Ultrabroadband THz Time-Domain Spectroscopy of a ... - IEEE Xplore

IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 4, NO. 4, JULY 2014

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Ultrabroadband THz Time-Domain Spectroscopy of a Free-Flowing Water Film Tianwu Wang, Pernille Klarskov, and Peter Uhd Jepsen, Member, IEEE

Abstract—We demonstrate quantitative ultrabroadband THz time-domain spectroscopy (THz-TDS) of water by application of a 17- m thick gravity-driven wire-guided flow jet of water. The thickness and stability of the water film is accurately measured by an optical intensity crosscorrelator, and the standard deviation of the film thickness is less than 500 nm. The cross section of the water film is found to have a biconcave cylindrical lens shape. By transmitting through such a thin film, we perform the first ultrabroadband (0.2–30 THz) THz-TDS across the strongest absorbing part of the infrared spectrum of liquid water using two different THz-TDS setups. The extracted absorption coefficient and refractive index of water are in agreement with previous results reported in the literature. With this we show that the thin free-flowing liquid film is a versatile tool for windowless, ultrabroadband THz-TDS with sub-100-femtosecond time resolution of aqueous solutions in transmission mode in the important cross-over region between vibrational and relaxational dynamics. Index Terms—Air plasma, photoconductive antenna, THz spectroscopy, ultrafast nonlinear optics, water hydrogen bond network.

I. INTRODUCTION

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ERAHERTZ time domain spectroscopy (THz-TDS) is a powerful experimental technique which complements Fourier-transform far-infrared (FT-FIR) spectroscopy [1], Raman spectroscopy [2], optical-heterodyne detected, Raman-induced Kerr effect spectroscopy (OHD-RIKES) [3], and optical Kerr effect (OKE) [4] measurements in the investigation of water hydrogen bond network dynamics. THz-TDS directly determines the broadband dielectric properties (real and imaginary part of the complex dielectric function) of the sample, and is thus a valuable tool for investigation of low-frequency infrared active modes of water and other liquids. In particular, many experiments have investigated the liquid structure and dynamics in water and aqueous solutions [5], [6]. These measurements were all performed with the sample in contact with a transparent window, either in reflection mode, in attenuated total reflection mode, or in transmission. At frequencies above a few THz, all commonly used window materials

Manuscript received December 13, 2013; revised February 18, 2014; accepted April 07, 2014. Date of publication May 29, 2014; date of current version June 26, 2014. This work was supported by the Danish Council for Independent Research under FNU Project THz-BREW and FTP Project HI-TERA. The authors are with the Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark (e-mail: ([email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TTHZ.2014.2322757

become increasingly opaque [7], with the notable exception of high-resistivity silicon [8] and diamond. However, the high refractive index of high-resistivity silicon will result in a large reflection loss and both the high-resistivity silicon and diamond windows will induce artifacts for time resolved experiments. It is therefore advantageous to perform spectroscopic measurements without windows surrounding the liquid. In ultrafast measurements of pump-induced THz dynamics in liquids [9], the presence of windows can lead to parasitic optical nonlinearities [10] as well as transient free carrier absorption in the THz range due to multiphoton excitation of the window material [11], resulting in limitations of the excitation intensity of the liquid. Pressure-driven jets of liquids with a thickness in the range of hundreds of micrometer, determined by a precision-tooled nozzle, can be employed to avoid the use of windows [12]. However, in the 10–20 THz region, the intermolecular hydrogen bond vibrational modes lead to optical densities in the range – for a 10- m path. Thus, the strong, broadband far-infrared absorption of liquid water makes the use of film thicknesses in the range of 10–20 m a requirement for sensitive spectroscopy in this frequency range. Free-flowing thin water films driven by gravity between thin metal wires [13] offer a solution to this problem, and is a standard technique in infrared (IR) absorption spectroscopy [14]. In the original characterization of such films by Tauber et al., thicknesses down to 6 m were demonstrated. However, a stable sub-20 m thick water film is rather difficult to produce, and most spectroscopic investigations based on this technique have been employing films with thicknesses in the 50–200 m range. Furthermore, the surface cross sectional shape of the water film has not been investigated in detail, in spite of its extensive use in ultrafast mid-infrared spectroscopic investigations [15]–[18]. Thus, the performance of such a thin water film and its influence on the beam path in a THz-TDS system needs to be characterized. In this paper, we generate a stable 17 m thick free-flowing water film and characterize its thickness profile using a customized intensity crosscorrelator. The stability and uniformity of the water film is investigated for accurate spectroscopic data analysis. We quantify the performance of the water film by performing the first ultrabroadband THz-TDS [19]–[21] of liquid water without any supporting material in transmission mode with an instrument based on air-plasma THz wave generation [22] and air-biased coherent detection (ABCD) [23] techniques, supplemented in the lowest frequency range by conventional THz-TDS based on photoconductive switches [24]. The transmission geometry has the distinct advantage in comparison to reflection geometries that the phase of the

