Optical microresonators have recently attracted a growing attention in the photonics community. 1 Their applications range from quantum electro-dynamics to sensors and filtering devices for optical telecommunication systems, where they are likely to become an essential building block. 2 The integration of nonlinear and electro-optical properties in the resonators represents a very stimulating challenge, as it would incorporate new and more advanced functionality. Lithium niobate is an excellent candidate material, being an established choice for electro-optic and nonlinear optical applications. Here we report on the first realization of optical microring resonators in submicrometric thin films of lithium niobate. The high index contrast films are produced by an improved crystal ion slicing and bonding technique using benzocyclobutene. The rings have radius R = 100 µm and their transmission spectrum has been tuned using the electro-optic effect. These results open new perspectives for the use of lithium niobate in chip-scale integrated optical devices and nonlinear optical microcavities.The established use of wavelength division multiplexed (WDM) for local area network systems has raised the demand for new filtering and switching functions. 3 In order to integrate these devices on a wafer scale, whispering gallery mode microresonators represent the most compact and efficient solution. They consist of a bus waveguide evanescently coupled to a micrometer-size ring resonator; the characteristic size-dependent frequency spectrum of the ring allows only selected wavelength channels to be transmitted or shifted to another waveguide. Small radii allow a large free spectral range -i.e. large separation between the filtered channels -but increase the propagation bending losses, 4 which can compromise the quality factor Q -i.e. the wavelength selectivity -of the device. To overcome this limitation, high refractive index contrast between the ring core and the surrounding materials is mandatory. A second, very important, requirement relates to the tunability. The possibility to electrically control the transmission spectrum, via electro-optic effect, would allow extremely compact and ultrafast modulation and switching. By integrating arrays of microring resonators on a single optical chip, the realization of complex functions would be feasible. 5 Besides, large-Q resonators based on non centro-symmetric materials would exploit the high amount of stored energy for enhancing the efficiency of nonlinear optical phenomena. 6 Several examples of microring resonators have been proposed and successfully realized in the last years in a variety of materials like semiconductors, 7-10 silica 11 and polymers. 12,13 The advanced structuring technology in semiconductor materials enables the realization of very high-Q resonators even for radii as small as 10 µm. Silicon-based resonators can be tuned by electrically-driven carriers arXiv:0705.2392v1 [physics.optics]
We show that electrostatic effects have a dramatic influence on thermal diffusion of charged micelles. In the dilute regime, the Soret coefficient strongly decreases with the solution ionic strength, and scales as the square of the Debye-Hückel length. Yet, collective effects yield a reversed scenario even at fairly low surfactant concentration. We find that single-particle behavior can be explained using an interfacial tension mechanism proposed by Ruckenstein, which also fairly accounts for collective effects and opens the way to a general picture of thermal diffusion in disperse systems.
We report on the refractive indices of Sn2P2S6 crystals in the wavelength range 550 - 2300 nm. The measurements are performed at room temperature using the minimum deviation method. The dispersion is described by a two oscillator model yielding the oscillator energies and strengths (Sellmeier parameters) for all polarization directions. The rotation of the indicatrix in the mirror plane and the direction of the optical axes have also been determined in the wavelength range lambda= 550-2200 nm.
We report on a technique for the fabrication of ridge optical waveguides on top of β-BaB2O4 (BBO) crystals. The BBO crystals were first implanted by He+ ions to form planar optical waveguides. In the second step, the femtosecond laser ablation technique was employed for micromachining of ridge-type optical waveguides. A thorough study of material-specific ablation parameters for BBO has been performed in order to achieve ablated structures with smooth sidewalls. A further process of Ar+ ion smoothing in a plasma chamber was used to reduce the sidewall roughness of the ablated ridges from 75 to 35 nm root mean square. We demonstrated optical waveguiding in these femtosecond-ablated plasma-treated waveguides and measured total propagation losses of less than 10 dB∕cm at 532 nm, making them suitable for nonlinear- and electro-optical applications.
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