Hongtao Lin

Carbonaceous Nanofiber Membranes for Selective Filtration and Separation of Nanoparticles

The assembly of 1D nanomaterials (e. g., nanowires, nanotubes, etc.) into 2D macroscopic membranes has attracted much research interest in recent years, [ 1–4 ] as these nanofi brous membranes possess many novel properties and have great potential applications in many fi elds, including as electromechanical actuators, [ 5 ] as optoelectronic devices, [ 6 ] as selective adsorbents for oil spill cleanup, [ 7 ] as gas sensors, [ 8 ] and as fi lters for gas or liquid fi ltration and separation. [ 9 ] In particular, when used as fi lters, the fi brous membranes have several unique advantages, i.e.. high porosity, good fl exibility, large surface area per unit volume, and interconnected open pore structure. [ 10 ] These characteristics make them attractive for fi ltration applications. The current fi brous fi ltering media were mostly based on non-woven polymer mats obtained by electrospinning techniques. [ 9 , 10 ] The diameters of electrospun fi bers are generally in the range of micrometers or sub-micrometers, thus fi brous membranes made by them have a large pore size and can only remove microparticles from solution. In view of a practical application, however, fi brous fi lters with nanometerscale pores are in demand because there are many nanometerscale pollutant particles and viruses needed to be removed from water. [ 11 , 12 ] In order to produce drinking water from ground and surface water with high performance, much effort is needed to fabricate membrane fi lters that consist of fi bers in the nanometer range ( < 100 nm). Another drawback of electrospun fi bers is their poor diameter control, which leads to a wide distribution of pore size of the fi lter. In fact, it was found that not only the average fi ber size but the distribution of the fi ber sizes is related to the fi ltering effi ciency. [ 13 ] These results indicate the necessity of the synthesis of highly uniform nanofi bers for constructing fi lters with a narrow pore size distribution. Although a few 1D nanostructures, such as carbon nanotubes, [ 11 , 14 ] inorganic nanowires, [ 4 , 15 , 16 ] and polymer nanowires, [ 17 ] have been used to construct membranes for the fi ltration of nanometer-scale particles or viruses from solution, it is diffi cult to regulate the diameter of the 1D nanostructure.

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.

Integrated flexible chalcogenide glass photonic devices

Photonic integration on plastic substrates enables emerging applications ranging from flexible interconnects to conformal sensors on biological tissues. Such devices are traditionally fabricated using pattern transfer, which is complicated and has limited integration capacity. Here we pioneered a monolithic approach to realize flexible, high-index-contrast glass photonics with significantly improved processing throughput and yield. Noting that the conventional multilayer bending theory fails when laminates have large elastic mismatch, we derived a mechanics theory accounting for multiple neutral axes in one laminated structure to accurately predict its strain-optical coupling behavior. Through combining monolithic fabrication and local neutral axis designs, we fabricated devices that boast record optical performance (Q=460,000) and excellent mechanical flexibility enabling repeated bending down to sub-millimeter radius without measurable performance degradation, both of which represent major improvements over state-of-the-art. Further, we demonstrate that our technology offers a facile fabrication route for 3-D high-index-contrast photonics difficult to process using traditional methods.

Flexible integrated photonics: where materials, mechanics and optics meet [Invited]

While the vast majority of integrated photonic devices are traditionally fabricated on rigid substrates, photonic integration of both passive and active photonic devices on flexible polymer substrates has been demonstrated in recent years, and its applications in imaging, sensing and optical interconnects are being actively pursued. This paper presents an overview of the emerging field of mechanically flexible photonics, where we examine material processing and mechanical design rationales dictated by application-specific optical functionalities. The examples include semiconductor nanomembranes which serve as the key enabling material for hybrid inorganic-organic flexible active photonics, and monolithically integrated passive photonic structures fabricated from semiconductors, polymers, or amorphous materials. Technical challenges and further research opportunities related to materials engineering and device integration on flexible substrates are also discussed.

Demonstration of high-Q mid-infrared chalcogenide glass-on-silicon resonators

We demonstrated high-index-contrast, waveguide-coupled As2Se3 chalcogenide glass resonators monolithically integrated on silicon fabricated using optical lithography and a lift-off process. The resonators exhibited a high intrinsic quality factor of 2×10(5) at 5.2 μm wavelength, which is among the highest values reported in on-chip mid-infrared (mid-IR) photonic devices. The resonator can serve as a key building block for mid-IR planar photonic circuits.

Double resonance 1-D photonic crystal cavities for single-molecule mid-infrared photothermal spectroscopy: theory and design

We propose a mid-infrared spectroscopic technique capable of detecting a single small molecule without using cryogenically cooled detectors. Such record sensitivity is attained by leveraging dramatically amplified photothermal effects in a pump-probe doubly resonant cavity.

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.

Si-CMOS compatible materials and devices for mid-IR microphotonics

CMOS compatible mid-Infrared (mid-IR) microphotonics including (1) broadband SOUP (Silicon on Oxide Undercladding Pedestal) waveguides; and (2) mid-IR transparent chalcogenide glass (ChGs) waveguides monolithically integrated with a PbTe thin film photodetector; are demonstrated. Using a pedestal undercladding geometry we obtain an optical loss for our Si waveguide which is 10 dB/cm lower compared to other waveguides using planar SiO2 cladding at λ = 5 µm, and a fundamental mode is seen over a broad mid-IR spectral range. To realize a fully integrated mid-IR on-chip system, in parallel, we develop PbTe thin film detectors that can be deposited on various mid-IR platforms through a thermal evaporation technique, offering high photoresponsivity of 25 V/W from λ = 1 µm to 4 µm. The detector can be efficiently integrated, using a suitable spacer, to an underlying Chalcogenide glass (ChGs) waveguide. Our results of low loss waveguides and integrated thin film detectors enable Si-CMOS microphotonics for mid-IR applications.

Demonstration of mid-infrared waveguide photonic crystal cavities

We have demonstrated what we believe to be the first waveguide photonic crystal cavity operating in the mid-infrared. The devices were fabricated from Ge23Sb7S70 chalcogenide glass (ChG) on CaF2 substrates by combing photolithographic patterning and focused ion beam milling. The waveguide-coupled cavities were characterized using a fiber end fire coupling method at 5.2 μm wavelength, and a loaded quality factor of ~2000 was measured near the critical coupling regime.

A Fully-Integrated Flexible Photonic Platform for Chip-to-Chip Optical Interconnects

We analyze a chip-to-chip optical interconnect platform based on our recently developed flexible substrate integration technology. We show that the architecture achieves high bandwidth density (100 Tbs/cm2), and does not require optical alignment during packaging. These advantages make the flexible photonics platform a promising solution for chip-to-chip optical interconnects. We further report initial experimental characterizations of the flexible photonics platform fabricated using thermal nanoimprint patterning of glass waveguides and III-V die bonding.