Search

We demonstrate the occurrence of spatial multistability using laterally confined microcavity exciton-polaritons. By coherently exciting with a blue detuned laser a series of confined polariton modes, we investigate the effects of multistability on the transmitted laser beam as a function of the excitation power. At each threshold of the hysteresis loop, a switching of the mode profile of the laser beam is associated with a significant energy jump of each of the confined polariton modes in the mesa. A simulation of this behavior is achieved with a multimode generalization of the Gross-Pitaevskii equations in the exciton photon basis. The mechanism behind the spatial multistability is identified as a repulsive cross interaction between polaritons in different modes.

We present the optical tomography of the probability density of microcavity polaritons, confined in three dimensions by cylindrical traps of various sizes. Collecting the photoluminescence emitted by the quasimodes under continuous nonresonant laser excitation, we reconstruct a three dimensional mapping of the photoluminescence, from which we can extract the spatial distribution of the confined states at any energy. We discuss the impact of the confinement shape and size on the probability density patterns.

We detail an innovative laser device based on a new relaxation mechanism for confined exciton-polaritons which preserves nonlinearity. We demonstrate the selective activation of polariton lasers at different wavelengths allowing to perform all-optical logic operations with fully coherent output [1]. (C) 2014 Optical Society of America

We have developed optically pumped passively mode-locked vertical-external-cavity surface-emitting lasers. We achieved as much as 950 mW of mode-locked average power in chirped 15-ps pulses, or 530 mW in 3.9-ps pulses with moderate chirp. Both lasers operate at a repetition rate of 6 GHz and have a diffraction-limited output beam near 950 nm. In continuous-wave operation, we demonstrate an average output power as high as 2.2 W. Device designs with a low thermal impedance and a smooth gain spectrum are the key to such performance. We discuss design and fabrication of the gain structures and, particularly, their thermal properties

Polaritons have often been proposed as promising candidates for the realization of all-optical devices due to the easy manipulation and control of their density and spin. In this paper we present a relaxation mechanism for confined polaritons and its application in a device based on polariton lasing in which the incoming monochromatic beam can be channeled into several polariton lasers at different wavelengths. Polaritons are injected in a 3 mu m diameter trap by an incoming beam slightly detuned with respect to a confined state, and the resulting optical bistability is then imprinted on the lower confined states through an efficient relaxation mechanism which combines phonon interaction and bosonic stimulation. Above threshold, all the confined states behave like lasers and the onset of coherence is demonstrated by spectral narrowing. Moreover, the initial polarization of the laser is conserved during the relaxation process allowing for spin logic operations. In this paper we demonstrate that, due to the nonlinear behavior, it is possible to switch ON and OFF the lasing from the confined states of the trap by tailoring the input beam. All the experimental findings are validated and very well reproduced in the framework of the generalized Gross-Pitaevskii equation.

We report what we believe to be the first demonstration of a SESAM (semiconductor saturable absorber mirror)-based femtosecond Cr3+:LiSrAlF6 laser pumped by narrow-stripe AlGaInP laser diodes that have diffraction-limited output beams. A highly asymmetric, four-mirror laser cavity design is described for which 57 fs duration pulses at an average output power of 6.5 mW are obtained for only 72 mW of incident pump power. Pump powers of this order were maintained for over 14 h using just six 1.5 V AA batteries as the electrical power source. We have shown that mode-locking can be sustained for incident pump powers as low as 21.5 mW.

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens-mode-locked Ti:sapphire laser with an average output power of 200 mW at a pulse repetition rate of 86 MHz. We have achieved a mode-locking buildup time of only 60 /spl mu/s, using a broad-band semiconductor saturable absorber mirror to initiate the pulse formation. The dispersion has been compensated with a prism pair in combination with improved double-chirped mirrors. The prism pair allows for the flexible adjustment of both the duration and the center wavelength of the pulse. The double-chirped mirrors show a high reflectivity better than 99.8% over the full bandwidth of 300 nm and a controlled group delay over more than 250 nm. The choice of a proper output coupler turns out to be critical for ultrashort pulse generation directly from the laser.

We generated self-starting sub-10-fs pulses from a Ti:sapphire laser using a combination of a broadband semi- conductor saturable absorber mirror (SESAM), novel double- chirped mirrors, and Kerr-lens modelocking. We present a de- tailed overview of broadband SESAMs used in Ti:sapphire lasers and discuss their design issues. Using such a SESAM to assist the KLM process, we have generated pulses with a duration of only 6: 5f s, the shortest to date directly from the laser. Pulses as short as 13 fs were generated by soliton modelocking alone. Higher-order dispersion compensation was achieved with novel double-chirped mirrors in combina- tion with intracavity prisms.

The characterization of sub-micron gratings is usually performed on scanning electron microscopes, although the dimensions of the features often make the observations difficult. We report here on a new method applicable to III - V and II - VI semiconductors: we use transmission electron microscopy on wedge-shaped samples oriented along a [111] zone axis. Grating parameters can be obtained with a precision of 1 nm, along with information on the shape and the regularity of the ridge profile, the surface quality as well as the local chemical composition.

Exciton polaritons have been shown to be an optimal system in order to investigate the properties of bosonic quantum fluids. We report here on the observation of dark solitons in the wake of engineered circular obstacles and their decay into streets of quantized vortices. Our experiments provide a time-resolved access to the polariton phase and density, which allows for a quantitative study of instabilities of freely evolving polaritons. The decay of solitons is quantified and identified as an effect of disorder-induced transverse perturbations in the dissipative polariton gas.

We report on the dynamics of confined zero-dimensional polaritons. We excite resonantly discrete polariton states optically with a tailored picosecond laser pulse and observe their emission time resolved in the two-dimensional k space or real space. We are able to distinguish between three regimes. When the laser excites only one state no dynamics are observed. When a small number of well separated states are excited, the dynamics are described in terms of interference. When a quasicontinuum of states is excited, the dynamics are described in terms of ballistically propagating two-dimensional (2D) polaritons which scatter elastically at the potential barriers. The measured propagation velocities are reproduced with the theory of 2D polaritons, which sustains our interpretation.

Control of the wave function of confined microcavity polaritons is demonstrated experimentally and theoretically by means of tailored resonant optical excitation. Three dimensional confinement is achieved by etching mesas on top of the microcavity spacer layer. Resonant excitation with a continuous-wave laser locks the phase of the discrete polariton states to the phase of the laser. By tuning the energy and momentum of the laser, we achieve precise control of the momentum pattern of the polariton wave function. This is an efficient and direct way for quantum control of electronic excitations in a solid.

We have developed a femtosecond-resolved near-field scanning optical microscope, using a diffraction-limited pump and near-field probe configuration, which allows us to measure carrier dynamics with a spatial resolution of ∼150 nm and a time resolution of ∼250 fs. This instrument is used for near-field degenerate pump–probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused-ion-beam (FIB) implantation. We find that lateral carrier diffusion across the nanometer-scale FIB pattern plays a significant role in the decay of the excited carriers.

Diode-pumped Yb:phosphate and Yb:silicate glass lasers have been passively mode locked for the first time to the authors’ knowledge. Reliable self-starting mode locking without critical cavity alignment has been achieved with intracavity semiconductor saturable-absorber mirrors and soliton mode locking. We generated pulses as short as 58??fs with the Yb:phosphate laser and 61??fs with the Yb:silicate laser at average output powers of 65 and 53??mW, respectively. The pulse repetition rate was 112??MHz. Additionally, we demonstrated tunability of femtosecond pulses from 1025 to 1065??nm for the Yb:phosphate and from 1030 to 1082??nm for the Yb:silicate glasses. The highest mode-locked output power was 405??mW, with 183-fs pulses from the phosphate glass. The diode pump power was 1.68??W, corresponding to 24% optical-to-optical efficiency. The highest cw output power was 510??mW at the same incident pump power.

We have demonstrated 175-fs pulses with 1 W and 300-fs pulses with 1.2 W of average output power at a pulse repetition rate of 117 MHz from a Nd:phosphate (Schott LG 760) glass laser pumped by a 1-cm-wide, 20-W diode laser bar. Stable soliton mode locking was achieved by use of an intracavity semiconductor saturable absorber mirror. We obtained more than 2 W of average power without mode locking. Using cylindrical cavity mirrors, we adapted the laser mode inside the Nd:glass to the highly elliptical pump beam in both dimensions (tangential and sagittal axes) while maintaining a nearly ideal circular TEM(00) output beam with M(2) approximately 1.2 . Overpumping the laser mode in the tangential plane and efficient unidirectional heat removal in the sagittal plane using a 0.8-mm thin Nd:glass also contributed to the good output-beam quality.

We demonstrate what is to our knowledge the first mode-locked Yb:KGd(WO(4))(2) laser. Using a semiconductor saturable-absorber mirror for passive mode locking, we obtain pulses of 176-fs duration with an average power of 1.1 W and a peak power of 64 kW at a center wavelength of 1037 nm. We achieve pulses as short as 112 fs at a lower output power. The laser is based on a standard delta cavity and pumped by two high-brightness laser diodes, making the whole system very simple and compact. Tuning the laser by means of a knife-edge results in mode-locked pulses within a wavelength range from 1032 to 1054 nm. In cw operation, we achieve output powers as high as 1.3 W.

We demonstrate a passively mode-locked thin-disk Yb:YAG laser that generates solitonlike pulses with durations that are continuously tunable in a very wide range from 3.3 to 89 ps or from 0.83 to 1.57 ps. The average powers are typically ~12 W . Previously [Opt. Lett. 25, 859 (2000)], only pulse durations in a narrow range near 0.7 ps could be obtained from such lasers because of the effect of spatial hole burning. We achieved this much wider range by constructing a laser cavity with two different angles of incidence on the thin disk, which greatly reduces the effect of spatial hole burning.

4.6 ps pulses at 910 MHz produced by a passively mode-locked VECSEL were amplified by ytterbium doped fiber amplifiers to average powers greater than 200 W and subsequently compressed to 430 fs duration.

We demonstrate three-dimensional spatial confinement of exciton-polaritons in a semiconductor microcavity. Polaritons are confined within a micron-sized region of slightly larger cavity thickness, called mesa, through lateral trapping of their photon component. This results in a shallow potential well that allows the simultaneous existence of extended states above the barrier. Photoluminescence spectra were measured as a function of either the emission angle or the position on the sample. Striking signatures of confined states of lower and upper polaritons, together with the corresponding extended states at higher energy, were found. In particular, the confined states appear only within the mesa region, and are characterized by a discrete energy spectrum and a broad angular pattern. A theoretical model of polariton states, based on a realistic description of the confined photon modes, supports our observations.

Fabrication and characterization of a current-injected InGaAs-GaAs quantum-wire gain-coupled distributed feedback (DFB) laser operating at 77 K at a wavelength of 923 nm are presented. Threshold current densities in broad area lasers were measured to be as low as 160 A/cm/sup 2/. The side-mode suppression ratio at twice threshold is 35 dB. A 4-/spl mu/m rib waveguide device has a threshold of 14 mA. The patterning process for the second-order DFB grating fabricated with deep UV holography and wet-chemical etching is described.

A diode-pumped Yb:YVO4 laser has been passively mode locked for the first time, to our knowledge. 120 fs pulses with an average output power of 300 mW and a peak power as high as 14.5 kW are obtained by use of a semiconductor saturable-absorber mirror for passive mode locking. The optical spectrum has a 10 nm bandwidth (full width at half-maximum) and is centered at 1021 nm.

We report on the efficient CW, Q-switched and mode-locked laser operation of a diode-pumped Yb:YVO4 laser. CW output power of about 1 W with slope efficiency of 59 % with respect to absorbed pump power was obtained. Passively Q-switched with a Cr4+:YAG saturable absorber, a Yb:YVO4 laser with Raman conversion was demonstrated. Q-switched 18.7-μJ and 3.6-μJ pulses with pulse durations of 17 ns and 6 ns were produced at 1018-nm fundamental and 1119.5-nm first-Stokes wavelengths, respectively. A diode-pumped Yb:YVO4 laser has been passively mode-locked using a semiconductor saturable-absorber mirror (SESAM). 120-fs pulses with an average output power of about 300 mW and a peak power as high as 14.5 kW were obtained at 1021 nm.

Summary form only given. Femtosecond lasers with high average output power are of great interest for numerous applications such as simple and efficient nonlinear frequency conversion or multi-photon spectroscopy. Such lasers can be realized by combining a diode-pumped thin-disk laser head with a semiconductor saturable absorber mirror (SESAM) for passive mode locking. In this paper we present the first passively mode-locked thin-disk laser which is based on the novel gain medium Yb:KY(WO/sub 4/)/sub 2/ (Yb:KYW). This laser crystal has a larger amplification bandwidth than Yb:YAG and is therefore suitable for significantly shorter pulses. We obtained 240-fs pulses at 1028 nm with 22-W average power and 3.3-MW peak power. The pulse repetition rate is 24.6 MHz, and the pulse energy is 0.9 /spl mu/J. The beam quality is close to the diffraction limit, with M/sup 2/ = 1.1. Focusing the beam to a spot diameter of 2 /spl mu/m would result in a peak intensity of 2 /spl middot/ 10/sup 14/ Wcm/sup -2/.

