Showing papers on "Femtosecond published in 1984"

Cherenkov Radiation from Femtosecond Optical Pulses in Electro-Optic Media

Abstract: It was recently suggested that under suitable conditions the propagation of femtosecond optical pulses in electro-optic materials should be accompanied by the radiation of an extremely fast electromagnetic transient [1,2]. This phenomenon, which arises from the inverse electro-optic effect [3], produces a Cerenkov cone of pulsed radiation having a duration of approximately one cycle and a frequency in the THz range.

Femtosecond Laser Interaction with Metallic Tungsten and Nonequilibrium Electron and Lattice Temperatures

Abstract: High-intensity, 75-fs optical pulses have been applied to observe multiphoton and thermally enhanced photoemission from a tungsten metal surface. Experimental results suggest the presence of anomalous heating, a transient nonequilibrium temperature difference between the electrons and lattice. Pump-probe measurements indicate an electron-phonon energy relaxation time of several hundred femtoseconds.

Amplified femtosecond optical pulses and continuum generation at 5-kHz repetition rate.

Abstract: Pulses of 90-fsec duration from a cavity-dumped colliding-pulse mode-locked laser have been amplified to microjoule energies at 5-kHz repetition rate using a copper-vapor-laser pump source. Near-diffraction-limited focusing and efficient femtosecond continuum generation are demonstrated.

Femtosecond studies of intraband relaxation in GaAs, AlGaAs, and GaAs/AlGaAs multiple quantum well structures

Abstract: Femtosecond intraband relaxation dynamics of hot carriers in highly excited states of GaAs, AlGaAs, and AlGaAs/GaAs multiple quantum well (MQW) structures are studied at room temperature using the equal‐pulse correlation technique. Initial carrier lifetimes of 35, 60, and 50 fs are measured for GaAs, Al0.32Ga0.68As, and MQW structures for excitation with 2.02‐eV photons at low carrier densities, and are in reasonable agreement with calculated scattering rates. The carrier‐density dependence of these lifetimes is measured for densities in the range 1.5×1017–5×1019 cm−3.

Novel transient scattering technique for femtosecond dephasing measurements.

Abstract: A novel transient scattering technique is proposed for femtosecond dephasing time measurements. This technique provides several advantages compared with existing techniques for time-domain dephasing measurements, including resolution below the pulse width, insensitivity to rapid energy relaxation, and clear demarcation between homogeneous and inhomogeneous broadening. We report experimental results for the dye Nile blue in solution; the dephasing time T2 is determined to be less than 20 fsec.

Analysis of quarter-wave dielectric-mirror dispersion in femtosecond dye-laser cavities.

Abstract: Analysis of dispersion produced by a lambda/4 multilayer dielectric mirror as a function of its construction parameters is presented. The amount of dispersion can be estimated from a careful inspection of the measured reflectivity curve. In some cases, which seem to be of practical relevance, the value of dispersion is such as to prevent pulses shorter than ~50 fsec from being produced in the mode-locked laser cavity.

Observed circuit limits to time resolution in correlation measurements with Si‐on‐sapphire, GaAs, and InP picosecond photoconductors

Abstract: We report cross‐correlation measurements of the response of photoconductor pulsers and sampling gates excited by a femtosecond laser. The photoconductors were fabricated in microstrip transmission line structures on Si‐on‐sapphire, semi‐insulating GaAs, and semi‐insulating InP wafers. The photoconductor sampling gates were ion beam damaged to produce short carrier lifetimes (<3 ps in one case). Damage was introduced with 6 MeV 20Ne on the Si‐on‐sapphire, 2 MeV 2H on the GaAs, and 2 MeV 4He on the InP. Doses in the range 1012–1015 cm−2 were used. Our results show circuit limits to the time resolution in correlation measurements from two sources: (a) RC time constants due to photoconductor gap capacitance and transmission line characteristic impedance and (b) dispersion in microstrip transmission lines.

