arXiv subjects

Christoph P. Hauri

Publications and source records attributed to Christoph P. Hauri.

16 recordsLinked to original sources

arXiv2017

Subcycle insulator-to-metal transition in vanadium dioxide by terahertz-field-driven tunneling

In vanadium dioxide, the interplay between coherent lattice transformation and electronic correlation drives an insulator-to-metal transition (IMT). This phase commutation can be triggered by temperature, pressure, doping or deposition of optical energy. Here we demonstrate that an atomically-strong terahertz electric field initiates a metastable ultrafast IMT in vanadium dioxide without a concomitant lattice transformation. The free-space terahertz field acts as off-resonant excitation with photon energy below the lattice phonons and the interband transitions. Differently from optical and infrared excitation, terahertz interaction leads to a full IMT by interband Zener tunneling with a negligible entropy deposition. In previous experiments the temporal dynamics of IMT in VO2 could be only indirectly inferred. We disentangle the electronic and lattice contributions to the IMT on a sub-picosecond timescale. Near the critical temperature the IMT becomes dissipative and the terahertz field concludes the lattice-assisted metallic nucleation initiated by heating. The method of strong-field induced phase transition presented here is applicable to a wide class of strongly correlated systems and will enable the discovery of novel metastable phases.

cond-mat.str-el↗

arXiv2016

Multi-octave spectrally tunable strong-field Terahertz laser

The ideal laser source for the emerging research field of nonlinear Terahertz (THz) spectroscopy should offer radiation with a large versatility and deliver both ultra-intense multi-octave spanning single-cycle pulses and user-selectable multi-cycle pulses at narrow linewidth. The absence of such a table-top source has hampered advances in numerous THz disciplines including imaging, nonlinear photonics and spectroscopy, selective out-of-equilibrium excitation of condensed matter and quantum systems. Here we introduce a highly versatile table-top THz laser platform providing single-cycle GV/m transients as well as spectrally narrow pulses tunable in bandwidth and central frequency across 5 octaves with hundreds of MV/m field strength. The compact scheme is based on optical rectification of a temporally modulated laser beam in organic crystals. It allows for the selection of THz oscillation cycles from 1 to >50 and central frequency tuning range from 0.5 to 7 THz by directly changing the modulation period of the driving laser. The versatility of the THz source is demonstrated by providing a broadband 5-octave spanning spectrum as well as a spectrally narrow line tunable across the full optical rectification phase-matching band with a minimum width of dv=30 GHz, corresponding to dE=0.13 meV and lambda^-1=1.1 cm-1. The presented table-top source shows performances similar or even beyond to that of a large-scale THz electron accelerator facility but offering in addition versatile multi-color and advanced femtosecond pump-probe opportunities at ultralow timing jitter.

physics.optics↗

arXiv2016

A multi-milliJoule femtosecond Raman laser emitting at 1.28 um

We report on the generation of broadband, high-energy femtosecond pulses centered at 1.28 um by stimulated Raman scattering in pressurized hydrogen cell. Stimulated Raman scattering is performed by two chirped and delayed pulses originating from a multi-mJ Ti:Sapphire amplifier. The Stokes pulse carries energy of 4.4 mJ and is recompressed down to 66 fs by reflective grating pair. We characterized the short-wavelength mid-infrared source in view of energy stability, beam profile and conversion efficiency at a repetition rate of 100 Hz and 10 Hz. The demonstrated laser will benefit intense THz generation applications from highly nonlinear organic crystals.

physics.optics↗

arXiv2016

Intense THz source based on BNA organic crystal pumped at Ti:Sapphire wavelength

We report on high energy terahertz pulses by optical rectification (OR) in the organic crystal N-benzyl-2-methyl-4-nitroaniline (BNA) directly pumped by a conventional Ti:Sapphire (Ti:Sa) amplifier. The simple scheme provides an optical to terahertz conversion efficiency of 0.25% when pumped by a collimated laser pulses with duration of 50 fs and central wavelength of 800nm. The generated radiation spans frequencies between 0.2 and 3 THz. We measured the damage threshold as well as the dependency of the conversion efficiency on the pump fluence, pump wavelength, and pulse duration.

cond-mat.mtrl-sci↗

arXiv2015

Femtosecond resolution timing jitter correction on a TW scale Ti:sapphire laser system for FEL pump-probe experiments

