THz Photonics

Electromagnetic radiation in the terahertz (THz) frequency range is a fascinating diagnostic tool that provides resonant access to fundamental modes, including the motions of free electrons, rotations of molecules, vibrations of crystal lattices and precessions of spins.

Our group focuses on the development, characterization and application of intense THz sources based on laser produced solid and gas density plasmas and organic crystals. Each source possess unique characteristics such as polarization, spectral width and thus caters to different applications.

THz sources

Angular beam profile of the THz radiation generated during high power-laser thin foil interaction [3]

Grafik: FSU Jena / IOQ

THz sources based on multi TW laser systems employ solid density plasmas generated during the intense laser-thin foil target interaction. Charge particle dynamics in the plasma generates bursts of broadband coherent terahertz radiation with hundreds of microjoules energy. So in a high power source, such as laser-driven particle accelerators, THz radiation can be employed as a non-invasive diagnostic tool to estimate the longitudinal duration of accelerated particle bunches and also for secondary particle acceleration schemes [1,2].

The advantage of solid density plasma based sources is that they can deliver broadband, high peak power THz pulses exceeding GWs. However such systems operate with low repetition rates. Thus laser based high average and moderate peak power THz sources are based on the excitation of plasma filaments in gases and polarization current in nonlinear crystals. In particular organic crystal based sources employing standard turnkey laser amplifiers are a promising way to generate THz radiation efficiently and they offer extremely simple geometries allowing compact and modular schemes.

Fig2: Beam profile of the THz beam generated by optical rectification of femtosecond laser pulses in a BNA crystal[4]. Inset presents the temporal duration of the THz pulse measured using single-shot electro-optic technique and the spectral content [5,6]

Grafik: FSU Jena / IOQ

Fig2: Beam profile of the THz beam generated by optical rectification of femtosecond laser pulses in a BNA crystal[4]. Inset presents the temporal duration of the THz pulse measured using single-shot electro-optic technique and the spectral content[5,6].

Novel diagnostic tools

For the detection and characterization of THz radiation we employ single shot and multi-shot electro-optic detection and magneto-optic techniques. They allow both spatial and temporal characterization of the radiation.

Fig3: Single-shot scheme for THz detection and the temporal waveform of the longitudinal (εz) and transverse (εy ) field components measured at the focus of the THz beam. Inset shows the corresponding spectral amplitudes [7].
Grafik: FSU Jena / IOQ

Focusing radially polarized, sub picosecond terahertz beams from multi TW laser-thin metal foil interaction can create multi-MV/cm longitudinally polarized beams [7]. EO scheme allowed unambiguous detection and characterization of longitudinally polarized terahertz beams enabling highly efficient acceleration of charged particles in free space, novel experiments in nonlinear spectroscopy of 2D materials etc.

Applications: Spectroscopy and Imaging

Intense terahertz pulses are employed for linear and nonlinear spectroscopy of materials, imaging low density media, particle acceleration and for longitudinal bunch characterization in laser-driven particle accelerators. With the generation of intense THz radiation, their propagation through materials can induce nonlinearity. For example, In centro-symmetric media, second-order susceptibility vanishes and third- or higher order prevails.

Fig4: Refractive index modulation and Kerr coefficient as a function of THz frequency in the spectral range 0.1 - 3 THz lactose and silicon respectively [8]
Grafik: FSU Jena / IOQ

In this scenario, it is possible to estimate the intensity-dependent nonlinear refractive index, absorption, and transmittance in materials. We employed a hybrid z-scan and single-shot EO detection technique for the investigation of THz induced non-linear effects in materials [8].

A. Gopal, T. May, S. Herzer, A. Reinhard, S. Minardi, M. Schubert, M. Kaluza, W. Ziegler, H-G. Meyer and G. G. Paulus, High energy T-ray pulses from table-top laser driven Ion accelerators, New Journal of Physics 14, 083012 (2012).
A. Woldegeorgis, S. Herzer, M. Almassarani, S. Marathappalli and A. Gopal, Modeling terahertz emission from the target rear side during intense laser-solid interactions, Phys. Rev E. 100, 053204 (2019).
A. Gopal, A. Woldegeorgis, S. Herzer, and M. Almassarani, Spatiotemporal visualization of the terahertz emission during high-power laser-matter interaction, Phys. Rev E. 100, 053203 (2019).
F. Roeder, M. Shalaby, B. Beleites, F. Ronnenberger and & A. Gopal, THz generation by optical rectification of intense near infra red pulses in organic crystal BNA, Opt. Express, 28, 36274 (2020).
A. Gopal, S. Herzer, A. Schmidt, P. Singh, A. Reinhard, W. Ziegler, D. Broemmel, A. Karmakar, P. Gibbon, U. Dillner, T. May, H-G. Meyer and G. G. Paulus, Observation of Gigawatt class THz pulses from a compact laser-driven particle accelerator, Physical Review Letters, 111, 074802 (2013).
S. Herzer A. Woldegeorgis, J. Polz, A. Reinhard, M. Almassarani, B. Beleites, F. Ronneberger, R. Grosse, G. G. Paulus, U. Hübner, T. May & A. Gopal, An investigation on THz yield at relativistic laser intensities from laser-produced solid density plasmas, New J. Phys. 20, 063019 (2018).
A. Woldegeorgis, T. Kurihara, M. Almassarani, B. Beleites, R. Grosse, F. Ronneberger & A. Gopal, Multi MV/cm longitudinally polarized THz pulses from laser thin foil interaction, Optica 5(11), 1474 (2018).
A. Woldegeorgis, T. Kurihara, B. Beleites, J. Bossert, R. Grosse, G. G. Paulus, F. Ronneberger, A. Gopal, THz induced nonlinear effects in materials at intensities above 26 GW/cm², Journal of Infrared and Millimeter Waves, 39, 667 (2018).