P. Uhd Jepsen

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.

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.

Electro-optic detection of THz radiation in LiTaO3, LiNbO3 and ZnTe

Freely propagating THz pulses are detected in electro-optic (eo) crystals by monitoring the phase retardation (PR) of an infrared femtosecond probe pulse. This technique permits the determination of the temporal shape of the THz pulse in the subpicosecond time domain. We present measurements in LiTaO3, LiNbO3, and ZnTe and compare their signal performance as eo crystals with theoretical calculations for the PR signal. ZnTe shows the best performance for eo detection.

Ultrafast carrier trapping in microcrystalline silicon observed in optical pump–terahertz probe measurements

We report on direct evidence of ultrafast carrier dynamics displaying features on the picosecond time scale in microcrystalline silicon (μc-Si:H). The dynamics of photogenerated carriers is studied by using above-band-gap optical excitation and probing the instantaneous carrier mobility and density with a THz pulse. Within the first picoseconds after excitation, the THz transmission transients show a fast initial decay of the photoinduced absorption followed by a slower decrease due to carrier recombination. We propose that the initial fast decay in the THz transients is due to carrier capture in the trapping states.

Ultrafast local field dynamics in photoconductive THz antennas

We demonstrate a new ultrafast pump‐probe technique using terahertz pulses to investigate carrier transport and screening in semiconductors. As an example we have studied the temporal evolution of the local electric field in a dipole antenna, used for generation of ultrafast terahertz pulses. Ultrafast screening effects are shown to be important for both carrier transport and the emission of THz radiation. At high carrier densities the external bias field is screened on a time scale comparable to the duration of the THz pulse, giving rise to changes in the shape and bandwidth of the radiated pulses.

Ultrabroadband terahertz conductivity of Si nanocrystal films

The terahertz conductivity of silicon nanoparticles embedded in glass with varying density is studied with ultra-broadband terahertz spectroscopy on picosecond time scales following fs optical excitation. The transition from relatively isolated charge carriers to densities which allow inter-particle transport is clearly observed. For the times immediately following carrier injection, we observe Drude-like long range transport that is rapidly replaced with a localized response on picosecond time scales. The localized response can be very well described by a phenomenological Drude-Smith model, verifying the applicability of this simple model to the conductivity of nanoparticle ensembles over the entire THz spectral window.

Interpretation of photocurrent correlation measurements used for ultrafast photoconductive switch characterization

Photocurrent correlation measurements used for the characterization of ultrafast photoconductive switches based on GaAs and silicon‐on‐sapphire are demonstrated. The correlation signal arises from the interplay of the photoexcited carriers, the dynamics of the bias field and a subsequent recharging of the switch. By using both photocurrent measurements and terahertz spectroscopy we verify the importance of space‐charge effects on the carrier dynamics. Photocurrent nonlinearities and coherent effects are discussed as they appear in the correlation signals. An analysis based on a simple model allows an estimate of the carrier lifetimes associated with the photoconductive switching process. We illustrate how pulse propagation can be studied sequentially using this technique and how a minor modification of the experimental setup enables the study of screening from long‐lived carriers. We emphasize in what ways the different techniques of measuring ultrashort electrical pulses are sensitive to different aspect...

Detection of High Power THz Pulses by Phase Retardation in an Electro-Optic Crystal

In the last decade much research work has been done on the generation, detection and application of short-pulse THz radiation, see for an overview.1

Photoconductive sampling of subpicosecond pulses using mutual inductive coupling in coplanar transmission lines

We demonstrate the generation and detection of subpicosecond pulses by photoconductive sampling in a three‐line coplanar transmission line structure. Due to a mutual inductive coupling between adjacent striplines a differential sampling is obtained. Voltage pulses as short as 390 fs have been measured in a long‐lifetime photoconductive switch. We emphasize that differential coupling can be used for the detection as well as the generation of subpicosecond pulses in long‐lifetime photoconductors.

Generation, Propagation and Detection of Terahertz Radiation from Biased Semiconductor Dipole Antennas

The use of ultrafast lasers to generate sub-picosecond pulses of electromagnetic radiation, THz pulses, has grown into a very active research field in the last decade. For reviews of different aspects of the THz field, see the reference list.1, 2, 3 The ultrafast interaction between a laserpulse and a material generates a DC-polarization, which is the source of the THz radiation. This polarization originates from either a simple flow of free carriers, or from a nonlinear χ2-process. The resulting THz pulse consists only of a few cycles of the electric field, and consequently have a very high bandwidth. With a center-frequency of 1 THz and a bandwidth extending beyond 5 THz, the pulses have proven to be very useful in time-domain spectrometers, and has also been applied as probes of phonon dynamics in crystals and as probes of fundamental carrier transport phenomena in semiconductors. In this work we focus on a detailed description of the elements in a typical THz time-domain spectrometer, as shown in Figure 1.