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light transmitted through the water film, relative to that of the light transmitted through air, is insensitive to small positioning errors of the film along the propagation axis, and thus precise values of the index of refraction can be extracted without the need for introducing small numerical corrections to the phase due to path differences in the data analysis. With the two methods in conjunction with the gravity-driven, wire-guided flow jet, we extend the spectral bandwidth exploited so far by THz-TDS on liquids by almost an order of magnitude, and present an ultrabroadband (0.2–30 THz) measurement of the absorption coefficient and index of refraction of water, thereby simultaneously covering the two intermolecular vibrational modes of the hydrogen bond network and the broadband relaxation processes at lower frequencies. The refractive index and absorption coefficient of water are compared with the data published in the literature [1], [25], [26]. Thus, we demonstrate that the gravity-driven wire-guided flow jet is a versatile tool for THz-TDS investigations of aqueous liquids.

Fig. 1. (a) Schematic diagram of thin film flow system. UR, LR: upper, lower reservoir, AF: aluminum frame, PP: peristaltic pump. Tubes of lengths connects the circuit. (b) Photograph of the water film. The water inlet tube cross section is visible in the top part of the photograph.

II. WATER FILM THICKNESS MEASUREMENT As shown schematically in Fig. 1(a), the water film is formed by gravity-assisted flow along two 18 m diameter metal wires separated by 4 mm (much larger than the employed THz spot sizes) as described by Tauber et al. [13]. The thickness of the water film depends on the flow rate, which is adjusted by the height difference between the upper reservoir (UR) and the top of the wires mounted in an aluminum frame (AF). The UR is connected to the AF with an mm, inner diameter 1.6 mm polyethylene (PE) tube. The tube is mounted to the AF and tungsten wires as indicated in the inset of Fig. 1(a). The water is collected in a drop collector (DC), and led to the lower reservoir (LR) through a tube (inner diameter 8.0 mm, length mm) and recirculated to the UR by a peristaltic pump (PP, Watson–Marlow Pumps, model 120 S) via a tube with lengths mm and mm before and after the PP, respectively (Watson–Marlow Bioprene, inner diameter 3.0 mm). A thin PE tube (outer diameter 3.0 mm) is inserted in the thicker PE tube between DC and LR to minimize bubble formation in the tube. Fig. 1(b) shows a photograph of a stable water film with a thickness of approximately 20 m. To stabilize the thin flowing water film, the upper reservoir is made with a dimension of 131 mm 131 mm. The drop collector helps to reduce evaporation. For a sub-20- m thick water film, the height difference between upper water reservoir and the top of water film can be stabilized for several days at 100 mm with a slow peristaltic pump speed of 40 rpm. A femtosecond background-free autocorrelator (AC) was used to measure the thickness of the water film. The beam in one of the arms of the AC was focused through the water film. By measuring the optical time delay caused by water film, the thickness of the water film can be calculated as , where is the optical time delay, is the speed of light, and is the group refractive index of water at 800 nm [27]. Fig. 2(a) shows two measured intensity AC signals of the 800 nm, 140 fs pulses recorded with and without (square and disk symbols, respectively) water film in the beam path. The dashed curves

Fig. 2. (a) Optical pulse intensity cross correlation traces. The filled red and open black symbols are the experimental data. The dashed curves are Gaussian fits. (b) The accuracy of water film thickness measurement in the center of the cross section of the water film. CI indicates the 95% confidence interval based on the student- distribution.

are Gaussian fits, used to accurately determine the pulse peak positions. Both instability of the water film and the statistical fluctuations of the measurement will contribute to the variation of the thickness which we estimate by calculating the variance of the thickness values recorded at regular 10 min intervals, as shown in Fig. 2(b). The mean thickness at the center of the film is 16.9 m, with a 95% confidence interval (CI) based on a student’s distribution of 0.62 m around the mean.