Tilted-pulse second-harmonic beam analysis for femtosecond to subnanosecond laser pulse-duration measurements

Abstract: The possibility of extending the second-harmonic beam (SHB) method proposed originally for picosecond and subpicosecond pulse-duration measurements to the femtosecond region is pointed out. This can be achieved by introducing a differential time delay of the pulse wave front corresponding to a tilting of the pulse in the direction other than that applied by Wyatt and Marinero, and also by Saltiel et al., who achieved extensions towards the subnanosecond region. The solution of the wave equations for noncollinear second-harmonic generation in the case of arbitrarily tilted pulses has been carried out. Simple formulae valid from the subnanosecond to the femtosecond region are presented.

Colliding Pulse Femtosecond Lasers and Applications to the Measurement of Optical Parameters

Abstract: Propagation of short coherent pulses through media poses new constraints on materials, as well as on the design of experimental set ups, in order to maintain the best possible temporal resolution. Hence the need for accurate and sensitive techniques to measure the change in pulse parameters transmitted through (or reflected off) optical samples. We show that the mode locked ring laser is in itself an accurate tool to investigate linear and nonlinear optical properties of its components. Measurements of transmission and reflection can be performed intra or extracavity. Intracavity measurements are more accurate because of the multiple passages through the sample. As in cw intracavity spectroscopy the enhancement of sensitivity when operating in the stationary mode-locking regime is of the order of the mean number of cycles in the cavity. The absorber jet is in ideal configuration to study the phase relaxation time of the dye by Degenerate Four Wave Mixing (DFWM). Because of the colliding pulse mode locking process, the two “pump pulses” for the DFWM interaction are simultaneously present in the jet, while the laser output itself can be used for the probe pulse. The “thin sample” condition required to obtain high temporal resolution in DFWM | l | is also met in such a laser.

Resolution of the Femtosecond Lifetime Species Involved in the Photodissociation Process of Hemeproteins and Protoheme

Abstract: Femtosecond absorption spectroscopy of photodissociated hemoproteins and protoheme indicates that a transient species with a 350 femtosecond lifetime is involved in the photodissociation process whatever the ligand is. The spectral properties of the deoxy-like species are analyzed.

3 MHz Amplifier for Femtosecond Optical Pulses

Abstract: We describe a system designed specifically for amplification of femtosecond optical pulses [1,2] at high (3 MHz) repetition rate. A key problem in high repetition rate amplification is that the energy of the available pump pulses is only a few microjoules as compared to the hundreds of millijoules available for low repetition rate amplifiers [2]. We resolve this problem, in part, by using multiple collinear passes to extract efficiently the pump pulse energy while also maintaining the transverse coherence and femtosecond duration of the amplified pulse. In addition we have for the first time cavity dumped the source laser, a colliding pulse mode-locked ring laser [3] while maintaining the femtosecond pulse duration. The pulse energy incident on the 3 MHz amplifier has thereby been increased by an order of magnitude, thus reducing the amplification requirements. Our amplifier, like previous high repetition rate amplifiers [4,5], employs a cavity dumped argon ion laser as the pump. The unique features of our system are the amplification of femtosecond, rather than picosecond, pulses and the use of an exactly collinear pumping geometry.

Integrated silicon photoconductors for picosecond pulsing and gating

Abstract: Integrated photoconductors were constructed on a crystalline Si substrate through standard integrated circuit fabrication techniques followed by shadow-masked ion-beam irradiation. Optoelectronic cross-correlation measurements were performed on these structures with a femtosecond colliding-pulse mode-locked dye laser system. Photoconductors processed as pulsers produced <2-ps rise time ∼ 200-mV pulses with full width at half maxima (FWHM) of 20 ps. Photoconductors processed as sampling gates demonstrated 3-dB measurement bandwidths between 5.3 and 7.6 GHz. Due to the absence of jitter, on-chip signal delays were measured with sub-picosecond precision.

