Experimental evidence of above-threshold photoemission in solids

Nonlinear photoemission from a silver single crystal is investigated by femtosecond laser pulses in a perturbative regime. A clear observation of above-threshold photoemission in solids is reported for the first time. The ratio between the three-photon above-threshold and the two-photon Fermi edges is found to be 10(-4). This value constitutes the only available benchmark for theories aimed at understanding the mechanism responsible for above-threshold photoemission in solids.     

Analysis of using femtosecond laser scanning system to impurity-induced disordering of InGaAsP quantum wells

This study is the first to demonstrate the selectivity quantum well intermixing process by using a femtosecond laser scanning-induced disordering technique. The advantages of the femtosecond laser are photochemical machining and the two-photon absorption mechanism. The femtosecond laser system can convert writing into the scan to create a nanostructure by adjusting the lens. The effect of power on the band gap shift during laser scanning was investigated. The band gap shift was small and unstable without the heating substrate. A wavelength shift higher than 77.3 nm for the InGaAsP MQW material was obtained at elevated temperatures.     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Impact of ice structure on ultrafast electron dynamics in D2O clusters on Cu(111)

The structure of D2O clusters on a Cu(111) surface and the femtosecond dynamics of photoexcited excess electrons are investigated by low-temperature scanning tunneling microscopy and two-photon photoemission spectroscopy. Two types of amorphous ice clusters, porous and compact, which exhibit characteristic differences in electron dynamics, are identified. By titration with Xe we show that in both structures solvated electrons preferentially bind on the cluster surface.     

Effects of absorption saturation in colloidal quantum dots as fluorophores for multiphoton fluorescence microscopy

It is shown that colloidal semiconductor nanocrystals (quantum dots), which are promising fluorophores for multiphoton fluorescence microscopy, exhibit a two-photon absorption saturation effect at moderate powers (not exceeding 10 mW) of femtosecond pumping radiation. An analytical expression for the power of two-photon fluorescence of quantum dots as a function of the average pumping power is obtained. With this expression, the deviation of the found dependence from the quadratic law is explained by two factors, i.e., a large two-photon absorption cross section of quantum dots and their slow (compared to the typical pumping pulse-repetition period) relaxation to an unexcited state. Using an LSM 510 Carl Zeiss laser scanning microscope equipped with a Ti:Sa tunable femtosecond laser, a series of model experiments is performed to reveal the saturation effect in a solution of commercially available quantum dots. Good agreement is obtained between the measured dependence of the power of two-photon fluorescence on the average pump power and the theoretical calculation results. It is also experimentally demonstrated that, under fluorescence saturation conditions, the spatial resolution of the method of multiphoton fluorescence microscopy is lost; this effect is analyzed numerically.     

Phase and population decay of image-potential states – the influence of defects

Quantum-phase and population decay of image-potential states have been investigated by two-photon photoemission with femtosecond time resolution. The influence of steps and defects on quasielastic and inelastic scattering processes is illustrated for a vicinal Cu(119) surface and diluted adsorbate layers of CO and Cu on Cu(001). Received: 19 April 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Femtosecond transfer dynamics of photogenerated electrons at a surface resonance of reconstructed InP(100)

Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.     

Femtosecond photoemission microscopy

We developed a novel experiment for time-resolved photoemission microscopy by combining a commercial photoemission electron microscope (PEEM) with a pulsed Ti:sapphire laser oscillator. The laser system, the setup of the delay stage for pump-probe experiments, and the interface between the PEEM and the laser system are discussed. We use self-organization of Ag islands and nanowires on Si(1 1 1) and 4° vicinal Si(0 0 1) to generate structures with a plasmon resonance that matches the photon energy of our laser (?ω = 3.1 eV after frequency doubling). In two-photon photoemission (2PPE) the photoemission yield then directly visualizes the plasmons in the nanostructures. Accordingly, the photoemission yield depends on the size and shape of the nanostructures, and on the polarization of the laser pulses as well. In Ag nanowires, we observe surface plasmon polariton (SPP) waves by a beating that is formed by interference of the SPP wave and the incident laser light. In a pump-probe experiment, we can directly visualize the propagation of the SPP on a femtosecond time scale.     

飞秒激光双光子微细加工技术及研究现状

飞秒激光双光子微细加工技术以其特有的高精度三维微加工优势,成为微型机械加工领域新的发展方向之一。介绍了飞秒激光双光子微细加工技术的原理和应用的现状。结合目前已有的微细加工技术,对双光子微细加工技术的特点加以评述。简要报道了利用飞秒激光双光子微细加工技术的一些研究进展。探讨了飞秒激光双光子微细加工技术今后的发展方向及其存在的基本问题。     

Ultrafast magnon generation in an Fe film on Cu(100)

We report on a combined experimental and theoretical study of the spin-dependent relaxation processes in the electron system of an iron film on Cu(100). Spin-, time-, energy- and angle-resolved two-photon photoemission shows a strong characteristic dependence of the lifetime of photoexcited electrons on their spin and energy. Ab?initio calculations as well as a many-body treatment corroborate that the observed properties are determined by relaxation processes involving magnon emission. Thereby we demonstrate that magnon emission by hot electrons occurs on the femtosecond time scale and thus provides a significant source of ultrafast spin-flip processes. Furthermore, engineering of the magnon spectrum paves the way for tuning the dynamic properties of magnetic materials.     