WANG et al.: THz-TDS OF A FREE-FLOWING WATER FILM

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Fig. 3. Characterization of the cross section of the water film. Inset shows a schematic diagram of the water film cross section.

when measured over 80 min. In practice, the film is stable at 20 m thickness for several hours without addition of surfactants, thus enabling the long recording time typically required in most THz-TDS experiments, and in particular in transient THz spectroscopy. The uniformity of the water film thickness is particularly important for precise terahertz spectroscopy, as the terahertz spot size is typically in the range of a few hundred m due to the long wavelengths involved. Therefore we measure the thickness of water film along its cross section by translation of the water film with respect to the laser probe spot, with results as shown in Fig. 3. The water film thickness varies significantly across the cross section, with a shape resembling that of a parabola (dashed curve in Fig. 3). The thickness increases from 17 m at the center to above 28 m at a position 1 mm off the center. If we only consider the central mm section (approximately the THz spot size at the lowest frequencies) of the film, the thickness variation from the middle point to the edge is less than 3 m. A sketch (not drawn to scale) of the cross section of the water film is shown in the inset of Fig. 3. The central part of the cross-sectional shape of the water film resembles that of a biconcave cylindrical lens with radius of curvature of each of the two surfaces of approximately 80–100 mm. The possible effect of the lens effect caused by the curvature will be discussed in Section III. III. EXPERIMENTAL SETUPS FOR ULTRABROADBAND THZ-TDS Two different THz-TDS setups were used to cover the spectral range from 0.2 to 30 THz. Fig. 4(a) illustrates the schematic diagram for an air photonics THz-TDS setup. A Ti:sapphire regenerative amplifier (SpectraPhysics Spitfire Pro) with central wavelength of 800 nm, 35 fs pulse duration, 3.5 mJ pulse energy, and 1 kHz repetition rate is used as the laser source. A fraction of the full beam power is used for the experiment, where the beam is split into pump (600 J) and probe (300 J) beams using a beam splitter. The pump beam is focused through a 100 m thick beta-barium borate ( -BBO) crystal to generate the second harmonic at 400 nm.

Fig. 4. THz-TDS setups. (a) Terahertz pulse is generated from laser-induced air plasma and detected with air biased coherent detection (ABCD) method. L1, L2: lens with focal length of 300 mm, BS: beam splitter, M1–M5: metallic silver mirror, BBO: -barium borate crystal, HP, DWHP: half-wave plate, dual-wavelength half-wave plate, SW: silicon wafer, P1-P4: off-axis parabolic mirror, HV: high voltage bias, PMT: photomultiplier tube. (b) Measured beam profile immediately after P2 (top) and at focal plane (bottom). (c) PCA THz-TDS system. PCA1, PCA2: photoconductive antenna, L1, L2: aspheric polymer lenses with 25.4 mm working distance.

The mixed fundamental and second harmonic beams have their polarizations aligned by a half-wave plate (HWP) and are then focused in dry nitrogen gas with a lens (effective focal length 300 mm) to generate an ionized plasma that emits a broadband THz pulse through a third-order nonlinear process. After being collimated and refocused by four parabolic mirrors P1–P4, the THz beam and the optical probe beam are recombined at the detection region, where an electric bias is applied to create a second harmonic local oscillator for coherent detection through a THz-field-induced second harmonic generation process. The second harmonic signal is detected with a photomultiplier tube (PMT) with a lock-in amplifier (300 ms time constant). The computer-controlled delay line M2–M3 is used to scan the temporal waveform of the detected THz transient. Spectroscopic data was recorded in the air photonics system with a time window of 10 ps, resulting in a spectral resolution of 0.1 THz, and time steps of 10 fs, corresponding to a Nyquist frequency of 50 THz. A 2 mm thick high resistivity silicon wafer is used to block the remaining optical pump beam. The water film is placed in the focal plane between the parabolic mirrors P2 and P3. The complete terahertz beam path is enclosed in a box and continuously purged with dry nitrogen gas to reduce water vapor absorption. We use a distributed inlet of the purge gas to minimize turbulence in the box which could disturb the stability of the water film. The water film apparatus is mounted on a 3-axis motion stage to control the position in the intermediate focal region of the THz beam path in the spectrometer.