Compression and Shaping of Femtosecond Pulses

Abstract: During the Past several years ultrashort pulse technology has advanced dramatically from the picosecond to the femtosecond time domain. The colliding-pulse modelocked (CPM) cw dye laser [1] is now a reliable source of pulses shorter than 100 fsec; and pulses from other laser systems are being compressed to similar durations by nonlinear optical fiber methods [2]. Fiber compression of amplified pulses from a CPM-based system has already been used to generate 30 fsec pulses [3]. In the first part of this paper we describe experiments in which this same combination of techniques has been extended to produce pulses as short as 16 fsec [4]. These pulses, with a center wavelength of about 625 nm, are comprised of only 8 optical cycles. Then, as an application for such ultrashort pulses, we describe tine-resolved reflectometry studies of multilayer dielectric mirror coatings. Dramatic pulse distortions can be observed after a single reflection from a broadband mirror. Finally, we discuss the use of transient four-wave mixing for pulse shortening and sharing at much higher power levels.

Imaging with Femtosecond Optical Pulses

Abstract: The interaction of short laser pulses with semiconductors has been studied by a variety of techniques including time-resolved reflectivity, [1–5] transmission, ]1–5] photoluminescence, [1–5] surface ellipsometry, [6] and surface second harmonic generation. [7] In the present work, we report an imaging technique used to obtain the first time-resolved photographs of a silicon surface at fixed time delays ranging from 100 fsec. to 600 psec following excitation with an intense ultrashort optical pulse. When the fluence E of 4he excitation pulse exceeds a threshold value ETH (approximately 0.1 J/cm2, under our experimental conditions) a rapid increase in surface reflectivity occurs which has been widely interpreted [8] as thermal melting. [1–5,9] The photographs depict the evolution of the surface reflectivity during and following melting with a time resolution of 100 fsec. and a spatial resolution of 5 µm. Using a movie camera and elementary synchronization electronics, we have also made a motion picture which shows the continuous sequence of melting, boiling, and material e fiction over a 600 psec period slowed in time by as much as a factor of 1013. The still photographs presented here depict the major events in this sequence.

Ultrafast Self-Phase Modulation in a Colliding Pulse Mode-Locked Ring Dye Laser

Abstract: In recent years, generation and application of ultrashort pulses from colliding pulse mode-locked (CPM) ring dye lasers [1–3] have attracted much attention for the study of ultrafast phenomena in the femtosecond region. We found that both spectral and temporal characteristics of these pulses are not simple but are largely affected by self-phase modulation (SPM), strongly depending on the concentration of the absorber dye.

Effects of cavity dispersion in femtosecond mode-locked dye lasers (A)

Abstract: Significant progress has recently been made in the generation of femtosecond dye laser pulses, in particular by means of the colliding pulse technique [1]. Some observed features of the mode-locked (ML) laser still remain largely unexplained, however. In a few cases, for instance, a strong negative frequency chirp has been experimentally observed [2] and attributed to a phenomenon of self-phase-modulation (SPM) arising from the saturation of the saturable absorber [3]. In other cases, the choice of the cavity mirrors [4] and their layout in the laser cavity [5] have been shown to influence the pulse duration.

Effects of Cavity Dispersion on Femtosecond Mode-Locked Dye Lasers

Abstract: Significant progress has recently been made in the generation of femtosecond dye laser pulses, in particular by means of the colliding pulse technique [1]. Some observed features of the mode-locked (ML) laser still remain largely unexplained, however. In a few cases, for instance, a strong negative frequency chirp has been experimentally observed [2] and attributed to a phenomenon of self-phase-modulation (SPM) arising from the saturation of the saturable absorber [3]. In other cases, the choice of the cavity mirrors [4] and their layout in the laser cavity [5] have been shown to influence the pulse duration.