Two-photon excited photoluminescence of photonic quantum ring laser structures

We report on the two-photon excited photoluminescence of photonic quantum ring laser structures using Ti:Sapphire femtosecond pulsed laser. Using two-photon excited photoluminescence microscopy, we were able to image the laser structures when optically excited and compare the results with previously obtained images on electrically pumped photonic quantum ring lasers. We also propose a method to evaluate the Rayleigh band on the circumference of these structures.     

Realization of a spin-polarized two-dimensional electron gas via image-potential-induced surface states

Electrons in image-potential-induced surface states form a two-dimensional electron gas in front of the surfaces. In the case of ferromagnets, their binding energies as well as lifetimes depend on the orientation of their spin magnetic moment with respect to the magnetization direction. Various experiments with inverse photoemission and two-photon photoemission to detect the spin dependence of image states are reviewed. A new and successful approach to achieve and detect a spin-polarized two-dimensional electron gas is presented, namely polarization-dependent and spin-resolved two-photon photoemission. Additional time resolution opens the way to study spin-dependent electron dynamics.     

Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO (110) surface

The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the elec- tronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO₂ single- crystalline surfaces. Our research into the CH₃OH/TiO₂ system exhibits complex dynamics, providing abundant informa- tion with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides.     

Dynamics of exciton formation at the Si(100) c(4 x 2) surface

Carrier recombination at the Si(100) c(4 x 2) surface and the underlying surface electronic structure is unraveled by a combination of two-photon photoemission and many-body perturbation theory: An electron excited to the silicon conduction band by a femtosecond infrared laser pulse scatters within 220 ps to the unoccupied surface band, needs 1.5 ps to jump to the band bottom via emission of optical phonons, and finally relaxes within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds.     

Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO2(110) surface

The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the electronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO₂ singlecrystalline surfaces. Our research into the CH₃OH/TiO₂ system exhibits complex dynamics, providing abundant information with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides.     

低强度飞秒激光激发下CdTe和CdTe/CdS核壳量子点的荧光上转换研究

利用400 nm和800 nm不同波长的低强度飞秒激光,对CdTe和CdTe/CdS核壳量子点溶胶进行激发,研究其稳态和时间分辨荧光性质.800 nm飞秒激光激发下,CdTe和CdTe/CdS核壳量子点产生上转换发光现象,上转换荧光峰与400 nm激发下的荧光峰相比蓝移最多达15 nm,而且蓝移值与荧光量子产率有关.变功率激发确认激发光功率与上转换荧光强度间满足二次方关系,时间分辨荧光的研究表明荧光动力学曲线服从双e指数衰减.提出表面态辅助的双光子吸收模型是低激发强度上转换发光的主要机理.CdTe和CdT 关键词： CdTe量子点 CdTe/CdS核壳量子点 时间分辨荧光 上转换荧光     

考虑光镊效应的飞秒激光双光子加工线宽

采用自由基浓度起伏理论结合光镊集聚效应,理论研究了飞秒激光双光子加工的线宽问题。根据双光子光聚合过程中自由基浓度随时间变化的关系,考虑光镊效应对自由基分布范围的影响,得到了飞秒激光双光子加工线宽的表达式。研究了线宽随扫描速度与激光功率的变化关系,并讨论了不同光引发剂对线宽的影响。得到了以自由基浓度起伏为基础,并考虑光镊效应的双光子加工线宽表达式,该结果与实验结果相符。研究结果为飞秒激光双光子加工的研究提供了新的思路,为光镊集聚效应对线宽影响的实验研究提供了理论依据。     

Multicontrast nonlinear optical microscopy with a compact and rapid pulse shaper

Homodyne detection can dramatically enhance measurement sensitivity for weak signals. In nonlinear optical microscopy it can make accessible a range of novel, intrinsic, contrast like nonlinear absorption and nonlinear phase contrast. Here a compact and rapid pulse shaper is developed, implemented, and demonstrated for homodyne detection in nonlinear microscopy with high-repetition rate mode-locked femtosecond lasers. With this method we generate two-photon absorption (TPA) and self-phase modulation images of gold nanostars in biological samples. Simultaneous imaging of two-photon luminescence and TPA also enables us to produce two-photon quantum yield images.     

Multielectron ionization of CdSe quantum dots in intense femtosecond ultraviolet light

Multielectron ionization of colloidal CdSe quantum dots under intense femtosecond UV excitation has been studied. By directly probing the absorption from the ionized electron, quantitative measurements of the yield and dynamics of the ionization have been made as a function of excitation fluence and variations of size and potential structure of quantum dots. The results have been explained by an ionization mechanism involving resonant two-photon absorption.     

Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO2 (110) surface

The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the electronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO₂ singlecrystalline surfaces. Our research into the CH₃OH/TiO₂ system exhibits complex dynamics, providing abundant information with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides.