Spencer Novak

Mid-infrared materials and devices on a Si platform for optical sensing

Abstract In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiNx waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Effect of annealing conditions on the physio-chemical properties of spin-coated As 2 Se 3 chalcogenide glass films

Thin film selenide glasses have emerged as an important material for integrated photonics due to its high refractive index, mid-IR transparency and high non-linear optical indices. We prepared high-quality As2Se3 glass films using spin coating from ethylenediamine solutions. The physio-chemical properties of the films are characterized as a function of annealing conditions. Compared to bulk glasses, as-deposited films possess a distinctively different network structure due to presence of Se-Se homo-polar bonds and residual solvent. Annealing partially recovers the As-Se3 pyramid structure and brings the film refractive indices close to the bulk value. Optical loss in the films measured at 1550 nm wavelength is 9 dB/cm, which was attributed to N-H bond absorption from residual solvent.

Composition dependence of the viscosity and other physical properties in the arsenic selenide glass system

The viscosity of the AsxSe100−x family of glasses has been measured for 10 ≤ x ≤ 40 using beam bending and parallel plate viscometry, and fit with the Vogel-Fulcher-Tamann (VFT) viscosity model. Measurement of other physical properties of the glasses, including the density, glass transition temperature, and coefficient of thermal expansion has been conducted in order to accurately calculate the viscosity as a function of temperature and glass composition. The variation in fragility of the glasses is explained in the context of frozen-in configurational entropy in the glasses. This configurational entropy has minima at the endpoints of the one-dimensional network of amorphous selenium and the fully three-dimensional network of As40Se60, and an apparent maximum at the composition As30Se70. The frozen-in configurational entropy can be well described by a modified entropy of mixing of two solid solutions model, implying that the topological contribution to configurational entropy is nearly constant across the...

Heterogeneous microring and Mach-Zehnder modulators based on lithium niobate and chalcogenide glasses on silicon

Thin films of lithium niobate are wafer bonded onto silicon substrates and rib-loaded with a chalcogenide glass, Ge(23)Sb(7)S(70), to demonstrate strongly confined single-mode submicron waveguides, microring modulators, and Mach-Zehnder modulators in the telecom C band. The 200 μm radii microring modulators present 1.2 dB/cm waveguide propagation loss, 1.2 × 10(5) quality factor, 0.4 GHz/V tuning rate, and 13 dB extinction ratio. The 6 mm long Mach-Zehnder modulators have a half-wave voltage-length product of 3.8 V.cm and an extinction ratio of 15 dB. The demonstrated work is a key step towards enabling wafer scale dense on-chip integration of high performance lithium niobate electro-optical devices on silicon for short reach optical interconnects and higher order advanced modulation schemes.

Incorporation of luminescent CdSe/ZnS core-shell quantum dots and PbS quantum dots into solution-derived chalcogenide glass films

CdSe/ZnS core-shell quantum dots (CSQDs) and PbS quantum dots (QDs) were synthesized using a colloidal method and incorporated into Ge23Sb7S70 glass films via a solution-derived approach to film formation. Photoluminescence (PL) from the QDs inside the glass matrix was observed in the visible (CdSe/ZnS) and near-IR (PbS) regions. Properties of the QDs were found to be environment dependent, with the amine solvent partially quenching the luminescence. The PL lifetime of the CdSe/ZnS CSQDs and PbS QDs in the glass film was decreased to varying degrees from that of the QDs in pure chloroform. Monitoring the steady-state PL intensity and luminescence lifetime of PbS doped films showed that appropriate heat treatment of the deposited film increases the luminescence efficiency by removing residual solvent from the glass matrix.

Low-loss, submicron chalcogenide integrated photonics with chlorine plasma etching

A chlorine plasma etching-based method for the fabrication of high-performance chalcogenide-based integrated photonics on silicon substrates is presented. By optimizing the etching conditions, chlorine plasma is employed to produce extremely low-roughness etched sidewalls on waveguides with minimal penalty to propagation loss. Using this fabrication method, microring resonators with record-high intrinsic Q-factors as high as 450 000 and a corresponding propagation loss as low as 0.42 dB/cm are demonstrated in submicron chalcogenide waveguides. Furthermore, the developed chlorine plasma etching process is utilized to demonstrate fiber-to-waveguide grating couplers in chalcogenide photonics with high power coupling efficiency of 37% for transverse-electric polarized modes.

Chalcogenide glass-on-graphene photonics

Two-dimensional (2D) materials are of tremendous interest to integrated photonics, given their singular optical characteristics spanning light emission, modulation, saturable absorption and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. Here, we present a new route for 2D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material that can be directly deposited and patterned on a wide variety of 2D materials and can simultaneously function as the light-guiding medium, a gate dielectric and a passivation layer for 2D materials. Besides achieving improved fabrication yield and throughput compared with the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light–matter interactions in the 2D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared waveguide-integrated photodetectors and modulators.Exploiting the peculiar properties of graphene, a series of high-performance glass-on-graphene devices, such as polarizers, thermo-optic switches and mid-infrared waveguide-integrated photodetectors and modulators are realized.

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films.

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge23Sb7S70 and As40S60, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 °C and 200 °C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As40S60 film as compared to Ge23Sb7S70. Based on the advantages of electrospray, direct printing of a gradient refractive index (GRIN) mid-IR transparent coating is envisioned, given the difference in refractive index of the two compositions in this study.

Direct Electrospray Printing of Gradient Refractive Index Chalcogenide Glass Films.

A spatially varying effective refractive index gradient using chalcogenide glass layers is printed on a silicon wafer using an optimized electrospray (ES) deposition process. Using solution-derived glass precursors, IR-transparent Ge23Sb7S70 and As40S60 glass films of programmed thickness are fabricated to yield a bilayer structure, resulting in an effective gradient refractive index (GRIN) film. Optical and compositional analysis tools confirm the optical and physical nature of the gradient in the resulting high-optical-quality films, demonstrating the power of direct printing of multimaterial structures compatible with planar photonic fabrication protocols. The potential application of such tailorable materials and structures as they relate to the enhancement of sensitivity in chalcogenide glass based planar chemical sensor device design is presented. This method, applicable to a broad cross section of glass compositions, shows promise in directly depositing GRIN films with tunable refractive index profiles for bulk and planar optical components and devices.

On-chip infrared spectroscopic sensing: Redefining the benefits of scaling

Infrared (IR) spectroscopy is widely recognized as a gold standard technique for chemical analysis. Recent strides in photonic integration technologies offer a promising route towards enabling miniaturized, rugged platforms for IR spectroscopic analysis. Here we show that simple size scaling by replacing bulky discrete optical elements used in conventional IR spectroscopy with their on-chip counterparts is not a viable route for on-chip infrared spectroscopic sensing, as it cripples the system performance due to the limited optical path length accessible on a chip. In this context, we discuss two novel photonic sensor designs uniquely suited for microphotonic integration. We leverage strong optical and thermal confinement in judiciously designed microcavities to circumvent the thermal diffusion and optical diffraction limits in conventional photothermal sensors and achieve parts-per-billion level gas molecule limit of detection. In the second example, an on-chip spectrometer design with Fellgetts advantage is proposed for the first time. The design enables sub-nm spectral resolution on a millimeter-sized, fully packaged chip without mechanical moving parts.