Simultaneous reference and differential waveform acquisition in time-resolved terahertz spectroscopy

Iwaszczuk, Krzysztof; Cooke, David G.; Fujiwara, Masazumi; Hashimoto, Hideki; Jepsen, Peter Uhd

doi:10.1364/oe.17.021969

Cited by 44 publications

(42 citation statements)

References 19 publications

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“…1(a)]. This is not possible with commonly used SiO 2 =Si substrates, which produce a large background signal in optical-pump terahertz-probe experiments [16]. We used z-cut crystalline quartz substrates and deposited 35-nm indium tin oxide (ITO) and 400-nm parylene-C thin films as the back-gate electrode and dielectric, respectively.…”

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confidence: 99%

“…In these measurements, the local detection time of the picosecond terahertz pulse was synchronized with the pump pulse such that the whole terahertz waveform experienced the same time delay after photoexcitation [29]. To reduce experimental errors due to laser drift, we simultaneously measured the transmitted terahertz electric field waveform E 0 ðtÞ without optical excitation and the optical-pump-induced change of the field ΔE τ ðtÞ via electro-optic sampling using a data acquisition card [16,17,30]. The resulting ratio −ΔE τ =E 0 (referred to as "differential field") approximately represents the PC, Δσ τ;1 (Refs.…”

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Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

et al. 2014

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We investigate the transient photoconductivity of graphene at various gate-tuned carrier densities by optical-pump terahertz-probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations can be accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Notably, we observe nonmonotonic fluence dependence of the photoconductivity at low carrier density. This behavior reveals the nonmonotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions. DOI: 10.1103/PhysRevLett.113.056602 PACS numbers: 72.80.Vp, 72.40.+w, 73.40.Qv, 78.20.−e Charge carriers in graphene mimic two-dimensional (2D) massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties [1]. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range [2]. Interband transitions in graphene give rise to pronounced optical absorption in the midinfrared to visible spectral range, where the optical conductivity is close to a universal value σ 0 ¼ πe 2 =2h [3]. Free-carrier intraband transitions, on the other hand, cause lowfrequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density [4]. In addition to this density dependence, the massless Dirac particles in graphene are predicted to exhibit a distinctive nonmonotonic temperature dependence of the intraband absorption strength, or Drude weight, due to their linear dispersion [5,6]. This behavior contrasts with the temperature-independent Drude weight expected in conventional systems of massive particles with parabolic dispersion [7,8]. Although the unique behavior of the Drude weight in graphene has been considered theoretically, experimental signatures are still lacking.The intrinsic properties of Drude absorption in graphene can be revealed by studying its dynamical response to photoexcitation. In particular, optical-pump terahertz-probe spectroscopy provides access to a wide transient temperature range via pulsed optical excitation, and allows measurement of the ac Drude conductivity by a timedelayed terahertz probe pulse [9]. This technique has been applied to study transient photoconductivity (PC) in graphene, but conflicting results have been reported [9][10][11][12][13][14][15]. Positive PC was observed in epitaxial graphene on SiC (Ref.[15]), while negative PC was seen in graphene grown by chemical vapor deposition (CVD) [11][12][13][14]. It has been argued that the opposite behavior in these samples arises from their different charge densities. Here we study...

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Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

et al. 2014

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“…The experimental method and setup have been described elsewhere [7,20]. In short, they consist of a Ti:sapphire regenerative fs laser amplifier delivering 45 fs, millijoule pulses at a 1 kHz repetition rate.…”

Section: Experimental Methodsmentioning

confidence: 99%

“…The transmitted THz pulse is recollimated and focused by a 3 00 focal length mirror onto a 515 μm h110i ZnTe crystal, where it copropagates with the sampling pulse for electro-optic detection. The reference and differential waveforms are measured simultaneously to reduce acquisition time and changes in the experimental conditions during measurements [20]. The measurements are carried out by fixing the pump and sampling pulses for a given t p , and the probe pulse is scanned.…”

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confidence: 99%

Finite-difference time-domain analysis of time-resolved terahertz spectroscopy experiments

2011

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In this paper we report on the numerical analysis of a time-resolved terahertz (THz) spectroscopy experiment using a modified finite-difference time-domain method. Using this method, we show that ultrafast carrier dynamics can be extracted with a time resolution smaller than the duration of the THz probe pulse and can be determined solely by the pump pulse duration. Our method is found to reproduce complicated two-dimensional transient conductivity maps exceedingly well, demonstrating the power of the time-domain numerical method for extracting ultrafast and dynamic transport parameters from time-resolved THz spectroscopy experiments. The numerical implementation is available online.

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“…19 The nanorods are photoexcited colinearly through a small hole in the last parabolic mirror before the sample by 400 nm fs laser pulses at a fluence of 570 μJ/cm 2 , unless otherwise stated. All measurements are performed at room temperature in a dry nitrogen environment.…”

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Picosecond dynamics of internal exciton transitions in CdSe nanorods

et al. 2013

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The picosecond dynamics of excitons in colloidal CdSe nanorods are directly measured via their 1s to 2p-like internal transitions by ultra-broadband terahertz spectroscopy. Broadened absorption peaks from both the longitudinal and transverse states are observed at 8.5 and 11 THz, respectively. The onset of exciton-LO phonon coupling appears as a bleach in the optical conductivity spectra at the LO phonon energy for times > 1 ps after excitation. Simulations show a suppressed exciton temperature due to thermally excited hole states being rapidly captured onto ligands or unpassivated surface states. The relaxation kinetics are manipulated and the longitudinal transition is quenched by surface ligand exchange with hole capturing pyridine.

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Simultaneous reference and differential waveform acquisition in time-resolved terahertz spectroscopy

Cited by 44 publications

References 19 publications

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

Finite-difference time-domain analysis of time-resolved terahertz spectroscopy experiments

Picosecond dynamics of internal exciton transitions in CdSe nanorods

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