Masazumi Fujiwara

Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal.

Widely tunable terahertz (THz)-wave generation using difference frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. To our knowledge, this is the first report of THz-wave generation by BNA DFG. Large, high-quality single crystals of BNA (phi 8 mm x 30 mm) were grown using the vertical Bridgman method. The nonlinear optical (NLO) coefficient d(33) of the BNA crystal is approximately 234 pm/V, which is the largest value reported for any yellow NLO material. The collinear phase-matching condition of the type-0 configuration is satisfied using a 0.7-1 microm band pump wavelength. We generated THz waves using an organic BNA crystal; the generation range is 0.1-15 THz.

Optimized terahertz-wave generation using BNA-DFG

An effective and widely tunable means of producing terahertz (THz) radiation using difference-frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated by optimizing the pump wavelengths. We calculated the (wideband) refractive index and coherence length mapping of BNA to establish the optimum phase-matching condition of the DFG configuration. To satisfy the phase-matching conditions over the range 0.1-20 THz, both pump wavelengths were varied from 780 to 950 nm, and the optical wavelength dependency of the THz-wave output was clearly observed. We expanded the range of THz-wave generation and increased the power output to ten times that obtained using previous methods.

Second Order Nonlinear Optical Properties of the Single Crystal of N-Benzyl 2-methyl-4-nitroaniline: Anomalous Enhancement of the d333 Component and Its Possible Origin

A single crystal of N-benzyl 2-methyl-4-nitroaniline (BNA) whose dimensions were 8×10 mm (diameter × length) was grown using a vertical Bridgman method with sufficient optical transparency for spectroscopic use. Polarized reflection spectra of the crystal were measured in order to determine its linear optical parameters (refractive indices and extinction coefficients). The relative magnitudes of the d-tensor components of the crystal were determined to be d333=(17±9)×d322, d322>d311, d311~0 from polarization dependencies of the second harmonic generation. These results are in disagreement with previously reported theoretical studies, in which the effects of intermolecular interactions were completely neglected. Here, we have included these intermolecular interactions in the calculations assuming the presence of supra-molecular clusters. We conclude that the anomalous enhancement of d333 is due to the interactions along the direction of the hydrogen bonds in the crystal and the resonance effect.

Simultaneous reference and differential waveform acquisition in time-resolved terahertz spectroscopy

We present a new method for data acquisition in time-resolved terahertz spectroscopy experiments. Our approach is based on simultaneous collection of reference and differential THz scans. Both the optical THz generation beam and the pump beam are modulated at two different frequencies that are not harmonic with respect to each other. Our method allows not only twice as fast data acquisition but also minimization of noise connected to slowly varying laser power fluctuations and timing instabilities. Our use of the nonlinear crystal N-benzyl-2-methyl-4-nitroaniline (BNA) enables time-resolved THz spectroscopy to beyond 5 THz, thereby highlighting that the presented method is especially valuable at higher frequencies where phase errors in the data acquisition become increasingly important.

All Organic Terahertz Electromagnetic Wave Emission and Detection Using Highly Purified N-Benzyl-2-methyl-4-nitroaniline Crystals

We studied terahertz (THz) electromagnetic wave emission and detection using highly purified N-benzyl-2-methyl-4-nitroaniline (BNA) crystals. In THz emission experiments, we compared the efficiencies of THz emission via optical rectification for BNA, ZnTe, and GaP crystals, and confirmed that the BNA crystal is the most efficient THz emitter among these three crystals within the frequency range limits for observation. In the THz detection experiment, we also compared the modulation efficiencies of the BNA and ZnTe crystals, and found that the ZnTe crystal has a higher efficiency than the BNA crystal. However, the BNA crystal has a higher efficiency than the ZnTe crystal in high-frequency regions. Since the BNA crystal is suitable for detecting a high-frequency region even when using thick crystals, it is expected that the BNA crystal as a THz detector will be easy to prepare and handle.

Determination of Refractive Indices and Absorption Coefficients of Highly Purified N-Benzyl-2-methyl-4-nitroaniline Crystal in Terahertz Frequency Regime

We have measured the refractive index and absorption coefficient of the N-benzyl-2-methyl-4-nitroaniline crystal using terahertz time-domain spectroscopy. We found that the refractive index had a value between 1.5 and 1.9 in the direction and between 1.9 and 2.3 in the direction. We also found that there were resonances at 1.6 and 3.2 THz in the direction, and 1.3, 2.2, and 3.6 THz in the direction within the observed spectral range between 0 and 5 THz. A Lorentz-type oscillator model is applicable to explain our experimental observations.

Four-wave mixing signals from β-carotene and its n = 15 homologue

The third-order nonlinear optical responses of β-carotene and its homologue having a conjugation-double bond nxa0=xa015 have been investigated using sub-20 fs ultra-short optical pulses in order to clarify the dissipation processes of excess energy. Using the four-wave mixing spectroscopy, we observed a clear coherent oscillation with a period of a few tens of femtoseconds. The spectral density of these molecules was estimated that allowed the theoretical linear and nonlinear optical signals to be directly compared with the experimental data. Calculations based on the Brownian oscillator model were performed under the impulsive excitation limit. We show that the memory of the vibronic coherence generated upon the excitation into the S2 state is lost via the relaxation process including the S1 state. The vibronic decoherence lifetime of the system was estimated to be 1xa0ps, which is about 5xa0times larger than the life time of the S2 state (∼150xa0fs) determined in previous studies. The role of coherence and the efficient energy transfer in the light-harvesting antenna complexes are discussed.

Coherent tunable monochromatic Terahertz-wave generation using N-Benzyl-2-methyl-4-nitroaniline (BNA) crystal

Widely tunable terahertz (THz) -wave generation using difference frequency generation (DFG) in an organic N-Benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. An organic nonlinear optical (NLO) BNA crystal is one of the promising materials for efficient and strong THz-wave generation because of its potential to have a sufficiently large enough second-order optical nonlinearity. Large and high quality single crystals of BNA (Φ8×30mm) were successfully grown by a vertical Bridgman method. The NLO coefficient d33 of BNA crystal is about 230pm/V. It is the largest value reported for any yellow-colored NLO materials. BNA has low refractive index dispersion between the optical and THz-wave region, therefore the colinear phase matching condition of the Type0 configuration is satisfied by using 0.7~1μm band pump wavelength. So, we developed a near-infrared dual-wavelength pump source for BNA-DFG. Two KTiOPO4 (KTP) crystals were mounted on galvano scanners inside a double-pass optical parametric oscillator (OPO). It is pumped using a frequency-doubled Nd:YAG laser (532 nm, 8 ns, 100 Hz). The signal wave of the KTP-OPO output was controlled independently and rapidly using a galvano scanner. We successfully generated THz-wave using organic BNA crystal. The THz-wave generation range is from 0.1 to 15THz, while the pumping dual-wavelength is controlled in the 0.8-0.9μm range.

Specific Channel of Energy Dissipation in Carotenoids: Coherent Spectroscopic Study

We investigate transient grating signals in β-carotene homologues by using sub-20-fs optical pulses. The results clearly show that shorter-chain carotenoids have a specific major channel of energy dissipation to the environment (the central C=C stretching mode) in the groundstate vibrational manifold, whereas the longer-chain carotenoids do not.

0.1–15THz generation using BNA (N-benzyl-2-methyl-4-nitroaniline) crystal

We demonstrate widely tunable terahertz-wave generation using difference frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal. The frequency tuning of the BNA-DFG was obtained between 0.1 and 15 THz.