Torsten Löffler

Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves.

Intense radiation in the terahertz (THz) frequency range can be generated by focusing of an ultrashort laser pulse composed of both a fundamental wave and its second-harmonic field into air, as reported previously by Cook et al. [Opt. Lett. 25, 1210 (2000)]. We identify a threshold for THz generation that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain our measurements. An additional THz field component generated in the type I beta-barium borate crystal used for second-harmonic generation has to be considered if one is to avoid misinterpretation of this kind of experiment. We conclude with a comparison that shows that the plasma emitter is competitive with other state-of-the-art THz emitters.

Continuous-wave all-optoelectronic terahertz imaging

We present an all-optoelectronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas. For a specific biological sample, we compare continuous-wave THz imaging and pulsed THz imaging at 1 THz with respect to data-acquisition time and signal-to-noise ratio, and discuss image formation from both amplitude and phase data. In addition, we introduce the application of hyperboloidal lenses which allow tighter focusing and a corresponding improvement in spatial resolution compared to off-axis paraboloidal mirrors.

Determination of the carrier-envelope phase of few-cycle laser pulses with terahertz-emission spectroscopy

The availability of few-cycle optical pulses opens a window to physical phenomena occurring on the attosecond timescale. To take full advantage of such pulses, it is crucial to measure1,2,3,4 and stabilize1,2 their carrier-envelope (CE) phase, that is, the phase difference between the carrier wave and the envelope function. We introduce an approach to determine the CE phase by down-conversion of the laser light to the terahertz (THz) frequency range by means of plasma generation in ambient air, an isotropic medium where optical rectification (down-conversion) in the forward direction is only possible if the inversion symmetry is broken by electrical or optical means5,6,7,8,9,10. We show that few-cycle pulses directly produce a spatial charge asymmetry in the plasma. The asymmetry, associated with THz emission, depends on the CE phase, which allows determination of the phase by measurement of the amplitude and polarity of the THz pulse.

Terahertz dark-field imaging of biomedical tissue.

We investigate dark-field imaging in the terahertz (THz) fre-quency regime with the intention to enhance image contrast through the analysis of scattering and diffraction signatures. A gold-on-TPX test structure and an archived biomedical tissue sample are examined in conventional and dark-field transmission geometry. In particular, the capability of the technique for tumor detection is addressed.

THz Active Imaging Systems With Real-Time Capabilities

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

Large-area electro-optic ZnTe terahertz emitters

We present a detailed experimental and theoretical study of terahertz (THz) generation and beam propagation in an optoelectronic THz system consisting of a large-area (ZnTe) electro-optic emitter and a standard electro-optic detector, and provide a comparison to typical biased GaAs emitters. As predicted by theory, in the absence of saturation the generated THz pulse energy is inversely proportional to the area of the optical pump beam incident on the emitter, although the detected on-axis electric field amplitude of the subsequently focused THz beam is practically independent of this area. This latter result promotes the use of larger emitter crystals in amplifier-laser-based THz systems in order to minimize saturation effects. Moreover, the generation of an initially larger THz beam also provides improved spatial resolution at intermediate foci between emitter and detector.

Generation of terahertz pulses by photoionization of electrically biased air

We present an experimental demonstration of the generation of far-infrared (terahertz) pulses by photoionization of electrically biased air with amplified laser pulses. The current surge following photoionization of the air with an applied bias field of 10.6 kV/cm leads to the emission of THz pulses with an intensity which can be almost as high as that of THz pulses radiated from a large-area intrinsic-field GaAs emitter. The spectra peak at higher frequency than those of biased large-area GaAs emitters.

All-optoelectronic continuous wave THz imaging for biomedical applications

We present an all-optoelectronic THz imaging system for ex vivo biomedical applications based on photomixing of two continuous-wave laser beams using photoconductive antennas. The application of hyperboloidal lenses is discussed. They allow for f-numbers less than 1/2 permitting better focusing and higher spatial resolution compared to off-axis paraboloidal mirrors whose f-numbers for practical reasons must be larger than 1/2. For a specific histological sample, an analysis of image noise is discussed.

Continuous-wave terahertz imaging with a hybrid system

The authors have developed a hybrid system for coherent raster-scan imaging at 0.6THz. It combines a high-power electronic source (a multiplied Gunn emitter) with a femtosecond Ti:sapphire laser in order to achieve a high dynamic range via electro-optic detection. The single-scan dynamic range of 60db at a lock-in time constant of 10ms is sufficient to permit detection of scattered terahertz radiation in addition to specularly reflected/transmitted light. Active synchronization of the electronic source and the laser is not needed because of the remarkably low jitter between the two radiation sources.

Indium-tin-oxide-coated glass as dichroic mirror for far-infrared electromagnetic radiation

We propose to utilize glass plates coated with indium tin oxide (ITO) as far-infrared dichroic mirrors, e.g., in optoelectronic terahertz spectroscopy for purposes such as electro-optic detection of terahertz radiation or in time-resolved experiments with pump pulses in the visible or near-infrared spectral range and probe pulses at terahertz frequencies. Measurements of the complex reflection and transmission coefficients in the spectral range from 0.1 to 2.8 THz indicate that commercially available ITO glass is suitable for this purpose.