High-field terahertz bulk photovoltaic effect in lithium niobate.

The terahertz (THz) response of the ferroelectric prototype material lithium niobate (LiNbO3) is studied in the nonperturbative regime of light-matter interaction. Applying two-dimensional THz spectroscopy with few-cycle pulses of an amplitude E≈100 kV/cm and a center frequency of 2 THz, we dissect the overall nonlinear response into different orders in the electric field. The underlying nonlinear current is of interband character and consists of a strong low-frequency shift current (SC) and higher harmonics of the THz fundamental. The SC component originates from the lack of inversion symmetry and the strong interband decoherence for long electron trajectories in k space as shown by theoretical calculations.

Perspective: structural dynamics in condensed matter mapped by femtosecond x-ray diffraction.

Ultrashort soft and hard x-ray pulses are sensitive probes of structural dynamics on the picometer length and femtosecond time scales of electronic and atomic motions. Recent progress in generating such pulses has initiated new directions of condensed matter research, exploiting a variety of x-ray absorption, scattering, and diffraction methods to probe photoinduced structural dynamics. Atomic motion, changes of local structure and long-range order, as well as correlated electron motion and charge transfer have been resolved in space and time, providing a most direct access to the physical mechanisms and interactions driving reversible and irreversible changes of structure. This perspective combines an overview of recent advances in femtosecond x-ray diffraction with a discussion on ongoing and future developments.

Internal motions of a quasiparticle governing its ultrafast nonlinear response.

A charged particle modifies the structure of the surrounding medium: examples include a proton in ice, an ion in a DNA molecule, an electron at an interface, or an electron in an organic or inorganic crystal. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle, known as the Fröhlich polaron. The basic properties of polarons and their drift motion in a weak electric field are well known. However, their nonlinear high-field properties--relevant for transport on nanometre length and ultrashort timescales--are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.

Electron transfer in a virtual quantum state of LiBH4 induced by strong optical fields and mapped by femtosecond x-ray diffraction.

Transient polarizations connected with a spatial redistribution of electronic charge in a mixed quantum state are induced by optical fields of high amplitude. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The prototype ionic material LiBH4 driven nonresonantly by an intense sub-40 fs optical pulse displays a large-amplitude fully reversible electron transfer from the BH4(-) anion to the Li+ cation during excitation. Our results establish this mechanism as the source of the strong optical polarization which agrees quantitatively with theoretical estimates.

Ultrafast soft-mode driven charge relocation in an ionic crystal.

Transient electron density maps of potassium dihydrogen phosphate (KH(2)PO(4), KDP) are derived from femtosecond X-ray powder diffraction patterns. Upon photoexcitation, the low-frequency TO soft mode is elongated impulsively and modulates the electronic charge distribution on the length scale of interatomic distances, much larger than the vibrational amplitude. The results demonstrate a charge transfer from the volumes around the P-atoms and K(+)-ions to those containing the O-HO units and a quadrupolar distortion of the K(+) charge distribution. This behavior reflects the interplay of nuclear motions and electric polarizations in the ionic crystal lattice.

The rotating-crystal method in femtosecond X-ray diffraction.

We report the first implementation of the rotating-crystal method in femtosecond X-ray diffraction. Applying a pump-probe scheme with 100 fs hard X-ray probe pulses from a laser-driven plasma source, the novel technique is demonstrated by mapping structural dynamics of a photoexcited bismuth crystal via changes of the diffracted intensity on a multitude of Bragg reflections. The method is compared to femtosecond powder diffraction and to Bragg diffraction from a crystal with stationary orientation.

Strong correlation of electronic and lattice excitations in GaAs/AlGaAs semiconductor quantum wells revealed by two-dimensional terahertz spectroscopy.

Coulomb-mediated interactions between intersubband excitations of electrons in GaAs/AlGaAs double quantum wells and longitudinal optical phonons are studied by two-dimensional spectroscopy in the terahertz frequency range. The multitude of diagonal and off-diagonal peaks in the 2D spectrum gives evidence of strong polaronic signatures in the nonlinear response. A quantitative theoretical analysis reveals a dipole coupling of electrons to the polar lattice that is much stronger than in bulk GaAs, due to a dynamic localization of the electron wave function by scattering processes.

High-field transport in an electron-hole plasma: transition from ballistic to drift motion.

The time evolution of high-field carrier transport in bulk GaAs is studied with intense femtosecond THz pulses. While ballistic transport of electrons occurs in an n-type sample, a transition from ballistic to driftlike motion is observed in an electron-hole plasma. This onset of friction is due to the holes, which are heated by THz absorption. Theoretical calculations, which reproduce the data quantitatively, show that both electron-hole scattering and local-field effects in the electron-hole plasma are essential for the time-dependent friction.

Femtosecond powder diffraction with a laser-driven hard X-ray source.

