Picosecond time-resolved fluorescence spectroscopy of hematoporphyrin derivative

The picosecond time-resolved fluorescence decaysI(t)and spectraI(lambda, t)for hematoporphyrin derivative (HPD) in a phosphate buffer saline aqueous solution at different concentrations (8.4 times 10^{-6}sim 8.4 times 10^{-3}M) are measured by a two-dimensional synchroscan streak camera with a mode-locked CW dye laser, and a new emission band (which we call theY-band) is found at high concentration. It is shown that the fluorescence decays composed of fast and slow components at high concentration are due to theY-band (120 ps lifetime) from head-to-tail aggregates including equilibrium dimer and stable dimer, and the usual band (3.6 ns lifetime) from monomer, respectively, and the latter band is dynamically quenched by the Förster type resonance energy transfer from the monomer to the aggregate. Furthermore, the measurement of static fluorescence spectra from human gastric cancers and the surrounding in vivo after HPD injection shows that a band corresponding to theY-band from the aggregate appears at only the cancerous cells.     

Picosecond spectroscopy of semiconductors

The time-resolved reflectivity of picosecond pulses from optically excited carrier distributions can provide important information about the energy relaxation rates of hot electrons and holes in semiconductors. The basic optical properties of non-equilibrium carrier distributions are discussed, and in the specific case of GaAs, a semi-empirical analysis of the reflectivity spectrum is described which estimates the contributions from the principal critical points of the band structure. Using Boltzmann factors to approximate the hot carrier distributions, it is found that the non-equilibrium reflectivity spectrum is a sensitive function of carrier temperature and that it can reverse its sign as the distribution relaxes. These results are in good qualitative agreement with recent experiments employing a mode-locked cw dye laser.     

Picosecond laser pulses

The broad bandwidth and long storage lifetimes of Nd³⁺: glass and ruby lasers have made possible the generation of picosecond laser pulses having peak powers it excess of one gigawatt and repetition rates in the microwave range. The numerous application areas of these pulses include research in nonlinear optics, transient response of atomic and molecular systems, optically generated plasmas, spectroscopy, ranging, optical information processing, and high-speed photography. This paper reviews several experimental techniques for generating, measuring, and utilizing these ultrashort laser pulses.     

Picosecond semiconductor laser radar with optical amplifier

An optical range finder, which uses the same semiconductor laser as both an optical pulse source and an optical preamplifier of the reflected pulse, is reported. This method has the advantage of reducing the number of optical components that must be aligned, which makes it suitable as a picosecond laser radar in aerospace applications. The device showed a maximum optical gain of 17 dB with a dynamic range of 33 dB, and the obtained resolution was of the order of 1 mm.     

Picosecond dynamics of a gain-switched InGaAsP laser

Optical pulses are observed, with subpicosecond resolution, from an InGaAsP laser driven with 17 ps wide electrical pulses. The temporal profile of these gain-switched pulses is analyzed to obtain the transient and steady-state injected carrier densities in the active layer for dc biases above and below threshold. Gain-switched pulses as short as 13 ps FWHM are presented, and empirical results are compared to computer simulations. The implications of these results to pulse code modulation at very high bit rates are also discussed.     

Picosecond magnetic spectroscopy of two- and three-dimensionaldiluted magnetic semiconductors

The authors have developed a novel type of magnetic spectroscopy which relies on an integrated multichip SQUID (superconducting quantum interference devices) microsusceptometer to obtain nearly quantum-limited spin resolution. The susceptometer, including two DC SQUIDs, pickup loops, and field coils, was fabricated using VLSI technology to achieve an unprecedented scale of magnetic sensitivity, optimized for the study of microscopic samples or thin layers. Furthermore, in combination with an ultrafast pulsed laser system, measurements can be made at picosecond time scales, providing an understanding of the dynamics of the magnetic phenomena in these materials and the effects of quantum confinement on magnetic behavior     

Picosecond pulse measurements using the active laser medium

A simple method for measuring the pulse lengths of synchronously pumped dye lasers which does not require the use of an external nonlinear medium, such as a doubling crystal or two-photon fluorescence cell, to autocorrelate the pulses is discussed. The technique involves feeding the laser pulses back into the dye jet, thus correlating the output pulses with the intracavity pulses to obtain pulse length signatures in the resulting time-averaged laser power. Experimental measurements were performed using a rhodamine 6G dye laser pumped by a mode-locked frequency-doubled Nd:YAG laser. The results agree well with numerical computations, and the method proves effective in determining lengths of picosecond laser pulses     

