Photo-thermal DeflectionPhoto-thermal DeflectionSpectroscopySpectroscopy

George Noid

LIGO SURF Student

SummarySummary

Introduction Theory Experiment Results

LIGOLIGO TESTTEST MASSESMASSES

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)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

PDSPDS

Probe– Does not disturb sample– Detected

Pump- More powerful- Modulated- Not detected

DeflectionDeflection

Deflection (cont.)Deflection (cont.)

Snells law: a sin A = b sin B (..) = arcsin { (n0)/(n(..)) * sin (pi/2 - )}

= laser angles = angle of deflection

– n0 = index of refraction of sapphire (1.862)

– n = index of refraction function caused by PDS

The Experiment: two laser The Experiment: two laser tablestables

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 TablePump Table

Nd YAG

chopper

5- axis fiberaligner

PolarizationSelecting filter

Injecting and MaximizingInjecting and Maximizing

Probe tableProbe table

Photo detector

sample

He Ne laser

Fiber conduit

telescope

Fiber chuck

Aspheric lensholder

1064 mirror

beam tube

XY stage

XYZstage

Probe Table: laser alignmentProbe 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

DataData

• 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 PeakRepresentative Peakfourier transform of photothermal deflection

0.00E+005.00E-061.00E-051.50E-052.00E-052.50E-053.00E-053.50E-054.

50E+

02

4.58

E+02

4.67

E+02

4.75

E+02

4.83

E+02

4.91

E+02

5.00

E+02

5.08

E+02

5.16

E+02

5.24

E+02

5.33

E+02

5.41

E+02

5.49

E+02

frequency(Hz)

sign

al(m

Vrm

s)

Series2