Gregor Klatt

Impulsive terahertz radiation with high electric fields from an amplifier-driven large-area photoconductive antenna

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 x 10(-3). This is achieved by photoexciting biased large-area photoconductive emitter with NIR fs pulses of microJ 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 10(4) signal-to-noise is obtained with an amplifier based system.

Terahertz emission from lateral photo-Dember currents.

The photo-Dember effect is known as source of impulsive THz radiation after excitation with femtosecond optical pulses. The origin of the emission is the ultrafast separation of electron and holes in strong carrier gradients due to different diffusion coefficients. For a simple semiconductor surface the time dependent polarization is oriented perpendicular to the excited surface which complicates efficient out coupling of THz radiation. We investigate a new scheme for generating strong carrier gradients parallel to the surface. In that case the photo-Dember currents are oriented parallel to the surface and the generated THz radiation can be easily out coupled. This concept can be scaled up so that multiple phase coherent photo-Dember currents contribute to the THz emission. These passive THz emitters reach electric field amplitudes comparable to high-efficiency externally biased photoconductive emitters.

Rapid-scanning terahertz precision spectrometer with more than 6 THz spectral coverage

We report a terahertz time-domain spectrometer with more than 6 THz spectral coverage and 1 GHz resolution based on high-speed asynchronous optical sampling. It operates at 2 kHz scan rate without mechanical delay stage. The frequency error of the system at 60 s acquisition time is determined by comparing a measured water vapor absorption spectrum to data reported in the HITRAN database. The mean error of 87 evaluated absorption lines is 142 MHz.

High-speed asynchronous optical sampling with sub-50fs time resolution

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 offset-stabilized using high-bandwidth feedback electronics operating at the tenth repetition rate harmonics. Definition of the offset frequency, i.e. the time-delay scan rate, in the range of a few kilohertz is accomplished using direct-digital-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.

Subharmonic Resonant Optical Excitation of Confined Acoustic Modes in a Free-Standing Semiconductor Membrane at GHz Frequencies with a High-Repetition-Rate Femtosecond Laser

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 systems quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.

Photo-Dember terahertz emitter excited with an Er:fiber laser

A terahertz emitter based on the lateral photo-Dember effect is shown to efficiently generateterahertz 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 μ m . 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.

High-Resolution Terahertz Spectrometer

Terahertz time-domain spectroscopy (THz-TDS) based on high-speed asynchronous optical sampling (ASOPS) with two offset-locked GHz femtosecond lasers requires no mechanical time-delay scanner. Consequently, measurements with 1-GHz frequency resolution are performed at intrinsically high scan rates in the multikilohertz range. This is at least one order, in most cases several orders of magnitude faster than conventional approaches employing mechanical time-delay scanners. We report a system offering a unique combination of high-frequency resolution (1 GHz) and high scan rate (2 kHz) with a spectral coverage of more than 6 THz. Its capabilities for high-precision spectroscopy are demonstrated by measuring the absorption spectrum of a mixture of H2O, D2O, and hydrogen deuterium oxide (HDO) vapor. H2O and HDO vapor absorption spectra are accurately tabulated in databases. However, D2 O absorption data are rare, because of residual H2O and HDO often present when measuring pure D2O. Here, we present a high-resolution absorption spectrum of D2O vapor numerically extracted from the absorption spectrum of the three-component mixture. In addition, we show that the high spectral resolution of the ASOPS THz-TDS system provides benefits in the analysis of frequency-selective surface sensors, which are promising candidates for biosensing applications in the THz regime.

Intense terahertz generation based on the photo-Dember effect

We demonstrate a new scheme for generating THz radiation based on the photo-Dember effect in lateral geometry. By micro-structuring a semiconductor surface we achieve strongly enhanced THz emission comparable to high-efficiency externally biased photoconductive emitters.

Asynchronous optical sampling with GHz repetition rate femtosecond lasers for high precision terahertz spectroscopy

We report a terahertz time-domain spectrometer with 6 THz spectral coverage and 1 GHz resolution which is based on high-speed asynchronous optical sampling. High-speed asynchronous optical sampling is based on two femtosecond lasers with approximately 1 GHz repetition rate. The two lasers are stabilized in their repetition rate to an off-set frequency of approximately 2 kHz. The time delay between a pump pulse exciting a photoconductive THz emitter and the probe pulses is hence scanned at 2 kHz scan rate without employing a mechanical delay stage. The timing jitter between the pump and the probe pulses is reduced to 40 fs over the full scan range giving a high time-resolution. We demonstrate the capabilities of this system for gas spectroscopy and the characterization of frequency selective surfaces with characteristic frequencies in the THz range.

High-speed ASOPS based THz time-domain spectrometer

High-speed asynchronous optical sampling with two offset-locked Ti:sapphire femtosecond oscillators permits time-domain THz spectroscopy with almost 7 THz bandwidth at 1 GHz resolution. The system operates at 2 kHz scan rate without moving mechanical parts. The mean fractional frequency error of absorption lines in a measured water vapor absorption spectrum with respect to the data published in the HITRAN database is 5.9×10−5, the maximum fractional error is 2.1×10−4. The systems potential for rapid sensing applications is demonstrated by reading out a frequency selective surface sensor with an uncertainty of 210 MHz within only 2 s of acquisition time.