Christopher T. Que

The complex refractive indices of the liquid crystal mixture E7 in the terahertz frequency range

We have used terahertz time-domain spectroscopy to investigate the complex optical constants and birefringence of a widely used liquid crystal mixture E7 in both nematic and isotropic phases (26°C–70°C). The extinction coefficient of E7 at room temperature is less than 0.035 and without sharp absorption features in the frequency range of 0.2–2.0 THz. The extraordinary (ne) and ordinary (no) indices of refraction at 26°C are 1.690–1.704 and 1.557–1.581, respectively, giving rise to a birefringence of 0.130–0.148 in this frequency range. The temperature-dependent (26°C–70°C) order parameter extracted from the birefringence data agrees with that in the visible region quite well. Further, the temperature gradients of the terahertz optical constants of E7 are also determined. The optical constants of E7 in the terahertz or sub-millimeter wave range are found to deviate significantly from values predicated by the usual extended Cauchy equations used in the visible and near-infrared.

Efficient electro-optic sampling detection of terahertz radiation via Cherenkov phase matching

We experimentally demonstrate an efficient electro-optic sampling scheme based on Cherenkov phase matching of broadband terahertz radiation with 800-nm femtosecond probe beam in a 0.5 mm-thick LiNbO3 crystal coupled to a Si prism. The electro-optic signal from a Cherenkov-phase-matched LiNbO3 crystal is found to be comparable to that with a 4 mm-thick ZnTe crystal using a collinear phase matching. The Cherenkov phase matching technique can be achieved with any probe wavelength and hence has an advantage over the collinear phase matching method.

Terahertz Radiation from InAs Films on Silicon Substrates Excited by Femtosecond Laser Pulses

Using ultrashort laser pulses, terahertz (THz) emission from InAs thin films grown on Si substrates is investigated. Results show that the measured radiation in transmission geometry exhibits an enhancement of the low frequency components and the strongest emission is from the thickest 520 nm film. Comparison of the emission from a 520 nm film and of bulk GaAs in reflection geometry reveals that the main THz radiation mechanism is the photo-Dember effect. Moreover, comparing the emission from bulk InAs, the thin films can also be categorized as strong THz emitters.

Terahertz emission from GaAs-AlGaAs core-shell nanowires on Si (100) substrate: Effects of applied magnetic field and excitation wavelength

Terahertz (THz) emission from GaAs-AlGaAs core-shell nanowires (CSNW) on silicon (100) substrates was investigated using THz time-domain spectroscopy. The applied magnetic field polarity dependence strongly suggests that THz emission originated from photo-carriers in the CSNWs. Optical excitation of the GaAs-AlGaAs core-shell yielded a wider THz emission bandwidth compared with that of just the GaAs core material. This result is currently attributed to faster carrier lifetimes in the AlGaAs shell. The THz emission spectral data are supported by time-resolved photoluminescence studies.

Fast-Scan Terahertz Time Domain Spectrometer Based on Laser Repetition Frequency Modulation

We propose a high-speed scanning terahertz (THz) time-domain spectrometer (TDS) based on optical sampling by repetition frequency modulation (OSREFM) using a repetition-frequency-tunable femtosecond fiber laser. Repetition frequency modulation is controlled by a highly stabilized external sweep generator. The fast scan of the time-delay between the pump and probe pulses is similar to an optical sampling with cavity tuning (OSCAT) THz-TDS system. However, the advantage of this technique is that it does not require physically moving components and uses only one laser system having an electronically modulated repetition rate. The time window and scanning time of this system depend on the modulation frequency range and the sweeping speed of the laser. In this study, we demonstrate the acquisition of a ~300 ps-window THz time waveform within 3 ms at a scan rate of 333 Hz.

Intense terahertz emission from molecular beam epitaxy-grown GaAs/GaSb(001)

Intense terahertz (THz) electromagnetic wave emission was observed in undoped GaAs thin films deposited on (100) n-GaSb substrates via molecular beam epitaxy. GaAs/n-GaSb heterostructures were found to be viable THz sources having signal amplitude 75% that of bulk p-InAs. The GaAs films were grown by interruption method during the growth initiation and using various metamorphic buffer layers. Reciprocal space maps revealed that the GaAs epilayers are tensile relaxed. Defects at the i-GaAs/n-GaSb interface were confirmed by scanning electron microscope images. Band calculations were performed to infer the depletion region and electric field at the i-GaAs/n-GaSb and the air-GaAs interfaces. However, the resulting band calculations were found to be insufficient to explain the THz emission. The enhanced THz emission is currently attributed to a piezoelectric field induced by incoherent strain and defects.

Terahertz Emission Enhancement in InAs Thin Films Using a Silicon Lens Coupler

Enhancement of the pulsed terahertz radiation generated from a lens-coupled InAs thin film excited by a femtosecond laser is reported. A Si hemispherical lens was used as a lens coupler and attached to the substrate-side of a 520-nm-thick InAs film, grown on a Si substrate. An enhancement factor of 7.5 times in the THz wave amplitude from the InAs film with the lens coupler was observed as compared with the bare InAs film. This enhancement is attributed to the improvement in the collimation condition of the radiated THz wave as it propagates through the index-matched lens coupler into free space.

Low-Frequency Coherent Raman Spectroscopy Using Spectral-Focusing of Chirped Laser Pulses

Vibrational spectroscopy is generally implemented using two schemes; that is, absorption spectroscopy and Raman spectroscopy. Conventionally, low frequency absorption spectroscopy is carried out using Fourier transform spectrometers equipped with a far infrared radiation source and a thermal detector. On the other hand, low frequency Raman spectroscopy is carried out by way of double or triple monochromaters and high-quality notch filters, whose performance determines the low frequency limit of the Raman spectrometer. In addition, in recent years, terahertz time-domain spectroscopy (THz-TDS) (Hangyo et al., 2005), utilizing femtosecond lasers as the excitation source, has been developed. THz-TDS enabled us to obtain absorption and dispersion spectra with a high signal-to-noise ratio in a frequency region less than 3 THz (100 cm-1) and can be applied for absorption spectroscopy of various substances (Kawase et al., 2009; Korter et al., 2006; Taday et al., 2003;Tani et al., 2004; Tani et al., 2010; Walther et al., 2003; Yamamoto et al., 2005; Yamaguchi et al., 2005) and imaging measurements (Kawase, 2004). There is a keen interest in low frequency vibrational spectroscopy for biomolecules since large amplitude, low frequency modes in macro biomolecules are believed to be associated with their respective function, as in the case of proteins (Chou, 1985, 1988). In order to fully understand the dynamics and function mechanisms of biomolecules, it is necessary to study their large amplitude and anharmonic low-frequency vibrational motions as these govern their thermal and physiochemical properties. A normal mode analysis of protein molecules revealed that large amplitude vibrational modes which are delocalized in the whole molecule lie within the THz region (< 120 cm-1) (Brooks & Karplus, 1985; Go et al., 1983). In addition, the calculations suggested that the entropy of the whole molecule, specifically its thermodynamic characteristics, is governed by the large amplitude vibrational modes in the sub-THz region (<30 cm-1). Consequently, important information related to the functions and dynamics of proteins can be derived by investigating THz vibration spectra.

Terahertz emission from Indium Oxide films grown on MgO substrates using sub-bandgap photon energy excitation

Indium oxide (In2O3) films grown by thermal oxidation on MgO substrates were optically excited by femtosecond laser pulses having photon energy lower than the In2O3 bandgap. Terahertz (THz) pulse emission was observed using time domain spectroscopy. Results show that THz emission saturates at an excitation fluence of ~400 nJ/cm2. Even as two-photon absorption has been excluded, the actual emission mechanism has yet to be confirmed but is currently attributed to carriers due to weak absorption from defect levels that are driven by a strain field at the interface of the substrate and the grown film.

Time-domain coherent Raman spectroscopy in THz frequency region

Time-domain coherent-Raman spectroscopy in the THz frequency region using a mode-locked femtosecond laser was studied. In this report, we attempted to detect the coherent Raman signal with an improved SNR owing to the averaging effect of the high-repetition frequency femtosecond laser.