Quadratic detection with two-photon quantum well infrared photodetectors

Two-photon quantum well infrared photodetectors (QWIPs) involving three equidistant subbands take advantage of a resonantly enhanced optical nonlinearity, which is six orders of magnitude stronger than in a bulk semiconductor. This approach results in a sensitive device to measure quadratic autocorrelation of mid-infrared optical pulses from modelocked quantum cascade lasers, nonlinear optical conversion, and free-electron lasers (FEL). We report on autocorrelation measurements at wavelengths in the mid-infrared and Terahertz regimes using ps optical pulses from the FEL at the Forschungszentrum Dresden Rossendorf. In particular, quadratic detection at wavelengths around 5.5 μm is still possible at room-temperature, which is crucial for applications in practical systems. We also report on a two-photon detector which works below the Reststrahlen band at 42 μm (7.1 THz).     

Pulse shaper assisted short laser pulse characterization

We demonstrate that a pulse shaper is able to simultaneously act as an optical waveform generator and a short pulse characterization device when combined with an appropriate nonlinear element. We present autocorrelation measurements and their frequency resolved counterparts. We show that control over the carrier envelope phase allows continuous tuning between an intensity-like and an interferometric autocorrelation. By changing the transfer function other measurement techniques, for example STRUT, are easily realized without any modification of the optical setup. PACS 42.65.Re; 42.30.Lr; 42.30.Rx     

Optical intersubband transitions and femtosecond dynamics in Ag/Fe(100) quantum wells

The optical intersubband transitions and femtosecond dynamics of electrons in quantum well states in Ag/Fe(100) are investigated by interferometric time-resolved two-photon photoemission. The quantum well wave functions and transition probabilities are evaluated from the two-photon photoemission resonance energies and intensities using an extended phase accumulation model. Direct femtosecond pump-probe correlation measurements elucidate the importance of interfaces in confined structures.     

Rapid pulse shaping with homodyne detection for measuring nonlinear optical signals

We have designed a common-mode interferometric acousto-optic pulse shaper that is capable of shaping individual pulses differently from a mode-locked laser. The design enables the measurement of weak nonlinear optical signals such as two-photon absorption and self-phase modulation at megahertz rates. The experimental apparatus incorporates homodyne detection as a means of resolving the phase of the detected signals. The fast data acquisition rate and the ability to perform measurements in scattering media make this experimental apparatus amenable to imaging applications analogous to measurements of two-photon fluorescence using a mode-locked laser.     

Parallel excitation pathways in ultrafast interferometric pump-probe correlation measurements of hot-electron lifetimes in metals

Hot electron (E-E_Fermi=0.75 to 1.55 eV) lifetimes for cesiated Cu(100) and Cu(111) surfaces are measured via interferometric time-resolved two-photon photoemission with a 19-fs intensity FWHM mode locked Ti:sapphire laser at 1.55 eV. The data are analyzed using the optical Bloch equations and a laser pulse characterized in situ via surface second-harmonic generation interferometric autocorrelation. It is found that the retrieved hot-electron lifetimes are unphysically fast, and have a strong dependence on the temperature of the sample and the polarization of the laser. A simple explanation for the data is that the measured signal consists of contributions from transitions through both virtual and real intermediate states. Received: 26 July 2000 / Accepted: 8 September 2000 / Published online: 12 October 2000     

All-reflective high fringe contrast autocorrelator for measurement of ultrabroadband optical pulses

We describe an all-reflective interferometric autocorrelator designed to measure ultrabroadband optical pulses in the UV through IR spectral regions. By carefully choosing the device geometry we are able to obtain approximations for the nonlinear autocorrelation functions that reduce computation times to values acceptable for use in iterative pulse reconstruction schemes. We describe the optical design, autocorrelation functions, and present proof-of-principle experimental results measuring 20.6 fs pulses with a transform limit of 9.6 fs.     

