Anu Agarwal

Fabrication and testing of planar chalcogenide waveguide integrated microfluidic sensor

We have fabricated and tested, to the best of our knowledge, the first microfluidic device monolithically integrated with planar chalcogenide glass waveguides on a silicon substrate. High-quality Ge(23)Sb(7)S(70) glass films have been deposited onto oxide coated silicon wafers using thermal evaporation, and high-index-contrast channel waveguides have been defined using SF(6) plasma etching. Microfluidic channel patterning in photocurable resin (SU8) and channel sealing by a polydimethylsiloxane (PDMS) cover completed the device fabrication. The chalcogenide waveguides yield a transmission loss of 2.3 dB/cm at 1550 nm. We show in this letter that using this device, N-methylaniline can be detected using its well-defined absorption fingerprint of the N-H bond near 1496 nm. Our measurements indicate linear response of the sensor to varying N-methylaniline concentrations. From our experiments, a sensitivity of this sensor down to a N-methylaniline concentration 0.7 vol. % is expected. Given the low-cost fabrication process used, and robust device configuration, our integration scheme provides a promising device platform for chemical sensing applications.

Si-CMOS-compatible lift-off fabrication of low-loss planar chalcogenide waveguides

We demonstrate, for the first time to the best of our knowledge, low-loss, Si-CMOS-compatible fabrication of single-mode chalcogenide strip waveguides. As a novel route of chalcogenide glass film patterning, lift-off allows several benefits: leverage with Si-CMOS process compatibility; ability to fabricate single-mode waveguides with core sizes down to submicron range; and reduced sidewall roughness. High-index-contrast Ge(23)Sb(7)S(70) strip waveguides have been fabricated using lift-off with excellent uniformity of loss propagation and the lowest loss figure of reported to date. We also show that small core Ge(23)Sb(7)S(70) rib waveguides can be fabricated via lift-off as well, with loss figures lower than 0.5 dB/cm. Additionally, we find through waveguide modal analysis that although overall transmission loss is low, the predominant source of this loss comes from scattering at the sidewalls.

Demonstration of chalcogenide glass racetrack microresonators

We have demonstrated what we believe to be the first chalcogenide glass racetrack microresonator using a complementary metal-oxide semiconductor-compatible lift-off technique with thermally evaporated As(2)S(3) films. The device simultaneously features a small footprint of 0.012 mm x 0.012 mm, a cavity Q (quality factor) of 10,000, and an extinction ratio of 32 dB. These resonators exhibit a very high sensitivity to refractive index changes with a demonstrated detection capability of Dn(As(2)S(3)=(4.5 x 10(-6)+/-10%) refractive index unit. The resonators were applied to derive a photorefractive response of As(2)S(3) to lambda=550 nm light. The resonator devices are a versatile platform for both sensing and glass material property investigation.

Optical loss reduction in high-index-contrast chalcogenide glass waveguides via thermal reflow

A thermal reflow technique is applied to high-index-contrast, sub-micron waveguides in As(2)S(3) chalcogenide glass to reduce the sidewall roughness and associated optical scattering loss. We show that the reflow process effectively decreases sidewall roughness of chalcogenide glass waveguides. A kinetic model is presented to quantitatively explain the sidewall roughness evolution during thermal reflow. Further, we develop a technique to calculate waveguide optical loss using the roughness evolution model, and predict the ultimate low loss limit in reflowed high-index-contrast glass waveguides. Up to 50% optical loss reduction after reflow treatment is experimentally observed, and the practical loss limiting factors are discussed.

