Nonlinear ultrafast modulation of the optical absorption of intense few-cycle terahertz pulses inn-doped semiconductors

Abstract

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,...

Intense ultrashort terahertz pulses: generation and applications

“…Nonlinear transmission studies in combination with modeling of the contribution of several of these effects have been reported [118] and strong absorption bleaching has been observed in a number of n-type semiconductors such as GaAs [119], InGaAs [120], Ge, Si [22] as well as in photoexcited GaAs [121]. In bulk samples, the increase in terahertz transmission can be almost one order of magnitude, making these materials suitable for use as saturable THz absorbers at room temperature [97].…”

Observation of nonequilibrium carrier distribution in Ge, Si, and GaAs by terahertz pump–terahertz probe measurements

“…Recently, the development of high-energy tabletop THz pulse sources 5,6 has enabled nonlinear transmission studies on semiconductors [7][8][9] as well as measurements with THz pump and near-IR ͑Ref. 10͒ or mid-IR ͑Ref.…”

Mobile source of high-energy single-cycle terahertz pulses

“…The same detector model was previously used by other research groups to measure 100-µJ THz pulses obtained with an accelerator-based source [3] and µJ-level pulses generated by optical rectification in ZnTe [5]. Below 1 THz, diffraction and the spectral dependence of the absorber placed on the pyroelectric crystal may result in a decrease in detector sensitivity, so that our measurements provide a lower limit of the absolute energy value in this frequency range.…”

“…Some of these applications require ultrashort THz pulses of high peak power, such as for nonlinear optics and spectroscopy in the THz frequency range and for recently developed time-resolved spectroscopy with THz pump [3][4][5][6][7]. To date, the highest THz peak power (100 MW) has been achieved with accelerator-based sources [3].…”