Gorazd Poberaj

Electro-optically active microring resonators in lithium niobate

Highly integrated photonic devices, which are predestined to replace electronic circuits for signal processing applications in the future, gained a growing interest in the last years. Among them, switchable microring resonators have the potential to be used in commercial optical integrated circuits, as they allow compact devices and a number of new functionalities. We present the fabrication of compact electro-optically tunable microring resonators in lithium niobate and discuss their performance in the telecommunication wavelength range around 1.55 µm.

Photonic crystal structures in ion-sliced lithium niobate thin films

We report on the first realization of photonic crystal structures in 600-nm thick ion-sliced, single-crystalline lithium niobate thin films bonded on a lithium niobate substrate using adhesive polymer benzocyclobutene (BCB). Focused ion beam (FIB) milling is used for fast prototyping of photonic crystal structures with regular cylindrical holes. Unwanted redeposition effects leading to conically shaped holes in lithium niobate are minimized due to the soft BCB layer underneath. A high refractive index contrast of 0.65 between the lithium niobate thin film and the BCB underlayer enables strong light confinement in the vertical direction. For TE polarized light a triangular photonic crystal lattice of air holes with a diameter of 240 nm and a separation of 500 nm has a photonic bandgap in the wavelength range from 1390 to 1500 nm. Experimentally measured transmission spectra show a spectral power dip for the GK direction of the reci ocal lattice with an extinction ratio of up to 15 dB. This is in good agreement with numerical simulations based on the three-dimensional plane wave expansion (PWE) and the finite-difference time-domain (FDTD) method.

Latent heat flux profiles from collocated airborne water vapor and wind lidars during IHOP_2002

Abstract Latent heat flux profiles in the convective boundary layer (CBL) are obtained for the first time with the combination of the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) water vapor differential absorption lidar (DIAL) and the NOAA high resolution Doppler wind lidar (HRDL). Both instruments were integrated nadir viewing on board the DLR Falcon research aircraft during the International H2O Project (IHOP_2002) over the U.S. Southern Great Plains. Flux profiles from 300 to 2500 m AGL are computed from high spatial resolution (150 m horizontal and vertical) two-dimensional water vapor and vertical velocity lidar cross sections using the eddy covariance technique. Three flight segments on 7 June 2002 between 1000 and 1300 LT over western Oklahoma and southwestern Kansas are analyzed. On two segments with strong convection, the latent heat flux peaks at (700 ± 200) W m−2 in the entrainment zone and decreases linearly to (200 ± 100) W m−2 in the lower CBL. A water vapor budget analysis reveals that ...

Optical microring resonators in fluorineimplanted lithium niobate.

We report on the production and characterisation of optical microring resonators and optical channel waveguides by using fluorine-ion implantation and planar structuring in lithium niobate. We demonstrate the production of single-mode planar waveguides by low fluence fluorine-ion implantation (?? = 2.5 x10(14) ions/cm(2)) into lithium niobate wafers. The waveguides are strongly confined by the amorphous 2-microm wide optical barrier induced by the implantation process. A refractive index contrast of Deltan(o) = 0.17 at the telecom wavelength lambda = 1.5 microm has been determined between the waveguide and the barrier. Planar structuring with ridge height of up to 1.2 microm has been achieved by laser lithography masking and Ar(+) sputtering. For TE waves, the channel waveguides exhibit propagation losses lower than 8 dB/cm. First ring resonators with 80-microm radius have been fabricated by planar structuring in fluorine-ion implanted lithium niobate. The measured resonance curves show an extinction ratio of 14 dB, a free spectral range of 2.0 nm and a finesse of 4.

Micromachining of ridge optical waveguides on top of He+-implanted β-BaB2O4 crystals by femtosecond laser ablation

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.

Second harmonic generation of continuous wave ultraviolet light and production of β-BaB2O4 optical waveguides

We report on the generation of continuous-wave (cw) ultraviolet (UV) laser light at λ=278nm by optical frequency doubling of visible light in β-BaB2O4 waveguides. Ridge-type waveguides were produced by He+ implantation, photolithography masking, and plasma etching. The final waveguides have core dimension of a few μm2 and show transmission losses of 5dB∕cm at 532nm and less than 10dB∕cm at 266nm. In our first experiments, a second harmonic power of 24μW has been generated at 278nm in an 8mm long waveguide pumped by 153mW at 556nm.

Free-Standing Lithium Niobate Microring Resonators for Hybrid Integrated Optics

We report on the fabrication of free-standing microrings using ion-sliced lithium niobate thin films bonded by benzocyclobutene on a lithium niobate substrate. The microrings can be detached from the benzocyclobutene layer using standard clean-1 solution and transferred onto any host substrate. This approach is suitable for building hybrid integrated optics devices with laterally and vertically coupled lithium niobate microring resonators. Gallium nitride was identified as a suitable material for the port waveguides in such a device due to the refractive index similar to lithium niobate. In our study, we transferred lithium niobate microrings on top of gallium nitride waveguides and aligned them for vertical coupling using a micropositioning tool. Transverse-electric-wave transmission spectra of the microring resonators with a radius of 20 ¿m exhibited a free spectral range of 8.1 nm and a finesse of ~ 23 at 1.55- ¿m wavelength. The resonance dips in these spectra showed an extinction ratio of up to 12 dB.

High-resolution laser lithography system based on two-dimensional acousto-optic deflection

We present an advanced high-resolution, compact laser lithography system for fast prototyping of complex integrated optics devices comprising microring resonators and photonic crystal structures. Precise and flexible structuring of photoresist patterns is achieved by combing three linear stages (xyz) for sample positioning and a two-dimensional acousto-optical deflector for laser beam steering and intensity control. A continuous wave diode laser operating at a wavelength of 375 nm is used to illuminate all types of photoresists including SU-8. Using a microscope objective with a numerical aperture of 1.40, structure widths of approximately 200 nm can be obtained. The write-field covered by acousto-optic deflection can be as large as 200x200 microm(2) when using an objective with a focal length of 4.5 mm. With a two-step lithography process, gaps as small as 150 nm between adjacent structures have been achieved, yielding superior photoresist masks for microring resonators with coupling ports.

Ultraviolet electro-optic amplitude modulation in β-BaB2O4 waveguides

The authors report on the realization of an electro-optical amplitude modulator based on β-BaB2O4 (BBO) waveguide operating in the ultraviolet wavelength range down to 257nm. The ridge waveguides are produced by He+ implantation, photolithography masking, and plasma etching. Electrodes are deposited by the standard lift-off technique. A minimum half-wave voltage times modulator length Vπ×1 of 43Vcm has been demonstrated at 257nm in a 6.5-mm-long modulator. Because of BBO’s broad transparency range (190–3500nm), such device could be employed in the deep ultraviolet with Vπ×1 estimated to reach 24Vcm at 200nm.

Optical waveguides in Sn(2)P(2)S(6) by low fluence MeV He+ ion implantation.

Optical waveguides in nonlinear crystals of Sn₂P₂S₆ have been produced by He⁺ ion implantation. Best results are obtained with a fluence about Phi = 0.5 middot 10¹⁵ ions/cm² and for hybrid _n1-mode. The depth of the induced optical barrier is Delta_n1 = -0.07 at lambda = 0.633mum.