Stevan Stanković

Silicon photonic devices and platforms for the mid-infrared

Due to its excellent electronic and photonic properties, silicon is a good candidate for mid-infrared optoelectronic devices and systems that can be used in a host of applications. In this paper we review some of the results reported recently, and we also present several new results on mid-infrared photonic devices including Mach-Zehnder interferometers, multimode interference splitters and multiplexers based on silicon-on-insulator, polysilicon, suspended silicon, and slot waveguide platforms.

Surface-Grating-Coupled Low-Loss Ge-on-Si Rib Waveguides and Multimode Interferometers

Germanium-on-silicon is a highly promising platform for planar photonics for the midinfrared, due to germaniums wide transparency range. In this letter, we report Ge-on-Si waveguides with record low losses of only 0.6 dB/cm, which is achieved using a 2.9-μm thick germanium layer, thus minimizing mode interaction with dislocations at the germanium/silicon interface. Using these waveguides, multimode interferometers with insertion losses of only 0.21 ± 0.02 dB are also demonstrated.

Silicon Photonic Waveguides and Devices for Near- and Mid-IR Applications

Silicon photonics has been a very buoyant research field in the last several years mainly because of its potential for telecom and datacom applications. However, prospects of using silicon photonics for sensing in the mid-IR have also attracted interest lately. In this paper, we present our recent results on waveguide-based devices for near- and mid-infrared applications. The silicon-on-insulator platform can be used for wavelengths up to 4 μm; therefore, different solutions are needed for longer wavelengths. We show results on passive Si devices such as couplers, filters, and multiplexers, particularly for extended wavelength regions and finally present integration of photonics and electronics integrated circuits for high-speed applications.

Mid-Infrared Thermo-Optic Modulators in SoI

We report experimental results for thermo-optic modulators in silicon-on-insulator (SoI) material operating at the wavelength of 3.8 μ m. These devices are based on asymmetric Mach-Zehnder interferometers (MZIs) with aluminum heaters placed above one MZI arm. The SoI rib waveguides with 400-nm Si device layer thickness are used. Devices with conventional straight MZI arm and spiral MZI arm geometries are investigated. Straight-arm MZIs exhibited higher modulation depths, of up to 30.5 dB, whereas spiral-arm MZIs required smaller switching powers, as low as 47 mW. Measured -3 dB bandwidths were up to 23.8 kHz and did not vary significantly with device configuration.

Silicon photonic waveguides for different wavelength regions

In this paper, we present our work on three silicon waveguide structures that are suitable for three different wavelength regions: near-, mid- and far-infrared. Design rules for standard rib SOI waveguides are given. Both single mode and polarization independence in these waveguides are discussed. A hollow-core waveguide suitable for gas-sensing applications in the mid-infrared wavelength region is also analysed. Finally, fabrication and experimental results for free standing waveguides, which may find application in the mid- and perhaps far-infrared wavelength regions, are presented.

Hybrid silicon lasers

Hybrid silicon lasers based on bonded III-V layers on silicon are discussed with respect to the challenges and trade-offs in their design and fabrication. Focus is on specific designs that combine good light confinement in the gain layer with good spectral control provided by grating structures patterned in silicon.

Si-rich silicon nitride for nonlinear signal processing applications

Nonlinear silicon photonic devices have attracted considerable attention thanks to their ability to show large third-order nonlinear effects at moderate power levels allowing for all-optical signal processing functionalities in miniaturized components. Although significant efforts have been made and many nonlinear optical functions have already been demonstrated in this platform, the performance of nonlinear silicon photonic devices remains fundamentally limited at the telecom wavelength region due to the two photon absorption (TPA) and related effects. In this work, we propose an alternative CMOS-compatible platform, based on silicon-rich silicon nitride that can overcome this limitation. By carefully selecting the material deposition parameters, we show that both of the device linear and nonlinear properties can be tuned in order to exhibit the desired behaviour at the selected wavelength region. A rigorous and systematic fabrication and characterization campaign of different material compositions is presented, enabling us to demonstrate TPA-free CMOS-compatible waveguides with low linear loss (~1.5 dB/cm) and enhanced Kerr nonlinear response (Re{γ} = 16 Wm−1). Thanks to these properties, our nonlinear waveguides are able to produce a π nonlinear phase shift, paving the way for the development of practical devices for future optical communication applications.