皮秒多脉冲泵浦KGW红外多波长拉曼激光器

报道了多脉冲机制泵浦KGW晶体的皮秒红外多波长拉曼激光器。建立数学模型研究了多脉冲泵浦机制对拉曼活性分子振动模式的影响,模拟结果显示了拉曼活性分子对多脉冲泵浦机制的响应相比于传统的单脉冲泵浦机制更加活跃与持久,从而促进衰减的分子振荡多次回到本征振动频率。多脉冲的增强效果有利于改善拉曼增益,降低拉曼阈值,提高拉曼转换效率。设计了皮秒多脉冲泵浦KGW拉曼晶体实验,结果表明,三脉冲泵浦机制将拉曼增益改善了2倍以上,拉曼阈值降低了50%以上,768 cm–1和901 cm–1拉曼模式的转换效率分别提高了16%和22%以上。基于三脉冲泵浦机制,设计了1 kHz毫焦级皮秒多脉冲泵浦KGW晶体红外多波长拉曼激光器,对于768 cm–1和901 cm–1两个振动模式,获得脉冲能量分别为1.39 mJ和1.38 mJ,最大拉曼转换效率分别为29.6%和25.7%。此外,对于两种拉曼模式,此拉曼激光器都可以同时输出多达8条红外拉曼光谱,涵盖范围800~1 700 nm。     

Picosecond pulse response of a travelling-wave semiconductor laser amplifier

The dynamic pulse response of a travelling-wave semiconductor laser amplifier has been investigated. Peak output powers greater than 100 mW, with a fibre-to-fibre gain of 10 dB, have been observed for narrow pulses (<50ps FWHM) at low repetition rates. The dynamic bandwidth of the amplifier has been shown to be at least 50 GHz.     

皮秒激光下超小型BPQDs非线性光学性质研究北大核心CSCD

近年来,黑磷量子点(Black Phosphorus Quantum Dots,BPQDs)线性和非线性光学(Nonlinear Optical,NLO)性质的研究取得了一定进展,但BPQDs尺寸依赖的NLO特性的研究仍有待开展。本文研究了5nm以下三种不同尺寸的超小型BPQDs的NLO行为,结果表明,尺寸为23nm的BPQDs比41nm的BPQDs的NLO吸收系数和极化率增加了近一倍,归因于量子限制效应和光致偶极矩的变化,时域有限差分(FDTD)仿真结果证实了这一原因。这项研究证实了调节超小型BPQDs的尺寸是增强其非线性光学效应的一种有效手段,并表明超小型BPQDs在光学和光电器件领域内具有潜在应用价值。     

Picosecond switching dynamics of a bistable-wavelength-latchtwo-segment distributed feedback laser

The bistable-wavelength-latch operation of a two-segment distributed-feedback laser was observed and characterized. Short electrical trigger pulses were used to demonstrate mode power switching in less than 200 ps with only 450-ps delay between set and reset pulses. Spectrally resolved measurements show transient spectral broadening during the bistable wavelength switching. Fast laser wavelength switching is important for wavelength-division-multiplex photonic switching systems     

乙酸乙酯溶液的时间分辨荧光光谱研究

为了研究模拟白酒环境中乙酸乙酯荧光衰减的影响,采用测得不同体积分数的乙酸乙酯乙醇水溶液的稳态及时间分辨荧光光谱结合量子化学计算的方法,进行了实验测量和理论分析。结果表明,乙酸乙酯的荧光峰值分别在407nm和431nm,荧光寿命均约为1.4ns;不同体积分数乙酸乙酯乙醇水溶液衰减过程中有两种荧光寿命,其中短寿命成分为乙酸乙酯,长寿命成分为乙酸乙酯和乙醇、水分子形成的多聚体结构;多聚体结构中分子间相互作用使得乙酸乙酯的平面性提高,这有利于辐射跃迁;水分子通过氢键和范德华力连接乙酸乙酯和乙醇的二聚体形成层状结构,阻碍了荧光体的运动,从而减少非辐射跃迁,使得荧光寿命延长。这一结果有助于丰富白酒中有机物的检测方法,也为研究溶剂环境对分子构象及光谱特性变化提供了一定的帮助。     

Picosecond optical pulse generation from mode-locked phased laser diode array

Actively mode-locked 26 ps optical pulses are generated from a gain-guided, 10-stripe, phased laser GaAs diode array with an external cavity consisting of two cylindrical lenses and a corner reflector. To our knowledge, this is the shortest pulse width yet demonstrated from a mode-locked phased laser diode array. The detuning bandwidth of the mode-locking is measured as 2.5 MHz.<>     

Picosecond synchronisation on mode-locked laser pulses by double constant-fraction triggering