X-ray powder diffraction with a femtosecond time resolution is introduced to map ultrafast structural dynamics of polycrystalline condensed matter. Our pump-probe approach is based on photoexcitation of a powder sample with a femtosecond optical pulse and probing changes of its structure by diffracting a hard X-ray pulse generated in a laser-driven plasma source. We discuss the key aspects of this scheme including an analysis of detection sensitivity and angular resolution. Applying this technique to the prototype molecular material ammonium sulfate, up to 20 powder diffraction rings are recorded simultaneously with a time resolution of 100 fs. We describe how to derive transient charge density maps of the material from the extensive set of diffraction data in a quantitative way.

Coherent ballistic motion of electrons in a periodic potential.

Electrons in bulk n-doped GaAs at a lattice temperature of 300 K are driven by ultrashort high-field transients of up to 300 kV/cm in the terahertz frequency range. In the lowest conduction band the carriers show coherent ballistic motion, which is detected via the THz field emitted by them. This partial Bloch oscillation is reproduced by a quantum-kinetic theory of coherent transport on ultrafast time scales.

Ultrafast spatiotemporal dynamics of terahertz generation by ionizing two-color femtosecond pulses in gases.

We present a combined theoretical and experimental study of spatiotemporal propagation effects in terahertz (THz) generation in gases using two-color ionizing laser pulses. The observed strong broadening of the THz spectra with increasing gas pressure reveals the prominent role of spatiotemporal reshaping and of a plasma-induced blueshift of the pump pulses in the generation process. Results obtained from (3+1)-dimensional simulations are in good agreement with experimental findings and clarify the mechanisms responsible for THz emission.

Phase-resolved two-dimensional spectroscopy based on collinear n-wave mixing in the ultrafast time domain.

We present a novel approach for femtosecond two-dimensional (2D) spectroscopy in the midinfrared combining a collinear beam geometry and phase-resolved detection. Two phase-locked pulses of variable time delay tau interact with the sample. The transmitted electric fields are measured in real time t by electro-optic sampling. 2D spectra are generated by Fourier transforming the signal along the two time axes tau and t. In the 2D spectra, nonlinear signals originating from different orders n in the electric field are separated. Such decomposition of the overall response is demonstrated by mapping the nonlinear response of intersubband transitions in GaAs/AlGaAs multiple quantum wells.

Ultrafast modulation of electronic structure by coherent phonon excitations.

Femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic source is used to map ultrafast changes of x-ray absorption by femtometer-scale coherent phonon displacements. In LiBH4, displacements along an Ag phonon mode at 10 THz are induced by impulsive Raman excitation and give rise to oscillatory changes of x-ray absorption at the Li K-edge. Electron density maps from femtosecond x-ray diffraction data show that the electric field of the pump pulse induces a charge transfer from the BH4- to neighboring Li+ ions, resulting in a differential Coulomb force that drives lattice vibrations in this virtual transition state.

Treatment of agoraphobia with group exposure in vivo and imipramine.

Seventy-six white agoraphobic women, 21 to 45 years old, were treated with combined group exposure in vivo and imipramine or placebo in a randomized double-blind study. A majority of the patients in both the placebo and imipramine groups showed moderate to marked improvement. However, imipramine therapy was significantly superior to placebo therapy on three of the four reported measures of improvement: primary phobia, spontaneous panic, and global improvement. There was a negative correlation between depression and outcome; ie, the more depressed patients fared worse on several outcome measures than those who were less depressed. A comparison of these patients with agoraphobic women previously treated with imipramine and imaginal desensitization showed a superiority of exposure in vivo midway in treatment, but no significant difference between the two groups at the completion of therapy.

Behavior therapy, supportive psychotherapy, imipramine, and phobias.

In a controlled outcome study of phobias, 111 adult patients (69% women, 31% men) received a course of 26 weekly treatment sessions consisting of (1) behavior therapy and imipramine hydrochloride (2) behavior therapy and placebo, or (3) supportive psychotherapy and imipramine. Patients were classified as agoraphobic, mixed phobic, or simple phobic. The great majority of patients in all groups showed moderate to marked global improvement (70% to 86%, depending on rater). In agoraphobics and mixed phobics (both groups experiencing spontaneous panic attacks), imipramine was significantly superior to placebo. There was no difference between behavior therapy and supportive therapy, both resulting in high improvement rates (76% to 100%, depending on rater). In simple phobic patients, there was a high rate of improvement with all treatment regimens (72% to 93%, depending on rater), with no significant difference between imipramine and placebo or between behavior therapy and supportive therapy. Of 88 moderately to markedly improved patients followed up for one year after completing treatment, 83% maintained their gains and 17% relapsed. No patients showed symptom substitution. Eighteen percent of the patients receiving imipramine hydrochloride showed marked stimulant side effects on from 5 to 75 mg/day.

Time course and biologic correlates of treatment response in first-episode schizophrenia.

