Showing papers by "Gerard Mourou published in 1991"

Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures

Abstract: Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time‐resolved‐reflectance techniques, a sub‐picosecond (<0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at ∼200°C and annealed at 600 °C. With the same material as a photoconductive switch we have measured electrical pulses with a full‐width at half‐maximum of 0.6 ps using the technique of electro‐optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of ∼120–150 cm2/V s. GaAs grown by MBE at 200 °C and annealed at 600 °C is also semi‐insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of subpicosecond photoconductive applications.

Terahertz attenuation and dispersion characteristics of coplanar transmission lines

Abstract: Experimental verification of analytic formulas for the dispersion and the attenuation of electrical transient signals propagating on coplanar transmission lines is presented. The verification is done in the frequency domain over a terahertz range although the experiments are in the time domain. The analytic formulas are obtained from fits to the full-wave analysis results. It is quantitatively verified that the full-wave steady-state solutions can be directly applied to the transient time-domain propagation experiments. Subpicosecond electrical pulses and an external electrooptic sampling technique are used to obtain the time-domain propagation data. From the Fourier transforms of the time-domain data both the attenuation and the phase information as a function of frequency are extracted. The dispersion and the attenuation characteristics are investigated for both coplanar waveguide and coplanar strip transmission lines. The investigation is carried out on both semiinsulating semiconductor and dielectric substrate materials. No observable losses caused by the semiconductor material are indicated. >

100-fs pulse generation and amplification in Ti:Al 2 O 3

Abstract: We have constructed a self-mode-locked Ti:Al(2)O(3) laser that produces 100-fs pulses at 750 nm. These pulses are then amplified in a Ti:Al(2)O(3) regenerative amplifier to the millijoule energy level. 105-fs, 10-GW pulses have been produced.

Multikilohertz Ti:Al 2 O 3 amplifier for high-power femtosecond pulses

Abstract: We amplified femtosecond pulses at repetition rates of as much as 7.2 kHz in a Ti:Al2O3 regenerative amplifier pumped by a frequency-doubled Q-switched Nd:YLF laser. Energies of as much as 1 mJ at 1 kHz and 85 μJ at 7.2 kHz in a 150-fs pulse have been obtained. This source was used to produce 1 W of femtosecond white light through spectral continuum generation.

Generation of 20-TW pulses of picosecond duration using chirped-pulse amplification in a Nd:glass power chain.

Abstract: Pulses of 20-TW peak power have been generated at 1064 nm using the chirped-pulse-amplification technique with a 90-mm output-aperture Nd:silicate glass amplification line. This system delivers 60 J of energy in a chirped pulse of 600-psec duration, with a capacity to maintain a 3.5-nm output bandwidth. These chirped pulses are compressed to 1.2 psec with an energy of 24 J by using large holographic gold-coated 1740-lines/mm diffraction gratings.

Subpicosecond time-resolved studies of coherent phonon oscillations in thin-film yba2cu3o6+x (x< 0.4)

Abstract: We report the results of the first time‐resolved observation of impulsively generated coherent optical phonon oscillations in the semiconducting cuprate compound YBa2Cu3O6+x (x<0.4). The oscillations, which were probed through time‐resolved transmissivity modulation, had a period of 237 fs at room temperature, corresponding to a Raman active mode of A1g symmetry at 142 cm−1. No oscillations were observed in the superconducting form of Y‐Ba‐Cu‐O either above or below Tc. The amplitude, frequency, and linewidth of this mode were measured over a temperature range from ∼7 K to room temperature.

Experimental characterization of external electrooptic probes

Abstract: The accuracy and invasiveness of various external LiTaO/sub 3/ electrooptic probe (EEP) geometries are investigated experimentally. The EEPs studied are used for microwave coplanar transmission-line measurements. It is shown that commonly employed EEP geometries can cause substantial inaccuracies in their measurements and that these inaccuracies can be related to specific geometric parameters. The major cause of distortions in pulse measurements was found to be attributable to electromagnetic radiation coupling into the LiTaO/sub 3/ crystal and resonating between the top and bottom crystal interfaces. The authors suggest eliminating the deleterious bulk resonance effects due to the EEP material bulk by using a thin LiTaO/sub 3/ crystal that will act as a single lumped element. The thickness that is expected to be optimal should be about the extent of the guided mode confinement near the conductors. A thicker than optimum cryst al will exhibit resonance, and a thinner one will have reduced sensitivity due to a reduced signal integration path. Thinned crystals possess the additional advantages of reduced thermal drift and reduced stray signal pickup from adjacent lines. >

1.4 ps rise‐time high‐voltage photoconductive switching

Abstract: We report on the generation of 825 V electrical pulses with 1.4 ps rise time and 4.0 ps duration using a pulse‐biased low‐temperature‐grown GaAs photoconductive switch triggered by an amplified femtosecond dye laser. Dependence of the pulse shape on both electric field and optical energy is observed and discussed.

