Dielectric II-VI and IV-VI Metal Chalcogenide Thin Films in Hollow Glass Waveguides (HGWs) for Infrared Spectroscopy and Laser Delivery Carlos M. Bledt * a, Daniel V. Kopp a, and James A. Harrington a a Department of Material Science & Engineering Rutgers, the State University of New Jersey October 18, 2011
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 Polyimide Coating Dielectric Film 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 Silver Film Silica Wall Inherent loss dependence in HGWs: Proportional to 1/a 3 (a is bore radius) Proportional to 1/R (R is bending radius)
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
Thin Film Interference Effect in HGWs Thin film effect in HGWs: Constructive interference maximum when individual films are of proper thickness Loss is still reduced when using non-optimized film thicknesses Additional dielectric layers reduce loss due to multiple surface reflections Multilayer dielectric stacks incorporate alternating low and high index materials Appearance of photonic band gaps with periodic layer structure Constructive thin film interference effect in quarter-wave dielectric multilayer stack (3 layers) with 180 phase change of composite reflected wave
Theoretical Loss in HGWs Losses in HGWs depend on: Propagating modes Dielectric thin film materials Thickness of deposited films Quality and roughness of films Number of films deposited Equation for Determining Theoretical Losses in HGWs α = u nm 2π 2 λ 2 a 3 n m n m 2 + k m 2 α = power attenuation coefficient u nm = propagating mode parameter λ = functional wavelength a = HGW bore radius size n m = refractive index of metal film k m = absorbance coefficient of metal film F film F film term dependence on: Propagating mode Dielectric film material Number of dielectric thin films Theoretical Losses for Lowest Order Hybrid Modes in PbS (n = 4.00) Coated HGWs HE 11 mode is lowest loss mode in dielectric coated HGWs
Metal Chalcogenides Metal chalcogenides are attractive dielectric materials for use as thin films in Hollow Glass Waveguides Advantages: IR transparent in NIR, MWIR, and LWIR regions Ability to deposit from aqueous solutions at STP Chemical and mechanical compatibility Disadvantages: Use of hazardous materials Slow deposition rates for certain metal chalcogenide films Attractive metal chalcogenide thin film dielectric materials include II-VI and IV-VI materials such as PbS, PbSe, CdS, CdSe, ZnS, and ZnSe Thin Film Material CdS (n = 2.27) PbS (n = 4.00) ZnS (n = 2.40) Current Process Established deposition procedure / High film quality Established deposition procedure / High film quality Difficulty in depositing thicker films / Low film quality
Multilayer Dielectric Stack HGWs Losses in HGWs decrease with increasing number of dielectric films due to constructive interference and coherent scattering Fabrication considerations Compatibility of dielectric materials High refractive index contrast Low surface roughness High film uniformity Advantages Reduced transmission losses Possible omnidirectionality Challenges Film quality decreases with increasing thickness of films Long deposition times required Increased surface roughness Polymer Coating Silver Film CdS Film PbS Film Silica Wall n Index profile
Experimental Approach Research objectives: Optimize CdS and PbS thin film deposition procedures Determine film growth kinetics of CdS and PbS films in HGWs Determine feasibility of depositing CdS / PbS multilayer stacks Analyze optical response of multilayer coated HGWs Experimental Approach 700 μm ID HGWs used in study Standard silver film deposition procedure used Determination of film thickness as function of deposition time for: CdS thin films (Thicknesses: 0.05 0.70 μm) PbS thin films (Thicknesses: 0.10 0.40 μm) Deposition of CdS / PbS based multilayer dielectric thin film stacks Characterization to include: FTIR spectroscopy Laser based attenuation measurements
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 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 DLPD Process Configuration 3. Dielectric Thin Film Cadmium Sulfide (CdS) Lead Sulfide (PbS)
Deposition of Metal Chalcogenide Films Deposition of metal sulfide films involves hydrolysis of thiourea in an alkaline medium containing complexed metal cation species Possible deposition mechanisms Competing homogeneous & heterogeneous (desired) deposition processes 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
Cadmium Sulfide Film Growth Kinetics Fabrication of Ag/CdS HGWs at deposition times from 155 400 min in 35 min intervals Deposition parameters: [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: 17.35 ml/min Highly linear film growth Growth rate: 1.43 nm/min Slight decrease in film uniformity at t > 450 min Best fit linear film growth: δa CdS = 1. 43 10 3 t CdS 0. 036
Lead Sulfide Film Growth Kinetics Fabrication of Ag/PbS HGWs at deposition times from 30 135 min in 15 min intervals Deposition parameters: [Pb(NO 3 ) 2 ] = 2.7 mm [SC(NH 2 ) 2 ] = 27.2 mm [NaOH] = 28.1 mm (ph 12.05) Volumetric Flow Rate: 17.35 ml/min Highly linear film growth Growth rate: 3.62 nm/min Slight decrease in film uniformity at t > 150 min Best fit linear film growth: δa PbS = 3. 62 10 3 t CdS + 0. 047
Practical Design of Multilayer HGWs Considerations in the practical design of multilayer HGWs: Correct individual film thickness for T max @ desired λ range Deposition times for films of desired thicknesses determined from film growth kinetics Individual films must be mechanically, thermally, and optically compatible Higher refractive index contrast of films (n H /n L ) yields lowest losses For a multilayer dielectric stack d c = j λ i i=1 4 n i 2 1 λ i = i th layer 1 st interference peak contribution n i = i th dielectric film refractive index d c = composite dielectric film thickness Optimal Wavelength Optimized CdS Thin Film (n = 2.27) Optimized PbS Thin Film (n = 4.00) λ = 1.0 μm 0.123 μm 0.065 μm λ = 2.0 μm 0.245 μm 0.129 μm λ = 5.0 μm 0.613 μm 0.323 μm λ = 10.6 μm 1.300 μm 0.684 μm Film thicknesses optimized for λ 0 = 2.0 μm Deposition times determined to be 165 min (CdS) & 26 min (PbS)
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
Conclusion DLPD parameter optimization for deposition of metal sulfide thin films Deposited films via DLPD process exhibited: Good uniformity & IR spectral response shift Chemical & mechanical structural stability Lower losses with > film index contrast Silver Film n 1 Film Future research: Increase film thicknesses in order to optimize multilayer designs for > λ Reduce necessary deposition times to reduce surface roughness scattering losses Further control of film thicknesses Incorporation of higher number of dielectric layers in multilayer stack HGWs Incorporation of novel dielectric thin film materials such as PbSe, ZnS, and CuS n n 2 Film Index profile
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