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

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

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

Spectroscopy From Hollas, Fig. 3.1, p. 42

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

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

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

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

Interferometers Interference can result in dispersion of wavelengths Remember oil slicks

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

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

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

Fabry-Perot Interferometer Resolving power: R = l/Dl½ R = l / (2l/mp√F) R = mp√F / 2 R > 106 is achievable  can resolve 0.0005 nm from l = 500 nm T 1 0.5 Dl½ = 2l/(mp√F) l

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

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

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

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

FTS from Hollas, Table 3.1, p. 60

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 = 0.025 nm at 500 nm