Interaction of a 1 psec laser pulse with solid matter

Abstract: Absorption and x‐ray emission results obtained with a 1 psec pulse incident on solid targets with an intensity between 1011 and 1016 W/cm2 are presented and discussed. For I 1015 W/cm2) an evaluation of the plasma parameters is obtained from high resolution keV spectra. Finally, the results are discussed in the light of 1‐D hydrodynamic simulations with time‐dependent atomic physics.

AMPLIFICATION OF ULTRASHORT PULSES WITH Nd:GLASS AMPLIFIERS PUMPED BY ALEXANDRITE FREE RUNNING LASER

Abstract: A system for producing ultra-high peak power pulses employs a plurality of solid state amplifying materials (13, 14, 15, 16), such as Nd:glass, alexandrite, and Ti:sapphire, to achieve stretching of a pulse (12) prior to amplification by a factor of approximately between 100 and 10,000. The time-stretched pulse is amplified by many orders of magnitude, illustratively 109. After time-stretching and amplification, the pulse is then compressed to its original duration (11). Pumping of the multiple solid-state elements (13, 14, 15, 16) is performed simultaneously using an alexandrite laser which is tunable between approximately 700 and 800 nm. The pumping energy has a pulse duration which is less than the fluorescence lifetime of the excited solid-state media.

Subpicosecond carrier llifetime in GaAs grown by molecular beam epitaxy at low temperatures

Abstract: (Received 3 1 May 1991; accepted for publication 23 September 1991) Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time-resolved-reflectance techniques, a sub- picosecond ( ~0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at -200°C and annealed at 600 “C. With the same material as a photoconductive switch we have measured electrical pulses with a full-width at half-maximum of 0.6 ps using the technique of electro-optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of - 120-150 cm”/V s. GaAs grown by MBE at 200 “C! and annealed at 600 “C is also semi-insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of . subpicosecond photoconductive applications. The development of ultrashort-pulse mode-locked la- ser systems has resulted in new techniques for the genera- tion and detection of picosecond and subpicosecond elec- trical transients. l-3 Among these, the use of semiconductor photoconductive switches are the most popular, because these devices can be used to efficiently generate signals and to generate and detect electrical transients in guided media or free space. Also, the semiconductor growth and process- ing techniques available for tailoring the properties of these materials enhance their versatility. The minimum attain- able electrical pulsewidth from a photoconductive element is limited by a number of factors such as the laser pulse- width, circuit parameters of the generation and detection site, and the carrier lifetime in the semiconductor. With the use of femtosecond lasers and photolithographically de- fined millimeter-wave co-planar structures, the limits to speed imposed by the first two factors can be reduced. To shorten the carrier lifetime of a semiconductor layer, im- purity doping of the semiconductor,4 growth of polycrys- talline or amorphous material,5 and damage by ion implantation6 can be used. Earlier we reported that photo- conductive switches based upon GaAs grown by molecular beam epitaxy (MBE) at low temperatures showed fast re- sponse (1.6 ps) and good responsivity in unoptimized structures.’ In this letter we extend our earlier study’ of the photoresponse of low-temperature (LT) GaAs using both a femtosecond transient reflectance technique and photo- conductive switching measurements. From both experi- ments we have observed a subpicosecond carrier lifetime for LT-GaAs grown at -200 “C. The 2+m-thick, (lOO)-oriented epitaxial tilrns dis- cussed here were grown by MBE at substrate temperatures of 400, 350, 300, 260,200, and 190 “C. For all the growths an As4 source was used, and the samples were mounted on the same MO block using In solder. The growth rate was 1.0 pm/h, and the As/Ga beam-equivalent-pressure ratio was 10. Pieces of the LT-GaAs samples were annealed inside the growth chamber under an As overpressure, just after the cmompletion of the growth, by raising the substrate temperature to 600 “C for 10 min. A number of papers have reported novel material properties of as-grown and annealed LT-GaAs layers, es- pecially those grown at -200 “C.“-” For photoconductive- switch applications, the most relevant properties of both as-grown and annealed 200 “C LT-GaAs are that the ma- terials are crystalline and yet contain a high density ( :> 10” cm 3 ) of point defects as As antisites, As inter- stitials, and Ga-related vacancies.“,” In addition to a high density of point defects, annealed 200 “C LT-GaAs grown in the Lincoln Laboratory MBE system also contains small ( < 5 nm) As precipitatesI at densities of -3~ lOI cm -3. The aforementioned point defects can act as recom- bination and trapping centers. Assuming simple Shockley- Read-Hall theory for the recombination mechanism of the photoexcited carriers, and using a density N- lOi cm ’ for the deep levels, a capture cross section u- lo- I3 cm2 (a typical value for deep levels in GaAs), and thermal velocity u,h at T = 300 K, we estimate that the carrier lifetime r = l/(Nmu,) in as-grown and annealed LT- GaAs to be less than 1 ps. Although as-grown LT-GaAs is relatively conducting (p- 10 s1 cm) at room temperature, annealed LT-GaAs is semi-insulating (p- 10’ fl cm).” Despite the high density of point defects and As precipitates, the Hall mobility at room temperature in annealed LT-GaAs is relatively high ( - 1000 cm”/V s).” Therefore, LT-GaAs grown at .- 200 “C and subsequently in situ annealed has the desired properties of a fast photoconductor; namely, a short carrier

Subpicosecond pulse propagation on coplanar waveguides: experiment and simulation

Abstract: Experimental results are presented concerning subpicosecond pulse propagation on normal metal, coplanar transmission line structures. The pulse distortion that occurs is modeled using a semiempirical curve fit to the full-wave analysis for the modal dispersion and quasi-static approximations for the conductor and radiation loss. Without using any adjustable parameters, very good agreement is obtained for the delay, rise time, and amplitude of the pulse for various propagation distances. For terahertz-bandwidth pulses on lines similar to the one studied, the modal dispersion and radiation losses are the dominant pulse-shaping mechanisms. >

Characteristics of an actively mode-locked 2-psec Ti:sapphire laser operating in the 1-μm wavelength regime

Abstract: We examine the performance of an actively mode-locked Ti:sapphire laser operating in the 1-μm wavelength regime. Pulse durations of 1.7 psec are produced with an average output power of 400 mW. With the use of external dispersion compensation this pulse width has been further reduced to 850 fsec.

Optical properties of high‐quality InGaAs/InAlAs multiple quantum wells

Abstract: We have measured the narrowest half‐width at half‐maximum photoluminescence linewidth of 2.8 meV, in 40‐period lattice‐matched In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells, grown by molecular‐beam epitaxy with growth interruption. A simple analysis of the linewidth suggests that the structure has near perfect interfaces. Temperature‐dependent photoluminescence linewidth data indicate impurity incorporation due to the growth interruption. However, the high quality of the multiple quantum well is not impaired as is seen in the room‐temperature absorption data, where excitonic features up to n=3 sublevel are clearly seen. Carrier lifetime in this multiple‐quantum‐well system has been measured, we believe for the first time, using the picosecond photoluminescence correlation technique. A lifetime of 860 ps is obtained, which is similar to the value obtained for high‐quality GaAs/AlGaAs and In0.53Ga0.47As/InP quantum wells. This further confirms the high quality obtained in this ternary material system...

Submillimetre wave response of superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ using coherent time-domain spectroscopy

Abstract: A coherent time-domain technique using freely propagating radiation bursts is used to study the submillimetre-wave response (extending to 1.4 THz) of a YBa2Cu3O7−x thin film above and below the critical temperature. From these measurements the complex dielectric response, and hence the complex conductivity, were directly obtained.

Effect of Growth Conditions on Optical Response of GaAs Grown at Low Substrate Temperature by MBE

Abstract: The effect of growth conditions on the properties of GaAs grown by molecular beam epitaxy at low substrate temperatures has been studied. It has been found that the response time to 100 fsec 830nm light pulses is a function of substrate temperature and arsenic flux. The reason for variation of optical response with growth conditions is related to the nature of the incorporation of excess arsenic. A recent model proposed by Warren and others is invoked to explain the change in optical response with growth conditions. Further substantiation of this model comes from experiments on the annealing of low substrate temperature GaAs which has been doped with silicon.

Ultrafast Properties and Applications of GaAs and InP Based Materials Grown by MBE at Low Temperatures

Abstract: The ultrafast carrier dynamics in GaAs, In0.52Al0 48As on InP, and In0.53Ga0.47As on InP, grown by molecular-beam-epitaxy (MBE) at low substrate temperatures, are investigated. A reduction in the carrier lifetime is observed with decreasing growth temperatures. The shortest carrier lifetimes of typically a picosecond (ps) are obtained at the lowest growth temperature range of 150–200 °C. Femtosecond optical absorption and reflectance measurements have been used to verify the sub-picosecond carrier lifetimes. Photoconductive switching measurements on these materials, measured using the technique of electro-optic sampling have further confirmed the sub-picosecond carrier lifetimes, and have also resulted in the generation of subpicosecond electrical signals. These short electrical pulses have been used for a variety of ultrafast optoelectronic applications.

