Simple route to strong-field coherent control

Coherent-control schemes to manipulate weak-field interactions are generally invalid at stronger fields, since strong-field interactions are accompanied by level power broadenings and level shifts that usually elude simple analytical treatments. Here we show that a broad subgroup of weak-field solutions (those with real fields, i.e., fields with only one quadrature in the complex plane) can be extended to the strong-field regime while retaining their properties. The salient feature of these fields is a symmetry that cancels out power broadening effects. Such fields can be generated from ultrashort coherent pulses or from incoherent broadband down-converted light. Weak-field coherent-control approaches based on these solutions can therefore be extended to the strong-field regime as we demonstrate in a two-photon absorption experiment in atomic cesium.     

Nonlinear spectrum effect on the coherent control of molecular systems

This work demonstrates that the detuning of the fs-laser spectrum from the two-photon absorption band of organic materials can be used to reach further control of the two-photon absorption by pulse spectral phase manipulation. We investigate the coherent control of the two-photon absorption in imidazole-thiophene core compounds presenting distinct two-photon absorption spectra. The coherent control, performed using pulse phase shaping and genetic algorithm, exhibited different growth rates for each sample. Such distinct trends were explained by calculating the two-photon absorption probability considering the intrapulse interference mechanism, taking into account the two-photon absorption spectrum of the samples. Our results indicate that tuning the relative position between the nonlinear absorption and the pulse spectrum can be used as a novel strategy to optimize the two-photon absorption in broadband molecular systems.     

Pico-second spectroscopy of the secondary radiation from resonantly excited excitonic molecules in CuCl

Pico-second time-resolved measurements of the two-photon resonant Raman scattering via excitonic molecules in CuCl were carried out for the first time. The Raman scattering leaving transverse excitons decays as fast as the laser light. When the energy of the incident light falls in the vicinity of the two-photon resonant absorption, both Raman and luminescence lines are simultaneously observed. In case of just resonant excitation, the transient response indicates that the secondary radiation can be decomposed into the Raman and the luminescence components as regards its temporal and spectral behavior.     

咔唑类化合物的光谱特性与三光子激射

测量了一系列新合成的咔唑类化合物的光谱特性。光谱研究结果发现端基团的选择引起双光子吸收截面的巨大变化。用1.3 μm强fs激光激发其中一双光子吸收截面高达617.3 GM的分子溶液,其波长和时间分辨光谱特性表明此分子的高方向性光发射是三光子诱导激射。[1 GM(Gppert-Mayer)=10-50 cm4·s·(photon)-1]。     

Adaptive pulse compression by two-photon absorption in semiconductors

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.     

Two- and three-photon excitation of Gd in CaAl12O19

We have employed two-photon excitation to study the higher energy levels of Gd³⁺ ions in CaAl₁₂O₁₉ and we compare the results with those obtained using conventional UV excitation techniques. Under two-photon excitation, the luminescence intensity exhibits an unusual temporal behavior, a very long build-up followed by a decrease by orders of magnitude, ascribed to a recombination-assisted luminescence excitation mechanism assuming photo-ionization of Gd³⁺ ions and trapping of free electrons on deep traps.

We also find that the two-photon excitation spectra contain an additional broadening contribution which can be attributed to homogeneous broadening of excitation levels caused by excited state absorption into the conduction band. We believe that this may be a general phenomenon whenever participating photons produce ionization of impurity ions from metastable excited states. The phenomenon can manifest itself also in two-photon ionization spectral hole burning and in up-conversion processes (in the latter case, the homogeneous broadening can be caused by an intra-ion excited-state absorption).     

