Torsten Löffler

Bio: Torsten Löffler is an academic researcher from Goethe University Frankfurt. The author has contributed to research in topics: Terahertz radiation & Laser. The author has an hindex of 21, co-authored 69 publications receiving 2587 citations.

Broadband THz emission from gas plasmas induced by femtosecond optical pulses: From fundamentals to applications

Abstract: We review the generation of broadband THz radiation from femtosecond photo-induced gas plasmas, with an emphasis on the highly efficient "AC-bias" case where the plasma is generated and driven by a superposition of fundamental and second-harmonic optical fields. The dependence on experimental parameters such as pulse energy, air pressure, polarization and focusing are presented, and compared to the predictions from semi-quantitative models for the THz generation process, namely (i) a microscopic photocurrent model and (ii) a four. wave mixing model. We also employ these models to the case of few-cycle pulses, where the observed THz emission is related directly to the carrier-envelope phase of the pulses, and hence provides a mechanism with which to measure this phase.

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

Abstract: 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 [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 β-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.

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

Abstract: 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.

Continuous-wave all-optoelectronic terahertz imaging

Abstract: 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.

Terahertz dark-field imaging of biomedical tissue

Abstract: We investigate dark-field imaging in the terahertz (THz) frequency 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.

Materials for terahertz science and technology

Abstract: Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules.

Terahertz spectroscopy and imaging – Modern techniques and applications

Abstract: Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Attosecond Physics

Abstract: Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

THz imaging and sensing for security applications—explosives, weapons and drugs

Abstract: Over the past 5 years, there has been a significant interest in employing terahertz (THz) technology, spectroscopy and imaging for security applications. There are three prime motivations for this interest: (a) THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation; (b) target compounds such as explosives and illicit drugs have characteristic THz spectra that can be used to identify these compounds and (c) THz radiation poses no health risk for scanning of people. In this paper, stand-off interferometric imaging and sensing for the detection of explosives, weapons and drugs is emphasized. Future prospects of THz technology are discussed.

Terahertz quantum-cascade lasers

Abstract: Six years after their birth, terahertz quantum-cascade lasers can now deliver milliwatts or more of continuous-wave coherent radiation throughout the terahertz range — the spectral regime between millimetre and infrared wavelengths, which has long resisted development. This paper reviews the state-of-the-art and future prospects for these lasers, including efforts to increase their operating temperatures, deliver higher output powers and emit longer wavelengths.