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Based on beam profiling investigations [28], we find that the THz beam profile originating from the two-color plasma has a characteristic emission cone angle [29], and thus a rather frequency-independent beam profile in the parallel sections of the beam path. Fig. 4(b) shows the donut-shaped beam profile immediately after mirror P2 (top) and in the focal plane (bottom), as measured with a broadband THz camera (NEC IRV-T0831, 320 240 pixels microbolometer array with spectral range 1–7 THz). In the intermediate focal region the donut-shaped beam profile is transformed to a single, central peak. The beam profile and its evolution through the optical system can be modeled by Gaussian beam propagation formalism [30]. The numerical aperture of the spectrometer at the sample position is 0.25. In the low frequency range from 0.2 to 2 THz, a fiber-coupled, commercial THz-TDS system (Picometrix T-Ray 4000) was used, as shown schematically in Fig. 4(b). The system uses fibercoupled femtosecond near-infrared fs pulses to excite photoconductive antennas (PCA) both for generating and coherently detecting the temporal profile of picosecond THz pulses, covering a typical spectral range from 0.05 to 2 THz. This system has a fast-scan time window of 320 ps and a temporal resolution of 78 fs (4096 data points) with an acquisition speed of 100 waveforms per second. The THz beam is focused tightly with a pair of polymer lenses with an effective focal length of 25.4 mm, corresponding to a numerical aperture of 0.62, with a resulting spot size of approximately 1.5 mm at 0.2 THz and 0.3 mm at 1 THz [31]. As in the air photonics setup, the water film apparatus could be accurately positioned in the focal region of the THz beam path. The concave cylindrical shape of the water film could lead to undesired modifications of the beam path in the spectrometers, which in turn could influence the quantitative spectroscopic results in an unpredictable manner. To address this issue, we have modeled an optical system closely mimicking the configuration of the real experiment depicted in Fig. 4(a), and find that the slightly concave water film located in the intermediate sample focal plane has negligible influence on the beam waist and radius of curvature of the transmitted THz beam at the detection point within the full frequency range 1–30 THz. The water film is modeled as a thin concave lens with strength , located in the focal plane between P2 and P3. The frequency dependent refractive index of water is taken from the measurements that will be discussed in Section IV. The frequency dependence of the index of refraction leads to significant variation of the focal length of the concave water film with frequency. However, due to the favorable position of the water film in the focal plane of the beam path, the water film lens effect has minimal effect on the propagation of the THz beam at all relevant frequencies, and the lens effect can thus be ignored in the subsequent spectroscopic analysis. In the case of the PCA-driven setup, the situation is slightly different. Here the relevant frequencies are much lower (0.2–2 THz), and the beam path includes a hyperhemispherical lens which focuses the THz radiation onto the photoconductive antenna in the detector [32]. Gaussian beam propagation through the optical system of the PCA-driven setup shows that the concave water film slightly disturbs the wavefront curvature at the

Fig. 5. THz pulses generated and detected in the THz-ABCD setup and transmitted through: (a) water; (b) air; and (c) corresponding amplitude spectra of the two pulses. Two time traces and their corresponding spectra are directly comparable on the plotted scales.

apex of the detector silicon lens, and thus the focusing power is slightly diminished, leading to a slightly larger focused spot size ( 10%) at the detection point, as well as a slightly modified Gouy phase shift ( 0.03 rad) at the focus point when compared to propagation through the setup without the water film. This defocusing may lead to a minor overestimation of the absorption coefficient of the water film, and the additional Gouy phase shift results in a slight overestimation of the index of refraction of the water film. The combination of these effects might be responsible for the slight deviation ( 10%) between our measured index of refraction at the lowest frequencies and corresponding literature values. IV. TERAHERTZ SPECTROSCOPY OF WATER Fig. 5(a) and (b) shows the time-domain traces of THz pulses through a 17 m water film and air, respectively, measured with the high-bandwidth air photonics setup. The frequency spectra of the pulses are displayed in Fig. 5(c). The reference spectrum covers the 1–30 THz range. The sharp dip in the spectral amplitude seen at 18.5 THz has been assigned to the bulk phonon combination band [33] in the silicon plate used to block the pump beam [see Fig. 4(a)]. After transmission through the water film, the peak amplitude of the terahertz pulse is reduced by a factor of approximately 2, and the pulse shape is somewhat broadened. The signal strength is low below 2 THz, and even though the absorption in the water film is very strong