Summary form only given. We performed a comprehensive four-wave-mixing (FWM) study of the optical dephasing in bulk GaAs and AlGaAs with broadband 16-fs laser pulses. In contrast to previous work, spectral resolution has allowed us to unambiguously relate the decay of the coherent emission to either continuum or excitonic transitions. We have extended earlier measurements of continuum dephasing to one order of magnitude lower carrier densities. We find much shorter dephasing times and a much weaker density dependence of the dephasing than expected from the extrapolation of the high-density results. Our data indicate that a quantum kinetics model for both Coulomb and LO-phonon scattering is required to describe continuum dephasing on the 10-fs time scale at lower densities.

Summary form only given. We investigated the single pass second harmonic generation of a passively mode-locked Nd:YAlO/sub 3/ laser operating at /spl lambda/ = 930 nm in a bulk periodically poled LiTaO/sub 3/ (PPLT) crystal. The Nd:YAlO/sub 3/ laser was passively mode-locked with a semiconductor saturable absorber mirror (SESAM) in a V-shaped cavity.

We report on the imaging of a molecular beam epitaxially grown GaAs/(AlAs)n(GaAs)m quantum well‐wire array by means of cross‐sectional scanning tunneling microscopy (XSTM) and scanning tunneling‐induced luminescence (STL). XSTM provides atomically resolved cross‐sectional images of sets of quantum well wires with chemical sensitivity within the group III species and electrical sensitivity to single dopant atoms. This permits the precise observation of growth mechanisms and the identification of defects responsible for inhomogeneities in the growth morphology, as well as the determination of dopant incorporation throughout the structure. STL permits the relative quantum efficiency of individual quantum wires to be quantified.

Summary form only given. We recently demonstrated for the first time passive mode locking of a vertical-external-cavity surface-emitting semiconductor laser (VEC-SEL). The device was pumped with a diode laser and mode-locked with a semiconductor saturable absorber mirror (SESAM) This technology is expected to lead to very powerful and compact pulsed sources, with multi-GHz repetition rates. Compared to previous results, we have now obtained a much higher average output power of 213 mW and at the same time a significantly reduced pulse duration of 3.2 ps. This amounts to a 20-fold enhancement of the peak power, which is now 30 W. Semiconductor laser sources have previously reached such power levels only with amplifier systems or with synchronous pumping of VEC-SELs, using a powerful mode-locked pump source. In edge-emitting semiconductor lasers the average and peak power is limited by the small mode area, which cannot be easily increased. With optically pumped VEC-SELs these constraints are eliminated, and much higher powers are possible with accordingly increased mode areas. Diffraction-limited output, as required for mode locking, is obtained with an external cavity.

Summary form only given. We have demonstrated a novel configuration of a passively mode-locked high-power thin disk laser, involving two different reflection angles on the thin disk. This allowed us to generate soliton-like pulses in a very wide range of pulse durations. In this way, we have greatly expanded the range of possible applications for this novel laser scheme, which is based on a power-scalable concept and thus promises to soon generate the by far highest average output powers both in the femtosecond and picosecond regime.

We have passively modelocked a diode-pumped Nd:YLF laser at 1.047 µm using semiconductor saturable absorber mirrors and achieved 1.3 W of average output power at 3.3 W pump power, which results in an optical efficiency of 39%. The pulse repetition rate was 50 MHz and pulse widths as short as 3.8 ps were obtained. We cavity-dumped single pulses from the circulating cw modelocked pulse train using a BBO Pockels cell and achieved 270 nJ pulses. The repetition rate was limited to below 2 kHz due to the Pockels cell driver electronics.

A surface-emitting semiconductor laser has been passively mode locked in an external cavity incorporating a semiconductor saturable absorber mirror. The gain medium consists of a stack of 12 InGaAs-GaAs strained quantum wells, grown above a Bragg mirror structure, and pumped optically by a high-brightness diode laser. The mode-locked laser emits pulses of 22-ps full-width at half maximum duration at 1030 nm, with a repetition rate variable around 4.4 GHz.

Femtosecond and picosecond pulses can find many applications if they can be produced with laser sources that are not only powerful and efficient but also compact and reliable. In continuous wave operation, diode pumping of solid-state lasers has allowed for a rapid progress towards powerful, compact and reliable sources, while the often used technique of Kerr lens modelocking for pulsed operation tends to be in conflict with requirements for diode-pumpable high power designs. Passive modelocking with semiconductor saturable absorber mirrors solves this problem as it relaxes the restrictions on the cavity design. We report on our recent achievements in this field. In particular we present a novel semiconductor device for dispersion compensation and various improved diode-pumped passively modelocked lasers. Also we show which laser parameters determine the stability of a passively modelocked lasers against Q-switching instabilities.