Intense ultrashort pulse lasers are used for fs resolution pumpprobe experiments more and more at large scale facilities, such as free electron lasers (FEL). Measurement of the arrival time of the laser pulses and stabilization to the machine or other sub-systems on the target, is crucial for high time-resolution measurements. In this work we report on a single shot, spectrally resolved, non-collinear cross-correlator with sub-fs resolution. With a feedback applied we keep the output of the TW class Ti:sapphire amplifier chain in time with the seed oscillator to ~3 fs RMS level for several hours. This is well below the typical pulse duration used at FELs and supports fs resolution pump-probe experiments. Short term jitter and long term timing drift measurements are presented. Applicability to other wavelengths and integration into the timing infrastructure of the FEL are also covered to show the full potential of the device.

physics.ins-det↗

arXiv2015

Simultaneous electronic and the magnetic excitation of a ferromagnet by intense THz pulses

The speed of magnetization reversal is a key feature in magnetic data storage. Magnetic fields from intense THz pulses have been recently shown to induce small magnetization dynamics in Cobalt thin film on the sub-picosecond time scale. Here, we show that at higher field intensities, the THz electric field starts playing a role, strongly changing the dielectric properties of the cobalt thin film. Both the electronic and magnetic responses are found to occur simultaneously, with the electric field response persistent on a time scale orders of magnitude longer than the THz stimulus

physics.optics↗

arXiv2015

The terahertz frontier for ultrafast coherent magnetic switching: Terahertz-induced demagnetization in ferromagnets

The transition frequency between nonthermal coherent magnetic precessions and ultrafast demagnetization is arguably the most sought after answer in magnetism science and technology nowadays. So far, it is believed to be in the terahertz (THz) range. Here, using an ultra-intense low frequency THz bullet, and thin magnetic layers, we report on experimental evidences that fully coherent nonthermal THz magnetic switching may never be reachable in conventional ferromagnetic thin films. At high excitation intensities, while the spins still coherently precess with the THz magnetic field, the deposited THz energy initiates ultrafast demagnetization and ultimately material damage. These series of phenomena are found to take place simultaneously. The reported experiments set fundamental limits and raise questions on the coupling between electronic and magnetic systems and the associated structural dynamics on the ultrafast time scale.

physics.optics↗

arXiv2015

Generation of 1.5-octave intense infrared pulses by nonlinear interactions in DAST crystal

Infrared pulses with large spectral width extending from 1.2 to 3.4 um are generated in the organic crystal DAST (4-N, N-dimethylamino-4-N-methylstilbazolium tosylate). The input pulse has a central wavelength of 1.5 um and 65 fs duration. With 2.8 mJ input energy we obtained up to 700 uJ in the broadened spectrum. The output can be easily scaled up in energy by increasing the crystal size together with the energy and the beam size of the pump. The ultra-broad spectrum is ascribed to cascaded second order processes mediated by the exceptionally large effective chi2 nonlinearity of DAST, but the shape of the spectrum indicates that a delayed chi3 process may also be involved. Numerical simulations reproduce the experimental results qualitatively and provide an insight in the mechanisms underlying the asymmetric spectral broadening.

physics.optics↗

arXiv2015

Air nonlinear dynamics initiated by ultra-intense lambda-cubic THz pulses

Air turns into a nonlinear medium for electromagnetic waves under exceptionally strong fields. However up to present, its minuscule nonlinear response has limited the exploration to the optical frequency regime owing to the availability of intense near-infrared lasers. Here, we report on the observation of large-amplitude nonlinearity in air induced by an extremely intense light bullet at Terahertz frequencies (0.1-10 THz) provoking strong air birefringence. The observed nonlinearity manifests itself as third order susceptibility. The presented nonlinear observations break the barrier for the entire exciting THz-induced nonlinear phenomena in air ranging from THz-induced self-focusing and self-phase modulation to THz solitons and filamentation.

physics.optics↗

arXiv2015

Anomalous visualization of sub-2 THz photons on standard silicon CCD and COMS sensors

We experimentally show that indirect light-induced electron transitions could lead to THz detection on standard CCD and CMOS sensors, introducing this well-established technological concept to the THz range. Unlike its optical counterpart, we found that the THz sensitivity is nonlinear. We imaged 1-13 THz radiation with photon energy less than 2% of the well-established band gap energy threshold. The unprecedented small pitch and large number of pixels uniquely allowed us to visualize the complex propagation of THz radiation, as it focuses down to the physical diffraction limit. Broadband pulses were detectable at a single shot. This opens a whole new field of real time THz imaging at the frame rate of the sensor.

