Carlos M. Bledt a , James A. Harrington a , and Jason M. Kriesel b

1. Multilayer Silver / Dielectric Thin-Film Coated Hollow Waveguides for Sensor and Laser Power Delivery ApplicationsTheory, Design, and Fabrication Carlos M. Bledta, James A. Harrington a, and Jason M. Krieselb a Dept. of Material Science & Engineering Rutgers, the State University of New Jersey b Opto-Knowledge Systems, Inc. January 21, 2012

2. Background on Hollow Glass Waveguides Polyimide Coating Dielectric Film • Used in the low loss broadband transmission from λ = 1 – 16 μm • Light propagation due to enhanced inner wall surface reflection • Structure of HGWs • SiO2 capillary tubing substrate • Ag film ~200 nm thick • Dielectric(s) such as AgI, CdS, PbS • Multilayer structures of interest Silica Wall Silver Film • Theoretical loss dependence * • 1/a3 (a is bore radius) • 1/R (R is bending radius) * Harrington, J. A., Infrared Fiber Optics and Their Applications 1/11

3. Attenuation Considerations in HGWs Ray Optics Attenuation Equation • Practical losses in HGWs: • Propagating modes • Dielectric thin film materials • Thickness of deposited films • Quality and roughness of films • Number of films deposited • ↑ Throughout ↓ mode quality α = power attenuation coefficient a = HGW inner radius size R = power reflection coefficient θ = angle of propagating ray • R(θ) term dependence on: • Angle of incidence • Thin film structure • Thin film materials • HE11 mode is lowest loss mode in metal / dielectric coated HWs • R(θ) is main design parameter 2/11

4. Motivation for Multilayer Designs Silver Film nL Film • Multilayer thin film designs • Alternating low (nL) and high (nH) refractive index films • Metal chalcogenides as film materials (compatible) • Periodic structure resulting in 1-D photonic band gap structure • Benefits of photonic band gap structures: • Ultra-low loss at discrete λ ranges • Omnidirectional properties (no bending loss) nH Film • Attenuation in multilayer HGWs * • Loss ↓ as NL+H ↑ (asymptotic behavior) • Loss ↓ as nH/nL (with nL > 1) • Issues with multilayer designs • Surface roughness ↑ with total film thickness (increased scattering losses) • Precise film thickness control necessary for photonic band gap structure n * Miyagi, M. and Kawakami, S. "Design theory of dielectric- coated circular metallic waveguides for infrared transmission Index profile 3/11

5. Spectral Simulation of Multilayer HGWs Simulation parameters for λT = 1.064 µm • Proposed PBG design • Use metal sulfide thin films: • PbS (nH≈ 3.80 – 4.10 at IR λ) * • CdS (nL≈2.25 – 2.45 at IR λ) * • Theoretical spectral calculations of 1-D PBG coated HGWs: • Ray-Transfer matrix Method • Multilayer structure design • High film index contrast (nH/nL) • nL > 1 for hollow air core HWs • Low κ materials at design λ • Careful film thickness control • Surface roughness excluded * Note: θi= 89.93° - HE11 Mode* * Palik, E. D. and Ghosh, G., Handbook of Optical Constants of Solids 4/11

6. Experimental Approach • Research objectives: • CdS (nL) / PbS(nH) alternating film pairs • Optimize CdS and PbS thin film deposition procedures • Determine film growth kinetics of CdS and PbS films in HGWs • Analyze optical response of multilayer coated HGWs • Optimize multilayer coated HGW to develop 1-D PBG structure • Experimental Approach • HGW dimensionality constant at ID = 700 μm • Film thickness as function of time for: • CdS on Ag & CdS on PbS thin films • PbS on Ag & PbS on CdS thin films • Deposition of CdS / PbS based multilayer dielectric thin film stacks • Characterization to include: • FTIR spectroscopy • Optical attenuation measurements 5/11

7. Fabrication Methodology • Films deposited via dynamic liquid phase deposition process (DLPD) [8] • The DLPD process: • Peristaltic pumps used to flow precursor solutions through HGW • Constant flow of solutions allows for deposition of films • Strong reaction kinetics temperature & concentration dependence • Advantages of DLPD process: • No solution concentration depletion • Flow speed adjusted to improve film quality x.xx rpm HGW Peristaltic Pump 1. Sn2+ Sensitization Step 2. Silver Film Deposition Waste Precursor Solution #1 Precursor Solution #2 3. Dielectric Thin Film Cadmium Sulfide (CdS) Lead Sulfide (PbS) 6/11

8. Deposition of Metal Chalcogenide Films • Deposition of metal sulfide films involves hydrolysis of thiourea in an alkaline medium containing complexed metal cation species [9,10,11,12] • Possible deposition mechanisms • Competing homogeneous & heterogeneous (desired) deposition processes [10,12] Homogeneous Growth Heterogeneous Growth • Ion by ion deposition • High overall film quality • Slow growth rate • Good film adherence • Low surface roughness • Cluster by cluster deposition • Low overall film quality • Rapid growth rate • Poor stability & high porosity • High surface roughness 7/11

9. Cadmium Sulfide Film Growth Kinetics • CdS on Ag HGW kinetics: • [Cd(NO3)2] = 7.49 mM • [SC(NH2)2] = 75 mM • [NH4OH] = 1.85 M (pH ≈ 11.75) • Volumetric Flow Rate: 17.35 mL/min • Growth rate: 3.62 nm/min • Temperature: 24 °C ± 0.5 • CdS on Ag / PbS HGW kinetics: • [Cd(NO3)2] = 7.49 mM • [SC(NH2)2] = 75 mM • [NH4OH] = 1.85 M (pH ≈ 11.75) • Volumetric Flow Rate: 17.35 mL/min • Growth rate: 4.79 nm/min • Temperature: 24 °C ± 0.5 8/11

10. Lead Sulfide Film Growth Kinetics • CdS on Ag HGW kinetics: • [Pb(NO3)2] = 2.72 mM • [SC(NH2)2] = 27.2 mM • [NaOH] = 37.5 mM (pH ≈ 12.05) • Volumetric Flow Rate: 17.35 mL/min • Growth rate: 3.62 nm/min • Temperature: 24 °C ± 0.5 • PbS on Ag / CdS HGW kinetics: • [Pb(NO3)2] = 2.7 mM • [SC(NH2)2] = 27.2 mM • [NaOH] = 37.5 mM (pH ≈ 12.05) • Volumetric Flow Rate: 17.35 mL/min • Growth rate: 6.71 nm/min • Temperature: 24 °C ± 0.5 9/11

11. CdS / PbS Multilayer Stack HGWs • High compatibility seen between CdS & PbS films • Characteristics spectral shift with additional layers • Surface roughness increase with time • Losses measured with Synrad CO2 laser emitting at λ = 10.6 μm • Drop in attenuation seen with successive layers up to 5 layers • Lower losses achieved relative to Ag/CdS & Ag/PbS only HGWs 10/11

12. Conclusion • Considerable progress achieved towards 1-D PBG structures in HGWs • Experimental Goals Achieved: • Theoretical calculations for 1-D CdS / PbS PBG structures • Film growth kinetics study for: • CdS films on Ag and PbS substrates • PbS films on Ag and CdS substrates • Deposition of CdS / PbS multilayers • Important considerations: • Substrate has pronounced effect on growth kinetics • Fabrication difficulty increases considerably with: • Total number of deposited films • Increasing individual film thickness • Future research: • Continue study of multilayer stacks – Incorporate novel materials • Optimize structure for appearance of PBG at NIR wavelengths • Study possibility of omnidirectional propagation – FDTD analysis 11/11

13. End of Presentation Thank you for your attention!