High-field high-bandwidth terahertz pulses: Generation in laser-excited gas plasmas and coherent detection in biased-air waveguides
Final Report Abstract
This project aimed to investigate the achievable limits of bandwidth and field strength for the terahertz-mid-infrared (THz-MIR) emission from gas plasmas with two-colour femtosecond excitation, based on table-top amplifier laser sources with sub-mJ energy. The key development for the generation was the use of ultra-broadband optical pump pulses – i.e. with a bandwidth sufficient for sub-20-fs pulse duration, obtained via spectral broadening of 150-fs amplifier laser pulses in a hollow-core fibre and subsequent recompression. Using this pump and optimised experimental parameters, we achieved state-of-the-art performance, with a continuous emission bandwidth extending from <1 to over 150 THz (i.e. well into the midinfrared), and pulse energies of several 100 nJ, such that peak focal fields of >10 MV/cm can be reached. We studied the role of the second-harmonic (SH) pulse both experimentally and with a theoretical propagation model, which indicated that the optimum emission corresponded to an incommensurate SH pulse which improves the pump field overlap in the plasma and promotes also a shift to higher emission frequencies. The role of the SH field and dependence on the pump pulse chirp indicate that even further improvements in the THz-MIR energy may be achievable, which motivates further systematic experimental and theoretical investigation. We performed spectrally resolved studies of the emission profile (which corresponds to a linearly polarised, conical emission due to phase-walk-off in the plasma between the optical-pump and THz-MIR waves), and used simulations to gauge its impact on the field distribution at subsequent focal regions for spectroscopy experiments. For the coherent detection of the ultra-broadband THz-MIR continuum, instead of applying time-domain sampling methods (as in conventional THz spectroscopy), we further developed the use of spectrogram measurements based on sum-frequency generation, which allowed us to measure the intensity spectrum and dispersion of the THz-MIR pulses with good sensitivity and no inherent detection bandwidth limit. A novel deconvolution algorithm was developed to retrieve the underlying THz-MIR pulses from such spectrograms in the case where the full phase-matching response of the detection medium has to be taken into account. The THz-MIR continuum pulses provide a probe for transient spectral signatures across the relevant ranges for dynamic conductivity, intra- and inter-molecular vibrations/phonons (as well other charge/spin excitations) simultaneously, and has been applied so far e.g. to a study of the dynamics of photoinduced charge carriers in common semiconductors. Moreover, the high focal peak fields coupled with the possibility for near-single-cycle THz-MIR pulses opens the way for applications in novel non-linear spectroscopies of systems with low-energy excitations. During the course of the project, several other groups have adopted this method to cover the full THz-MIR range (with reported bandwidths now reaching 200 THz, i.e. essentially covering the entire range up to the near-IR), and we anticipate that this source will be employed by a growing community, allowing both THz and MIR spectroscopists to cover both spectral ranges with one methodology in experiments.
Publications
- Terahertz white-light pulses from an air plasma photo-induced by incommensurate two-color optical fields, Opt. Express 18, 23173-23182 (2010)
M. D. Thomson, V. Blank, H. G. Roskos
- Broadband terahertz spectroscopy: principles, fundamental research and potential for industrial applications, Eur. J. Phys. 34, S179 (2013)
W. Zouaghi, M. D. Thomson, K. Rabia, R. Hahn, V. Blank and H. G. Roskos
- Optimization of single-cycle terahertz generation in LiNbO3 for sub-50 femtosecond pump pulses, Opt. Express 21, 6826-6836 (2013)
M. Kunitski, M. Richter, M. D. Thomson, A. Vredenborg, J. Wu, T. Jahnke, M. Schöffler, H. Schmidt-Böcking, H. G. Roskos, R. Dörner
(See online at https://doi.org/10.1364/OE.21.006826) - Recovery of ultra-broadband terahertz pulses from sum-frequency spectrograms using a generalised deconvolution method, Eur. Phys. J. Web Conf. 41, 09011 (2013)
M. D. Thomson, V. Blank, H. G. Roskos
- Spatio-spectral characteristics of ultrabroadband THz emission from two-colour photoexcited gas plasmas and their impact for nonlinear spectroscopy, New J. Phys. 15, 075023 (2013)
V. Blank, M. D. Thomson, H. G. Roskos
(See online at https://doi.org/10.1088/1367-2630/15/7/075023) - Terahertz frequency upconversion via relativistic Doppler reflection from a photoinduced plasma front in a solid-state medium, Phys. Rev. B 87, 085203 (2013)
M. D. Thomson, S. M. Tzanova, H. G. Roskos