Mode-locking of the ground-state transition in neodymium-doped laser hosts in combination with external frequency doubling is a possibility to realize a coherent blue-light source in the blue spectral region. Therefore we made an investigation of the mode-locking properties of a Nd:YAlO/sub 3/ laser on the ground-state transition. We observed nearly transform-limited solitonlike pulses as short as /spl Delta//spl tau/=1.9 ps. Soliton mode-locking was achieved by introducing a semiconductor saturable absorber mirror (SESAM) and a Gires-Tournois-Interferometer (GTI) into the cavity.

We report on a significant improvement in average output power of a diode-pumped Nd:glass laser, achieving >1 W mode locked and 2 W continuous wave. We used a 1-cm-wide, 806-nm 20-W diode bar and applied optimized mode matching (OMM) in the gain material in both the fast (i.e., sagittal) axis and the slow (i.e., tangential) axis of the diode laser, following the cavity design first demonstrated by Kopf et al. (1995) for Cr:LiSAF. Applying OMM to both axes results in a highly elliptical laser mode inside the gain medium, because the pump beam can be focused much smaller in the diffraction-limited fast axis compared to the slow axis. The highly elliptical pump mode allows for improved heat removal by using a very thin 0.8-mm-thick Nd:glass material and by actively cooling only the upper and lower side of the glass.

We demonstrate tunable diode-pumped femtosecond Yb-doped phosphate (Kigre QX/Yb) and silicate (Kigre Q246/Yb) glass lasers. The shortest pulses were ≈60 fs for both glasses and the tuning range for femtosecond pulses extended from 1025 nm to 1065 nm for the phosphate glass laser and from 1030 nm to 1082 nm for the silicate glass laser.

We present the optical tomography of the probability density of quasiparticles, the microcavity polaritons, confined in three dimensions by cylindrical traps. Collecting the photoluminescence emitted by the quasimodes under continuous nonresonant laser excitation, we reconstruct a three-dimensional mapping of the photoluminescence, from which we can extract the spatial distribution of the confined states at any energy. We discuss the impact of the confinement geometry on the wave function patterns and give an intuitive understanding in terms of a light-matter quasiparticle confined in a box.

We demonstrate what is to our knowledge the first passively mode-locked thin-disk Yb:KY(WO(4))(2) laser. The laser produces pulses of 240-fs duration with an average power of 22 W at a center wavelength of 1028 nm. At a pulse repetition rate of 25 MHz, the pulse energy is 0.9microJ , and the peak power is as high as 3.3 MW. The beam quality is very close to the diffraction limit, with M(2)=1.1 .

A passively modelocked diode-pumped surface-emitting semiconductorlaser at 950nm with a 2GHz repetition rate is reported. Compared tothe first device of this kind, which the authors recently reported, agreatly improved average output power of 213mW and a reduced pulseduration of 3.2ps are achieved. The device consists of an opticallypumped semiconductor gain structure and a semiconductor saturableabsorber mirror (SESAM) in an external cavity. Introduction: Pulsed laser sources with multi-GHz repetition rates andaverage powers > 100mW are required for applications such as opticalclocking of integrated circuits, optical testing of semiconductor elec-tronics and telecommunication components, particle accelerators, andanalogue-to-digital conversion. Edge-emitting semiconductor lasershave been passively modelocked for quite some time [1] and ultra-shortpulses [2] and extremely high repetition rates [3] have been reported.The average and peak power is limited in these devices, because stablemodelocking requires fundamental transverse mode operation, whichrestricts the size of the facet. With optically pumped vertical-external-cavity surface-emitting semiconductor lasers (VECSELs [4]) these con-straints are eliminated, and much higher powers are possible withaccordingly increased mode areas, while diffraction-limited output isenforced by the external cavity. Pulsed operation has been achieved withsynchronous pumping [5, 6] and with active modelocking [7]. Recently[8], we demonstrated the first VECSEL which is continuously pumpedand passively modelocked with a semiconductor saturable absorber mir-ror (SESAM [9, 10]). With a new gain structure optimised for a lowthermal impedance and a smooth gain spectrum we have obtained tentimes more average power and six times shorter pulse duration. Gain structure: In the previously described gain structure [8], the gener-ated heat is removed through the GaAs substrate, which introduces asignificant thermal impedance. We have eliminated this problem by fab-ricating a structure where the active region is separated from the copperheat sink only by a semiconductor Bragg mirror. This is achieved by epi-taxial lift-off (ELO): first, we grew the gain structure in reverse order ona GaAs substrate with three intermediate etch-stop layers. We used afluxless indium soldering process similar to the one published by So etal . [11] to solder a dice to a copper heat sink and finally removed theGaAs substrate by etching. The additional effort of the epitaxial lift-off is justified by two impor-tant improvements. First, the reduced thickness of the semiconductormaterial leads to a greatly reduced thermal impedance, and the nearlyone-dimensional heat flow into the heat sink makes the device power-scalable: the output power can, for example, be doubled by applyingtwice the pump power to twice the mode area without raising the tem-perature of the gain structure, as long as the heat sink is able to removethe additional power. The second improvement is that we eliminate thepreviously observed etalon effect from the interference of reflectedwaves from the Bragg mirror and the soldered surface. This effect lim-ited the bandwidth and disturbed the modelocking in the previousdevice. The gain structure consists of a bottom mirror, an active region, andan anti-reflective section grown by metal-organic chemical vapour depo-sition (MOCVD). For the bottom mirror we used seven pairs of GaAs/AlAs and 23 pairs of Al

