Gerard Mourou

Bio: Gerard Mourou is an academic researcher from École Polytechnique. The author has contributed to research in topics: Laser & Ultrashort pulse. The author has an hindex of 82, co-authored 653 publications receiving 34147 citations. Previous affiliations of Gerard Mourou include University of Michigan & San Diego State University.

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

Fivefold compression of 250-TW laser pulses

Abstract: The pulse spectrum at the laser output was broadened due to self-phase modulation in fused silica and then the pulse was compressed by chirped mirrors. It was demonstrated that with an optimal choice of mirror dispersion a pulse with an energy of 17 J can be compressed from 70 to 14 fs. This compression after compressor approach has undoubted merits: simplicity, low cost, negligible pulse energy losses, and applicability to any high-power laser.

Applications of femtosecond lasers in corneal surgery

Abstract: We investigated potential applications of ultrashort (femtosecond) pulsed laser technology in corneal refractive surgery. When compared with longer pulsewidth nanosecond or picosecond laser radiation, femtosecond laser-tissue interactions are characterized by significantly smaller and more deterministic photodisruptive energy thresholds, as well as reduced shock waves and smaller cavitation bubbles. Femtosecond laser technology may be able to perform a variety of corneal refractive procedures with high precision, offering advantages over current mechanical and laser devices and techniques, enabling entirely new approaches for refractive surgery. An analytically solvable shell model has been developed to predict the results of completely intrastromal incisionless surgery for myopic and hyperopic refractive corrections.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

A roadmap for graphene

Abstract: Recent years have witnessed many breakthroughs in research on graphene (the first two-dimensional atomic crystal) as well as a significant advance in the mass production of this material. This one-atom-thick fabric of carbon uniquely combines extreme mechanical strength, exceptionally high electronic and thermal conductivities, impermeability to gases, as well as many other supreme properties, all of which make it highly attractive for numerous applications. Here we review recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.

Black Hole Explosions

Abstract: QUANTUM gravitational effects are usually ignored in calculations of the formation and evolution of black holes. The justification for this is that the radius of curvature of space-time outside the event horizon is very large compared to the Planck length (Għ/c3)1/2 ≈ 10−33 cm, the length scale on which quantum fluctuations of the metric are expected to be of order unity. This means that the energy density of particles created by the gravitational field is small compared to the space-time curvature. Even though quantum effects may be small locally, they may still, however, add up to produce a significant effect over the lifetime of the Universe ≈ 1017 s which is very long compared to the Planck time ≈ 10−43 s. The purpose of this letter is to show that this indeed may be the case: it seems that any black hole will create and emit particles such as neutrinos or photons at just the rate that one would expect if the black hole was a body with a temperature of (κ/2π) (ħ/2k) ≈ 10−6 (M/M)K where κ is the surface gravity of the black hole1. As a black hole emits this thermal radiation one would expect it to lose mass. This in turn would increase the surface gravity and so increase the rate of emission. The black hole would therefore have a finite life of the order of 1071 (M/M)−3 s. For a black hole of solar mass this is much longer than the age of the Universe. There might, however, be much smaller black holes which were formed by fluctuations in the early Universe2. Any such black hole of mass less than 1015 g would have evaporated by now. Near the end of its life the rate of emission would be very high and about 1030 erg would be released in the last 0.1 s. This is a fairly small explosion by astronomical standards but it is equivalent to about 1 million 1 Mton hydrogen bombs. It is often said that nothing can escape from a black hole. But in 1974, Stephen Hawking realized that, owing to quantum effects, black holes should emit particles with a thermal distribution of energies — as if the black hole had a temperature inversely proportional to its mass. In addition to putting black-hole thermodynamics on a firmer footing, this discovery led Hawking to postulate 'black hole explosions', as primordial black holes end their lives in an accelerating release of energy.

Materials for terahertz science and technology

Abstract: Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules.