Time-domain Spectroscopy Measurements Research Articles

Metallic and 3D-printed dielectric helical terahertz waveguides.

We investigate guidance of Terahertz (THz) radiation in metallic and 3D-printed dielectric helical waveguides in the frequency range from 0.2 to 1 THz. Our experimental results obtained from THz time-domain spectroscopy (THz-TDS) measurements are in very good agreement with finite-difference time-domain (FDTD) simulations. We observe single-mode, low loss and low dispersive propagation of THz radiation in metallic helical waveguides over a broad bandwidth. The 3D-printed dielectric helical waveguides have substantially extended the bandwidth of a low loss dielectric tube waveguide as observed from the experimental and simulation results. The high flexibility of the helical design allows an easy incorporation into bench top THz devices.

Lattice Vibrations of Natural Seraphinite Gemstone Probed by Terahertz Time-Domain Spectroscopy

In this paper, we report the first observation of terahertz (THz) frequency lattice vibrations in naturally occurring seraphinite gemstones. Seraphinite is a particular form of clinochlore minerals in the chlorite group, and the THz frequency response of any chlorite minerals has been unknown. Here, we show, based on our THz time-domain spectroscopic measurements, that there are three absorption modes at 0.8, 0.96, and 1.2 THz , respectively. The 0.96 THz mode is, in particular, strong and narrow $(Q=8)$ , comparable to the previously reported 0.53 THz mode in $\alpha$ -lactose monohydrate. Also, a polarization-dependent study reveals the $A_{u}(z')$ -symmetry of the 0.8 and 1.2 THz modes and the $B_{u}(x',y')$ -symmetry of the 0.96 THz mode. The anisotropy of the phonon-polariton dispersions shows an excellent agreement with theoretical analysis based on Kurosawa formula.

Scheme to extend the range of electro-optic sampling of an intense terahertz field

Generation of intense terahertz (THz) radiation requires an efficient method to determine the field intensity with a wide range of intensity response and accuracy. A new scheme is proposed to extend the limited response range of available time-domain spectroscopy measurement. The method applies two pieces of thin ZnTe crystals with a fixed angle between the (001) axes. At the optimized angle, the linear response range of the THz field can reach 31 MV/cm.

Tunable Terahertz Filter and Modulator Based on Electrostatic MEMS Reconfigurable SRR Array

This paper demonstrates a tunable terahertz (THz) filter and a modulator based on an electrostatic micro electro mechanical systems (MEMS) reconfigurable split ring resonator (SRR) array. The device uses tunable metamaterial concept, in which an RF-MEMS (radio frequency-MEMS) capacitor is embedded in each SRR unit cell to tune the electromagnetic resonant frequency. By utilizing THz time domain spectroscopy (THz-TDS) measurement and through the voltage actuation, we have controlled the THz transmission of the metamaterial device, and thus demonstrated a tunable filter. Since the low loss material of fused quartz is used as the substrate, the device shows a high contrast ratio THz switch at a given frequency by controlling the capacitance of the RF-MEMS capacitor. The dynamic performance of the switch, i.e., the modulation ability, is also characterized by THz-TDS. The measurement results show up to 2 kHz of the modulation speed with the TDS operated. The device is developed by the surface micromachining process with three photolithography steps.

Spin reorientation transition process in single crystal NdFeO3

The spin reorientation transition in single crystal NdFeO3 is studied using AC magnetic susceptibility, hysteresis loops, and polarized terahertz (THz) time domain spectroscopy measurements. Different frequency dependence behaviors of AC susceptibility reflect that the dynamic response of magnetization inside the spin reorientation region differs from the phase outside the transition region. The magnetization hysteresis loops at different temperatures reveal that domains formed during the spin reorientation process, which coincides with the abrupt increase of AC magnetic susceptibility during the transition. In addition, temperature dependent THz wave excitation of quasi-antiferromagnetic mode indicates the process of spin reorientation as a continuous rotation of Fe3+ spins rather than a mixed phase of Γ4 and Γ2.

Terahertz dynamics of water before and after water shedding from reverse micelles

Picosecond relaxation dynamics of water before and after water shedding from reverse micelles are studied using temperature-dependent terahertz time-domain spectroscopy measurements to clarify the confinement effect of the reverse micelle on the dynamics of water. Before water shedding, the relaxation process due to the collective motion of bulk-like water in the core of the reverse micelle exhibits a considerable slowing down in comparison with that in liquid water as the temperature decreases. This is attributed to the confinement effect of reverse micelles. Moreover, the time constant abruptly drops at the water shedding, and its temperature behavior is similar to that of liquid water afterwards. These results demonstrate that the water extracted from the reverse micelle attains the property of liquid water by reduction in the confinement imposed on the motion of the water.

Controllable terahertz conductivity in single walled carbon nanotube/polymer composites

Terahertz (THz) conductivity of single walled carbon nanotube (SWNT)/poly-vinyl alcohol (PVA) composites has been studied in the frequency window of 0.3–2.0 THz. SWNT/PVA composite films with a constant thickness of 300 ± 20 μm are grown by dispersing required amount of SWNT in PVA solution via a slow drying process at room temperature under ambient condition. THz time domain spectroscopic measurements have been performed in transmission geometry at room temperature under N2 atmosphere and THz conductivity spectra have been extracted from the time domain data. It is found that conductivity of these samples can be efficiently tuned by changing the length of the SWNTs and also the SWNT weight fraction. For the highest weight fraction at a frequency of 1.5 THz, longer SWNT sample (average length ∼ 15 μm) showed 80% increased conductivity than its shorter counterpart (average length ∼ 2 μm) of the same diameter (1–2 nm). Shielding effectiveness of the samples has also been engineered by simply changing the effective length of SWNT inclusion in the polymer matrix. A modified Universal Dynamic Response model is applied to analyze the conductivity spectra of the samples.

Terahertz broadband polarizer using bilayer subwavelength metal wire-grid structure on polyimide film

A terahertz (THz) broadband polarizer using bilayer subwavelength metal wire-grid structure on both sides of polyimide film is simulated by the finite-difference time-domain method. We analyze the effect of film ­thickness, material loss, and lateral shift between two metallic gratings on the performance of the THz ­polarizer. Bilayer wire-grid polarizers are fabricated by a simple way of laser induced and non-electrolytic plating with copper. The THz time-domain spectroscopy measurements show that in 0.2-1.6 THz frequency range, the extinction ratio is better than 45 dB, the average extinction ratio reaches 53 dB, and the ­transmittance exceeds 67%, which shows great advantage over conventional single wire-grid THz polarizer.

Low temperature grown photoconductive antennas for pulsed 1060 nm excitation: Influence of excess energy on the electron relaxation

We investigate properties of MBE grown photoconductive terahertz (THz) antennas based on the InGaAs/InAlAs/InP material system aimed for an excitation wavelength of approx. 1060 nm. Therefore, we analyze several different approaches concerning growth parameters, layer and material compositions as well as doping. The carrier dynamics are probed via transient white-light pump-probe spectroscopy as well as THz Time Domain Spectroscopy (TDS) measurements. We find that the electron capture probability is reduced for higher electron energies. By adjusting the material band gap this can be resolved and lifetimes of 1.3 ps are obtained. These short lifetimes enable the detection of THz TDS spectra with a bandwidth exceeding 4 THz.

Graphene-based tunable metamaterial terahertz filters

We propose and describe a micro-machined tunable metamaterial terahertz filter based on graphene. The device structure consists of periodic metallic rings with several gaps where tunable graphene stripes are located. We demonstrate that the filter resonance frequency can be adjusted easily by varying the conductivity of graphene and implement this by changing the number of stacked graphene layers. Moreover, the proposed design is scalable, in the sense that the resonance frequency tuning can be controlled by scaling the inner and outer radius of the metal rings. Using numerical simulations and terahertz time-domain spectroscopy measurements of the fabricated samples, we show that the resonance frequency of the structure can be altered by 40% (i.e., from ∼0.2 THz to ∼0.12 THz) by simply tuning the conductivity of graphene. Importantly, the active area of the device is ≪0.1% of the total unit cell area, which can boost the device speed upon electrostatic actuation.

Determination of refractive index and thickness of a multilayer structure with a single terahertz time domain spectroscopy measurement.

A processing technique for the determination of the average refractive index and thickness of a two-layer system is presented. It is based on a single measurement with a standard terahertz time-domain spectrometer and the multilayer system thickness. The technique relies on the interference caused by the main pulse with the echoes produced in each material. This approach allows noninvasive inspection of double-layer compound products.

Electrical characterization of silver nanowire-graphene hybrid films from terahertz transmission and reflection measurements

We determined the electrical sheet conductivity of silver nanowire-graphene hybrid films from transmission and reflection terahertz time-domain spectroscopy measurements. The sheet resistance extracted from noncontact terahertz measurement is in good agreement with one measured with a classical 4-point-probe technique. The conductivity is well described by a Drude-Smith model and is calculated to peak around 10 THz.

Time-domain modeling of dispersive and lossy liquid-crystals for terahertz applications

A numerical framework based on the finite-difference time-domain method is proposed for the rigorous study of electro-optically tunable terahertz devices based on liquid crystals. The formulation accounts for both the liquid-crystal full-tensor anisotropy and the dispersion of its complex refractive indices, which is described via modified Lorentzian terms. Experimentally characterized liquid-crystalline materials in the terahertz spectrum are fitted and modeled in benchmark examples, directly compared with reference analytical or semi-analytical solutions. In addition, the efficiency of broadband time-domain modeling of the proposed technique is also demonstrated by accurately reproducing time-domain spectroscopy measurements.

Continuous-wave terahertz system based on a dual-mode laser for real-time non-contact measurement of thickness and conductivity

Terahertz (THz) waves have been exploited for the non-contact measurements of thickness and refractive index, which has enormous industrial applicability. In this work, we demonstrate a 1.3-μm dual-mode laser (DML)-based continuous-wave THz system for the real-time measurement of a commercial indium-tin-oxide (ITO)-coated glass. The system is compact, cost-effective, and capable of performing broadband measurement within a second at the setting resolution of 1 GHz. The thickness of the glass and the sheet conductivity of the ITO film were successfully measured, and the measurements agree well with those of broadband pulse-based time domain spectroscopy and Hall measurement results.

Coherent Vibrational Dynamics and High-resolution Nonlinear Spectroscopy: A Comparison with the Air/DMSO Liquid Interface

We present a comparative study on the C—H stretching vibrations at air/DMSO (dimethyl sulfoxide) interface with both the free-induction decay (FID) coherent vibrational dynamics and the sub-wavenumber high resolution sum-frequency generation vibrational spectroscopy measurements. In principle the frequency-domain and time-domain spectroscopic measurements should generate identical information for a given molecular system. However, when the molecular systems are with several coupled or overlapping vibrational modes, obtaining detailed spectroscopic and coherent dynamics information is not as straightforward and rather difficult from either the time-domain or the frequency domain measurements. For the case of air/DMSO interface that is with moderately complex vibrational spectra, we show that the frequency-domain measurement with sub-wavenumber high-resolution sum- frequency generation vibrational spectroscopy is probably more advantageous than the time- domain measurement in obtaining quantitative understanding of the structure and coherent dynamics of the molecular interface.

Possible Bose-condensate behavior in a quantum phase originating in a collective excitation in the chemically and optically doped Mott-Hubbard system UO2+x

X-ray pair distribution function (pdf) and U L${}_{3}$ extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO${}_{2}$ show U(VI)-oxo moieties with x rays for mixed valence U${}_{4}$O${}_{9}$ and U${}_{3}$O${}_{7}$, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO${}_{2}$ are essentially identical at 30, 100, and 200 K, those of the UO${}_{2+x}$ compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO${}_{2+x}$ compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U${}_{4}$O${}_{9}$ does not behave as a linear combination of the UO${}_{2}$ and U${}_{3}$O${}_{7}$ fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump--1.57 eV probe reflectivity on UO${}_{2}$ that initially populates the U 6$d$-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-\ensuremath{\mu}sec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5$f$-dominated portion of the UHB shows an analogous 2.8-\ensuremath{\mu}sec peak but also a plateau bracketing this peak that ends in a cusp at 50--60 K and an abrupt change in the hardening rate of a novel 12--15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5--1.1 THz range with a decrease in temperature from \ensuremath{\sim}30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer--phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) \ensuremath{\leftrightarrow} U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

Giant rotary power of a fishnet-like metamaterial

We show experimentally and numerically that cholesteric-type metal-dielectric structures, made of twisted sub-wavelength hole elliptical–shaped arrays, exhibit strong optical activity and circular dichroism. The experimental demonstration is carried out with terahertz time-domain spectroscopy measurements on a three layered structure operating around 0.5 THz, leading to a rotary power as high as 1000°/wavelength. The contribution of the chirality factor (κ), permittivity (ɛ), and permeability (μ) is discussed after the retrieval of effective parameters from the complex scattering ones.

Capturing inhomogeneous broadening of the –CN stretch vibration in a Langmuir monolayer with high-resolution spectra and ultrafast vibrational dynamics in sum-frequency generation vibrational spectroscopy (SFG-VS)

While in principle the frequency-domain and time-domain spectroscopic measurements should generate identical information for a given molecular system, the inhomogeneous character of surface vibrations in sum-frequency generation vibrational spectroscopy (SFG-VS) studies has only been studied with time-domain SFG-VS by mapping the decay of the vibrational polarization using ultrafast lasers, this due to the lack of SFG vibrational spectra with high enough spectral resolution and accurate enough lineshape. Here, with the recently developed high-resolution broadband SFG-VS (HR-BB-SFG-VS) technique, we show that the inhomogeneous lineshape can be obtained in the frequency-domain for the anchoring CN stretch of the 4-n-octyl-4'-cyanobiphenyl (8CB) Langmuir monolayer at the air-water interface, and that an excellent agreement with the time-domain SFG free-induction-decay can be established. We found that the 8CB CN stretch spectrum consists of a single peak centered at 2234.00 ± 0.01 cm(-1) with a total linewidth of 10.9 ± 0.3 cm(-1) at half maximum. The Lorentzian contribution accounts only for 4.7 ± 0.4 cm(-1) to this width and the Gaussian (inhomogeneous) broadening for as much as 8.1 ± 0.2 cm(-1). Polarization analysis of the -CN spectra showed that the -CN group is tilted 57° ± 2° from the surface normal. The large heterogeneity in the -CN spectrum is tentatively attributed to the -CN group interactions with the interfacial water molecules penetrated/accommodated into the 8CB monolayer, a unique phenomenon for the nCB Langmuir monolayers reported previously.

Low-Bias Active Control of Terahertz Waves by Coupling Large-Area CVD Graphene to a Terahertz Metamaterial

We propose an hybrid graphene/metamaterial device based on terahertz electronic split-ring resonators directly evaporated on top of a large-area single-layer CVD graphene. Room temperature time-domain spectroscopy measurements in the frequency range from 250 GHz to 2.75 THz show that the presence of the graphene strongly changes the THz metamaterial transmittance on the whole frequency range. The graphene gating allows active control of such interaction, showing a modulation depth of 11.5% with an applied bias of 10.6 V. Analytical modeling of the device provides a very good qualitative and quantitative agreement with the measured device behavior. The presented system shows potential as a THz modulator and can be relevant for strong light-matter coupling experiments.

Influence of structural anisotropy to anisotropic electron mobility in a-plane InN

This study reports on the anisotropic electron transport properties and a correlation between the electron mobility (μ) and the stacking faults (SFs) in the a-plane InN film. Electron mobilities measured by terahertz time-domain spectroscopy and Hall effect measurement along the in-plane [1¯100] (c⊥) orientation were much higher than those of the in-plane [0001] (c∥) orientation. This result shows a sharp contrast to higher defect density for the c⊥ orientation as measured by x-ray diffraction. The electrons transporting through the planar SFs aligned along the c⊥ direction are expected to experience more scattering by defects, resulting in lower μ for the c∥ orientation.