Properties of pulsed entangled two-photon fields

The dependence of one- and two-photon characteristics of pulsed entangled two-photon fields generated in spontaneous parametric down-conversion on the pump-pulse properties (shape of the pump-pulse spectrum and its internal structure) is examined. It is shown that entangled two-photon fields with defined properties can be generated. A general relation between the spectra of the down-converted fields is established. As a special case the interference of two partially overlapping pulsed two-photon fields is studied. The fourth-order interference pattern of entangled two-photon fields is investigated in the polarization analog of the Hong-Ou-Mandel interferometer. Received: 23 December 1998 / Received in final form: 14 April 1999     

染料掺杂聚合物薄膜的光谱下转换研究

在玻璃基片表面制作了Coumarin 500、Coumarin 540和Rhodamine 6G 3种染料掺杂的聚合物薄膜,并以市售LED灯和氙灯为光源,实验研究了单一染料掺杂和混合染料掺杂的聚合物薄膜的吸收光谱、荧光光谱及其光谱下转换的特征。研究结果表明：这种多染料掺杂的聚合物薄膜能够吸收350~550 nm波长范围内的光子并将其转换至波长为580 nm左右的光子。在一定的浓度范围内,较高的染料浓度能够获得较高的下转换光子输出。由于Nd³⁺/Yb³⁺共掺玻璃中Nd³⁺离子在 580 nm 附近有一个较强的吸收峰,这种染料掺杂聚合物薄膜的光谱下转换有助于提高Nd³⁺/Yb³⁺共掺玻璃对太阳光中蓝绿光的吸收并通过共振能量转移激发Yb³⁺离子的近红外发光,对提高单晶硅太阳能电池的综合光伏转换效率有潜在的应用价值。     

Polarization and phase control of two-photon absorption in an isotropic molecular system

<正>We theoretically and experimentally study the polarization and phase control of two-photon absorption in an isotropic molecular system.We theoretically show that the two-photon transition probability decreases when the laser polarization changes from linear through elliptical to circular,and the laser polarization does not affect the control efficiency of two-photon transition probability by shaping the spectral phase.These theoretical results are experimentally confirmed in coumarin 480.Furthermore,we propose that the combination of the laser polarization with the spectral phase modulation can further increase the control efficiency of the two-photon absorption.     

Manipulating quantum states with aspheric lenses

We present an experimental demonstration to manipulate the width and position of the down-converted beam waist. Our results can be used to engineer the two-photon orbital angular momentum (OAM) entangled states (such as concentrating OAM entangled states) and generate Hermite–Gaussian (HG) modes entangled states.     

Resonant transfer of one- and two-photon excitations in quantum dot–bacteriorhodopsin complexes

Light-sensitive protein bacteriorhodopsin (BR), which is capable of electrical response upon exposure to light, is a promising material for photovoltaics and optoelectronics. However, the rather narrow absorption spectrum of BR does not allow achieving efficient conversion of the light energy in the blue and infrared spectral regions. This paper summarizes the results of studies showing the possibility of extending the spectral region of the BR function by means of the Förster resonance energy transfer (FRET) from CdSe/ZnS quantum dots (QDs), which have a broad spectrum of one-photon absorption and a large twophoton absorption cross section (TPACS), to BR upon one- and two-photon excitation. In particular, it is shown that, on the basis of QDs and BR-containing purple membranes, it is possible to create electrostatically associated bio-nano hybrid systems in which FRET is implemented. In addition, the large TPACS of QDs, which is two orders of magnitude larger than those of BR and organic dyes, opens up a means for selective two-photon excitation of synthesized bio-nano hybrid complexes. On the basis of the results of this work, the spectral region in which BR converts the light energy into electrical energy can be extended from the UV to near-IR region, creating new opportunities for the use of this material in photovoltaics and optoelectronics.     

Highly selective population of two excited states [2mm]in nonresonant two-photon absorption

A nonresonant two-photon absorption process can be manipulated by tailoring the ultra-short laser pulse. In this paper, we theoretically demonstrate a highly selective population of two excited states in the nonresonant two-photon absorption process by rationally designing a spectral phase distribution. Our results show that one excited state is maximally populated while the other state population is widely tunable from zero to the maximum value. We believe that the theoretical results may play an important role in the selective population of a more complex nonlinear process comprising nonresonant two-photon absorption, such as resonance-mediated (2+1)-three-photon absorption and (2+1)-resonant multiphoton ionization.     

