BNA Yields Widely Tunable THz Source
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BNA Yields Widely Tunable THz Source The terahertz (THz) region has attracted much attention in recent years for its numerous potential spectroscopic applications in astronomy, earth science, biology, and medicine, as well as applications in communications and national security. However, in most cases, THz sources are limited in their tunability requiring a relative large number of different source systems when trying to cover a wide frequency range. Only organic nonlinear optical (NLO) materials have shown promise for wide tunability. K. Miyamoto and H. Minamide at RIKEN Sendai, M. Fujiwera and H. Hashimoto at Osaka City University, and H. Ito at RIKEN Sendai and Tohoku University, Sendai, have demonstrated a widely tunable THz-wave generation system using difference-frequency generation (DFG) in N-benzyl- 2-methy l-4-nitroaniline (BNA) which has a large tuning capability from 0.1 THz to 15 THz and the highest NLO coefficient d33 for any yellow NLO material with 234 pm/V. As described in the February 1 issue of Optics Letters (p. 252), researchers at RIKEN Sendai, Japan prepared large BNA crystals (ϕ8×30 mm) using the vertical Bridgman method. BNA is an N-derivative of 2-methyl-4-nitroaniline (MNA). BNA was developed to replace MNA because of the difficulty in growing large crystals of MNA due to its unfavorable morphology and thermal decomposition behavior as well as its hygroscopicity. With the development of BNA, the researchers were able to eliminate these drawbacks. In addition, BNA exhibits large second-order optical nonlinearity and low refractive index dispersion between the optical and the THz region. The researchers excited DFG in BNA using a double-crystal KTiOPO4 (KTP) optical parametric oscillator (OPO) pumped by a frequency-doubled Nd:YAG laser. The maximum energy was measured at 1.1 mJ/pulse at dual wavelength (887/902 nm). The conversion efficiency was nearly 11%, corresponding to a quantum efficiency of 19% with a peak power of 160 kW and an oscillation threshold of 2.5 mJ/pulse (~5 MW/cm2). By controlling the KTP OPO at high speed, the BNA can jump to any THz frequency without scanning through the intermediate wavelengths and at the same time permits continuous-frequency scanning in the 0.1–15 THz range by mixing the output of the KTP OPO over 860–902 nm. The researchers said that an ultrawide band (0.1–40 THz) THz-wave source is achievable by combining BNA
(0.1–15 THz) and 4-dimethylamino-Nmethyl-1-4-stilbazolium-tosylate (DAST) (which the researchers had previously shown to be tunable over the range 1.5–40 THz) organic nonlinear optical crystals. ALFRED A. ZINN
Microstructured Monolithic Dielectric Surface Provides High Optical Reflectivity and Low Mechanical Loss Optomechanical systems have attracted more and more research interest in many fields of physics. The interface between the light field and the solid-state matter needs to provide high reflectivity and low mechanical losses to enable applications such as laser cooling of mechanical oscillators, optical traps for mirrors, generation o
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