Speaker
Description
Between microwaves and the infrared, terahertz (THz) radiation was long hard to produce and to measure. Yet it is the natural frequency range of many low-energy excitations in condensed matter: lattice vibrations, spin waves and the motion of free carriers all leave their signatures there. A THz pulse can both probe these excitations and drive them, on a subpicosecond timescale [1]. Thanks to femtosecond lasers, such experiments no longer require large-scale facilities: they fit on an optical table, around a Ti:Sapphire laser that many ultrafast laboratories already own.
This talk surveys what two complementary machines of this kind make possible. A 1 kHz amplifier delivers energetic pulses and therefore intense THz fields; an 80 MHz oscillator delivers far weaker pulses, but many more of them, which favours sensitivity. The laser largely dictates the source. With the amplifier, optical rectification in lithium niobate reaches field strengths of several hundred kV/cm, enough to drive matter into the nonlinear regime [2]. Organic crystals pumped by an optical parametric amplifier (OPA) push such fields to higher frequencies [3], and difference-frequency mixing at the OPA output yields multicycle THz pulses [4]. Spintronic emitters, metal films a few nanometres thick, radiate over an exceptionally broad band with a polarization set by a magnet, and work with either laser [5]. With the oscillator, photoconductive antennas remain the reference for compact, stable spectroscopy [6], while on-chip devices guide the wave through micrometre-scale structures and open up samples too small for a free-space beam [7].
Detection involves the same trade-offs. Electro-optic sampling, available with both lasers, records the amplitude and phase of the field directly [8]; with the amplifier, air-biased coherent detection extends the usable bandwidth considerably [9]. In between, passive components - spectral filters, polarizers, waveplates - select a frequency window and control the polarization state, which matters above all for magnetic studies.
Most of these approaches will be illustrated with results obtained on such systems, in particular pump-probe experiments on ultrafast magnetization dynamics, on the coupling between nonlinear optical modes, and on subcycle modulation in plasmonic systems. The aim is to give non-specialists practical guidance in matching laser, source and detector to their physical question, and to show that this spectral range is now within reach of most ultrafast optics laboratories.
References
[1] T. Kampfrath, K. Tanaka, K. A. Nelson, Nat. Photonics 7, 680 (2013).
[2] H. Hirori, A. Doi, F. Blanchard, K. Tanaka, Appl. Phys. Lett. 98, 091106 (2011).
[3] C. P. Hauri, C. Ruchert, C. Vicario, F. Ardana, Appl. Phys. Lett. 99, 161116 (2011).
[4] A. Sell, A. Leitenstorfer, R. Huber, Opt. Lett. 33, 2767 (2008).
[5] T. Seifert et al., Nat. Photonics 10, 483 (2016).
[6] D. H. Auston, K. P. Cheung, P. R. Smith, Appl. Phys. Lett. 45, 284 (1984).
[7] A. M. Potts et al., Nano Lett. 23, 3835 (2023).
[8] Q. Wu, X.-C. Zhang, Appl. Phys. Lett. 67, 3523 (1995).
[9] J. Dai, X. Xie, X.-C. Zhang, Phys. Rev. Lett. 97, 103903 (2006).
| Type of presence | Presence online |
|---|