Sudipta Mondal

Intense terahertz radiation and their applications

In this paper, we will review both past and recent progresses in the generation, detection and application of intense terahertz (THz) radiation. We will restrict the review to laser based intense few-cycle THz sources, and thus will not include sources such as synchrotron-based or narrowband sources. We will first review the various methods used for generating intense THz radiation, including photoconductive antennas (PCAs), optical rectification sources (especially the tilted-pulse-front lithium niobate source and the DAST source, but also those using other crystals), air plasma THz sources and relativistic laser–plasma sources. Next, we will give a brief introduction on the common methods for coherent THz detection techniques (namely the PCA technique and the electro-optic sampling), and point out the limitations of these techniques for measuring intense THz radiation. We will then review three techniques that are highly suited for detecting intense THz radiation, namely the air breakdown coherent detection technique, various single-shot THz detection techniques, and the spectral-domain interferometry technique. Finally, we will give an overview of the various applications that have been made possible with such intense THz sources, including nonlinear THz spectroscopy of condensed matter (optical-pump/THz-probe, THz-pump/THz-probe, THz-pump/optical-probe), nonlinear THz optics, resonant and non-resonant control of material (such as switching of superconductivity, magnetic and polarization switching) and controlling the nonlinear response of metamaterials. We will also provide a short perspective on the future of intense THz sources and their applications.

MV/cm terahertz pulses from relativistic laser-plasma interaction characterized by nonlinear terahertz absorption bleaching in n-doped InGaAs

We have developed a tabletop intense broadband terahertz (THz) source in the medium frequency range (≤ 20 THz) based on the interaction of a high-intensity femtosecond laser with solid targets at relativistic laser intensities. When an unpolished copper target is irradiated with a high-intensity femtosecond laser, a maximum of ~2.2 μJ of THz pulse energy is collected and detected with a calibrated pyroelectric detector. The THz spectrum was measured by using a series of bandpass filters, showing a bandwidth of ~7.8 THz full-width at half-maximum (FWHM) with a peak at ~6 THz. With tight focusing to reach high field strengths, we have demonstrated THz nonlinearity exemplified by THz absorption bleaching in a heavily n-doped InGaAs thin film, which enabled us to estimate the peak electric field of the THz pulses. We simulated the experimentally observed bleaching by employing a THz pulse having a bandwidth similar to that measured in our experiments and a temporal profile recoded in single-shot electro-optic detection. Through the simulations, we estimate a peak electric field associated with the THz pulses to be 2.5 MV/cm.

Intense THz-coherent transition radiation from laser solid plasma interaction

We investigate intense broadband terahertz radiation generation based the interaction of high intensity ultrashort laser with solid plasma. THz pulse with electric field of hundreds of MV/cm are generated using laser with intensity of about 1018 W/cm2. Theoretical model, simulations agree well with experiments, and reveal that the THz radiation is coherent transition radiation by hot electrons produced in laser-plasma interaction. We have studied both planar and nanorod array targets, latter of which efficiently enhance the intensity and directionality of the THz source.

Control of THz pulse emission in laser plasma interaction by plasma shaping

High-energy, few-cycle THz pulse generation by high-intensity (∼1018 W/cm2) femtosecond laser with plasma mirrors have received significant attention in recent years [1-3]. This novel THz source has the potential of generating large bandwidth millijoule pulse energies, opening up applications in THz nonlinear optics, single-shot imaging and spectroscopy [4]. Solid density plasma shaping has already been observed [5, 6] to yield sub-cycle control in optimizing high harmonic generation from plasma mirrors. Here, we demonstrate that plasma shaping can be a prospective avenue for enhancing and controlling THz emission from plasma mirrors. In this study, we have used two different cases of plasma shaping.

High field broadband THz pulses by ultrashort laser-plasma interaction

Broadband THz radiation with electric field up to hundreds of MV/cm is achieved using femtosecond laser interaction with solid target. Particle-in-cell simulations reveal that the THz pulses are coherent transition radiation (CTR) of hot electrons produced in laser-plasma interaction. Due to the square law dependence of CTR energy on the electron number, laser-plasma based strong THz sources can reach unprecedented intensity comparing to other methods. Besides planar targets, nano-grating targets have also been investigated, which improve the THz sources both in intensity and directionality. Theory model and simulations agree very well with experimental observations.

Suppression of resonant-induced harmonics of tin with tunable laser wavelengths

We investigate the behavior of resonant-induced harmonics from tin using driving lasers with tunable wavelengths. The intensity of the resonant harmonic is suppressed by the tuning laser wavelength around 1.8μm to understand the interaction dynamics of continuum electron with the autoionizing states.

Highly efficient high-order harmonics with large cutoff from carbon molecules using various laser wavelengths

Carbon molecules are used to generate intense high-order harmonics using driving lasers with 0.8 μm-1.71 μm wavelengths. By driving plasma of reduced size (~200μm) with 1.71μm laser, we could extend the cutoff to ~70eV, while reducing the peak intensity by only ~31%.

Ultra Broadband THz and IR (0.1-200THz) Pulse Generation from Laser Plasma Interaction

We present THz and infrared optical pulse generation from the interaction of high intensity femtosecond laser with solid density plasmas. We report the enhancement of THz pulse energy up to 28 times by using aligned nanorod targets.