Carlito Ponseca

Transmission of terahertz radiation using a microstructured polymer optical fiber

A hollow-core microstructured polymer optical fiber was analyzed in the terahertz (THz) region. Spectral analysis of time domain data shows propagation of THz waves in both the hollow-core and the microstructured cladding with a time delay of approximately 20 ps. The frequency range and shift of the transmission bands between different sized waveguides suggested photonic bandgap or resonant guidance. Finite-difference time domain calculations agree relatively well to the experimental transmission results. Propagation losses were estimated to be as low as 0.9 dB/cm.

Low-loss single-mode terahertz waveguiding using Cytop

A polymer-based, Cytop planar photonic crystal waveguide (PPCW) was designed for guiding terahertz radiation. Results indicate that the propagation and coupling losses in the 0.2–1.1THz range are relatively small for a sheetlike thickness design. Spectral analysis of the transmission data reveals frequency selectivity of the PPCW. Calculations of the spatial distribution of the terahertz electric field through the waveguide show evidence of single-mode propagation at a 0.45THz central frequency. The highly transparent nature of Cytop from deep ultraviolet to the far infrared region indicates its potential use as an integral component in hybrid optics.

Magnetic-field-induced fourfold azimuthal angle dependence in the terahertz radiation power of (100) InAs

The azimuthal angle dependence in the terahertz radiation power of (100) InAs under 1T magnetic field is presented. Results show that although the dominant radiation mechanism is surge current, azimuthal-angle-dependent radiation due to the nonlinear effect is also observed. The twofold symmetry of the p-polarized terahertz radiation power was modified to a fourfold symmetry with the transverse magnetic field. Moreover, results exhibited fourfold symmetry for the s-polarized terahertz power even with no applied field. The anisotropic intervalley scattering of photocarriers is tentatively proposed as the origin of quadrupole response and the fourfold emission symmetry.

Birefringence of β-BaB2O4 crystal in the terahertz region for parametric device design

β-BaB2O4 (BBO) was found to have huge birefringence in the 0.1–1.1THz frequency region. Polarized terahertz transmission spectroscopy revealed a Δn∕n value of about 0.12±0.02. This result could be related to a polarization-dependent absorption that was observed at about 0.65THz; in close agreement with first principles-calculations, showing this to be due to low-frequency phonon modes of the [B3O6]3-rings. These findings suggest the utilization of the birefringent properties of BBO for optical parametric devices including amplifiers and oscillators, operating in the terahertz regime.

Low-frequency dynamics of biological molecules studied by terahertz time-domain spectroscopy

By terahertz (THz) time-domain spectroscopy we have measured low-frequency spectra of amino acid (glycine; Gly), short peptides ((Gly)3 and (Gly)4), six globular proteins and bacteriorhodopsin (BR). From the analysis of the THz spectra we defined and obtained the reduced absorption cross sections for these cases, which are proportional to the vibrational density of states. We observed anharmonic behaviors in the low-frequency modes of the short peptides. The globular proteins we investigated show a universal feature in the low-frequency spectra. BR shows the dynamical transition in the temperature dependence of the THz spectrum when the sample is hydrated.

Transmission characteristics of lens-duct and photonic crystal waveguides in the terahertz region

Transmission characteristics of lens-duct, Cytop planar photonic crystal waveguide (PPCW) and hollow-core microstructured optical fiber were analyzed in the terahertz (THz) region. The lens duct was able to channel and couple the THz radiation into a Teflon waveguide with a loss of about 0.7 dB. Single-mode propagation and frequency-selective properties were achieved using PPCW whose central frequency is at 0.45 THz. Results of time-domain spectra of the hollow-core microstructured polymer optical fiber showed a difference of about 20 ps between the THz waves that propagated in the core and cladding. Frequency shift of the transmission bands between waveguides of different size suggested photonic bandgap guidance. Finite-difference time domain calculations agreed relatively well to the experimental results of PPCW and hollow-core fibers.

Transverse magnetic field polarity effects on the terahertz radiation from GaAs/AlGaAs modulation-doped heterostructures with varying AlGaAs spacer-layer thickness

We report the effect of changing the polarity of a magnetic field parallel to the surface plane of GaAs/AlGaAs modulation-doped heterostructures (MDHs) with various spacer thicknesses on the terahertz radiation power and its spectral characteristics. Results show that flipping the direction of the transverse 1 T magnetic field modifies the extent of field-induced terahertz radiation enhancement. The observations are analyzed in the context of junction electric field strength, carrier confinement and mobility, and the AlGaAs/GaAs interface roughness. This terahertz method may prove as an efficient tool to qualitatively evaluate the characteristics of MDH layers.

Numerical Approach for Determination of Cutback Length to Estimate Dispersion and Loss Parameters of Terahertz Waveguides

We present a numerical scheme for the determination of the optimum fiber length in the cutback method to estimate the dispersion and loss parameters in terahertz waveguides. From a numerical simulation study and observations of the propagation of terahertz waves in wave guides, we found that the optimum fiber length giving the most accurate estimates of absorption and/or dispersion parameters obeys the empirical power law. We expect that the characterization of terahertz waveguides using this scheme will aid the precise control of the dispersion and loss parameters of waveguides in large volume fabrication.

Low-frequency spectra of amino acids and peptides probed by terahertz time-domain spectroscopy

Low-frequency spectra of amino acids and peptides of L-alanine and glycine were analyzed from the power law behavior of its absorption cross section. For amino acids the exponent around two were observed and interpreted as a collection of harmonic vibrations. However for peptides with longer chain i.e. (Gly)4 and (L-ala)5 exponents were reduced to 1.43 and 1.34, respectively. This behavior shows chain length dependence of the low frequency dynamics.

Terahertz birefringence of β-BaB2O4 (BBO) crystal

Birefringence was observed for a beta-BaB2O4 crystal in the 0.1 to 1.1 terahertz region. The calculated Deltan value was ~0.296. Moreover, an angle-dependent absorption band observed at ~0.65 terahertz is attributed to low-frequency phonon modes.