Clemens Ruchert

Strong-field single-cycle THz pulses generated in an organic crystal

We present high-power single-cycle carrier-envelope phase locked THz pulses at a central frequency of 2.1 THz with MV/cm electric field strengths and magnetic field strengths beyond 0.3 T. The THz radiation is generated by optical rectification in an organic salt crystal 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate called DAST pumped with the signal wavelength of a powerful optical parametric amplifier. Conversion efficiencies of more than 2% are reported.

Off-resonant magnetization dynamics phase-locked to an intense phase-stable terahertz transient

Controlling ultrafast magnetization dynamics by a femtosecond laser is attracting interest both in fundamental science and industry because of the potential to achieve magnetic domain switching at ever advanced speed. Here we report experiments illustrating the ultrastrong and fully coherent light-matter coupling of a high-field single-cycle THz transient to the magnetization vector in a ferromagnetic thin film. We could visualize magnetization dynamics which occur on a timescale of the THz laser cycle and two orders of magnitude faster than the natural precession response of electrons to an external magnetic field, given by the Larmor frequency. We show that for one particular scattering geometry the strong coherent optical coupling can be described within the framework of a renormalized Landau Lifshitz equation. In addition to fundamentally new insights to ultrafast magnetization dynamics the coherent interaction allows for retrieving the complex time-frequency magnetic properties and points out new opportunities in data storage technology towards significantly higher storage speed.

Scaling submillimeter single-cycle transients toward megavolts per centimeter field strength via optical rectification in the organic crystal OH1

We present the generation of high-power single-cycle terahertz (THz) pulses in the organic salt crystal 2-[3-(4-hydroxystyryl)-5.5-dimethylcyclohex-2-enylidene]malononitrile or OH1. Broadband THz radiation with a central frequency of 1.5 THz (λ(c)=200 μm) and high electric field strength of 440 kV/cm is produced by optical rectification driven by the signal of a powerful femtosecond optical parametric amplifier. A 1.5% pump to THz energy conversion efficiency is reported, and pulse energy stability better than 1% RMS is achieved. An approach toward the realization of higher field strength is discussed.

Ultrabroadband TW-class Ti:sapphire laser system with adjustable central wavelength, bandwidth and multi-color operation

We demonstrate a versatile tunable and highly stable ultrabroadband Ti:sapphire chirped pulse amplification system with a compressed pulse energy of 20 mJ at 100 Hz repetition rate. High power Ti:Sa systems in principle do not offer wavelength tunability due to gain narrowing. Here we demonstrate transform limited pulse generation from 15 fs to 94 fs with tunable central wavelength (λc from 755 nm to 845 nm) and bandwidth (130 nm<Δλ<16 nm) as well as multi-color, time synchronized, sub-100 fs pulses with user defined central wavelength separation. The unique wavelength tunability capabilities have been expanded into the UV and deep-UV by second and third harmonic generation with excellent energy stability. Enhanced energy stability is achieved by multiplexing six ultrastable diode-based solid state pump lasers.

Generation of broadband THz pulses in organic crystal OH1 at room temperature and 10 K

We studied the effects of cryogenic cooling of a 2-[3-(4-hydroxystyryl)-5, 5-dimethylcyclohex-2-enylidene] malononitrile (OH1) crystal on the generation of broadband THz pulses via collinear optical rectification of 1350 nm femtosecond laser pulses. Cooling of the OH1 crystal from room temperature to 10 K leads to a ~10% increase of the pump-to-THz energy conversion efficiency and a shift of the THz pulse spectra to a higher frequency range. Both effects are due to the temperature variation of the THz absorption and the refractive index of the OH1 crystal. This conclusion has been verified by temperature dependent measurements of the linear absorption in the THz frequency region. An approach to obtain a stronger increase of the THz generation efficiency at cryogenic cooling of the OH1 crystal is discussed.

Intense THz radiation produced in organic salt crystals for high-field applications

Organic stilbazolium salt crystals pumped by intense, ultrashort mid-infrared laser have been investigated for efficient THz generation by optical rectification. In this paper we present our latest results in view of the generation of single-cycle and high-field THz transient in the THz gap (0.1-10 THz). The organic rectifiers like DAST, OH1 and DSTMS combine extremely large optical susceptibility with excellent velocity matching between the infrared pump and the THz radiation. Our simple collinear conversion scheme provides THz beams with excellent focusing properties and single cycle electric field larger than 1.5 MV/cm and magnetic field strength beyond 0.5 Tesla. The source can potentially cover the full THz gap at field strength which is barely provided by other THz sources. The THz pulse is carrier-envelope phase stable and the polarity of the field can be easily inverted.

Terahertz-driven non-resonant magnetization dynamics in cobalt

We demonstrate non-resonant magnetization dynamics in the ferromagnetic cobalt thin film induced by a high-field Terahertz pulse. The magnetization dynamics are coherent and exactly follow the THz carrier oscillations.

Multi-octave MV/cm pulses filling the THz gap

Single-cycle THz pulses with electric field strength of MV/cm are required for wide range of applications from physics to biology and medicine [1]. Presently, high fields are still challenging to produce in the THz gap (0.1-10 THz) where important condensed matter resonances are expected (i.e. magnons, phonons, electromagnons). We present our latest results on the generation of 1.5 MV/cm electric field in this frequency range. Broadband THz pulse are achieved by optical rectification (OR) in large organic crystals such as DAST, OH1 and DSTMS [2-4]. These materials present an OR susceptibility one order of magnitude larger than the room-temperature inorganic nonlinear crystals. The THz radiation is phase-matched with IR pump allowing for photon conversion larger than 200% and pulse energy of tens of μJ. The beam can be focused to nearly diffraction-limit for the realization of the highest field. For 0.5 mm thick DSTMS, electric field larger than 1.5 MV/cm is achieved in 350 μm FWHM spot. The electro-optical sampling trace, the corresponding 5-octave spectrum (0.15-5 THz) and the THz focus are shown in Fig. 1. Fields of the 1 MV/cm are obtained also in DAST and OH1. We show that proper dispersive materials can be used to control absolute phase of the THz transient without significant losses.

Highly efficient generation of single-cycle MV/cm THz pulses in organic crystals

We present the generation of high-power single-cycle THz pulses in organic salt crystals. Broadband THz radiation with MV/cm electric field strength is produced by optical rectification driven with a powerful femtosecond optical parametric amplifier.

Laser-driven generation of intense single-cycle THz field

We report on laser-based, high power single-cycle THz source. The THz radiation is generated by four-wave mixing in plasma and by optical rectification in organic salt crystal pumped by powerful optical parametric amplifier. The first approach permits the generation of electric field of hundreds of kV/cm at central frequency of 0.7 THz. The second technique allows the synthesis of an electric field exceeding 1 MV/cm paired with an unprecedented conversion efficiency of more than 2%, at frequency of 2 THz. The presented sources can be focused to a diffraction-limited spot and are suitable-versatile tool for time resolved THz experiment.