Photo-thermal Deflection Spectroscopy George Noid LIGO SURF Student

Summary Introduction Theory Experiment Results

LIGO TEST MASSES Fused Silica – Currently uses – Thermal Conductivity is 1.38 W/mK Synthetic Sapphire – Proposed for LIGO II – Crystal Systems – Thermal Conductivity is 23.1 parallel to optical axis

Photothermal Deflection Spectroscopy (PDS) PDS First used: 1979 Two Lasers: – Pump – Probe Temperature Gradient => Gradient in the index of refraction Useful as a non-destructive spectroscopic method – Can observe microscopic impurities in a crystal – Ex: trace metals in a sapphire crystal

PDS Probe – Does not disturb sample – Detected Pump - More powerful - Modulated - Not detected

Deflection

Deflection (cont.) Snells law: a sin A = b sin B  (..) = arcsin { (n 0 )/(n(..)) * sin (pi/2 -  )} –  = laser angles –  = angle of deflection – n 0 = index of refraction of sapphire (1.862) – n = index of refraction function caused by PDS

The Experiment: two laser tables Pump table – 700 mW Nd YAG laser – Chopper – Fiber coupler – Fiber Probe table – 5 mW He Ne – Fiber – Telescope – 1064 mirror – Biconvex lens – Sapphire crystal – Quad cell Detetector

Pump Table Nd YAG chopper 5- axis fiber aligner Polarization Selecting filter

Injecting and Maximizing

Probe table Photo detector sample He Ne laser Fiber conduit telescope Fiber chuck Aspheric lens holder 1064 mirror beam tube XY stage XYZ stage

Probe Table: laser alignment Small angle theta in the y direction – Increases interaction length Beam waists: 80 microns Nd Yag – Magnified, collimated in telescope – Reflected off mirror – Focused in biconvex lens into sapphire He - ne – Passes through 1064 mirror – Focused in biconvex lens into sapphire

Data Initial results were inconclusive with lock- in amplification. Observed PDS with signal analyzer Pump power incident: 300 mw Chopping frequency: 500 Hz Maximum signal to noise ratio: 27 to 3 Crystal mapping ongoing

Representative Peak