Multilayer thin film coatings for reduced infrared loss in hollow glass waveguides

1. ICO-22 Paper #2286190 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. Krieselb bOpto-Knowledge Systems, Inc. 19805 Hamilton Ave., Torrance CA 90502 August 19, 2011

2. Background on Hollow Glass Waveguides Polyimide Coating Dielectric Film • 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 Silver Film • Inherent loss dependence in HGWs: • Proportional to 1/a3 (a is bore radius) • Proportional to 1/R (R is bending radius) • Decreases with increasing number of layers

3. 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

4. Theoretical Loss in HGWs Ray optics equation for calculating loss • 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 Ag/AgI HGW • HE11 mode is lowest loss mode in dielectric coated HGWs • Loss contribution due to surface roughness (scattering)

5. Experimental Approach nH Film (AgI, CdS, PbS) Silver Film • 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 Tmax @ λ = 10.6μm • Fabricate dual layer HGWs and determine efficacy in reducing loss nL Film (PS) • 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 Index profile

6. Fabrication Methodology x.xx rpm x.xx rpm • 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 HGW HGW Peristaltic Pump Peristaltic Pump Ag, AgI, CdS, and PbS Film Deposition System Configuration PS Film Deposition System Configuration Polystyrene Solution • 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 Waste Micro-Bore Tubing Precursor Solution #1 Precursor Solution #2

7. 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 AgI Thin Films CdS Thin Films • Spectral response shifts to longer wavelengths with increase in dielectric film thickness • Establish deposition kinetics to deposit films of desired thickness PbS Thin Films

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

9. 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 = 1.58 @ 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

10. Practical Design of Multilayer HGWs • Considerations in the practical design of multilayer HGWs: • Correct compound film thickness for Tmax @ 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 λi = ith layer 1st interference peak contribution ni = ith dielectric film refractive index dc = composite dielectric film thickness • Desired secondary PS film thickness: ~ 0.15 μm (3 wt % PS sol. used) • First interference peak of dual layer HGW designed @ λ = 4 μm ± 10%

11. Silver Iodide / Polystyrene Coated HGWs Silver Iodide n = 2.10 Silver Iodide n = 2.10 • Waveguide loss reduced by a factor of 1.88 • Polystyrene film was of good uniformity alongsample length Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack Silver Film Silver Film Polystyrene n = 1.58 Diagram representation of Ag/AgI HGW before and after PS addition

12. Cadmium Sulfide / Polystyrene Coated HGWs Cadmium Sulfide n = 2.27 Cadmium Sulfide n = 2.27 • Waveguide loss reduced by a factor of 1.97 • Polystyrene film was of good uniformity alongsample length Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack Silver Film Silver Film Polystyrene n = 1.58 Diagram representation of Ag/CdS HGW before and after PS addition

13. Lead Sulfide / Polystyrene Coated HGWs Lead Sulfide n = 4.00 Lead Sulfide n = 4.00 • Waveguide loss reduced by a factor of 2.11 • Polystyrene film was of decent uniformity alongsample length Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack Silver Film Silver Film Polystyrene n = 1.58 Diagram representation of Ag/PbS HGW before and after PS addition

14. Conclusion Silver Film n1 Film • 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 n2 Film • 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 n Index profile