Cold Stress Dynamic Thermography for Evaluation of Vascular Disorders in Hand- Arm Vibration Syndrome

Exposure to hand-transmitted vibration can cause a variety of disorders collectively known as the Hand-Arm Vibration Syndrome (HAVS). Its neurovascular component is Vibration-induced White Finger (VWF), a type of secondary Raynaud’s phenomenon (RP), which manifests itself as episodic blanching of the fingers in response to cold. Due to the episodic nature of this condition, an occupational health physician rarely observes the blanching; thus, VWF has often been diagnosed based only on patient’s history together with a history of occupational exposure to vibration and the exclusion of other known causes of RP . For the assessment of VWF, measurements of finger skin temperature (FST) and finger systolic pressure (FSP) in response to cold stress are most widely used . The FST-based assessment relies on the principle that the pattern of FST following cooling reflects the degree of cold-induced vasoconstriction in the digital blood vessels. The rise in FST during recovery reflects the increase in blood flow in the investigated skin area. Abnormal rewarming times indicate different patterns of vascular dilatation following vasomotor responses to cooling. A lower FST is expected to reflect a persistent abnormality of blood flow in patients with HAVS 2, . Merla et al. described a technique that used infrared imaging to record the thermal recovery and produced images which visualized the τ times of individual pixels, assuming an exponential rewarming process ( τ being the time needed for rewarming to 63% of the total temperature change). The damaged areas exhibited a slower recovery and longer τ time. This was a novel approach in that it used dynamic parameter (Tau) imaging, but the parameter was derived as a cutoff-value, not from all the available rewarming data. Using a non-imaging infrared-based device, Foerster et al. also reported that a rewarming pattern could be described using the τ value. According to Darton and Black , thermographic images of the hands and rewarming curves after cold provocation show characteristic differences among patients with primary and secondary RP, and normal subjects. We propose a novel method of dynamic infrared thermography for assessing the dynamic response of the microcirculation during rewarming.

Material and optical properties of low-temperature NH3-free PECVD SiNx layers for photonic applications

SiN x layers intended for photonic applications are typically fabricated using LPCVD and PECVD. These techniques rely on high-temperature processing (>400 °C) to obtain low propagation losses. An alternative version of PECVD SiN x layers deposited at temperatures below 400 °C with a recipe that does not use ammonia (NH3-free PECVD) was previously demonstrated to be a good option to fabricate strip waveguides with propagation losses <3 dB cm−1. We have conducted a systematic investigation of the influence of the deposition parameters on the material and optical properties of NH3-free PECVD SiN x layers fabricated at 350 °C using a design of experiments methodology. In particular, this paper discusses the effect of the SiH4 flow, RF power, chamber pressure and substrate on the structure, uniformity, roughness, deposition rate, refractive index, chemical composition, bond structure and H content of NH3-free PECVD SiN x layers. The results show that the properties and the propagation losses of the studied SiN x layers depend entirely on their compositional N/Si ratio, which is in fact the only parameter that can be directly tuned using the deposition parameters along with the film uniformity and deposition rate. These observations provide the means to optimise the propagation losses of the layers for photonic applications through the deposition parameters. In fact, we have been able to fabricate SiN x waveguides with H content <20%, good uniformity and propagation losses of 1.5 dB cm−1 at 1550 nm and <1 dB cm−1 at 1310 nm. As a result, this study can potentially help optimise the properties of the studied SiN x layers for different applications.

Grating coupled low loss Ge-on-Si waveguides and multimode interferometers for the mid-infrared

Germanium-on-silicon is a promising platform for planar photonics over the entire mid-infrared range. We report here grating coupled Ge-on-Si waveguides with record low losses of 0.6dB/cm, multimode interferometers and Mach-Zehnder interferometers.