M. Tondusson

Three-dimensional terahertz computed tomography of human bones

Three-dimensional terahertz computed tomography has been used to investigate dried human bones such as a lumbar vertebra, a coxal bone, and a skull, with a direct comparison with standard radiography. In spite of lower spatial resolution compared with x-ray, terahertz imaging clearly discerns a compact bone from a spongy one, with strong terahertz absorption as shown by additional terahertz time-domain transmission spectroscopy.

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.

Ultrafast Dynamics of Metal Complexes of Tetrasulphonated Phthalocyanines

A promising material in medicine, electronics, optoelectronics, electrochemistry, catalysis, and photophysics, tetrasulphonated aluminum phthalocyanine (AlPcS(4)), is investigated by means of steady-state and time-resolved pump-probe spectroscopies. Absorption and steady-state fluorescence spectroscopy indicate that AlPcS(4) is essentially monomeric. Spectrally resolved pump-probe data are recorded on time scales ranging from femtoseconds to nanoseconds. The nature of these fast processes and pathways of the competing relaxation processes from the initially excited electronic states in aqueous and organic (dimethyl sulfoxide) solutions are discussed. The decays and bleaching recovery have been fitted in the ultrafast window (0-10 ps) and later time window extending to nanoseconds (0-1 ns). While the excited-state dynamics have been found to be sensitive to the solvent environment, we were able to show that the fast dynamics is described by three time constants in the ranges of 115-500 fs, 2-25 ps, and 150-500 ps. We were able to ascribe these three time constants to different processes. The shortest time constants have been assigned to vibrational wavepacket dynamics. The few picosecond components have been assigned to vibrational relaxation in the excited electronic states. Finally, the 150-500 ps components represent the decay from S(1) to the ground state. The experimental and theoretical treatment proposed in this paper provides a basis for a substantial revision of the commonly accepted interpretation of the Soret transition (B transition) that exists in the literature.

Coherent combination of four laser beams in a multi-axis Fourier cavity using a diffractive optical element

Thermal aberrations in diode-pumped solid-state laser media are well known. To avoid these effects we propose to end-pump separated zones of a laser crystal with elementary beams of moderate power. We present a multi-axis laser cavity with a single-gain medium longitudinally pumped by four fibre-coupled laser diodes. The four elementary laser beams are coherently combined inside the cavity by a diffractive optical element. 82% of the laser power is focused in a single diffraction limited beam.

Terahertz radiation generated and detected by Br+-irradiated In0.53Ga0.47As photoconductive antenna excited at 800nm wavelength

The authors investigate the generation and the detection of terahertz radiation by Br+-irradiated In0.53Ga0.47As photoconductive antennas using an optical excitation at 800nm wavelength. The detected temporal wave form presents a full width at half maximum of 650fs. The signal-to-noise ratio and frequency range of the emitted terahertz radiations are similar to those emitted by low-temperature-grown GaAs photoconductive antennas. The dependences of terahertz wave form amplitude on optical pump power for both Br+-irradiated In0.53Ga0.47As and low-temperature-grown GaAs photoconductive antennas are investigated and compared.

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.

Time-domain terahertz spectroscopy of spin state transition in [Fe(NH2 − trz)3]2+ spin crossover compounds

We have measured the evolution of the THz spectrum of iron(II) spin crossover compounds within the low-spin/high-spin thermal hysteresis loop in the 0.6–6 THz frequency range. This study enabled to follow both the variations of the refractive optical index and absorption during the spin state transition. Marked absorptions centered ∼2 – 3 THz and ∼5 THz shifting with the spin state are revealed. Our work provides a means to store optically information and to read it out in the THz domain and also offers indications about the structural evolution occurring during the spin state transition.

High-Frequency Response in Ferroelectric BaSrTiO3 Thin Films Studied by Terahertz Time-Domain Spectroscopy

We have measured the dielectric properties of BaTiO3–SrTiO3 thin films, deposited on fused silica substrates, in the MHz and THz frequency regions. Different experimental Chemical Vapor Deposition parameters were analyzed to improve dielectric properties of these thin films. In particular, we clarify pressure and oxygen percentages during the deposition of the thin layer. The complex permittivity function of the film was accurately determined between 100 GHz and 1 THz. We show that the real part of the dielectric behavior at very high frequencies follows the same tendency of the low frequency dielectric behavior also measured. These results are correlated to X-ray diffraction and RBS patterns, which show that the cationic ratio (Ba+Sr)/Ti increases from about 0.6 to 0.9 as sputtering pressure changes from 1 to 5 Pa, leading to better dielectric properties.

Optimized terahertz generation via optical rectification in ZnTe crystals

We report on optimal control of the output energy of terahertz (THz) waves generated by optical rectification of femtosecond pulses in ZnTe crystals. An enhancement by a factor up to 2.4 is obtained with chirped pump pulses. The optimized THz wave also displays a spectral broadening. The influence of the optical pulse shaping on the pump pulse propagation, and consequently on the THz generation efficiency, is numerically investigated and discussed.

Direct observation of low frequency confined acoustic phonons in silver nanoparticles: Terahertz time domain spectroscopy

Terahertz time domain spectroscopy has been used to study low frequency confined acoustic phonons of silver nanoparticles embedded in poly(vinyl alcohol) matrix in the spectral range of 0.1-2.5 THz. The real and imaginary parts of the dielectric function show two bands at 0.60 and 2.12 THz attributed to the spheroidal and toroidal modes of silver nanoparticles, thus demonstrating the usefulness of terahertz time domain spectroscopy as a complementary technique to Raman spectroscopy in characterizing the nanoparticles.