Masaya Nagai

Attenuated Total Reflection Spectroscopy in Time Domain Using Terahertz Coherent Pulses

We present a novel technique for time-domain attenuated total reflection spectroscopy in the terahertz (THz) frequency region. The advantage of this technique is that it can be used to access samples with various shapes and optical properties without changing the optical setup. The technique is applied to determine the complex dielectric constants in InAs and distilled liquid water.

Generation and detection of terahertz radiation by electro-optical process in GaAs using 1.56μm fiber laser pulses

We report the generation and detection of a terahertz wave with a nonresonant electro-optical crystal using 1.56μm femtosecond laser pulses. Using a 0.5-mm-thick (110)-oriented GaAs wafer for optical rectification and electro-optical modulation, we detected spectral sensitivity below 3THz. We also estimated the availability of GaAs from the viewpoint of an electro-optical coefficient and phase match condition.

Enhancement of terahertz wave generation by cascaded χ (2) processes in LiNbO 3

We demonstrate, to the best of our knowledge, a novel generation process of a terahertz (THz) electromagnetic wave via a noncollinear χ(2) process in LiNbO3 with a phase-front-controlled femtosecond Ti:sapphire laser pulse. Systematic measurements of both the THz electric field and the excitation transmitted through an LiNbO3 crystal at different excitation powers show that the maximum amplitude of the THz electric field increases nonlinearly above the critical input powers. This enhancement of optical rectification efficiency is attributed to the distortion of the excitation pulse shape via electro-optic (EO) phase modulation by the emitted THz wave. Actually, spectral line shapes of both the THz wave and the excitation pulse become broadened above the threshold, and they are well reproduced by numerical simulations with equivalent one-dimensionally mapped coupled equations in the frequency domain. This generation scheme shows potential optimization of further generation efficiency.

Ultrabroadband coherent electric field from far infrared to 200 THz using air plasma induced by 10 fs pulses

We generated an ultrabroadband infrared pulse ranging from far infrared to 200 THz through a plasma by focusing a hollow-fiber compressed intense 10-fs pulse and its second harmonic in air. We coherently detected the signal up to 100 THz with electro-optic sampling and clarified its drastic dependence on the orientation of the second harmonic crystal in a range of 100–200 THz with an HgCdTe detector. From these, we confirmed the whole frequency components originated from the AC biased plasma and were phase locked. This result opens the possibility of a pump-probe spectroscopy which covers the whole infrared range.

THz Nonlinear Spectroscopy of Solids

We present a review of the recent progress in the generation methods of intense terahertz (THz) single-cycle pulses and their application to THz nonlinear spectroscopy in condensed matters. Special attentions are paid to various aspects of nonlinearity in semiconductors including dynamical Franz-Keldysh effect, ballistic acceleration of free carriers, and coherent control of the exciton system.

Broadband and high power terahertz pulse generation beyond excitation bandwidth limitation via χ (2) cascaded processes in LiNbO 3

We proposed a novel THz generation technique beyond the limitation of the input optical pulse width, based on phase modulation via cascaded chi((2)) process. When intense THz electric field generated by optical rectification lies in electro-optic (EO) crystal, emitted THz field gives phase modulation to the optical excitation pulse. The phase modulation causes excitation pulse narrowing and consequently gives rise to the enhancement of conversion efficiency and THz wave bandwidth broadening. We experimentally realize this generation technique with high chi((2)) EO crystal LiNbO(3) and with subpicosecond pulse from Yb-doped fiber laser. It opens new concept of THz technologies.

Destructive interference effect on surface plasmon resonance in terahertz attenuated total reflection

We investigate the surface plasmon resonance at the interface between air and n-type (100) oriented-InAs as an active material with a time-domain attenuated total reflection technique with coherent terahertz pulses. The characteristic spectra of the attenuated total reflectivity and phase shift caused by surface plasmon are observed in the Otto configuration. The surface plasmon resonance frequency and the phase jump strongly depend on the wave vector of the evanescent wave, the refractive index of the prism, and the incident angle of the terahertz pulses and the distance between the prism and active material. These features can no longer be explained with conventional Ottos approximation. We show that the interference effect between the electromagnetic wave reflected at the prism-air interface and that reemitted from excited surface plasmon plays a key role in the surface plasmon resonance.

Dynamical Franz–Keldysh effect in GaAs/AlGaAs multiple quantum wells induced by single-cycle terahertz pulses

We studied absorption spectra near the band gap of GaAs/AlGaAs multiple quantum wells subjected to strong single-cycle terahertz pulses. With an increasing terahertz electric field, the heavy-hole 1s exciton energy near the band gap redshifts, reaches a maximum, and turns to blueshift. The blueshift increases proportionally to the ponderomotive energy, i.e., the terahertz electric field squared, which differs from the response expected from the Frantz–Keldysh effect (FKE) in a dc electric field. This fact suggests that the observed ponderomotive energy dependence of the blueshift can be ascribed to the dynamical FKE.

Evaluation of effective electric permittivity and magnetic permeability in metamaterial slabs by terahertz time-domain spectroscopy.

We established a novel method to evaluate effective optical constants by terahertz (THz) time domain spectroscopy and suggested a strict definition of optical constants and an expression for electromagnetic energy loss following the second law of thermodynamics. We deduced the effective optical constants of phosphor bronze wire grids in the THz region experimentally and theoretically. The results depend strongly on the polarization of the THz waves. When the electric field is parallel to the wires, we observed Drude-like electric permittivities with a plasma frequency reduced by a factor of 10(-3), whereas when the field is perpendicular, the sample behaved as a simple dielectric film. We also observed unexpected magnetic permeabilities, which originate from the non-resonant real magnetic response of finite size-conductors.

Extremely Thin Metamaterial as Slab Waveguide at Terahertz Frequencies

We investigate the waveguiding properties of a planar metamaterial slab using terahertz time-domain attenuated total reflection spectroscopy. The enhancement of evanescent waves is observed for transverse electric and transverse magnetic excitation and is caused by resonant excitation of waveguide modes in the slab. Our calculation describes the experimental results and justifies the extremely small effective thickness. We also studied the dispersion relations of the waveguide modes of the slab by theoretical calculation.