Cavity-Enhanced IR Absorption in Planar Chalcogenide Glass Microdisk Resonators: Experiment and Analysis

Planar microdisk optical resonators fabricated from Ge23Sb7S70 chalcogenide glass on a silicon substrate are applied for cavity-enhanced spectroscopic measurement of chemical molecular absorption fingerprint. A 0.02 cm- 1 detection limit for these devices is demonstrated. This detection limit represents a threefold improvement as compared to a straight waveguide sensor, while the physical device length is reduced by 40-fold. The reduction in device footprint with enhanced sensitivity makes the structure attractive for ldquosensor-on-a-chiprdquo device applications. We also present a design optimization approach for cavity-enhanced IR absorption spectroscopy using traveling-wave resonators, which indicates that further performance improvement can be achieved in optimally coupled, low-loss resonant cavities.

Low-loss high-index-contrast planar waveguides with graded-index cladding layers.

We experimentally demonstrate, for the first time, propagation loss reduction via graded-index (GRIN) cladding layers in high-index-contrast (HIC) glass waveguides. We show that scattering loss arising from sidewall roughness can be significantly reduced without compromising the high-index-contrast condition, by inserting thin GRIN cladding layers with refractive indices intermediate between the core and topmost cover of a strip waveguide. Loss as low as 1.5 dB/cm is achieved in small core (1.6 mum x 0.35 mum), high-index-contrast (Deltan = 1.37) arsenic-based sulfide strip waveguides. This GRIN cladding design is generally applicable to HIC waveguide systems such as Si/SiO2.

Development of novel integrated bio/chemical sensor systems using chalcogenide glass materials

This paper reviews ongoing progress in the design and fabrication of new, on-chip, low loss planar molecular sensors. We report the details of device design, material selection and manufacturing processes used to realise high-index-contrast (HIC), compact micro-disk resonators. These structures have been fabricated in thermally evaporated As- and Ge-based chalcogenide glass films with PDMS (polydimethylsiloxane) micro-fluidic channels using standard UV lithography. Discussed are findings that demonstrate that our novel chalcogenide-based micro-fluidic device can be used as highly sensitive refractive index sensors.

Low-loss integrated planar chalcogenide waveguides for microfluidic chemical sensing

Chalcogenide glasses are an ideal material candidate for evanescent biochemical sensing due to their mid and far-infrared transparency. We have fabricated and tested, to the best of our knowledge, the first microfluidic device monolithically integrated with planar chalcogenide glass waveguides. High-index-contrast channel waveguides have been defined using plasma etching in thermally evaporated Ge23Sb7S70 films, followed by microfluidic channel patterning in photocurable resin (SU8) and channel sealing by a polydimethylsiloxane (PDMS) cover. Using this device, N-methylaniline can be detected using its well-defined absorption fingerprint of the N-H bond near 1496 nm. Our measurements indicate linear response of the sensor to varying N-methylaniline concentrations and a sensitivity of this sensor down to N-methylaniline concentration of 0.7 vol. %. Thermal reflow has been employed as an effective method to smooth chalcogenide waveguide sidewall roughness from 6.1 nm to 0.56 nm. Given the low-cost fabrication process and robust device configuration, our integration scheme provides a promising device platform for infrared chemical sensing applications.

Design, Fabrication and Integration of HIC Glass Waveguides on a Silicon Platform

We demonstrate Si-CMOS-compatible lift-off fabrication of chalcogenide glass waveguides monolithically integrated on a silicon platform. As a novel route of glass film patterning, lift-off allows several benefits: leverage with Si-CMOS process compatibility; ability to fabricate single-mode waveguides with core sizes down to submicron range; reduced sidewall roughness; and wide applicability to other non-silica glass compositions. High-index-contrast (HIC) single-mode strip waveguides have been fabricated with from several glass target compositions including Ge23Sb7S70, As2S3, As36Ge6S58, As36Sb6S58 and TeO2. We measured Ge-Sb-S waveguides with low loss and excellent wafer-scale uniformity. We have experimentally demonstrated propagation loss reduction via graded-index (GRIN) cladding layers in HIC glass waveguides. These efforts have shown that scattering loss arising from sidewall roughness can be significantly reduced without compromising the high-index-contrast condition by inserting thin GRIN cladding layers with refractive indices intermediate between the core and topmost cover of a strip waveguide.