1. Optical Characterization Techniques
Reflection
Microscopy
Ellipsometry
Reflectance
Emission PL
UPS
Incident
Photons
Absorption
Photoconductivity
Inelastic
Scattering
Raman
Brillouin
Transmission
Transmittance
FTIR

2. Optical Characterization Techniques
Measurement (reflected or transmitted light)
Photometry
Intensity
Spectroscopy
Wavelength /
Energy
Interferometry
Phase
Ellipsometry
Polarization

3. Optical Characterization Techniques
Advantages :
Non-destructive
No contacts required
High sensitivity,
< 1012 cm-3 impurity detection

4. Spectroscopy
From Hollas, Fig. 3.1, p. 42

5. Spectroscopy Need a dispersive element to separate wavelengths
4 ways of performing spectroscopy
Prism
Grating
Interferometer
Michelson
Interferometer
(FTS)
Fabry-Perot
Interferometer

6. Resolving power, R = b dn/dl
Prisms
Remember Newton
Wavelengths are separated by the wavelength dependence of refractive index
Dispersion, dn/dl
Resolving power, R = b dn/dl
From Hollas, Fig. 3.3, p. 44

7. Grating Monochromator
Wavelengths are separated spatially by a diffraction grating
Requires a lock-in amplifier for good S/N
LaPierre, Ph.D. thesis

8. Grating Monochromator
Resolving power:
R = l/Dl = mN
m = diffraction order
N = # of grooves
R ~ 104
 can resolve
0.05 nm from l = 500 nm
Usually used in the uv, visible and near-infrared region
Mid- and far-infrared dispersion is more effectively performed by FTS

9. Interferometers
Interference can result in dispersion of wavelengths
Remember oil slicks

10. Fabry-Perot Interferometer
Constructive interference occurs in transmission
for wavelengths satisfying (normal incidence):
2d = mlo/n1
multi-wavelength source, Dl
plate spacing
d = mlo/2n1 n1 I
detector l lo

11. Fabry-Perot Interferometer
The F-P transmission is given by the Airy function
F = 4R/(1-R)2
= 4pn1d / lo (normal angle of incidence)
R = mirror reflectivity 1
1 + Fsin2(d/2)
T = T d

12. Fabry-Perot Interferometer
Want a high coefficient of finesse for a narrow transmission
F = 4R/(1-R)2
 Want high reflectivity mirrors T d

13. Fabry-Perot Interferometer
Resolving power:
R = l/Dl½
R = l / (2l/mp√F)
R = mp√F / 2
R > 106 is achievable  can resolve
nm from l = 500 nm T 1
0.5
Dl½ = 2l/(mp√F) l

14. Fourier-Transform Spectroscopy (FTS)
Michelson
interferometer BS
source M1 P
frequency
spectrum
source
interferogram I I
laser
(long lc) w
mirror position I I
LED
(short lc) w
mirror position
large lc (e.g., laser)  wide interferogram
short lc (e.g., LED)  narrow interferogram

15. Fourier-Transform Spectroscopy (FTS)
The frequency spectrum is the Fourier transform
of the interferogram
Fourier transform
frequency
spectrum
source
interferogram I I
laser
(long lc) w
mirror position I I
LED
(short lc) w
mirror position

16. P FTS Reference spectrum is taken with sample removed from system
Sample is placed in one arm of interferometer
Intensity (interferogram) is measured at detector, P, as a function of mirror position
Fourier transform (FFT) of interferogram gives spectral content of input (includes absorption spectrum from sample) M2
sample
input
wavelengths
(broadband source) M1 BS P

17. FTS
FTS usually used in the near-, mid-, and far-infrared wavelength range
Also called Fourier transform infrared spectroscopy (FTIR spectroscopy)
Strong absorption by H2O; must purge optical path with N2
Windows in the system must be transparent to the wavelengths of interest

18. FTS
from Hollas, Table 3.1, p. 60

19. FTS
Advantages of FTS compared to monochromator or F-P :
Whole spectrum is measured at once (Fellget advantage)
Large energy throughput; large input/output aperture (Jacquinot advantage)
Better S/N ratio
Fast measurement
Resolving power limited by mirror displacement (d) :
R = l / Dl = 2d / l
e.g., mirror displacement of d = 0.5 cm  R = 20000
 Dl = nm at 500 nm