Femtosecond Dephasing Measurements Using Transient Induced Gratings

Abstract: Transient four-wave mixing techniques have been developed in the past several years for the investigation of ultrafast deohasina of electronic transitions in condensed matter [1,2]. These experiments involve self-diffraction of two noncollinear pulses from an optically induced absorption grating. We have recently reported an improved scheme utilizing three separate input pulses [3]. Similar three pulse grating experiments [4] can be used to study processes such as spatial diffusion, orientational relaxation and excited state relaxation; but the applicability to dephasing studies had not been previously recognized. Compared to the two pulse scattering, our three pulse experiment provides several important advantages, including resolution below the pulsewidth, clear demarcation between homogeneous and inhomogeneous broadening, and separation of energy relaxation (T1) effects from the phase relaxation (T2) effects.

Degenerate Four-Wave Mixing of Femtosecond Pulses in the Saturable Absorber of a Ring Dye Laser

Abstract: The saturable absorber dye jet of a femtosecond ring dye laser provides an ideal candidate for the study of fast transients in de- generate four-wave mixing (DFWM). Since the counterpropagating pulses in the ring laser meet at the absorber jet, the problem of timing the DFWM pump pulses is solved. The energy of the pulses inside the cavity is such that the absorber is saturated. This meets one of the conditions for efficient DFWM in saturable media.1 Since the dye jet is thin, fast transients in DFWM are not masked by propagation effects.

FEMTOSECOND DYNAMICS OF NONEQUILIBRIUM CORRELATED ELECTRON-HOLE PAIR DISTRIBUTIONS IN ROOM-TEMPERATURE GaAs MULTIPLE QUANTUM WELL STRUCTURES

Abstract: The GaAs multiple quantum well structure (MQWS) is a new and unique nonlinear optical material [1]. The reduced dimensionality of the electron-hole system in the ultra-thin (100 A) layers gives rise to clear excitonic absorption peaks which are observed even at room temperature [2]. The GaAs MQWS may form the basis for future high speed optoelectronic devices operating at room temperature.

Femtosecond Studies of Intraband Relaxation of Semiconductors and Molecules

Abstract: In this talk we present a summary of our studies of the ultrafast relaxation dynamics of hot carriers in semiconductors [1] and of highly excited dye molecules in solution [2] using the equal pulse correlation technique [3]. These intraband processes occur on timescales comparable to the pulsewidths generated by current femtosecond (fs) technology, and hence present difficulties in extracting their relaxation times due to the presence of slow relaxation processes and of the coherent artifact.

Picosecond And Femtosecond Diagnostics Of Semiconductors

Abstract: Ultrashort light pulses are used to study carrier dynamics in highly excited semiconductor materials. Picosecond pulses from a cw modelocked Nd:YAG laser create carriers and probe nonlinear (Auger) recombination in InGaAs and InGaAsP epilayers. Femtosecond continuum pulses from a dye oscillator/amplifier system monitor the spectral dynamics of free excitons in CdSe following optical excitation.© (1984) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Femtosecond Multiphoton Photoelectron Emission from Metals

Abstract: Multiphoton photoelectron emission from metals has been the subject of experimental and theoretical investigation for several years [1]. The development of high intensity ultrashort pulse laser sources has made possible the extension of these studies into the picosecond time regime [2,3]. For intense pulses of short enough duration, it has been postulated that a transient thermal nonequilibrium between the electrons and phonons may be generated [4]. This phenomenon has been termed anomalous heating and is predicted when the laser pulse durations are comparable to or shorter than the electron-phonon energy relaxation time. Because of the smaller heat capacity of the electron gas, heating of the electrons to temperatures in excess of the lattice melting temperature would then be possible. Previous experimental investigations of photoelectron emission have been performed with high intensity picosecond pulses in attempts to observe anomalous heating [2,3]. To date, none have achieved the temporal resolution necessary for such an observation. However, recently, indirect evidence of transient heating has been reported using picosecond reflectivity measurements in copper [5].