Chap 4 Optical Measurement

Transcription

1 Chap 4 Optical Measurement

2 4.1 Light Solid Interaction

3 E-M Wave permittivity, permeability Refractive index, extinction coefficient propagation absorption Refraction Absorption Scattering, Rayleigh Scattering target is smaller than wavelength Coefficients, reflection, absorption, transmission

4 Classical Description in Metals and Insulators Drude, Lorentz Lorentz Oscillator electrons in an insulator solution induced dipole Macroscopic polarization With assumption T A R T Permanent Ionic Electron Dipole Lattice Vib. Displacement

5 For Metal Drude is derive from Lorentz Oscillator, local field =0, ω o = 0 damping is coming from resistivity, Γ = 1/τ

6 Quantum Description, Semiconductor Oscillator Strength Joint Density of States Absorption in indirect semiconductor

7 4.2 Optics Basics and Components

8 Basic Terminology in Optics Focal Length single lens compound lens F NN = f d Numerical Aperture nn = sssα α 가작으면 nn d 2f = 1 2F NN

9 Basic Terminology in Optics Point Spread Function Optical Transfer Function f x, y = 1 + cos 2ππx g x, y = 1 + H ν sin 2ππx H(ν) : optical transfer function at ν.

10 Lasers

11 Photon Detectors Saleh, Teich (Chap 17) Thermal detector : eg) bolometer Photoelectric detectors Photoemission (photomultiplier, PMT) Micro-Channel Plate (MCP)

12 Semiconductor-Based Photon Detectors Quantum Efficiency: η = 1 R 1 exp αd, reflection coef. Absorption coef. Depth # of electron-hole pair per photon Rameo Theorem (response) i t = Q v(t) width, drift velocity w

13 Semiconductor-Based Photon Detectors p-n photodiode Only near junction, reverse pulse p-i-n photodiode Wider depletion region

14 Semiconductor-Based Photon Detectors Schottky Barrier photodiode Charge-coupled device PtSi/Si Schottky

15 Semiconductor-Based Photon Detectors Avalanche photodiode Boxcar Averager Photon Counting

16 Polarization and Polarizer

17 Beam Splitter

18 Window and Filters

19 Neutral Density Filter

20 Spectrometer

21 Fiber Optics Elements Multi or Single Mode Fiber Optics Adapter Fiber Optics Alignment Mount Fiber Optics Collimator

22 Other Optics Tools Beam Expander Diffractive Optical Elements Theta-Scanning Lens Gaussian to Top-hat Converter

23 Fourier Optics

24 4.3 Optics Experiments

25 Atomic Absorption Spectroscopy water

26 Raman Spectroscopy Micro-Raman

27 CARS Microscopy for Biological System

28 Z-Scan Measurement

29 Detecting Nanoparticles

30 Non-linear Optical Property Z-scan technique in the nano- and picosecond regime

31 Second Harmonic Generation Second harmonic generation experimental setup (SHG): BS1, BS2-beam splitters, Ph1, Ph2-photodiodes, λ/2-half wave plate, P-Glan polarizer, A-Glan analyser, L-lens, RS-rotation stage, F-filter/s, PMTphotomultiplier tube

32 Atom Trap When a cesium atom intersects the cooling laser beams, it experiences laser cooling which reduces its velocity considerably and cools it to a few micro Kelvin.

33 Optical Twitzer

34 Kerr Effect Measurement34

35 Optical Parametric Oscillation

36 Photoluminescence

37 Pump Probe Method

38 Heterodyne Method High frequency neglected

39 Holography

40 4.4 Raman Spectroscopy

41 Instrumentation

42 History Raman Spectroscopy Strokes and anti-strokes lines in the Raman spectrum of CCl 4 -mercury are excitation, nm ( cm -1 ) History 1928, Raman Discovery 1930, Nobel Prize Many Chemical Structure Study 1940~1950 * Rapid development of Commercial Infrared Instrumentation * No major advances in Raman Instruments * Perkin-Elmer Model 21 : Vibrational spectroscopic data from IR Mid ~100mW CW He-Ne Laser 1980 Monochromators, Detectors, Amplifier, Minicomputer to study vibrational spectroscopic works

43 Definition Raman Spectroscopy Monochromatic light E = E 0 cos 2πυot induces polarizable electric field, that results in Molecular dipole moment. p = αe α : polarizability of the material = αe 0 cos 2 πν 0t where α is linked to the normal mode. α α = α 0 + θk θk α = α 0 + θ 0k cos 2πνkt θk M = α 0E 0 cos 2πν 0t α + E 0 θ 0k[cos 2π ( ν 0 + νk) + cos 2π ( ν 0 νk)] θk The difference is from the induced dipole moments by Boltzman distribution of state populations. Px = αxxex + αxyey + αxzez Px = αyxex + αyyey + αyzez Px = αzxex + αzyey + αzzez Difference between Raman Spectroscopy and IR Spectroscopy Polarizability Raman Raman scattering : molercular motion produces a change in the polarizability of the molecule IR dipole transition Infrared absorption : normal mode of vibration in which the diople moment of the molecule varies with time * Appearance of bands differences in band intensities in the spectra * With increasing molecular symmetry, the difference increases. Raman : Symmetric vibrations and nonpolar groups Infrared : Antisymmetric vibrations and polar groups