Fig. 6. THz time-domain waveforms measured using PCA setup transmitted through: (a) water; (b) air, and (c) corresponding amplitude spectra of the signals. The two time traces and their spectra are directly comparable on the plotted scales.

in the 10–25 THz region, the useful frequency range for spectroscopy extends to 30 THz. The spectral amplitude of the THz signal vanishes rapidly below 2 THz in the ABCD setup. Thus, for completeness and calibration purposes, the frequency range below 2 THz was additionally covered with the PCA-based THz-TDS system. Measurements were again performed in a purged box to reduce the influence of water evaporation. We accumulated the average of 10 000 temporal waveforms automatically (10 ms/waveform, 100 s total acquisition time), resulting in a useful spectroscopic range 0.2–2 THz. The time-domain traces for transmission through the water film and through air are shown in Fig. 6(a) and (b), respectively. Due to the lower absorption of liquid water at these lower THz frequencies, the pulse is not attenuated as much as observed in Fig. 5. The amplitude spectra of the time domain traces are displayed in Fig. 6(c), covering the 0.2–2 THz range with weak water vapor absorption lines clearly present in both spectra due to the water film apparatus inside the purge box. The standard method to extract the optical properties (here represented by the complex-valued index of refraction) of a sample in a transmission THz-TDS measurement is to compare the spectral content of a THz pulse which has propagated through the sample to that of a THz pulse recorded without the sample to get the complex-valued spectral transfer function. The amplitude and phase of the transfer function is then

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Fig. 7. (a) Refractive index and (b) absorption coefficient of water. Blue and red dot curves are our extracted parameters; the error bars are the standard variance intervals calculated from the individual measurements. Literature values are represented by the black, dotted curve [26], blue, long-dashed curve [25] and blue short-dashed curve [37].

used to calculate the refractive index and absorption coefficient based on a more or less elaborate model of the transmission of the field through the sample. The thin water film considered here must be analyzed taking multiple reflections as well as the complex-valued transmission coefficients at the interfaces into account. As time windowing techniques for separation of multiple reflections are not practical for such thin samples, more advanced analysis techniques must be applied. A numerical approach taking such considerations into account was introduced by Duvillaret et al. [34] and later further refined by Pupeza et al. [35]. By iteratively minimizing the difference between the magnitude and phase of the model and experimental transfer functions, we accurately determine the optical parameters of the sample. In Fig. 7(a) and (b), we show the absorption coefficient and the refractive index of liquid water at room temperature 22 2 C in the frequency range from 0.2 to 30 THz. The insets in each panel show a magnified view of the 0–3 THz range where there is spectral overlap between the data sets from the two experimental setups. The red and black symbols represent our extracted water parameters from the PCA and air photonics setup, respectively. The standard errors indicated by the gray error bars were individually calculated from 11 (PCA) and 5 (air photonics) independent measurements for the two setups. The reproducibility of the data between the two setups is confirmed by the good quantitative agreement between the data measured with the two setups near 2 THz (see insets). In Fig. 7(a), we clearly observe

