Shinsuke Umegaki

Surface-emitting distributed-feedback dye laser of a polymeric multilayer fabricated by spin coating

A distributed-feedback laser was demonstrated by using a fluorescent dye-doped polymer. The periodic structure of a one-dimensional photonic crystal was fabricated through spin coating and alternate stacking of high and low refractive-index polymers. The rhodamine-6G-doped multilayer showed a photonic band gap in the transmissivity spectrum at the fluorescent wavelength range. Laser emission from the surface of the multilayer was attained at the band edge for a pump of the second harmonic of a neodymium-doped yttrium aluminum garnet laser. Wavelength tuning was possible by spin coating a multilayer onto a concave substrate.

A Nonlinear Optical Organic Crystal for Waveguiding SHG Devices: (-)2-(α-Methylbenzylamino)-5-Nitropyridine (MBANP)

A nonlinear optical organic material, (-)2-(α-methylbenzylamino)-5-nitropyridine (MBANP), has been comprehensively investigated with the objective of application to frequency doubling of laser diodes. Its molecular hyperpolarizability is as large as that of p-nitroaniline. MBANP crystallizes with a highly asymmetric molecular arrangement. The largest nonlinear optical coefficient, d22, is measured to be 60 pm/V. The corresponding figure of merit for frequency doubling at 1.064 µm is 3.5 times larger than that for d33 of LiNbO3. Cerenkov-radiation-type, phase-matched second-harmonic generation has been demonstrated utilizing the d22 coefficient by making use of an appropriate waveguiding structure.

Generation of blue coherent light from a continuous‐wave semiconductor laser using an organic crystal‐cored fiber

Efficient frequency doubling of a continuous‐wave semiconductor laser was attained by using a nonlinear optical fiber with a single crystal core of an organic 3,5‐dimethyl‐1‐ (4‐nitrophenyl)pyrazole. The second‐harmonic power generated with a 15 mm length of the fiber reached 0.16 mW from the incident 16.6‐mW fundamental laser power at 884 nm. Nonlinear optical properties of the crystal and a fabrication technique of the fiber are also reported.

Characteristics of optical second-harmonic generation due to Čerenkov-radiation-type phase matching

Cerenkov-radiation-type optical second-harmonic generation is analyzed by using the Green function of the Helmholtz equation, including the nonlinear-polarization term. The numerical results agree well with those of previously reported experiments. A formulation is given that provides a means of designing frequency doublers, which make use of channel waveguides and optical fibers.

Theoretical analysis of Cerenkov-type optical second-harmonic generation in slab waveguides

Optical second-harmonic generation in the Cerenkov-radiation scheme in slab waveguides has been theoretically investigated for all the possible type I configurations. By solving the nonlinear wave equations analytically, a simple general expression for the second-harmonic power has been obtained. Characteristics of the Cerenkov-type second-harmonic generation and optimum conditions for attaining high conversion efficiencies are discussed on the basis of the theoretical analysis. It is shown that, in order to attain high conversion efficiencies, off-diagonal elements of the nonlinear optical coefficient tensor should be utilized taking advantage of birefringence. >

Diffraction efficiency increase by corona discharge in photoinduced surface-relief gratings on an azo polymer film

The diffraction efficiency of a surface-relief grating (SRG) was dramatically increased by corona discharge. We fabricated this SRG by applying surface modulation to azo polymer films, using laser interference fringes. The electric charge was deposited upon the SRG by a corona discharge in an oven. The first-order diffraction efficiency measured before and after corona discharge above the glass-transition temperature increased from ~2% to ~40% in a SRG written with a circularly polarized beam. The relief depth of the SRG as measured with an atomic-force microscope also increased, from ~130 nm to ~450 nm. The increase in diffraction efficiency was independent of the writing polarization.

Refractive Indices of 2-Methyl-4-Nitroaniline (MNA)

We report on the results of ellipsometric measurements of refractive indices and their dispersion over the wavelength range 500 nm-1100 nm on a single-crystal thin film of 2-methyl-4-nitroaniline (MNA). The refractive indices show large anisotropy attributable to the orientation of the molecular dipole moment.

Multiple-Reflection Effects in Optical Second-Harmonic Generation

We report the first observation of multiple-reflection effects in optical second-harmonic generation. The observation has been made possible by the use of a single-mode laser diode as a light source. The experimental results agree excellently with a theoretical analysis which takes account of the multiple reflections of both the fundamental and harmonic waves.

Modified method of electric‐field induced second‐harmonic generation

Second‐order hyperpolarizabilities β of organic nonlinear optical materials were measured by a modified method of the electric‐field induced second‐harmonic generation. Since this method avoided interference between two second‐harmonic waves from a solution and cell‐window glass, β of a dilute solution was easily determined and at the same time, a third‐order hyperpolarizability γ of a solvent was also evaluated. The obtained β values were constant in a wide range of the concentration.

Theory of optical second-harmonic generation in crystal-cored fibers based on phase matching of Čerenkov-type radiation

A general formulation of optical second-harmonic generation in crystal-cored fibers is described. Although pump depletion of the fundamental wave and anisotropy of the core crystal are ignored, the theory and its numerical results provide a practical means of designing frequency doublers that make use of nonlinear-optical fibers. Comparison with previous results is also provided.