Manuel Koechlin

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.

Electro-optic single-crystalline organic waveguides and nanowires grown from the melt

Organic nonlinear optical materials have proven to possess high and extremely fast nonlinearities compared to conventional inorganic crystals, allowing for sub-1-V driving voltages and modulation bandwidths of over 100 GHz. Compared to more widely studied poled electro-optic polymers, organic electro-optic crystals exhibit orders of magnitude better thermal and photochemical stability. The lack of available structuring techniques for organic crystals has been the major drawback for exploring their potential for photonic structures. Here we present a new approach to fabricate high-quality electro-optic single crystal waveguides and nanowires of configurationally locked polyene DAT2 (2-(3-(2-(4-dimethylaminophenyl)vinyl)-5,5-dimethylcyclohex-2-enylidene)malononitrile). The high-index-contrast waveguides (delta(n) = 0.54 +/- 0.04) are grown from the melt between two anodically bonded borosilicate glass wafers, which are structured and equipped with electrodes prior to bonding. Electro-optic phase modulation is demonstrated for the first time in the non-centrosymmetric DAT2 single crystalline channel waveguides at a wavelength of 1.55 microm. We also show that this technique in combination with DAT2 material allows for the fabrication of single-crystalline nanostructures inside large-area devices with crystal thicknesses below 30 nm and lengths of above 7 mm.

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.

Electro-optic tuning and modulation of single-crystalline organic microring resonators

We present, for the first time to our knowledge, the fabrication and electro-optic (EO) tuning of single-crystalline organic microring resonators. In recent years, optical microring resonators have proven to be highly suitable building blocks for the realization of very large-scale integrated photonic circuits. In particular, microresonators based on organic materials are very promising for ultrafast EO applications, due to the electronic nature of the EO response preserving the modulation performances beyond 100 GHz. In contrast to polymer waveguiding structures realized previously, our crystalline thin-film devices feature an excellent long-term stability of the chromophore orientation and superior photochemical stability, and they do not require high-field poling prior to operation. The introduced thin-film fabrication method significantly reduces fabrication complexity of organic crystalline EO waveguides, compared to previously developed techniques. We have fabricated crystalline COANP (2-cyclo-octylamino-5-nitropyridine) microring resonators with resonance contrast up to 10 dB, ring waveguide propagation losses of about 10 dB/cm, a free spectral range of 1.6 nm, a finesse of up to 20, and a corresponding Q-factor of about 20,000, measured in the telecom wavelength range around 1.55 μm. We have demonstrated resonance wavelength tuning at the rate of 0.13 GHz/V(1.1 pm/V).

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.

Integrated electro-optic devices of melt-processable single-crystalline organic films

Organic electro-optic (EO) materials are the materials of choice for high speed optical modulators with modulation frequencies greater than 100 GHz. This is due to the large EO effects observed and a low material dispersion of the dielectric constant resulting in a very small velocity mismatch between the optical and electrical waves. However, the implementation of organic materials into real devices has been hindered by several factors such as an insufficient long-term thermal and photochemical stability of the widely investigated poled polymers or the lack of available structuring techniques for the inherently superior organic EO crystalline materials. Here we report on the realization of integrated organic EO single-crystalline Mach-Zehnder modulators by a recently developed melt based channel growth technique. The main fabrication concept is to grow the organic EO singlecrystals from the melt directly in pre-structured and electroded waveguide channels, which were obtained by standard optical lithographic techniques and wafer bonding. By this method single crystal structure details with a size below 30 nm have been achieved and the growth of single-crystalline Mach-Zehnder modulators has been successfully demonstrated, where we have chosen DAT2 (2-(3-(2-(4-dimethylaminophenyl)vinyl)-5,5- dimethylcyclohex-2-enylidene)malononitrile) as EO material. The half-wave voltage × length product determined in the DAT2 based Mach-Zehnder modulators has been found to be 78 ± 2 Vcm for TE-modes and 60 ±1 Vcm for TM-modes at a wavelength of 1.55 μm. The accuracy and reproducibility of the process allowed also for the realization of the first EO single-crystalline microring resonator in an organic material.

Microring Resonators and Photonic Crystal Structures in Ion-Sliced LiNbO 3 Thin Films

We report on the realization of electro-optically tunable microring resonators and photonic crystal structures in ion-sliced lithium niobate thin films.

Single-crystalline organic electro-optic microring filters and modulators

Polymeric materials as well as organic crystals have generally higher and faster nonlinearities compared to the inorganic standard lithium niobate and are therefore ideally suited for high-speed modulators. Compared to poled polymers, organic single crystals are advantageous because of superior long-term thermal and photochemical stability combined with a higher chromophore concentration. Despite of several promising microfabrication techniques developed for organic single crystals (for a recent review see [1]), their growth as well as their structuring with adequate quality and precision for optical applications has remained challenging. We recently developed a new fabrication technique in which the melt of the organic material flows into predefined channels by capillary force and crystallizes there upon cooling. By this method it has already been shown that the growth of high-quality single-crystalline phase modulators from the melt is possible [2].