Wavelength scaling of high harmonic generation efficiencyUsing longer wavelength laser drivers for high harmonic generation is desirable because the highest extreme ultraviolet frequency scales as the square of the wavelength. Recent numerical studies predict that high harmonic efficiency falls dramatically with increasing wavelength, with a very unfavorable Àð5À6Þ scaling. We performed an experimental study of the high harmonic yield over a wavelength range of 800-1850 nm. A thin gas jet was employed to minimize phase matching effects, and the laser intensity and focal spot size were kept constant as the wavelength was changed. Ion yield was simultaneously measured so that the total number of emitting atoms was known. We found that the scaling at constant laser intensity is À6:3AE1:1 in Xe and À6:5AE1:1 in Kr over the wavelength range of 800-1850 nm, somewhat worse than the theoretical predictions.

We demonstrate the generation muJ-level, single-cycle terahertz pulses by optical rectification from a large-aperture ZnTe single crystal wafer. Energies up to 1.5 muJ per pulse and a spectral range extending to 3 THz were obtained using a 100 Hz Ti:sapphire laser source and a 75-mmdiameter, 0.5-mm-thick, (110) ZnTe crystal, corresponding to an average power of 150 muW and an energy conversion efficiency of 3.1 x 10(-5). We also demonstrate real-time imaging of the focused terahertz beam using a pyroelectric infrared camera.

Nonlinear transient absorption bleaching of intense few-cycle terahertz (THz) pulses is observed in photoexcited GaAs using opticalpump--THz-probe techniques. A simple model of the electron transport dynamics shows that the observed nonlinear response is due to THz-electric- field-induced intervalley scattering over sub-picosecond time scales as well as an increase in the intravalley scattering rate attributed to carrier heating. Furthermore, the nonlinear nature of the THz pulse transmission at high peak fields leads to a measured terahertz conductivity in the photoexcited GaAs that deviates significantly from the Drude behavior observed at low THz fields, emphasizing the need to explore nonlinear THz pulse interactions with materials in the time domain.

We use an open-aperture Z scan technique to show how intense few-cycle terahertz pulses can experience a nonlinear bleaching of absorption in an n-doped semiconductor due to terahertz-electric-field-driven intervalley scattering of electrons in the conduction band. Coherent detection of the transmitted terahertz pulse wave form also allows the nonlinear conductivity dynamics to be followed with subpicosecond time resolution. Both the Z scan and time-domain results are found to be in agreement with our theoretical analysis. DOI: 10.1103/PhysRevB.79.193204 PACS number͑s͒: 78.47.Fg, 73.50.Fq Ultrafast nonlinear processes have been extensively studied in the visible and near infrared 1 but have remained basically unexplored in the terahertz spectral region because of the lack of sources delivering high-energy, few-cycle terahertz pulses. Nowadays, these sources are becoming available, 2-6 allowing the possibility of investigating novel and exciting aspects of radiation-matter interaction.Over the past 25 years, few studies have explored the nonlinear dynamics of semiconductors using terahertz ͑THz͒ sources with microsecond or nanosecond pulse durations. [7][8][9] Only very recently, however, coherently detected intense terahertz pulses with picosecond pulse durations have become available to study the nonlinear response of different physical systems. 10-14 For example, Hebling et al. 10 explored ionic nonlinearities in LiNbO 3 crystals, observing significant lattice anharmonicities induced by strong terahertz electric fields. Gaal et al.11 reported a nonlinear electronic response in n-type GaAs, where a long-lived coherent THz emission, centered around 2 THz and driven by intense THz pulses has been observed and attributed to stimulated emission from impurities exhibiting a population inversion. These two studies are good examples of how broad the interest can be associated with ultrafast nonlinear spectroscopy in the terahertz region since it can give access to either ionic or electronic information, depending on the properties of the excited system. In principle, this new spectroscopy can also explore drift-velocity-based nonlinearities of free carriers in semiconductors since the low terahertz photon energy makes multiphoton interband effects negligible even in narrow-bandgap semiconductors. 15 This kind of free-carrier nonlinearities has been studied in the past in the microwave region, 16,17 but the availability of ultrashort intense terahertz pulses can now unveil the dramatic dynamics of these processes with an unprecedented time resolution.In order to explore these new nonlinearities we have decided to borrow one of the most common and straightforward nonlinear optical characterization techniques, namely, open-aperture Z scan, 18 and apply it to the THz regime. It consists of scanning the sample transmission through the focus of an intense optical beam ͓see Fig. 1͑a͔͒, and thus it gives access to nonlinear ͑i.e., intensity dependent͒ transmission properties. Widely used in multiphoton absorption studies,...

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