44 Merit and Demerit Merit * Visible region of the spectrum * Optic is simple * Entire spectrum is obtained in the same instrument and cell * Sample preparation is simple * Intractable polymers * Single crystal * Aqueous solution - Demerit 10~400 cm -1 far IR 400~4000 cm -1 mid IR 4000~10,000 cm -1 near IR * Intrinsic weakness of the Raman Effect(10-6 of the original line) * Cannot be used for highly colored or fluorescing samples

45 Raman and IR Sample handling Raman General applicability 95% 99% Infrared Sample limitations Color : fluorescence Single crystals ; metals; aqueous solutions Easy of sample preparation Very simple Variable Liquids Very simple Very simple Powders Very simple More difficult Single crystals Very simple Very difficult Polymers Very simple (Sample limitations) More difficult Single fibers Possible Difficult Gases and vapors Now possible Simple Cells Very simple (glass) More complex (alkali halide) Micro work Good ( < 1 μg ) Good ( < 1 μg ) Trace work Sometimes Sometimes High and low temperature Moderately simple Moderately simple Application Raman Infrared Fingerprinting Excellent Excellent Best vibrations Symmetric Asymmetric Assignment work Group frequencies Aqueous solutions Quantitative analysis Low frequency modes Excellent Excellent Very good More difficult Excellent Very good Excellent Very difficult Good Difficult

46 Instrumentation and active modes Instrumentation Raman Infrared * Active modes in Raman and IR Relative complexity Moderate Slightly greater - exclusive C 5h, C 5d, D 5h, D 6d, S 8 Source Laser Blackbody or diode laser Detector Photomultiplier tube Thermal, pyroelectric, bolometers Resolution ca cm -1 ca cm -1 Principal limitation Energy Energy - Some vibration appeared in only one of the spectra D 2, C 4, C 3h, D 2d, C 4v, D 4, C 3h, C 6v, T, T d, O, C ν - No IR active ( totally symmetric representation) D 2, C 3h, D 3, D 2d Wavenumber range Purge requirement Photometry 10~4000 cm ~4000 cm -1 (one instrument) 10~400 cm -1 (second instrument or new beamsplitter, source and detector) No Scattering single beam Yes Absorption double beam - All IR, Raman active : C 1 - In crystals each component may be isolated Raman active unlike liquid. - The following three symmetries are important Unit cell symmetry Site symmetry of each molecule in the cell Molecular point group

47 Application to Organic Chemistry IR and. Raman : characteristic frequencies, useful in chemical applications Raman : bond is polarizable IR : dipole moment change Raman -C-C- -N=N- -C=C- IR Raman Complementary IR C=O P=O NO 2 Raman C=C stretching vibrations generally occur near 1640Cm -1 Symmetric vibration Raman spectrum is strong with sulfur

48 Characteristic Wave Numbers Characteristic Wavenumbers in Raman Spectra From Carbon-Carbon Double Bond Stretching

49 Microprobe Microprobe - Organic, Inorganic species - Polymers - Salts of organic acids - Cholesterol (biological importance) - Solid state reaction - Analysis of microfossils - Semiconductor device manufacturing control Schematic diagram of the Raman microprobe commercial instrument measured by Instruments SA.

50 Micro-Raman Triple Bond Stretching C C, C N Light microscope Infrared and Raman spectra of α-chloroacetonitrile. Due to α-carbon halogenation, intensity of C N stretch is drastically reduced in the IR but retained in the Raman.

51 4.5 IR Spectroscopy

52 Definition The development of FT-IR Spectrometers began with the invention of the two-beam interferometer by Michelson Michelson and Lord Rayleigh recognized that it was theoretically possible to obtain spectra from the interference pattern generated by the interferometer (now called the interferogram) through the computation of its Fourier transform * FTIR : Fourier Transform Infrared Spectrometer 1964 Cooley and Tukey : Fase Fourier Transform(FFT) algorithm 1975 FTIR spectrometry accepted for measuring high-quality infrared spectra * Grating IR : most widely used tool for the identification of organic compound or others(vibrational and Rotational spectroscopy)

53 Michelson Interferometry Michelson, two-beam interferometer If we consider a single wavelength, λ, then If M 3 B -M 2 B= (n+½)λ Þ destructive interference If mirror M 3 is moved with constant speed, v, towards B then the optical signal (in the absence of a sample) due to the recombined beam will vary sinusoidally. Since the mirror is moving with constant speed, then the sinusoidal variation can be mapped as a function of path-length difference or time.

54 Michelson Interferometry The signal recorded by the detector as a function of time is then fed into a computer which performs a FT to convert it into the frequency regime. If our beam consists of two well defined wavelengths The signal vs. time spectrum is known as the "interferogram" If we measure a signal V(t) over a total Measurement time of T then we may write the FT as : For a polychromatic source the interferogram is more complex:

55 Logic for FTIR The key to FTIR spectroscopy is to perform two experiments one with and one without the sample present. The difference between the two computed spectra is due to the sample. Finiteδ D( δ ) = 1 = 0 if if δ δ or δ Box car truncation function B( υ) = I( δ ) D( δ )cos 2πυδ dδ I( δ ) = const, Single spcetral line of B( υ) = 2 sin C(2πυ ) B( υ) = 2B( υ1)sin 2π ( υ υ1) freq.

56 Application * Chemistry First overtone band of solid parahydrogen Far-IR when mass is big (with NMR) Inorganic molecule heavy atom Bending Mode, Stretching mode * Physics light transmission Reflection Emission light scattering band gap superconducting gap dielectric const excitation Electron rotation, vibration...