宽带低相干光单次时间相干性测量研究

传统测量光束时间相干性的方法是通过机械扫描的方式实现的,这种方法不能够实现单次测量,而且对于相干时间较短的宽带光测量误差较大。本文提出了一种单次时间相干性测量的新方法,通过给迈克尔逊干涉仪的反射镜引入楔角,使光束波前产生随位置变化的延迟差,可从单次测量的一幅干涉图中计算提取出光场完整的时间相干性信息。实验中测量了不同宽带入射光的时间相干性,均与理论结果吻合较好。单次时间相干性测量的方法将为高功率宽带激光装置提供更为方便的时间相干性测量手段,提高实验测量效率。     

Quantum interference and diffraction of parametric down-converted biphotons

We present two-photon diffraction and interference experiments utilizing parametric down-converted photon pairs (biphotons) and a transmission grating. The biphoton exhibits a diffraction-interference pattern equivalent to an effective single particle with half wavelength of the constituent photons.     

Broadband absorption spectroscopy via excitation of lossy resonance modes in thin films

It is shown that broadband absorption spectroscopy utilizing thin lossy film configurations can be optimally facilitated when applied to metallic and insulating materials. For metallic films, the zero-order highly lossy resonance mode, characterized by ultra wideband absorption behavior under normal incidence, can be shifted, under parallel-polarization oblique incidence, toward a narrow band light-wavelength surface plasmon resonance condition. Higher order low-loss modes, however, occur in thin insulating films, exhibiting Debye relaxation behavior, typical for many aqueous solutions and biological substances. They can be excited in a highly scalable and sensitive manner in various frequency bands, between light and radio frequencies.     

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation

Improving the up-conversion luminescence efficiency of rare-earth ions via the multi-photon absorption process is crucial in several related application areas. In this work, we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er~(3+) ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components.So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. Moreover, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width(or laser spectral bandwidth), final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earthions.     

Direct shaping of tunable UV ultra-short pulses

Time shaping of ultra-short UV pulses has been performed using a specially designed acousto-optic programmable dispersive filter in KDP material. A time shaping window up to 5 ps has been obtained in the 260–410 nm range. A temporal/spectral characterization of shaped pulses on this broad UV spectral range is presented. The efficiency at shorter wavelength is limited by two-photon absorption. However several µJ is obtained at the output of the device allowing chemistry coherent control experiment.     

Electrical noise thermometer

Comparing the recombination light intensity produced in GaAs by a 1.06 μm pulse inducing two-photon absorption, with the intensity produced by a 0.53 μm pulse, we obtain directly the two-photon absorption cross-section. This method can be generalized to other materials.     

Independence of Maximum Single Molecule Fluorescence Count Rate on the Temporal and Spectral Laser Pulse Width in Two-Photon FCS

We investigate the fluorescence emission characteristics of standard dye tetramethylrhodamine (TMR) in two-photon fluorescence correlation spectroscopy for different temporal and spectral properties of the femtosecond excitation pulses. After determining the second-order dispersion of our setup, including the microscope objective, a pulse stretcher was employed to control the temporal width at the location of the specimen. As expected, the fluorescence per molecule and therefore the signal-to-noise ratio of an FCS-measurement can be improved at constant average excitation power by altering either the temporal or spectral width of the excitation pulses. We found however, that the maximum achievable molecular brightness is largely independent of the temporal and spectral width in the regime analyzed. This observation confirms the current working hypothesis for two-photon fluorescence correlation spectroscopy that bleaching and saturation, and thus, the inherent properties of the dye system, are the dominant effects limiting the quality of measurements. As a practical consequence, elaborate optimization of temporal and spectral laser pulse width, e.g. by introducing pulse stretchers in the beam path, is less critical than previously expected.     

Fine structure of two-photon absorption in CdS

We have observed the fine structure of the P-excitons (A, n = 2) in Cds through two-photon absorption. The strengths of absorption peaks of P-excitons and their dependence on the polarization of the light agree with predictions of a four-band model of two-photon absorption. Additional two-photon absorption peaks were identified as phasematched two-step two-photon processes involving second harmonic polarizations.