Generation of sub-4-fs pulses via compression of a white-light continuum using only chirped mirrors

Using noble gases as a nonlinear medium, it has become possible to compress energetic laser pulses into the sub-10-fs regime. Hollow fiber capillaries can serve to increase the effective interaction length of the pulses, and impressive white-light continua have been reported as a result. With demonstrated bandwidths exceeding the optical octave, this method holds the potential for generating single-cycle optical pulses. On the practical side, however, it becomes very difficult to compensate for dispersive effects and to fully exploit the enormous bandwidth. We will discuss chirped mirrors as one means for pulse compression. A further challenge lies on the characterization side. Utilizing advanced characterization schemes, we were able to demonstrate compression of a white-light continuum down to a pulse duration of 3.8 fs, which corresponds to only about 1.6 cycles at the carrier wavelength. These are the shortest pulses in the visible/near-infrared wavelength range that have ever been produced with a non-adaptive approach to dispersion compensation. Moreover, these are the shortest pulses generated using chirped mirrors, which compares favorably to previous results that were achieved with much more elaborate and lossier adaptive compression schemes. PACS 42.65.Re; 42.65.Wi     

Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter

Femtosecond pulses with center wavelengths between 470 and 750 nm are generated in a single-stage type I BBO optical parametric amplifier pumped by a frequency-doubled 1-kHz Ti:sapphire amplifier. A high-quality white-light continuum is used as the seed. Pulse durations as short as 16 fs and pulse energies of as much as 11 microJ are observed. The quantum efficiency is ~25% for both 7- and 40-microJ pump pulses. This unique combination of ultrashort pulse duration and high conversion is made possible by noncollinear phase matching that permits a sufficiently large amplification bandwidth. Simultaneously the group velocities of the signal and the idler are effectively matched. As a result widely tunable sub-20-fs pulses can be generated in a nonlinear crystal as thick as 2 mm.     

基于光纤中脉冲压缩与受激喇曼散射的超短光脉冲产生方法

本文系统地综述了基于单模光纤中脉冲压缩与超快受激喇曼散射过程的超短光脉冲产生方法、原理及其进展,报道了我们最近的研究结果,讨论了在光纤中实现高效率超短光脉冲产生的途径.     

基于平衡光学互相关方法的超短脉冲激光相干合成技术

相干合成技术是超快光学领域的重要研究方向之一.当单路脉冲激光的连续谱超过一个倍频程时,精确控制其光谱相位(色散管理)是获得亚周期超短脉冲激光的关键.由于常见的脉冲压缩系统存在光谱带宽限制,因此多通道相干合成技术受到了广泛的关注.本文将充气空心光纤展宽后的超倍频程连续光谱分波段独立压缩,并利用平衡光学互相关方法锁定子脉冲之间的相位延迟,获得了4.1 fs的合成脉冲.实验结果表明相干合成技术在高能量亚周期超快光场调控中存在优势.     

Advanced time-resolved absorption spectroscopy with an ultrashort visible/near IR laser and a multi-channel lock-in detector

Ultrashort visible-near infrared (NIR) pulse generation and its applications to ultrafast spectroscopy are discussed. Femtosecond pulses of around 800 nm from a Ti:sapphire laser are used as a pump of an optical parametric amplifier (OPA) in a non-collinear configuration to generate ultrashort visible (500–780 nm) pulses and deep-ultraviolet (DUV, 259–282 nm) pulses. The visible-NIR pulses and DUV pulses were compressed to 3.9 fs and 10.4 fs, respectively, and used to elucidate various ultrafast dynamics in condensed matter with a sub-10 fs resolution by pump-probe measurements. We have also developed a 128-channel lock-in amplifier. The combined system of the world-shortest visible pulse from the OPA and the lock-in amplifier with the world-largest channel-number can clarify the sub-10 fs-dynamics in condensed matter. This system clarified structural changes in an excited state, reaction intermediate, and a transition state. This is possible even during molecular vibration and reactions via a real-time-resolved vibronic spectrum, which provides molecular structural change information. Also, ultrafast dynamics in exotic materials like carbon nanotubes, topological insulators, and novel solar battery systems have been clarified. Furthermore, the carrier-envelope phase in the ultrashort pulse has been controlled and measured.     