A novel technique is introduced for the synchronisation with better-than-30 ps jitter of electrical pulses to ultrafast light pulses at high repetition rate (74 MHz and more). The technique makes it possible to perform time-correlated single-photon counting measurements at picosecond resolution using mode-locked lasers without cavity dumping.<>     

Picosecond internal Q-switching mode correlates with laser diode breakdown voltage

The record picosecond power density recently achieved with a current-pumped laser diode turned our attention to a still unexplained 50-year-old phenomenon termed “internal Q-switching”. The correlation found experimentally here between the relatively high breakdown voltage (～5–11 V) in a heavily doped single-heterostructure laser diode and its high-power picosecond lasing provides a means for solving the puzzle. Together with the experimental fact that picosecond lasing occurs from the p-n junction, this implies that internal Q-switching is determined by the compensated layer rather than by “traditional” single-heterostructure waveguide. This finding is valid for various growth technologies independently of whether the high break-down voltage and picosecond lasing are achieved by exact compensation of shallow donors by shallow acceptors, or by doping profile gradients.     

基于微纳光纤脉冲压缩器的皮秒脉冲掺镱光纤激光器

1.0 m波段的超短脉冲激光器在激光加工、光学精密测量和生物医学等领域具有重要应用价值,但由于掺镱光纤激光器工作在全正色散区域,激光器直接输出的脉冲通常宽度较大。文中利用改变微纳光纤尺寸可以使其在1.0 m波段提供反常色散的特点,将微纳光纤作为色散补偿元件在掺镱光纤激光器腔外对脉冲进行压缩来获得超短脉冲。实验中,自主拉制的微纳光纤锥腰直径为3 m,锥腰长度为5 cm。掺镱光纤激光器直接输出脉冲宽度为37.6 ps,经微纳光纤压缩后脉冲宽度为8.5 ps。该结果提供了一种更简便低廉的压缩脉冲方法。     

Application of linear prediction least-squares fitting to time-resolved optical spectroscopy

Linear prediction least-squares fitting is shown to be a powerful tool for the quantitative analysis of pump-probe-type experiments. When used correctly, it is capable of extracting multiple exponential or exponentially damped sinusoidal components from noisy data. Furthermore, it can be used to analyze data that are not simply the response of the system being studied, but rather the convolution of a nonnegligible instrument function with the desired response. In pump-probe experiments, it can be used to deduce time constants that are considerably shorter than the laser pulse itself.     

Picosecond optical properties of a grating surface emittingtwo-dimensional coherent laser array

The picosecond dynamical properties of a two-dimensional grating surface emitting (GSE) laser array are investigated by recording with a streak camera the optical emission under low duty cycle, 200-ns pulsed operation. The elements of the two-dimensional GSE arrays are index-guided ridge lasers. The array (a 10×2 array) consists of two groups of ten laterally coupled laser elements which are coupled longitudinally by an intervening grating, and gratings on either end of the two groups which provide the necessary feedback for the laser cavities. A variety of nonlinear dynamic behavior regimes, ranging from quiescent through periodic, to erratic is observed during the course of a single-drive current pulse. The transition from one regime to another may be due to transient heating during the pulse     

Picosecond pulse analysis of gain-switched 1.55 µm InGaAsP laser

Results of computer simulation of gain-switched pulses from InGaAsP lasers emitting at 1.55 μm are reported. Multimode rate equations are solved for short triangular current pulses of large amplitude superimposed on a dc bias close to threshold. Lasers of conventional length (200μm) and with short cavity (50 μm) are analyzed. The effect of wavelength chirp on time-resolved and the time-averaged multimode spectra is demonstrated. Optical pulses as short as 10 ps are predicted for short-cavity lasers with reflective coatings. While the number of longitudinal modes in gain-switched spectrum is efficiently reduced for short-cavity lasers, the dynamic linewidth of each mode remains of a similar magnitude (up to 140 GHz) as for standard-cavity lasers. It is expected that gain saturation effects could reduce the calculated values of dynamic linewidth.     

Picosecond and femtosecond pulse generation in a regenerativelymode-locked Ti:sapphire laser

The authors have produced transform-limited pulses ranging from 100 ps to 40 fs duration from a Ti:sapphire laser. Output powers in excess of 1 W and peak powers of 0.5 MW have been observed. They describe the technique of regenerative mode locking and present evidence that a transient with a peak power of more than 10 kW is required to initiate mode locking. The role of group velocity dispersion is highlighted and a value of -750 fs² is measured for the group delay dispersion in an operating laser. The authors describe the limits on both the power and pulsewidth obtainable from this laser and present pulse compression experiments which produce 17 fs pulses with 70 mW of average power