Tunable THz Emission Research Articles

Intense multicycle THz pulse generation from laser-produced nanoplasmas

We present a novel scheme to obtain robust, narrowband, and tunable THz emission using a nano-dimensional overdense plasma target, irradiated by two counter-propagating detuned laser pulses. So far, no narrowband THz sources with a field strength of GV/m-level have been reported from laser-solid interaction (mostly half-or single-cycle THz pulses with only broadband frequency spectrum). From two- and three-dimensional particle-in-cell simulations, we find that the strong plasma current generated by the beat ponderomotive force in the colliding region, produces beat-frequency radiation in the THz range. Here we report intense THz pulses (fsimeq 30 ;THz) with an unprecedentedly high peak field strength of 11.9 GV/m and spectral width (Delta f/fsimeq 5.3%), which leads to a regime of an extremely bright narrowband THz source of TW/cm^{2}, suitable for various ambitious applications.

Intracavity THz Polariton Source Using a Shallow-Bounce Configuration

We demonstrate a frequency-tunable THz polariton source based on stimulated polariton scattering (SPS) in MgO:LiNbO3. The system produces THz emission using an intracavity shallow-bounce configuration. Continuously tunable THz emission across the frequency range of 1.05–2.89 THz is demonstrated, and a maximum average THz power of 67.5 μ W is detected at 1.34 THz, when pumped with 15.1 W from a continuous-wave laser diode at 880 nm. In contrast to conventional linear configurations, this shallow-bounce configuration offers enhanced THz output power across the systems' entire THz frequency-tuning range, especially at higher frequencies. This is achieved through careful consideration of the THz generation volume, and locating it as close as possible to the emitting surface of the polariton-active crystal, thereby minimizing the deleterious effect of absorption within the SPS crystal. In addition to experimental data, the degree of THz enhancement offered using this configuration, in contrast to a linear configuration is also investigated mathematically.

Coherent and Tunable THz Emission Driven by an Integrated III–V Semiconductor Laser

We demonstrate coherent and tunable THz emission by excitation of a unitraveling-carrier photodiode by a dual-frequency III–V semiconductor laser emitting up to 80 mW of optical power around 1 $\mu$ m. The laser is an optically-pumped vertical-external-cavity surface-emitting laser that operates simultaneously on two transverse Laguerre–Gauss modes. Modes frequency difference is driven by thermal effects, band-filling effects and/or phase masks, allowing THz emission from 50 GHz to few THz. To reach THz emission from a pigtailed photodiode, we detail quantitatively how orthogonal transverse modes can be coupled within a single-mode fiber, leading to more than 20% beat efficiency. Coherent THz emission spectrum is presented with a linewidth of about 150 kHz for 3-ms acquisition time, and an output power limited by the photodiode (typically 1 $\mu$ W at 300 GHz). Frequency noise is measured for the optical transverse modes along with the THz signal. The latter presents a frequency noise that is about 20-dB lower than the optical ones, thus proving that the dual-frequency concept allows frequency noise reduction by correlating part of the technical noise of the two modes.

CEP-stable tunable THz-emission originating from laser-waveform-controlled sub-cycle plasma-electron bursts.

We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.

Continuous terahertz emission from dipolaritons

We propose a scheme of continuous tunable THz emission based on dipolaritons, mixtures of strongly interacting cavity photons and direct excitons, where the latter are coupled to indirect excitons via tunneling. We investigate the property of multistability under continuous wave (CW) pumping, and the stability of the solutions. We establish the conditions of parametric instability, giving rise to oscillations in density between the direct-exciton and indirect-exciton modes under CW pumping. In this way, we achieve continuous and tunable emission in the THz range in a compact single-crystal device. We show that the emission frequency can be tuned in a certain range by varying an applied electric field and pumping conditions. Finally, we demonstrate the dynamic switching between different phases in our system, allowing rapid control of THz radiation.

THz emission control by tuning density profiles of neutral gas targets during intense laser-gas interaction

Ionization currents generated from two-color or few-cycle intense femtosecond laser pulses interaction with neutral gas targets can emit strong THz waves. Here it is found that the initial non-uniformity of the gas density can significantly affect the ionization currents and subsequent THz emission both in amplitude and in spectrum. Density profile effects on the forward and backward emissions have been studied in details by particle-in-cell simulations, in which the field ionization module is included. Increasing the gas density gradient length, the emitted forward THz spectrum shifts from high to low frequency, and the spectrum width reduces, which offers a way to obtain a tunable THz emission source by laser-gas interaction.

Layered Compound Semiconductor GaSe and GaTe Crystals for THz Applications

AbstractThe single crystal growth of layered semiconductors GaSe and GaTe by vertical Bridgman technique using zone refined selenium (Se), tellurium (Te) and high purity (HP) gallium (Ga) have been described. The grown crystals (2.5 cm diameter and ∼10 cm long) have demonstrated efficient broadband tunable THz emission and as sensitive THz detectors. The crystals have shown promising characteristics with good optical quality, high dark resistivity, wide band gap (GaSe-2.01 eV and GaTe-1.66 eV at 300 K), good anisotropic (parallel, p & perpendicular, pa) electrical properties (σ∥ vs σ⊥ and μ∥ vs σ⊥) and long term stability. Different steps involved in processing GaSe and GaTe crystals as THz sources and sensors are described.