锁模光纤激光器时变动力学

锁模激光器除了可以产生稳定的超短脉冲以外,还可产生一系列重要的非平衡态动力学过程。这些快速变化的动力学过程有助于理解超快激光器和相关非线性系统的动力学,也对超快激光器的稳定性设计有重要指导意义。随着超快探测技术的发展,锁模激光器超快动力学的研究取得了一系列突破。介绍了锁模激光器几个典型的非平衡态动力学过程,包括锁模启动过程,孤子分子动力学,呼吸子超快激光,以及孤子、呼吸子爆炸动力学。这些研究不仅揭示了超快激光器中新的物理机制,也将进一步促进超快激光器、孤子及呼吸子相关理论的发展。     

Single-shot time-frequency imaging spectroscopy using an echelon mirror

We demonstrate single-shot time-frequency imaging spectroscopy with an echelon mirror for measuring ultrashort laser pulses as well as ultrafast responses of materials using the same optical setup. The echelon mirror produces a spatially encoded time delay for the probe pulse whereby both the probe and pump pulses are focused on samples with small spot size. Using the optical Kerr gate apparatus, we successfully mapped the time-frequency images of ultrashort laser pulses and subsequently evaluated the chirp characteristics with the phase-retrieval procedure on a single-shot basis. By simply replacing the Kerr medium with samples, we could also visualize the phonon-polariton oscillations in ferroelectric LiNbO3.     

Ultrafast solid-state laser oscillators: a success story for the last 20 years with no end in sight

Ultrashort lasers provide an important tool to probe the dynamics of physical systems at very short time-scales, allowing for improved understanding of the performance of many devices and phenomena used in science, technology, and medicine. In addition ultrashort pulses also provide a high peak intensity and a broad optical spectrum, which opens even more applications such as material processing, nonlinear optics, attosecond science, and metrology. There has been a long-standing, ongoing effort in the field to reduce the pulse duration and increase the power of these lasers to continue to empower existing and new applications. After 1990, new techniques such as semiconductor saturable absorber mirrors (SESAMs) and Kerr-lens mode locking (KLM) allowed for the generation of stable pulse trains from diode-pumped solid-state lasers for the first time, and enabled the performance of such lasers to improve by several orders of magnitude with regards to pulse duration, pulse energy and pulse repetition rates. This invited review article gives a broad overview and includes some personal accounts of the key events during the last 20 years, which made ultrafast solid-state lasers a success story. Ultrafast Ti:sapphire, diode-pumped solid-state, and novel semiconductor laser oscillators will be reviewed. The perspective for the near future indicates continued significant progress in the field.     

强超短脉冲抽运下半导体光放大器中的非线性过程

建立了包含载流子浓度脉动(CDP)、自由载流子吸收(FCA)、受激辐射(SE)、双光子吸收(TPA)、光谱烧孔(SHB)和超快非线性折射(UNR)过程的半导体光放大器(SOA)理论模型,通过与已报道的实验结果的比较对模型进行了验证,实现了对已有 SOA模型的修正,并对UNR,FCA和TPA效应对强超短光脉冲传输特性的影响进行了分析.当脉宽为几个皮秒的强光脉冲注入工作于透明电流下的SOA时,其强度特性主要受FCA和TPA效应的影响.由于加入了FCA效应,使模型对200fs脉冲强度传输特性的仿真结果与实验结果 关键词： 非线性过程 强超短光脉冲 SOA理论模型 增益透明     

双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.     

Ultrafast structural dynamics with table top femtosecond hard X-ray and electron diffraction setups

The following tutorial review is directed to graduate students willing to be part of the emerging field of ultrafast structural dynamics. It provides them with an introduction to the field and all the very basic assumptions and experimental tricks involved in femtosecond (fs) diffraction techniques. The concept of stroboscopic photography and its implication in ultrafast science are introduced. Special attention is paid to the generation of ultrashort electron and hard X-ray pulses in table top setups, and a direct comparison in terms of brightness and temporal resolution between current table top and facility-based methodologies is given for proper calibration. This review is focused on ultrafast X-ray and electron diffraction techniques. The progress in the development of fs-structural probes during the last twenty years has been tremendous. Current ultrafast structural probes provide us with the temporal and spatial resolutions required to observe atoms in motion. Different compression approaches have made it possible the generation of ultrashort and ultrabright electron pulses with an effective brightness close to that of fs-hard X-ray pulses produced by free electron lasers. We now have in hand a variety of ultrafast structural cameras ready to be applied for the study of an endless list of dynamical phenomena at the atomic level of inspection.     