Electric-field reconstruction of femtosecond laser pulses from interferometric autocorrelation using an evolutionary algorithm

We present an evolutionary algorithm for reconstructing a femtosecond laser pulse from its interferometric autocorrelation trace and laser spectrum. The algorithm is optimized for the intensity and phase characterization of several-cycle optical pulses. We tested this algorithm with numerically-generated femtosecond pulses and then applied it to experimental data. In the experiment, a negatively chirped 31-fs pulse and a sub-10-fs pulse containing high-order phase distortion were characterized. Frequency-resolved optical gating measurements, performed for comparison, confirm the reliability of our technique.     

Generation of smooth, ultra-broadband spectra directly from a prism-less Ti:sapphire laser

Ultra-broad and smooth spectra are generated directly from a mirror-dispersion-controlled Kerr-lens mode-locked Ti:sapphire oscillator. The full width at half maximum of the spectrum is 277 nm. The pulse width is evaluated as 6.5 fs by using both interferometric autocorrelation and spectral phase interferometry for electric-field reconstruction. This compact, user-friendly source opens the door to routine implementation of ultra-high-resolution as well as spectroscopic optical coherence tomography in a clinical environment. PACS 42.60.By; 42.65.Re     

一种多用途的光电自动微调器

本文介绍一种多用途光电自动微调器,它是由单片微机控制的具有自动反馈功能的光电子器件,适用于各种利用微调方式对光路进行实时自动反馈式平衡准直和精密测量.     

Theoretical analysis of third order interferometric autocorrelation signals for enhanced sensitivity towards pulse chirp and asymmetry

In this paper, we report theoretical analysis of third order interferometric autocorrelation to achieve enhanced sensitivity towards pulse chirp and asymmetry. The analysis is based on interferometric correlative envelope (ICE) functions and ICE difference signals derived from interferometric autocorrelation signals. The third order ICE signals are compared with second order ICE signals obtained from a second order interferometric autocorrelation signals. It is shown that one out of six third order ICED signals may be used to obtain simultaneous detection and measurement of pulse chirp as well as pulse asymmetry of the chirped ultrashort laser pulse. This is in contrast to use of two out of three second order ICED signals for simultaneous detection of pulse chirp and asymmetry.     

Femtosecond Kerr-lens autocorrelation

An autocorrelation measurement of femtosecond laser pulse duration using the Kerr-lens mechanism is demonstrated. This technique can also be used as a sensitive and absolutely calibratable method for measuring ultrafast optical nonlinearities. A method that uses an electronic spectral-filtering scheme is proposed for determining the frequency chirp of pulses by interferometric autocorrelation.     

Ultrasensitive femtosecond two-photon detector with resonantly enhanced nonlinear absorption

We report on two-photon detection based on nonlinear absorption between subbands in quantum wells. Resonantly enhanced nonlinear absorption, 6 orders of magnitude higher than that of typical bulk semiconductors, leads to a threshold power density for quadratic detection as low as 0.1 W/cm2 and to femtosecond time resolution. The approach facilitates dynamic characterization of the optical light field of infrared emitters with unprecedented sensitivity. We also present a new method for determining the intersubband and intrasubband scattering times by means of autocorrelation measurements.     

Ex vivo characterization of sub-10-fs pulses

An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within a custom nonlinear optical microscope. Mouse tail tendon is used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an ideal method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Compensated autocorrelation traces are presented for biologically relevant objective lenses.     

Two-color cross-correlation of fs-laser pulses by two-photon induced photoconductivity for near and far field optical measurements

Two-color two-photon induced photoconductivity in a GaAsP diffusion type photodiode is demonstrated by measuring femtosecond cross-correlation functions for widely separated wavelength pairs of 775 and 1300 nm. Results are obtained for a range of tunable wavelengths and average powers of the incident lasers by measuring the two-photon induced photocurrent as a function of the optical delay between the pulses. The temporal autocorrelation of femtosecond laser pulses in the near-field of a small diameter aluminum coated optical fiber tip is also obtained with the same photodiode method for single colors.     

Sustained saturation in a free-electron laser oscillator at perfect synchronism of an optical cavity.