We experimentally demonstrate that adaptive feedback optical pulse shaping can be used to control ultrafast semiconductor nonlinearities. The control scheme is based on an evolutionary algorithm, which directs the modulation of the spectral phase of 20 fs laser pulses. The algorithm has optimized the broadband semiconductor continuum nonlinearity measured in differential transmission experiments. Our results show that insight into light–semiconductor interaction is obtained from the optimum laser pulse shape even if the interaction is too complex to predict this shape a priori. Moreover, we demonstrate that adaptive feedback control can substantially enhance ultrafast semiconductor nonlinearities by almost a factor 4.

Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens mode-locked Ti:sapphire laser with 200-mW average output power at a pulse repetition rate of ~86 M Hz. This is to our knowledge the shortest pulse ever generated directly from a laser. For dispersion compensation we used a prism pair in combination with double-chirped mirrors, which balances the higher-order dispersion of the prism pair and therefore flattens the average total group-delay dispersion in the laser cavity. For self-starting mode locking we used a broadband semiconductor saturable-absorber mirror.

Well-characterized pulses in the two-optical-cycle regime are demonstrated using a Kerr-lens modelocked (KLM) Ti:sapphire laser and phase-sensitive diagnostics. The oscillator employs double-chirped mirrors (DCMs) for dispersion compensation, spectral shaping via wavelength-dependent output coupling, and a semiconductor saturable absorber mirror (SESAM) for reliable operation at up to 300 mW output power and with a repetition rate of ∼90 MHz.In the discussion of the experimental results we emphasize how spectral shaping can be used most efficiently in the generation of the shortest possible pulse supported by the Ti:sapphire gain bandwidth.

We present an extensive comparison of techniques capable of characterizing sub-6-fs pulses from a laser. Apart from the traditional characterization by autocorrelation and measuring the spectrum, we use novel variants of spectral interferometry for direct electric field reconstruction (SPIDER) and frequency-resolved optical gating (FROG) that have been optimized for use with pulses of extreme bandwidth. Both of these techniques allow for a full characterization of the pulses, i.e., they reconstruct pulse amplitude and phase.

The incorporation of In in the growth of crescent-shaped In0.12Ga0.88As quantum wires embedded in (AlAs)(4)(GaAs)(8) superlattice barriers is studied in atomic detail using cross-sectional scanning tunneling microscopy. It is found that the In distribution in both the surface and the first subsurface layer can be atomically resolved in the empty- and filled-state images, respectively. Strong In segregation is seen at the InGaAs/GaAs interfaces, but neither an expected enhancement of the In concentration at the center of the quantum wire compared to the planar quantum well nor In clustering beyond the statistical expectation is observed. (C) 1995 American Institute of Physics.

We discuss challenges arising from the quest for high average powers from passively mode-locked diode- pumped lasers. The recently obtained detailed understand- ing of Q-switching instabilities in passively mode-locked lasers turns out to be a crucial element on the way towards higher powers. We give an overview on results achieved with Nd:YAG (10: 7W , 16 ps and 27 W, 19 ps), Yb:YAG (8: 1W , 2: 2p sand 16 W, 0: 7p s )a ndNd:glass (1: 4W , 275 fs).