Femtosecond pulse parametric amplification at narrowband high power gas laser pumping

In ultrashort pulse amplification a narrowband gas pump pulse laser has been used for the first time. An all-stage optical parametric chirped pulse amplifier (OPCPA) was driven by a single-shot iodine photodissociation laser. For the first time a broadband amplification was achieved in potassium dihydrogen phosphate crystal at 800 nm seeding. Ti:sapphire laser pulses stretched from 12.5 fs to 250 ps were amplified and compressed to 27 fs at a 0.5 TW output power. The results suggest using narrowband high power gas lasers as OPCPA drivers to generate petawatt beams.     

Generation of femtosecond X-ray pulses via laser–electron beam interaction

The generation of femtosecond X-ray pulses will have important scientific applications by enabling the direct measurement of atomic motion and structural dynamics in condensed matter on the fundamental time scale of a vibrational period. Interaction of femtosecond laser pulses with relativistic electron beams is an effective approach to generating femtosecond pulses of X-rays. In this paper we present recent results from proof-of-principle experiments in which 300 fs pulses are generated from a synchrotron storage ring by using an ultrashort optical pulse to create femtosecond time structure on the stored electron bunch. A previously demonstrated approach for generating femtosecond X-rays via Thomson scattering between terawatt laser pulses and relativistic electrons is reviewed and compared with storage-ring based schemes.     

Two-dimensional materials-decorated microfiber devices for pulse generation and shaping in fiber lasers

Two-dimensional(2D) materials have been regarded as a promising nonlinear optical medium for fabricating versatile optical and optoelectronic devices. Among the various photonic applications, the employment of 2D materials as nonlinear optical devices such as saturable absorbers for ultrashort pulse generation and shaping in ultrafast lasers is one of the most striking aspects in recent years. In this paper, we review the recent progress of 2D materials based pulse generation and soliton shaping in ultrafast fiber lasers, and particularly in the context of 2D materials-decorated microfiber photonic devices. The fabrication of 2D materials-decorated microfiber photonic devices, high performance mode-locked pulse generation, and the nonlinear soliton dynamics based on pulse shaping method are discussed. Finally, the challenges and the perspective of the 2D materials-based photonic devices as well as their applications are also discussed.     

Ultrafast optical power limiting in free-standing Pt–polyvinyl alcohol nanocomposite films synthesized in situ

Free-standing platinum–polyvinyl alcohol nanocomposite films have been prepared by a simple in situ method. By thermal annealing, Pt nanoparticles of different sizes and shapes have been obtained. Their optical nonlinearity is measured using ultrafast (100 fs) laser pulses at 404 nm, in the absorption wing region. A strong optical power limiting is found in the films. The timescale of this limiting action is ultrafast, as it happens within the incident laser pulsewidth. Experimental results and numerical simulation indicate that the sign of the nonlinearity can be controlled by varying the film composition and annealing temperature. Use of ultrashort laser pulses in the free-standing film configuration permits a direct and unambiguous determination of the electronic nonlinearity of the material, since accumulative effects occur at later times lying outside the sharp measurement window.     

