的特点,因此是玻璃焊接的理想热源。介绍了超快激光焊接玻璃材料的原理和作用机制,详细分析了钠钙玻璃、硅酸盐玻璃和石英玻璃激光焊接的工艺特性和成形规律,通过玻璃焊接的模拟仿真分析了激光焊接玻璃的成形机制,并指出了超短脉冲激光焊接玻璃所面临的的挑战和发展方向。