Multilayer thin film coatings for reduced infrared loss in hollow glass waveguides Carlos M. Bledt* a, Daniel V. Kopp a, and James A. Harrington a a Dept. of Material Science & Engineering Rutgers, the State University of New Jersey Jason M. Kriesel b b Opto-Knowledge Systems, Inc Hamilton Ave., Torrance CA August 19, 2011 ICO-22 Paper #

Background on Hollow Glass Waveguides Hollow Glass Waveguides (HGWs) are used in the low loss broadband transmission of infrared radiation ranging from λ = 2 – 16 μm HGWs function due to enhanced reflection of incident IR radiation Structure of HGWs: –Silica capillary tubing substrate –Silver (Ag) film ~200 nm thick –Dielectric film(s) such as AgI, CdS, PbS, PS –Current research on multilayer structures Silica Wall Dielectric Film Polyimide Coating Silver Film Inherent loss dependence in HGWs: –Proportional to 1/a 3 (a is bore radius) –Proportional to 1/R (R is bending radius) –Decreases with increasing number of layers

Advantages of Hollow Glass Waveguides Advantages of HGWs include: –High laser damage threshold –Broadband IR transmission –Customizability of optical response –No end reflection losses –Low production costs –Low losses at IR wavelengths –Small beam divergence –Single-mode radiation delivery –Higher order-mode filtering Applications of HGWs include: –Surgical laser delivery –IR chemical & gas sensing –Thermal imaging –IR spectroscopy

Theoretical Loss in HGWs Losses in HGWs depend on: –Propagating modes –Dielectric thin film materials –Thickness of deposited films –Quality and roughness of films Losses calculated using both wave and ray optics theories α = power attenuation coefficient a = HGW inner diameter size R = power reflection coefficient θ = angle of propagating ray Power reflection coefficient, R(θ), depends on incident angle as well as on film materials & structure Ray optics equation for calculating loss HE 11 mode is lowest loss mode in dielectric coated HGWs Loss contribution due to surface roughness (scattering) Ag/AgI HGW

Experimental Approach Research objectives: –Theoretically determine the loss of HGWs incorporating dual layer structures –Develop a deposition kinetics model for the growth of AgI, CdS, and PbS films –Optimize dual layer HGWs incorporating secondary PS films for T λ = 10.6 μm –Fabricate dual layer HGWs and determine efficacy in reducing loss Experimental Approach –700 μm ID HGWs used in study –Film growth kinetics determined –Growth kinetics used to determine optimal AgI, CdS, and PbS film thicknesses –Separate study of PS film thickness as a function of solution concentration –Optimization of films for dual dielectric structure with secondary PS film –Comparison of measured losses for dual layer HGWs with theory and single layer HGWs n L Film (PS) n H Film (AgI, CdS, PbS) Silver Film n Index profile

Fabrication Methodology Films deposited in HGWs from precursor solutions via dynamic liquid phase deposition (DLPD) The DLPD process: –Peristaltic pumps used to flow precursor solutions through HGW –Constant flow of precursor solutions allows for deposition of films Ag, AgI, CdS, and PbS Film Deposition System Configuration Precursor Solution #1 Precursor Solution #2 HGW Waste Peristaltic Pump x.xx rpm Polystyrene Solution HGW Peristaltic Pump Micro-Bore Tubing x.xx rpm No depletion of solution concentrations in DLPD DLPD setup depends on precursor solution viscosity DLPD process used for Ag, AgI, CdS, PbS, and PS thin films in HGWs PS Film Deposition System Configuration

FTIR Analysis of Single Dielectric Thin Films FTIR analysis from λ = 2 – 15 μm used to determine mid-IR spectral response of HGWs as a function of dielectric film deposition time Spectral response shifts to longer wavelengths with increase in dielectric film thickness Establish deposition kinetics to deposit films of desired thickness CdS Thin FilmsAgI Thin Films PbS Thin Films

