Development of polycrystalline silicon waveguides by laser crystallization

Silicon (Si) is an excellent material for integrated photonics devices as its high refractive index allows for small device footprints. To date, most of the work in this area has leveraged the single crystal silicon-on-insulator platforms, which are relatively expensive to produce and thus drive up component costs. Here we propose an alternative method to fabricate crystalline silicon waveguides by laser processing of an amorphous starting material. As well as reducing production costs, this approach has the added advantage of removing the substrate dependence so that more flexible alternatives can be considered. This method has previously been applied to a-Si wires grown inside silica capillaries and shown to produce very large crystallites [1]. Here we demonstrate preliminary results of laser-induced crystallization of a-Si films and micro-patterned wires produced by chemical vapor deposition (CVD) on SiO2 substrates. The samples have been crystallized using a c.w. argon-ion laser at 488nm. Crystallized tracks have been written by scanning the focused beam across the samples using different laser intensities and scanning speeds. The resulting material quality is then studied using Raman spectrometry, optical and electronic microscopy and X-ray diffraction. For the planar films, we have produced crystallite sizes on the order of hundreds of nanometers to a few microns; similar to those obtained via conventional pulsed Excimer laser crystallization [2]. However, for the micro-patterned samples, we have found that it is possible to grow crystals that almost cover the entire width of the wire, over lengths of up to 18µm, considerably larger than what is typically reported for polysilicon waveguide devices [3]. Furthermore, this laser crystallization method has been observed to reform the surface of the Si wires resulting in very smooth sidewall profiles (as shown in Fig. 1) which is very important for low loss optical transmission in photonic devices

A silicon/lithium niobate hybrid photonic material platform produced by laser processing

Silicon (Si) and lithium niobate (LiNbO3) are two materials that are synonymous with the electronics and photonics industries respectively and are supported by a significant amount of technological know-how. It has been suggested and demonstrated recently that Si could also be used for the production of integrated photonic devices, however its performance can be limited by the transmission cutoff at short wavelengths, a relatively high two-photon absorption, and a zero second order nonlinear optical susceptibility. LiNbO3 on the other hand is a very good dielectric material with very little electronic functionality and high second order nonlinearity. Thus, as these two materials have complementary properties, there is significant merit in combining them into a single hybrid system that will benefit from the properties of its constituents, as demonstrated via direct bonding in [1]. Here we propose a route for producing such a hybrid material system via local laser processing of a low cost, easy to produce amorphous silicon (a-Si) film deposited onto a single crystal LiNbO3 substrate. This research is based on recent encouraging results of a laser based crystallization process obtained in a-Si core optical fibres that not only produced crystallites with very large aspect ratios, but also allowed for tuning of the Si bandgap [2].The emphasis of this laser-processing route has been on achieving structures with large crystals and low surface roughness in order to obtain good photonic and electronic device performance. Interestingly it was revealed that, apart from the expected local crystallization of the a-Si film, this particular system exhibited a plethora of interesting and potentially useful effects including the direct formation of optical waveguides in LiNbO3, enabled ferroelectric domain reversal and the spontaneous formation of periodic structural features on the Si film, shown in the figure below

Laser processing of amorphous silicon on lithium niobate for photonic applications

Silicon (Si) and lithium niobate (LiNbO3) are two materials that are synonymous with the electronics and photonics industries respectively and are supported by a significant amount of technological know-how. It has been suggested and demonstrated recently that Si could also be used for the production of integrated photonic devices, however its performance can be limited by the transmission cutoff at short wavelengths, a relatively high two-photon absorption, and lack of second order nonlinear optical susceptibility. LiNbO3 on the other hand is a very good dielectric material with high second order nonlinearity but with very little electronic functionality. It can be envisaged however that these two materials have complementary properties therefore there is significant merit in combining them into a single hybrid system that will benefit from the properties of its constituents as demonstrated in [1] on a directly bonded single crystal hybrid. In this contribution we will present results on laser processing of amorphous silicon films deposited on LiNbO3 and other substrates suggesting a new route for the fabrication of Si based photonic circuits. This research is based on recent encouraging results of a laser based crystallization process obtained in a-Si core optical fibres that not only obtained crystallites with very large aspect ratio but also allowed for tuning of the Si bandgap [2]. &more..