Picosecond High-Voltage Pulses Generation Using Low-Temperature (LT) GaAs

Abstract: We report on the generation of 825 V electrical pulses with 1.4 ps rise time and 4 ps Full-Width-at-Half-Maximum using pulse biased Low-Temperature-grown GaAs photoconductive switch triggered by submicrojoule, 150 fs laser pulses. Dependence of the temporal pulse shape on both the electric field and the optical energy is observed and discussed.

Tunable high-power femtosecond pulses from Ti:Al 2 O 3 and alexandrite

Abstract: We will present here a unique high power, all solid-state femtosecond source that is tunable, demonstrates high efficiency, and has a very low background. This source, based on Ti:Al2O3 and alexandrite, produces 10–300 GW, 100-fs pulses. To the best of our knowledge this is the first all solid-state source, excluding pump lasers, producing high power, femtosecond pulses. We believe that this kind of system is more powerful and even more reliable then femtosecond dye systems and has the potential of becoming the workhorse for both femtosecond spectroscopy and high intensity physics.

Generation and Amplification of High-Peak-Power Femtosecond Pulses in Ti:Al2O3/Alexandrite and Ti:Al2O3/Nd:Glass Systems

Abstract: We present recent developments in tunable femtosecond oscillators and amplifiers using Ti:Al2O3. These systems produce gigawatt pulses at wavelengths compatible with large Alexandrite or glass amplifiers.

Mode-Locking of Ti:Al2O3 Lasers Using Self-Focusing

Abstract: We show that femtosecond pulses can be obtained using self-focusing in cwTi:Al2O3 lasers. The role of gain guiding and gain saturation is presented.

TEM and HREM Studies of As-Grown Low-Temperature Molecular Beam Epitaxial In0.52Al0.48As/InP Heterolayers

Abstract: Transmission and High Resolution Electron Microscopy have been used to study the dramatic changes in crystalline quality which occur in As-rich In0.52Al0.48As/InP layers when the growth temperature is lowered. We have found that for temperatures as low as 200°C and for a flux ratio of 20, the layers can be of high quality. For the lowest growth temperature of 150°C, pyramidal defects as well as hexagonal As grains are found which are characteristic of the breakdown of the monocrystalline growth in these layers. A mechanism for the formation of these defects is proposed based on their crystallographic structure. The fast photo response previously observed in these layers must be related to the presence of these defects.

Waveguides : Experiment and Simulation

Abstract: Abstruct-ExperimentaI results are presented for subpicosecond pulse propagation on normal-metal, coplanartransmission-line structures. The pulse distortion that occurs is modelled using a semiempirical curve fit to the fullwave analysis for the modal dispersion and quasi-static approximations for the conductor and radiation loss. Without using any adjustable parameters, very good agreement is obtained for the delay, rise time and amplitude of the pulse for various propagation distances. For terahertz-bandwidth pulses on lines similar to the one studied, the modal dispersion and radiation losses are the dominant pulse-shaping mechanisms. ICOSECOND pulses propagating on normal-metal, P coplanar-circuit interconnects are degraded by the dispersion and attenuation characteristics of these guided-wave structures [ 13 - [3]. The dispersion of a coplanar transmission line arises primarily due to the dielectric inhomogeneity at the surface [4] and the complex surface impedance of the electrodes. The attenuation is due to the conductor losses, substrate conductivity losses and radiation or surface wave losses [5]. Modeling of picosecond pulse propagation on coplanar lines needs to incorporate the previous effects, while trying to retain simplicity in computation through the use of approximations verified via simulations and experiments. In this letter, we present experimental results on subpicosecond pulse propagation on normal-metal coplanar-waveguide (CPW) transmission lines defined on a semi-insulating GaAs substrate. We have considerably extended the bandwidth as compared to previously published results [ 13 - [3], and data for various propagated distances are presented. Simulation is done using a semiempirical curve fit to the fullwave analysis for modal dispersion [4], and quasi-static approximation for the conductor [l] and radiation loss [6]. Excellent agreement is obtained for the important pulse parameters-propagation delay, rise time and pulse amplitude -without the use of any adjustable parameters. For the typical dimensions of the coplanar lines and the frequency bandwidth of the pulse considered here, the distortion of the pulse shape is dominated by the modal dispersion (caused by the dielectric mismatch at the surface), and the loss is domi