physics.optics↗

arXiv2014

Temporal and Spectral Shaping of Broadband THz Pulses in a Photoexcited Semiconductor

Transmission through photoexcited semiconductors is used to temporally and spectrally shape a Terahertz pulse. By adjusting the optical pump-THz probe delay, we experimentally introduce a polar asymmetry in the pulse profile as high as 92%. To shape the spectrum, we apply the same technique after strongly chirping the Terahertz pulse. This led to significant reshaping of the spectrum with a 52% upshift of spectrum median. Pulse shaping techniques introduced here are of particular importance for temporal and spectral shape-sensitive THz nonlinear experiments.

physics.optics↗

arXiv2014

Terahertz Magnetic Modulator based on Magnetically-Clustered Nanoparticles

Random orientation of liquid-suspended magnetic nanoparticles (Ferrofluid) gives rise to zero net magnetic orientation. An external magnetic field tends to align them into clusters, leading to a strong linear dichroism on a propagating wave. Using 10 nm-sized Fe3O4, we experimentally realize a polarization-sensitive magnetic modulator operating at terahertz wavelengths. We reached a modulation depth of 66% using a field of 35 mT. The proposed concept offers a solution towards fundamental terahertz magnetic modulators.

physics.optics↗

arXiv2014

Tailoring single-cycle electromagnetic pulses in the 2-9 THz frequency range using DAST/SiO2 multilayer structures pumped at Ti:sapphire wavelength

We present a numerical parametric study of single-cycle electromagnetic pulse generation in a DAST/SiO2 multilayer structure via collinear optical rectification of 800 nm femtosecond laser pulses. It is shown that modifications of the thicknesses of the DAST and SiO2 layers allow tuning of the average frequency of the generated THz pulses in the frequency range from 3 to 6 THz. The laser-to-THz energy conversion efficiency in the proposed structures is compared with that in a bulk DAST crystal and a quasi-phase-matching periodically poled DAST crystal and shows significant enhancement.

physics.optics↗

arXiv2014

Terahertz brightness at the extreme: demonstration of 5 GV/m, 17 T low frequency λ3 terahertz bullet

The brightness of a light source defines its applicability to nonlinear phenomena in science. Bright low frequency terahertz (< 5THz) radiation confined to a diffraction-limited spot size is a present hurdle due to the broad bandwidth and long wavelengths associated with single-cycle terahertz pulses as well as due to the lack of terahertz wavefront correctors. Here, using a present-technology system, we employ a new concept of terahertz wavefront manipulation and focusing optimization. We demonstrate a spatio-temporal confinement of terahertz energy at its physical limits to the least possible 3-dimensional light bullet volume of lambda cubic. This leads to a new regime of extremely bright terahertz radiation reaching 40 PW/m2 intensity. The presented work is focused on the sub-5 THz range using small aperture organic crystals DSTMS and OH1. The obtained peak field of up to 5 GV/m and 17 Tesla is order of magnitude higher than any reported single-cycle field oscillation in the entire THz range from a laser-based system and surpassing large scale accelerator systems. The presented results are foreseen to have a great impact on future nonlinear terahertz applications in different science disciplines.

physics.optics↗

arXiv2014

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 the temperature variation of 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.

cond-mat.mtrl-sci↗

arXiv2013

Design of efficient single stage chirped pulse difference frequency generation at 7 μm driven by a dual wavelength Ti:sapphire laser

We present a design for a high-energy single stage mid-IR difference frequency generation adapted to a two-color Ti:sapphire amplifier system. The optimized mixing process is based on chirped pulse difference frequency generation (CP-DFG), allowing for a higher conversion efficiency, larger bandwidth and reduced two photon absorption losses. The numerical start-to-end simulations include stretching, chirped pulse difference frequency generation and pulse compression. Realistic design parameters for commercially available non linear crystals (GaSe, AgGaS2, LiInSe2, LiGaSe2) are considered. Compared to conventional un-chirped DFG directly pumped by Ti:sapphire technology we report a threefold increase of the quantum efficiency. Our CP-DFG scheme provides up to 340 μJ pulse energy directly at 7.2 μm when pumped with 3 mJ and supports a bandwidth of up to 350 nm. The resulting 240 fs mid-IR pulses are inherently phase stable.

physics.optics↗