Polycrystalline silicon waveguides for integrated photonics

Silicon photonics is an expanding domain of research since its booming a decade ago. Leveraging decades of research and development from the electronics industry, silicon photonics is the ideal candidate to overcome the bottleneck that microelectronics is facing with regard to the interconnect bandwidth limitations. In addition to being a platform compatible for both photonics and electronics, silicon is transparent in the mid-infrared regime, has a high refractive index for tight light confinement (i.e., small footprints), and presents a high nonlinear refractive index that is of high interest for optical signal processing applications. However, the integration of a silicon photonic layer is still challenging due to either the deposition flexibility or the material quality. In this thesis, a technique is presented to integrate a silicon layer with the deposition flexibility of amorphous silicon and the material quality of crystalline silicon, whilst being low-cost and having a thermal budget of < 450 °C making it compatible with the CMOS technology. Using a laser treatment, the as-deposited amorphous silicon is locally crystallised into polycrystalline silicon, a composite material of made of crystalline silicon crystallites surrounded by an amorphous silicon matrix. Both film and wire structures are processed and the material quality is assessed through optical microscopy, Raman spectroscopy, and X-ray diffraction. The optical quality of the polycrystalline wire waveguides is also investigated in the linear and nonlinear regime. In parallel to the planar silicon photonics geometry, silicon core fibres are also investigated in this work. These novel fibres offer the capability to integrate the functionality of silicon photonics platforms directly inside fibre architectures. Amorphous core fibres can be drawn with the lowest losses but as for the planar geometry, the material lacks electronics capabilities. On the other hand, polycrystalline silicon core fibres, which are suitable for both optical and electrical applications, can be drawn but their propagation losses remain high. In this work, two silicon core fibre material improvement methods, based on laser recrystallisation and tapering, are presented. To assess the material improvement, fibres are analysed through optical microscopy, Raman spectroscopy and X-ray diffraction. Finally, the optical losses of the improved fibres are measured on an optical transmission setup

Strategies for wideband light generation in nonlinear multimode integrated waveguides

In this paper we discuss two strategies to achieve wideband light generation through intermodal nonlinear parametric processes in multimode integrated waveguides. We outline how the interplay among intermodal interactions and high dispersion may lead to the generation of light characterized by substantial power spectral density if compared to supercontinuum sources. These results are valid independently of the waveguide material, however our numerical simulations focus on silicon waveguides, which are nowadays at the core of integrated photonics. The long-term vision is that, by exploiting an adequate number of intermodal processes, widely tunable radiation having high-power spectral density can be generated in a broad portion of the transparency window of silicon

Laser-assisted material composition engineering of SiGe planar waveguides

We report the compositional engineering of silicon-germanium planar microstructures through laser processing. The effects of the laser treatment are assessed through microscope imaging and Raman spectroscopy. Our results reveal that the laser-exposed regions display a significant change in the material composition

Hot-wire chemical vapor deposition low-loss hydrogenated amorphous silicon waveguides for silicon photonic devices

We demonstrate low-loss hydrogenated amorphous silicon (a-Si:H) waveguides by hot-wire chemical vapor deposition (HWCVD). The effect of hydrogenation in a-Si at different deposition temperatures has been investigated and analyzed by Raman spectroscopy. We obtained an optical quality a-Si:H waveguide deposited at 230°C that has a strong Raman peak shift at 480 cm−1, peak width (full width at half-maximum) of 68.9 cm−1, and bond angle deviation of 8.98°. Optical transmission measurement shows a low propagation loss of 0.8 dB/cm at the 1550 nm wavelength, which is the first, to our knowledge, report for a HWCVD a-Si:H waveguide. </p

CO₂ laser-induced directional recrystallization to produce single crystal silicon-core optical fibers with low loss

Reduced losses in silicon-core fibers are obtained using CO2 laser directional recrystallization of the core. Single crystals with aspect ratios up to 1500:1 are reported, limited by the scan range of the equipment. This processing technique holds promise for bringing crystalline silicon-core fibers to a central role in nonlinear optics and signal processing applications

Laser processed semiconductors for integrated photonic devices

We report results of laser processing of amorphous silicon and silicon-germanium semiconductor materials for the production of integrated photonic platforms. As the materials are deposited and processed at low temperatures, they are flexible, low cost, and suitable for multi-layer integration with other photonic or electronic layers. We demonstrate the formation of waveguides via crystallization of pre-patterned silicon components and functional microstructures through crystallization and compositional tuning of silicon-germanium alloy films. These results open a route for the fabrication of high density, multi-functional integrated optoelectronic chips