The first observation of sustained saturation in a free-electron laser (FEL) oscillator at perfect synchronism of an optical cavity is presented. A simultaneous measurement of FEL power and absolute detuning length of the optical cavity ( deltaL) has clearly shown that the FEL efficiency becomes maximum at deltaL = 0.0+/-1 microm, although it has been considered that only a transient state exists at deltaL = 0 due to the well-known laser lethargy effect. The observed efficiency detuning curve is well reproduced by our numerical simulation including a small shot-noise effect.     

单光子和双光子激发的超歧化聚合物的光谱学特性研究

研究了二种新型超歧化聚合物在氯仿和二甲基甲酰胺溶剂中的飞秒激光单光子和双光子激发的光谱学特性．这二种聚合物是以苯为核、以对二甲氧基苯为连接单元，分别以二甲基苯胺为端基或以特丁基苯为端基的超歧化共轭聚合物．结果表明这种超歧化聚合物具有较大的溶剂极性相关的飞秒双光子吸收截面，不同的端基不仅影响分子内的电荷转移，而且导致超歧化分子共轭程度的变化．作为一种潜在的光学非线性材料，利用这种大的双光子吸收截面的超歧化聚合物可实现了飞秒双光子直写的三维高密度数据存储．     

Measurement of the temporal and spatial phase variations of apulsed free electron laser amplifier

We report temporal and spatial phase measurements of a pulsed free electron laser amplifier (FEL) operating in combined wiggler and axial guide magnetic fields. The system uses a mildly relativistic electron beam (750 kV, 90-300 A, 30 ns), and is excited by a high power 33.39 GHz magnetron source. The phase during the voltage pulse is measured by an interferometric technique from which frequency chirping is determined     

High-speed phase- and group-delay scanning with a grating-based phase control delay line

A rapid-scanning optical delay line that employs phase control has several advantages, including high speed, high duty cycle, phase- and group-delay independence, and group-velocity dispersion compensation, over existing optical delay methods for interferometric optical ranging applications. We discuss the grating-based phase-control delay line and its applications to interferometric optical ranging and measurement techniques such as optical coherence domain reflectometry and optical coherence tomography. The system performs optical ranging over an axial range of 3 mm with a scanning rate of 6m/s and a repetition rate of 2 kHz. The device is especially well suited for applications such as optical coherence tomography that require high-speed, repetitive, linear delay line scanning with a high duty cycle.     

FEL光学速调管的升级性能分析

通过三维磁场的有限元计算,给出了自由电子激光（FEL）研究用光学速调管升级后的磁参数。国家同步辐射实验室合肥光源（HLS）电子储存环能量可以日常运行在200～800 MeV间,为了与电子储存环能量匹配,并在较高束电子能量下进行实验和得到较多的相干辐射光子,光学速调管从原来的对称结构升级成非对称结构,用于HLS储存环谐波产生FEL实验。给出了升级后非对称光学速调管的几组匹配磁参数,用于在HLS储存环注入能量和可以运行的最高能量下进行谐波FEL实验。初步计算表明,HLS 储存环电子束性能优越,能散很低,FEL实验用最高能散仅为2.05×10-4,相应FEL辐射的能散修正因子在0.96以上,可以忽略不计。     

取代效应对脂滴检测NAPBr染料分子的双光子吸收和激发态性质影响的理论探究

双光子荧光染料分子在生物医学成像中具有广阔的应用前景,但取代效应对分子结构以及光物理性质影响的探求相对匮乏. 本文设计并研究了一系列脂滴检测染料分子,分析了分子的光学性质以及无辐射跃迁等. 通过分子内弱相互作用和电子- 空穴布居分析,阐述了其内在机理. 结果表明,所研究的分子均具有优良的光物理性能、高效荧光量子产量、大的斯托克斯位移以及显著的双光子吸收截面等. 本工作合理地解释了实验现象并阐述了取代效应对脂滴检测NAPBr染料分子的双光子吸收和激发态性质的影响,这为设计新型的高效有机分子提供了理论指导.