Deposition Kinetics of Dielectric Thin Films Film thickness of dielectric films can be calculated from FTIR spectra AgI Thin Films CdS Thin Films For a single dielectric layer λ p = 1 st interference peak position n d = dielectric film refractive index d f = dielectric film thickness d f as a function of deposition time determined for AgI, CdS, PbS Growth Rate: 3.00 nm/sec Growth Rate: 1.59 nm/min PbS Thin Films Growth Rate: 3.13 nm/min

Polystyrene Thin Films Polystyrene (PS) can be used as low index thin film in HGWs Polystyrene thin films can be deposited via DLPD process Advantages of PS dielectric thin films –Low refractive index material (n = λ = 10.6 μm) –Ability to be deposited from aqueous solution –Inexpensive and non-toxic material –Ability to deposit uniform thin films Control of PS film thickness: –Solution concentration –Deposition pump rate –Volume of solution –Drying/curing process –Deposition time independent

Practical Design of Multilayer HGWs Considerations in the practical design of multilayer HGWs: –Correct compound film thickness for T desired λ range –Careful design of individual layer thicknesses using kinetics studies –Individual films must be mechanically, thermally, and optically compatible –Higher refractive index contrast of films yields lowest losses For a multilayer dielectric stack λ i = i th layer 1 st interference peak contribution n i = i th dielectric film refractive index d c = composite dielectric film thickness Dielectric Material Deposition Time Dielectric Film Thickness (µm) AgI55 sec0.40 CdS330 min0.41 PbS55 min0.23 Desired secondary PS film thickness: ~ 0.15 μm (3 wt % PS sol. used) First interference peak of dual layer HGW λ = 4 μm ± 10%

HGW Structure Δn (|n L -n H |) Loss (dB/m) Dielectric Film Thickness (µm) Ag/AgIN/A Ag/AgI/PS HGW Structure Δn (|n L -n H |) Loss (dB/m) Dielectric Film Thickness (µm) Ag/AgIN/A Ag/AgI/PS Polystyrene n = 1.58 Silver Iodide n = 2.10 Silver Film Silver Iodide n = 2.10 Silver Film Silver Iodide / Polystyrene Coated HGWs Waveguide loss reduced by a factor of 1.88 Polystyrene film was of good uniformity along sample length Diagram representation of Ag/AgI HGW before and after PS addition Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack

HGW Structure Δn (|n L -n H |) Loss (dB/m) Dielectric Film Thickness (µm) Ag/CdSN/A Ag/CdS/PS HGW Structure Δn (|n L -n H |) Loss (dB/m) Dielectric Film Thickness (µm) Ag/CdSN/A Ag/CdS/PS Polystyrene n = 1.58 Cadmium Sulfide n = 2.27 Silver Film Cadmium Sulfide n = 2.27 Silver Film Cadmium Sulfide / Polystyrene Coated HGWs Waveguide loss reduced by a factor of 1.97 Polystyrene film was of good uniformity along sample length Diagram representation of Ag/CdS HGW before and after PS addition Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack

HGW Structure Δn (|n L -n H |) Loss (dB/m) Dielectric Film Thickness (µm) Ag/PbSN/A Ag/PbS/PS HGW Structure Δn (|n L -n H |) Loss (dB/m) Dielectric Film Thickness (µm) Ag/PbSN/A Ag/PbS/PS Polystyrene n = 1.58 Lead Sulfide n = 4.00 Silver Film Lead Sulfide n = 4.00 Silver Film Lead Sulfide / Polystyrene Coated HGWs Waveguide loss reduced by a factor of 2.11 Polystyrene film was of decent uniformity along sample length Diagram representation of Ag/PbS HGW before and after PS addition Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack

Conclusion Successful fabrication of dual layer HGWs incorporating PS thin films Deposited films via DLPD process exhibited: –Good uniformity & IR spectral response shift –Reduced loss with addition of PS film –Chemical & mechanical structural stability –Lower losses with > film index contrast n 2 Film n 1 Film Silver Film n Index profile Future research: –Incorporation of novel materials such as ZnS & ZnSe as dielectric thin films –Fabrication of HGW structures with larger number of alternating index films –Possibility of photonic bandgap structure with larger number of alternating layers –Multilayer dielectric designs in gradually tapered HGWs

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