S. R. Keiding

Generation and detection of terahertz pulses from biased semiconductor antennas

We propose a simple model based on the Drude–Lorentz theory of carrier transport to account for the details of the ultrashort terahertz pulses radiated from small photoconductive semiconductor antennas. The dynamics of the bias field under the influence of the space-charge field from the accelerated carriers is included in the model. We consider in detail the optical system used to image the terahertz radiation onto the terahertz detector, and we calculate the frequency-dependent response of the detector. The proposed model is compared with several different experiments, each focusing on different parameters of the model. Agreement between experiment and model is found in all cases, supporting the validity of this simple and appropriate model.

Coherent anti-Stokes Raman scattering microscopy with a photonic crystal fiber based light source

Coherent anti-Stokes Raman scattering (CARS) microscopy is demonstrated using a light source consisting of the output from a photonic crystal fiber pumped by a standard Ti:sapphire oscillator and the fundamental oscillator beam.

THz reflection spectroscopy of liquid water

Abstract We report an investigation of the temperature-dependent far-infrared spectrum of liquid water. We have employed a new experimental technique based on ultrashort electromagnetic pulses (THz pulses). This technique allows for fast and reliable data of both index of refraction and absorption coefficient for highly absorbing liquids. The temperature dependence reveals an enthalpy of activation corresponding to 2.5 kcal/mol, in agreement with recent Raman experiments, but lower than the enthalpy observed in dielectric relaxation experiments. This demonstrates that part of the orientational relaxation in liquid water takes place without breaking of hydrogen bonds with bonding energy of 5 kcal/mol, as suggested in recent theoretical model.

Supercontinuum generation in a photonic crystal fiber with two zero dispersion wavelengths

We demonstrate supercontinuum generation in a highly nonlinear photonic crystal fiber with two closely lying zero dispersion wavelengths. The special dispersion of the fiber has a profound influence on the supercontinuum which is generated through self-phase modulation and phasematched four-wave mixing and not soliton fission as in the initial photonic crystal fibers. The supercontinuum has high spectral density and is extremely independent of the input pulse over a wide range of input pulse parameters. Simulations show that the supercontinuum can be compressed to ultrashort pulses.

THz time-domain spectroscopy of nonpolar liquids

A newly developed terahertz time-domain spectrometer based on photoconductive dipole antennas driven by femtosecond laser pulses was used to measure the absorption and index of refraction for benzene, carbon tetrachloride, and cyclohexane in the far infrared. The spectra cover the region of 3-66 cm/sup -1/, where the dielectric response of the nonpolar liquids is dominated by collision induced dipole moments. With terahertz time-domain spectroscopy, information on both the real and imaginary part of the frequency response is obtained in a range difficult to access by standard techniques. >

Far infrared properties of electro-optic crystals measured by THz time-domain spectroscopy

In this article we report an experimental investigation of the far-infrared properties of several nonlinear crystals, LiNbO3, LiTaO3, ZnTe and CdTe. Using Terahertz Time-Domain Spectroscopy (THz-TDS) we have measured the complex frequency response, i.e. both index of refraction and absorption up to 3 THz (100 cm−1) for the electro-optic crystals at room temperature. The single Lorentzian oscillator model is used to describe the aquired data. Additional resonance features are observed, especially in the II-VI compounds.

TWO-PHOTON DISSOCIATION AND IONIZATION OF LIQUID WATER STUDIED BY FEMTOSECOND TRANSIENT ABSORPTION SPECTROSCOPY

The photodissociation and photoionization of liquid water following two-photon absorption at 266 nm is studied in the spectral range from 213 to 1108 nm with subpicosecond time resolution. Probing in the UV enables the first direct simultaneous observation of the photoproducts eaq−, Haq, and OHaq. This makes it possible to follow the geminate recombination kinetics between the photoproducts and to determine the relative yields of the dissociation and ionization channels. The concentration of hydrated electrons deduced from the visible and near-infrared transient absorption measurements decays by 40%±2% within the first 90 ps due to recombination with OHaq and H3O+. Analyzing our measurements of the hydrated electron concentration using the independent reaction time approximation results in the relative yields of 82%±3% and 18%±3% for recombination with OHaq and with H3O+, respectively. This is in excellent agreement with the relative yield of 82%±10% for recombination with OHaq determined directly from ou...

Low frequency spectroscopy of liquid water using THz-time domain spectroscopy

In this paper we present an investigation of the dielectric response of H2O(1) and D2O(1) as a function of temperature from a supercooled state to near the boiling point. The data are obtained using THz-time domain spectroscopy, and covers the frequency range from 0 1-2 THz corresponding to 3 to 66 cm(-1). Before presenting the experimental results, we give a short introduction to some of the properties of liquid water, in particular the inter- and intramolecular motions responsible for the low frequency spectra, and brief introduction to the THz-time domain technique and its application as reflection spectroscopy is given. We also address recent theoretical and experimental contributions to the ongoing discussion regarding the microscopic interpretation of the low frequency modes of liquid water

Radiation patterns from lens-coupled terahertz antennas

We report on investigations of the angular distribution of the radiation emitted from a terahertz antenna system equipped with a truncated spherical silicon lens. The pattern is calculated by wide-angle interference principles and Huygens-Fresnel diffraction theory. Experimental determination of the radiation pattern is performed by spatially resolved terahertz time-domain spectroscopy. Good agreement between theory and experiment is obtained, and we find that the terahertz beam can be represented by a Gaussian beam emitted from a circular aperture equal to the diameter of the lens.

Far-infrared properties of DAST

Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorption coefficient of the organic ionic salt 4-N, N-dimethylamino-4?-N?-methyl-stilbazolium tosylate (DAST). This promising organic electro-optic material exhibits strong absorption and dispersion for frequencies above 1 THz at both room temperature and 83 K. No reduction in the absorption strength is observed when DAST is cooled, suggesting a single-phonon origin of the resonances. A simple vibration of the anion and cation of the salt is suggested as the origin of the exceptionally strong far-infrared absorption and the high-frequency electro-optic properties of DAST.