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 In(0.53)Ga(0.47)As. 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.
We propose sub-harmonic 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.In recent years, nanophononic and nanomechanical systems have emerged as intriguing subjects for studying mechanics, heat transfer and opto-mechanical coupling on a nanometer scale [1][2][3]. From a fundamental point of view, they provide a route to study mechanical excitations and their interactions with other elementary excitations [1,4]. ¿From an applied perspective they have opened a pathway for high sensitivity sensors in the zeptogram mass range and in the attonewton force range [5,6]. In established experimental methods these systems are driven electrically, magnetically, thermoelastically [7], via radiation pressure from continuous wave lasers [8], or via other optical non-radiation-pressurebased schemes [9,10]. The frequencies of typical systems investigated so far are in the megahertz to gigahertz frequency range [7]. The investigation of higher frequencies is strongly restricted by the driving and detection methods. Here, we report a new method for the investigation of a vibrational system by sub-harmonic resonant excitation with a high-repetition rate femtosecond laser. This excitation scheme can be regarded as tuning the separation of modes of the frequency comb of a femtosecond laser [11] to a commensurable of the frequency of the phononic system. By sweeping the comb spacing of the femtosecond laser, resonant impulsive excitation of the mechanical oscillator can be achieved, which allows the determination of its quality factor in the gigahertz to terahertz frequency range with femtometer sensitivity for the mechanical amplitude. We demonstrate the amplification of the fundamental eigenmode of a free-standing silicon membrane at 19 GHz by a factor of 20 compared to the off-resonant case and determine its quality factor.The dynamical properties of the free-standing silicon membranes were investigated by performing fs resolution pump-probe experiments using the recently developed high-speed asynchronous optical sampling (ASOPS) method [12,13]. This method is based on two asynchronously linked femtosecond Ti:sapphire ring lasers of repetition rate f R ∼ 1 GHz. One laser provides the pump beam and the second laser the probe beam. In this technique the time delay between pump-/probepulse pairs of the two pulse trains is realized through an actively stabilized 10 kHz repetition-rate-offset ∆f R between the tw...
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.
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.
Abstract-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 muItikilohertz 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 H 2 0, D 2 0, and hydrogen deuterium oxide (HDO) vapor. H20 and HDO vapor absorption spectra are accurately tabulated in databases. However, D 2 0 absorption data are rare, because of residual H 2 0 and HDO often present when measuring pure D2 O. Here, we present a high-resolution absorption spectrum of D 2 0 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 benefit~ in the analysis of frequency-selective surface sensors, which are promising candidates for biosensing applications in the THz regime.
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