High-power ultrafast thin disk laser oscillators and their potential for sub-100-femtosecond pulse generation

Ultrafast thin disk laser oscillators achieve the highest average output powers and pulse energies of any mode-locked laser oscillator technology. The thin disk concept avoids thermal problems occurring in conventional high-power rod or slab lasers and enables high-power TEM₀₀ operation with broadband gain materials. Stable and self-starting passive pulse formation is achieved with semiconductor saturable absorber mirrors (SESAMs). The key components of ultrafast thin disk lasers, such as gain material, SESAM, and dispersive cavity mirrors, are all used in reflection. This is an advantage for the generation of ultrashort pulses with excellent temporal, spectral, and spatial properties because the pulses are not affected by large nonlinearities in the oscillator. Output powers close to 100 W and pulse energies above 10 μJ are directly obtained without any additional amplification, which makes these lasers interesting for a growing number of industrial and scientific applications such as material processing or driving experiments in high-field science. Ultrafast thin disk lasers are based on a power-scalable concept, and substantially higher power levels appear feasible. However, both the highest power levels and pulse energies are currently only achieved with Yb:YAG as the gain material, which limits the gain bandwidth and therefore the achievable pulse duration to 700 to 800 fs in efficient thin disk operation. Other Yb-doped gain materials exhibit a larger gain bandwidth and support shorter pulse durations. It is important to evaluate their suitability for power scaling in the thin disk laser geometry. In this paper, we review the development of ultrafast thin disk lasers with shorter pulse durations. We discuss the requirements on the gain materials and compare different Yb-doped host materials. The recently developed sesquioxide materials are particularly promising as they enabled the highest optical-to-optical efficiency (43%) and shortest pulse duration (227 fs) ever achieved with a mode-locked thin disk laser.     

Femtosecond high-sensitivity, chirp-free transient absorption spectroscopy using kilohertz lasers

A simple method is demonstrated for high-sensitivity, chirp-free measurements of femtosecond (fs) transient absorption over the entire bandwidth of a white-light continuum probe. This technique uses phase-sensitive detection, with spectral scanning and simultaneous adjustment of the time delay between pump and probe pulses; it permits a direct measurement of spectra undistorted by chirp at all time scales, limited only by the resolution of the fs source. The method is applied to study the ultrafast relaxation dynamics of pi-conjugated oligomers and semiconductor nanocrystals.     

Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR

Noncollinearly phase-matched optical parametric amplifiers (NOPAs) pumped by the blue light of a frequency-doubled Ti:sapphire regenerative amplifier are a convenient source of continuously tunable ultrashort pulses in the visible and near infrared for spectroscopic experiments. We present the underlying principles, report recent improvements and describe the experiences gained from the routine use of a number of NOPAs in our laboratories. We find that the setup can easily be optimized for the given experimental requirements. Typical output-pulse energies in the visible are 5 to 10 μJ and a few μJ in the NIR from 200 μJ regenerative-amplifier pulses at 800 nm. From 460 to 700 nm, pulse lengths between 10 and 20 fs are routinely achieved, while the length increases monotonically from about 20 fs at 900 nm to just below 50 fs at 1600 nm. In all cases this corresponds to a dramatic shortening compared to the length of the pump pulses of around 100 fs. First results show that the 700 to 900 nm region can be accessed with sub-50-fs pulse lengths by use of an intermediate white-light generator in a two-stage setup. Received: 29 November 1999 / Published online: 5 July 2000     

Generating a high-extinction-ratio pulse from a phase-modulated optical signal with a dispersion-imbalanced nonlinear loop mirror

We demonstrate a method for generating ultrashort pulses from a phase-modulated optical signal by using a dispersion-imbalanced nonlinear loop mirror instead of the traditional linear dispersion medium. The extinction ratio of the pulses is greatly improved at the same time. By controlling the bandwidth of the phase-modulated signal and the dispersion map of the dispersion-imbalanced nonlinear loop mirror, we can control the pulse width from several picoseconds to hundreds of femtoseconds.     

Controlling dielectric properties of cBN by an ultrashort double-pulse light

The optical responses of wide band gap materials can be manipulated by a purposely designed ultrashort double-pulse light, based on the real-time time-dependent density functional theory. Taking the cubic boron nitride (cBN) as a typical example, we calculate the energy transfer from an ultrashort double-pulse light field to dielectric materials. We show that the purposely designed ultrashort double-pulse light and pulse train can reversibly manipulate the dielectric properties of cBN without damage. By tuning the parameters of light pulse, the cBN will change between isotropic, elliptic and hyperbolic for a particular light frequency at certain time points during the pulse light. We propose that the cBN with reversible but deep nonlinear optical responses excited by the external light field can be used in the ultrafast all-optical signal processing and optical-field-effect devices.