2015, Liang, Qijun, Klatt, Gregor, Kraub, Nico, Kukharenko, Oleksii, Dekorsy, Thomas

We demonstrate theoretically and experimentally how changes of a terahertz (THz) beam induced by the sample affect the accuracy of the determination of THz dielectric properties in THz time-domain transmission spectroscopy (TDTS). We apply a Gaussian beam and the ABCD matrix formalism to describe the propagation of the THz beam in a focused beam setup. The insertion of the sample induces a focus displacement which is absent in the reference measurement without a sample. We show how the focus displacement can be corrected. The THz optical properties after focus displacement correction reported in this Letter are in quantitative agreement with those obtained using collimated beam THz–TDTS in previous work.

2011, Klatt, Gregor

2010, Gebs, Raphael, Klatt, Gregor, Janke, Christof, Dekorsy, Thomas, Bartels, Albrecht

We report an ultrafast time-domain spectroscopy system based on high-speed asynchronous optical sampling operating without mechanical scanner. The system uses two 1 GHz femtosecond oscillators that are offsetstabilized using high-bandwidth feedback electronics operating at the tenth repetition rate harmonics. Definition of the offset frequency, i.e. the timedelay scan rate, in the range of a few kilohertz is accomplished using directdigital-synthesis electronics for the first time. The time-resolution of the system over the full available 1 ns time-delay window is determined by the laser pulse duration and is 45 fs. This represents a three-fold improvement compared to previous approaches where timing jitter was the limiting factor. Two showcase experiments are presented to verify the high time-resolution and sensitivity of the system.

2010, Beck, Matthias, Schäfer, Hanjo, Klatt, Gregor, Demsar, Jure, Winnerl, Stephan, Helm, Manfred, Dekorsy, Thomas

We report on the generation of impulsive terahertz (THz) radiation with 36 kV/cm vacuum electric field (1.5 mW average thermal power) at 250 kHz repetition rate and a high NIR-to-THz conversion efficiency of 2×10−3. This is achieved by photoexciting biased large-area photoconductive emitter with NIR fs pulses of μJ pulse energy. We demonstrate focussing of the THz beam by tailoring the pulse front of the exciting laser beam without any focussing element for the THz beam. A high dynamic range of 104 signal-to-noise is obtained with an amplifier based system.

2013, Dekorsy, Thomas, Klatt, Gregor, Bastian, Georg, Huska, Klaus

The invention concerns a passive terahertZ radiation source configured to emit electromagnetic radiation having frequency in the range of 10 GHZ to 50 THZ and a method for generating a terahertZ radiation. The passive terahertz radiation source comprises: a source of a pulsed excitation light; an emitter comprising one or more emitter elements, each emitter element comprising a semiconductor layer being arranged such that at least a portion of first major Surface of said semiconductor layer is exposed to the excitation light, wherein each emitter element is configured such that upon exposure to the excitation light, a gradient of the charge carrier density is generated in the semiconductor layer in the area of transition betWeen first area of the semiconductor layer and a second area of the semiconductor layer, the gradient being substantially parallel to the first major surface of the semiconductor layer.

2011, Bruchhausen, Axel, Gebs, Raphael, Hudert, Florian, Issenmann, Daniel, Klatt, Gregor, Bartels, Albrecht, Schecker, Olivier, Waitz, Reimar, Erbe, Artur, Scheer, Elke, Huntzinger, Jean-Roch, Mlayah, Adnen, Dekorsy, Thomas

We propose subharmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a subharmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the system’s quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.

2010, Klatt, Gregor, Stephan, Daniel, Beck, Matthias, Demsar, Jure, Dekorsy, Thomas

Intense terahertz emitters are one of the most important components of terahertz (THz) time-domain spectroscopy systems. In this presented report, the development of THz emitters over the last two decades is reviewed, and an outlook for future THz emitters is given. The physical principle behind the THz generation process is discussed for two types of emitters: state-of-the-art large-area photoconductive emitters are compared to THz emitters based on the photo-Dember effect. The latter do not require an external bias voltage. This passive character of the photo-Dember emitters has several advantages which are outlined.

2011, Klatt, Gregor, Surrer, Bernhard, Stephan, Daniel, Schubert, Olaf, Fischer, Milan, Faist, Jérôme, Leitenstorfer, Alfred, Huber, Rupert, Dekorsy, Thomas

A terahertz emitter based on the lateral photo-Dember effect is shown to efficiently generate terahertz radiation with a peak frequency of 0.7 THz and an electric field amplitude up to 5 V/cm when excited by 90 fs pulses centered at 1.55 μm. A thin layer of In_0.53 Ga_0.47 As grown on InP provides the substrate material in which unidirectional lateral photo-Dember currents are excited. Since photo-Dember terahertz emitters do not require an external bias, they do not suffer from high dark currents limiting the application of biased InGaAs photoconductive terahertz emitters.

2010, Kampfrath, Tobias, Sell, Alexander, Klatt, Gregor, Pashkin, Alexej, Mährlein, Sebastian, Dekorsy, Thomas, Wolf, Martin, Fiebig, Manfred, Leitenstorfer, Alfred, Huber, Rupert

Ultrafast charge and spin excitations in the elusive terahertz regime of the electromagnetic spectrum play a pivotal role in condensed matter. The electric field of free-space terahertz pulses has provided a direct gateway to manipulating the motion of charges on the femtosecond timescale. Here, we complement this process by showing that the magnetic component of intense terahertz transients enables ultrafast control of the spin degree of freedom. Single-cycle terahertz pulses switch on and off coherent spin waves in antiferromagnetic NiO at frequencies as high as 1 THz. An optical probe pulse with a duration of 8 fs follows the terahertz-induced magnetic dynamics directly in the time domain and verifies that the terahertz field addresses spins selectively by means of the Zeeman interaction. This concept provides a universal ultrafast means to control previously inaccessible magnetic excitations in the electronic ground state.

2010, Klatt, Gregor, Hilser, Florian, Qiao, Wenchao, Beck, Matthias, Gebs, Raphael, Bartels, Albrecht, Huska, Klaus, Lemmer, Ulrich, Bastian, Georg, Johnston, Michael, Fischer, Milan, Faist, Jérôme, Dekorsy, Thomas

The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In0.53Ga0.47As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.