Multilayer Silver / Dielectric Thin-Film Coated Hollow Waveguides for Sensor and Laser Power Delivery Applications

Transcription

1 Multilayer Silver / Dielectric Thin-Film Coated Hollow Waveguides for Sensor and Laser Power Delivery Applications Theory, Design, and Fabrication Carlos M. Bledt a, James A. Harrington a, and Jason M. Kriesel b 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 Used in the low loss broadband transmission from λ = 1 16 μm Light propagation due to enhanced inner wall surface reflection Polyimide Coating Dielectric Film Structure of HGWs SiO 2 capillary tubing substrate Ag film ~200 nm thick Dielectric(s) such as AgI, CdS, PbS Multilayer structures of interest Silver Film Silica Wall Theoretical loss dependence * 1/a 3 (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 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 Ray Optics Attenuation Equation 2α θ = 1 R θ 2a cot θ 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 HE 11 mode is lowest loss mode in metal / dielectric coated HWs R(θ) is main design parameter 2/11

4 Motivation for Multilayer Designs Multilayer thin film designs Alternating low (n L ) and high (n H ) 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) Silver Film n L Film Attenuation in multilayer HGWs * Loss as N L+H (asymptotic behavior) Loss as n H /n L (with n L > 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 H Film n * Miyagi, M. and Kawakami, S. "Design theory of dielectriccoated circular metallic waveguides for infrared transmission Index profile 3/11

5 Spectral Simulation of Multilayer HGWs Proposed PBG design Use metal sulfide thin films: PbS (n H at IR λ) * CdS (n L at IR λ) * Theoretical spectral calculations of 1-D PBG coated HGWs: Ray-Transfer matrix Method Simulation parameters for λ T = µm CdS Film PbS Film N δ 131 nm 68 nm n κ N/A 0 N/A 0 * Note: θ i = HE 11 Mode* E z F z = N i=1 A i C i B i D i E 0 F 0 Multilayer structure design High film index contrast (n H /n L ) n L > 1 for hollow air core HWs Low κ materials at design λ Careful film thickness control Surface roughness excluded * Palik, E. D. and Ghosh, G., Handbook of Optical Constants of Solids 4/11

6 Experimental Approach Research objectives: CdS (n L ) / PbS (n H ) 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 x.xx rpm Advantages of DLPD process: Peristaltic Pump HGW No solution concentration depletion Flow speed adjusted to improve film quality 1. Sn 2+ Sensitization Step Waste 2. Silver Film Deposition 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 Cluster by cluster deposition Low overall film quality Rapid growth rate Poor stability & high porosity High surface roughness Ion by ion deposition High overall film quality Slow growth rate Good film adherence Low surface roughness 7/11

9 Cadmium Sulfide Film Growth Kinetics CdS on Ag HGW kinetics: [Cd(NO 3 ) 2 ] = 7.49 mm [SC(NH 2 ) 2 ] = 75 mm [NH 4 OH] = 1.85 M (ph 11.75) Volumetric Flow Rate: ml/min Growth rate: 3.62 nm/min Temperature: 24 C ± 0.5 δa CdS = t CdS CdS on Ag / PbS HGW kinetics: [Cd(NO 3 ) 2 ] = 7.49 mm [SC(NH 2 ) 2 ] = 75 mm [NH 4 OH] = 1.85 M (ph 11.75) Volumetric Flow Rate: ml/min Growth rate: 4.79 nm/min Temperature: 24 C ± 0.5 δa CdS = t CdS /11

10 Lead Sulfide Film Growth Kinetics CdS on Ag HGW kinetics: [Pb(NO 3 ) 2 ] = 2.72 mm [SC(NH 2 ) 2 ] = 27.2 mm [NaOH] = 37.5 mm (ph 12.05) Volumetric Flow Rate: ml/min Growth rate: 3.62 nm/min Temperature: 24 C ± 0.5 δa PbS = t PbS PbS on Ag / CdS HGW kinetics: [Pb(NO 3 ) 2 ] = 2.7 mm [SC(NH 2 ) 2 ] = 27.2 mm [NaOH] = 37.5 mm (ph 12.05) Volumetric Flow Rate: ml/min Growth rate: 6.71 nm/min Temperature: 24 C ± 0.5 δa PbS = t PbS /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 CO 2 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!