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the two strong, broad absorption bands caused by intermolecular vibrations present near 6 and 22 THz. As could be expected, the error bars are bigger near 18.5 THz as a consequence of the lower dynamic range due to the strong phonon combination band absorption in the silicon plate. In Fig. 7(b), the rather strong dispersion of water across the THz region is seen. The refractive index changes from 3 to 1.06 throughout the frequency range. The main part of this strong dispersion is due to the two parallel Debye-like relaxation processes with time constants of 8.2 and 0.3 ps which determine the optical properties below 2 THz [26], [36]. In order to benchmark our spectroscopic results against accepted literature values we directly compare our measured data with selected authoritative results in Fig. 7(a) and (b). The dotted curve is the reported data at 19.15 C in the earlier study by Rønne et al. [26] who investigated the temperature dependence of dielectric relaxation in water by reflection THz-TDS [see inset of Fig. 7(a) and (b)]. The absorption agrees well with our present data, whereas our measurement of the index of refraction is slightly higher than the literature values in the 1–2 THz range, possibly due to the slight defocusing of the THz beam by the concave water film in the PCA setup. The second set of literature data between 0.2 and 30 THz (the blue, long-dashed curve) was compiled by Segelstein [25] from a range of already published Fouriertransform spectroscopy measurements, including those of Afsar and Hasted [1]. Our measured absorption coefficient and index of refraction compares quantitatively with the Segelstein data compilation across the full frequency range. This benchmarking against previous measurements performed by dispersive FTIR reflection spectroscopy shows that transmission THz-TDS with a gravity-driven wire-guided flow is a viable technique even in the strongest absorption region of the infrared spectrum of water. We also compare our data with a recent and very comprehensive molecular dynamics simulation of the hydrogen bond network dynamics of water published by Heyden et al. [37] [blue, short-dashed curve in Fig. 7(a)]. We have divided their reported values of by our measured , without further scaling of the values and also here find good agreement.

V. CONCLUSION AND PERSPECTIVES In conclusion, we have demonstrated ultrabroadband THz-TDS of deionized water by employing a gravity-assisted wire-guided flowing film with a thickness of 17 m. The water film remains stable over several hours with sub- m precision. THz air photonics and more conventional photoconductive antennas were used independently to cover the broad spectrum from 0.2 to 30 THz. As quantitative spectroscopy with air photonics technology is still a relatively new technique, and the lensing effect due to the concave shape of the water film so far has not been described in detail, we benchmarked our results with authoritative literature values, and find very good agreement. As an important perspective on the results reported here, we note that ultrabroadband THz-TDS has the unique capability

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Tianwu Wang was born in Hei Longjiang Province, China, in 1984. He received the B.E. degree in optoelectric engineering and the M.S. degree in optical engineering from National University of Defense Technology (NUDT), Changsha, China, in 2007 and 2009, respectively, and is working toward the Ph.D. degree in the Department of Optical Engineering, NUDT, since 2010. In 2011, he joined the Department of Photonics Engineering, Technical University of Denmark (DTU), first as a guest Ph.D. student, and in 2013 as full-time Ph.D. student, working on terahertz broadband relaxtion dynamics of electons in water, supported by the Danish Council for Independent Research. His research areas include ultrabroadband, time-resolved THz spectroscopy and ultrafast laser technology. Mr. Wang won the second award in 2007 China undergraduate Challenge Cup contest in Extracurricular Academic and Technology Works.

Pernille Klarskov received the B.Sc. degree in physics from University of Copenhagen, Denmark, in 2009, and the M.Sc. degree in physics and nanotechnology from the Technical University of Denmark, in 2011, and is currently working toward the Ph.D. degree in high-intensity terahertz sources for 2D spectroscopy from Department of Photonics Engineering, Technical University of Denmark, since 2012. From 2011 to 2012, she was research assistant with photonic crystal fibers for biomedical imaging at the Department of Photonics Engineering, Technical University of Denmark. In 2014, she received the prestigious Elite Research Travel Award from the Danish Council for Independent research, which will support her research stay in the group of Professor X.-C. Zhang at Institute of Optics, University of Rochester, NY, USA, during the Spring 2014. Her research interests include nonlinear spectroscopy with intense terahertz sources and molecular dynamics in crystalline materials.

Peter Uhd Jepsen (M’11) received the M.Sc. degree in physics and chemistry from Odense University, Denmark, in 1994 , and the Ph.D. degree in natural sciences from Århus University, Denmark, in 1996. He was with the University of Freiburg, Germany, from 1996 to 2004, working with terahertz time-domain spectroscopy. From 2005, he was Associate Professor at the Technical University of Denmark (DTU), and since 2008, Full Professor and Head of the Terahertz Science and Technology group at DTU. He was visiting professor at Osaka University in 2008. His research areas include photonics-based THz technology, ultrabroadband spectroscopy, time-resolved THz spectroscopy, THz imaging and non-destructive testing, and nonlinear THz science. He has authored and co-authored more than 80 peer-reviewed papers on terahertz science and technology. Dr. Jepsen is Associate Editor for Optics Express, a Topic Editor for IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, and a member of the Editorial Board of Journal of Infrared, Millimeter and THz Waves.