OBJECTIVE: To examine the course and potential predictors of treatment response in the early phase of schizophrenia. DESIGN: Prospective study of an inception cohort. SETTING: Psychiatric division of an academic medical center with a suburban metropolitan catchment area. PATIENTS AND INTERVENTION: Seventy first-episode patients who had undergone four biologic assessment procedures (brain magnetic resonance imaging, behavioral response to methylphenidate hydrochloride, growth hormone levels, eye tracking) were treated with a standardized antipsychotic drug protocol until recovery. Response was measured in terms of psychopathology and degree of remission. RESULTS: Using survival analysis, the proportion of patients remitting by 1 year was estimated at 83%. Mean and median times to remission were 35.7 weeks and 11 weeks, respectively. No baseline demographic or psychopathologic measure significantly predicted time to or level of remission. However, males tended to be nonresponders to treatment and have diagnoses of schizophrenia rather than schizoaffective disorder. Brain pathomorphology and abnormal basal growth hormone significantly predicted time to remission. CONCLUSIONS: These results indicate that the antipsychotic treatment response of first-episode schizophrenics is better than chronic multiepisode patients and suggest that specific pathobiologic markers reflect pathophysiologic processes that mediate antipsychotic treatment response.

Prediction of relapse in schizophrenia.

Despite the proven efficacy of neuroleptic drugs in the acute and maintenance pharmacotherapy of schizophrenia, practical methods for identifying patients who require neuroleptic treatment to prevent relapse are lacking. This study evaluated the use of a methylphenidate challenge test to predict the outcome in 34 stable outpatients with schizophrenia receiving neuroleptic treatment. Patients received two infusions, one of methylphenidate and one of placebo, in randomized order one week apart while receiving neuroleptic treatment and again three weeks after drug withdrawal. Behavioral, cardiovascular, and neurologic responses were evaluated before and after infusion under double-blind conditions. Patients were then followed up without medication for 52 weeks or until symptom recurrence. The results indicate that specific measures, including behavioral response to methylphenidate, presence of tardive dyskinesia, and, under specific pharmacologic conditions, tardive dyskinesia, blink-rate, and pulse-rate responses to methylphenidate, are associated with time and propensity to relapse following neuroleptic withdrawal. These measures may be potentially useful in the identification of candidates for neuroleptic withdrawal and/or dosage-reduction treatment strategies.

Treatment of phobias. II. Behavior therapy and supportive psychotherapy: are there any specific ingredients?

Systematic hierarchical desensitization (behavior therapy [BT]) was compared with supportive psychotherapy (ST) during a 26-week treatment trial of patients with agoraphobia, mixed phobia, or simple phobia. We found essentially no difference in effectiveness, not because patients treated with BT had done badly, but because patients receiving ST had done well. A detailed methodologic review concludes that this finding is entirely compatible with the major critical reviews of the psychotherapy literature. Is the active ingredient in psychotherapy simply the generation of hopeful expectancies? This is necessary but not sufficient. For phobics, the psychotherapy session serves primarily as an instigator. The specific corrective activity occurs outside of the formal session in the form of maintained exposure in vivo. Supposed differences between therapies may be entirely due to the rapidly with which the instigational function becomes effective.

Treatment of phobias. I. Comparison of imipramine hydrochloride and placebo.

In a controlled-outcome study of phobias, 218 adult phobic patients (147 women and 71 men) received a course of 26 weekly treatment sessions that consisted of behavior therapy (BT) and imipramine hydrochloride, BT and placebo, or supportive psychotherapy and imipramine. The BT consisted of systematic desentization using fantasy and assertiveness training. Patients were classified as agoraphobic, mixed phobic, or simple phobic. Although the conditions of most patients in each group showed moderate to marked improvement, the effects of imipramine were significantly superior to those of placebo in patients with spontaneous panic attacks, ie, patients with agoraphobia or mixed phobia. In patients with simple phobia, who do not experience spontaneous panic, there was not a significant difference between imipramine and placebo. This study clearly distinguished those phobic patients who experienced spontaneous panic from those who did not in terms of pharmacologic benefits.

Low-dose neuroleptic treatment of outpatient schizophrenics. I. Preliminary results for relapse rates.

In an attempt to begin to establish minimum effective dosage requirements for the maintenance treatment of schizophrenia, we undertook a double-blind comparison of low-dose fluphenazine decanoate (1.25 to 5.0 mg/2 wk) with the standard-dose regimen (12.5 to 50.0 mg/2 wk) in outpatient schizophrenics. For the first 126 patients studied, cumulative relapse rates at one year for the low dose were 56% and for the standard dose 7%, a significant difference. Despite the fact that very little dyskinetic symptomatology developed in the sample as a whole, the low-dose treatment appeared to have a significant advantage in producing fewer early signs of tardive dyskinesia. Severity of relapse and total cumulative dosage were also considered.