J. Degert

Observation of Coherent Transients in Ultrashort Chirped Excitation of an Undamped Two-Level System

The effects of coherent excitation of a two-level system with a linearly chirped pulse are studied theoretically and experimentally [in Rb (5s-5p)] in the low field regime. The coherent transients are measured directly on the excited state population on an ultrashort time scale. A sharp step corresponds to the passage through resonance. It is followed by oscillations resulting from interferences between off-resonant and resonant contributions. We finally show the equivalence between this experiment and Fresnel diffraction by a sharp edge.

Mechanism for optical switching of the spin crossover [Fe(NH2-trz)3](Br)2·3H2O compound at room temperature

This paper reports on phase transition photo-induced by a nanosecond laser pulse in the molecular spin crossover material [Fe(NH(2)-trz)(3)] (Br)(2).3H(2)O (with NH(2)trz = 4-amino-1,2,4-triazole) around room temperature and in the close vicinity of the thermal hysteresis loop. The measurements are carried out using a time-resolved pump-probe experiment and by recording the reflectivity change at various temperatures and laser intensities. The dynamics of the optically induced reflectivity changes are presented and discussed. We propose a simple model that describes well the recorded phenomena. It takes into account the physical and optical properties of the sample that directly impact the amplitude and the dynamics of the laser-induced heating of the compound.

Room temperature study of the optical switching of a spin crossover compound inside its thermal hysteresis loop

We have studied the low-spin to high spin state phase transition induced by a single or a sequence of nanosecond laser pulses within the thermal hysteresis loop of the [Fe(NH2-trz)3](NO3)2-H2O spin crossover compound. We demonstrate that the final state that is photoinduced can be finely controlled by changing the central wavelength and the energy of the laser pulses. A simple model accounts for the observed phenomena and paves the way for the practical applications to optical data storage at room temperature of spin state transition compounds.

Femtosecond optical pulse shaping for tunable terahertz pulse generation

We report on the generation of phase-locked terahertz pulse pairs and tunable narrow-band terahertz pulses by means of optical pulse shaping combining a liquid crystal spatial light filter and optical rectification in ZnTe. They are generated through simple sinusoidal and/or triangular phase modulations and do not require any optimization algorithm. The corresponding spectra are tunable between 0.5 and 2.5 THz with a spectral bandwidth as narrow as 140 GHz.

Impact of dispersion, free carriers, and two-photon absorption on the generation of intense terahertz pulses in ZnTe crystals

We study the evolution of the energy and the spectrum of a terahertz wave generated by optical rectification of an ultrashort laser pulse in ZnTe crystals with different thicknesses. As the pump intensity increases, we observe a shift in the terahertz spectrum toward lower frequencies. Moreover, at high pump intensities, in disagreement with common sense, thin crystals have a better conversion efficiency than thicker ones. These phenomena are accounted for by the pump depletion induced by two-photon absorption, pulse broadening and the impact of the photoinduced free carriers on the complex refractive index of the crystal in the terahertz range.We study the evolution of the energy and the spectrum of a terahertz wave generated by optical rectification of an ultrashort laser pulse in ZnTe crystals with different thicknesses. As the pump intensity increases, we observe a shift in the terahertz spectrum toward lower frequencies. Moreover, at high pump intensities, in disagreement with common sense, thin crystals have a better conversion efficiency than thicker ones. These phenomena are accounted for by the pump depletion induced by two-photon absorption, pulse broadening and the impact of the photoinduced free carriers on the complex refractive index of the crystal in the terahertz range.

Study of the fast photoswitching of spin crossover nanoparticles outside and inside their thermal hysteresis loop

We have studied the low spin to high spin phase transition induced by nanosecond laser pulses outside and within the thermal hysteresis loop of the [Fe(Htrz)2 trz](BF4)2-H2O spin crossover nanoparticles. We demonstrate that, whatever the temperature of the compound, the photo-switching is achieved in less than 12.5 ns. Outside the hysteresis loop, the photo-induced high spin state remains up to 100 μs and then relaxes. Within the thermal hysteresis loop, the photo-induced high spin state remains as long as the temperature of the sample is kept within the thermal loop. A Raman study indicates that the photo-switching can be completed using single laser pulse excitation.

One-dimensional tunable photonic crystals with spin crossover material for the terahertz range

The authors use a spin crossover material as a defect inserted into a periodic structure of alternating layers of glass and air. A single defect mode of this Bragg filter designed for the submillimeter wavelength can be tuned over 15GHz by inducing the spin transition of the defect. This shows potential applications of spin crossover materials at terahertz frequencies.

Dielectric characterization of [Fe(NH2−trz)3]Br2•H2O thermal spin crossover compound by terahertz time domain spectroscopy

The complex optical index refraction of an iron (II) spin-crossover coordination polymer is measured in the Terahertz frequency range by Terahertz time-domain spectroscopy (THz-TDS). By scanning the temperature from 288 to 333 K, we have recorded the evolution of the THz spectrum within the low spin - high spin thermal hysteresis loop. We were able to simultaneously infer the refractive index and absorption variations. The low spin-high spin transition has a marked spectral signature in the millimeter wavelength. In the 0.1–0.6 THz frequency range, the variation of real and imaginary part of the index of refraction is 6% and 20%, respectively. A marked absorption is observed above 500 GHz. The THz-TDS provides a clear direct fingerprint of this class of materials, which are interestingly potential candidates for optical data storage and processing devices.

Tuning and focusing THz pulses by shaping the pump laser beam profile in a nonlinear crystal

Spatially shaped femtosecond laser pulses are used to generate and to focus tunable terahertz (THz) pulses by Optical Rectification in a Zinc Telluride (ZnTe) crystal. It is shown analytically and experimentally that the focusing position and spectrum of the emitted THz pulse can be changed, in the intermediate field zone, by controlling the spatial shape of the near-infrared (NIR) femtosecond (fs) laser pump. In particular, if the pump consists of concentric circles, the emitted THz radiation is confined around the propagation axis, producing a THz pulse train, and focusing position and spectrum can be controlled by changing the number of circles and their diameter.

Switching of spin-state complexes induced by the interaction of a laser beam with their host matrix

We demonstrate that the interaction of a CO2 laser with a host matrix, in which spin-crossover compounds are diluted, makes it possible to induce a low-spin-to-high-spin transition. The phenomenon is demonstrated for two different compounds diluted in a commercial polymer. The photo-induced transition is due to laser-induced heating of the host-matrix, which results in a localized modification of the properties of the sample. This phenomenon is reversible, and its kinetics are well accounted for by a simple model. This process makes it possible to write optical information at a high speed and to erase it in such materials.