optical methods typical components - 연세대학교...

by Prof. Myeong Hee Moon

Ch. 7 Components of Optical Instruments

• Optical Methods

AbsorptionFluorescencePhosphorescenceScatteringEmissionChemiluminescence

• Typical Components

1. Stable Source of Radiation2. Sample Cell (transparent)3. Detector (convert to signal)4. Signal Processor & Read out

Components of Optical Spectroscopy

source lampof heated solid

sample holder

wavelengthselector

photoelectrictransducer

signal processorand readout

h h h I

sample holder

wavelengthselector

Absorption

fluorescencephosphorescence

wavelengthselector

emission & chemiluminescence

7B. Sources of Radiation

Continuum

7B-1. Continuum Sources

For absorption & fluorescence Spectroscopy

7B-2. Line Sources

For atomic absorption spec. : atomic & molecular fluorescenceRaman spectroscopy

D2, Ar, Xe, Hg filled arc lamps

w filament

inert solids

hollow cathode lamp

7B-3. Laser Sources

LASER: 1960s

Extremely high intensity narrow band width (<0.01nm)Become very important tool in UV, VIS, IR region

Typical source: Rubby (GaAs): organic dye: Ar or Kr

Light Amplification by Stimulated Emission of Radiation

a. pumping :

10-13~10-15 s life time

laser excitation by electrical discharge, passage of electric currents to an intense radiant source

1. Mechanism of Laser actionPumping – spontaneous – stimulated – absorption

emission emission

7B-3. Laser Sources

b. spontaneous emission : random process, incoherent radiation but monochromatic

7B-3. Laser Sources

c. stimulated emission :

d. absorption :

same direction with phase – coherent emission

7B-3. Laser Sources

2.Light amplification

>number of photons

Lost by absorption

number of photons by stimulated

emission

Population inversion

Population inversionfavored in 4-level

system

3. three- and four-level laser systems

7B-3. Laser Sources

4. Useful Lasers

• solid state lasers : 1st type & wide usage, 3-level device- ruby (Al2O3 + 0.05% Cr(III) doped in Al(III) lattice)

Cr(III) : active lasing material, 694 nm

- Nd : YAG, most common in solid statesNeodymium in yttrium aluminum garnet, 4-level

give very high power at 1064nmpossible to double at 532nm

used for pumping tunable dye lasers

7B-3. Laser Sources

• gas lasers :

-He/Ne (632.8nm): most common, reliablelow power consumption

-Ar ion (514nm-green, 488.0nm-blue) – 4-levelAr+ : by e or RF discharge

4P 4S : fluorescence & Raman spec.-N2 (337.1nm) : pulsed mode by spark

used for exciting fluorescencepumping dye lasers (CO2– 10.6m)

-Excimer lasers (XeF: 351nm, KrF: 248nm, ArF: 193nm)

gaseous mixtures of He, F2, and one of (Ar, Kr, Xe) to form (ArF*, KrF*, XeF* --- excimers) by I (current)

Excimers: stable only at excited states.

7B-3. Laser Sources

• Dye lasers :

With organic compound fluorescing UV, VIS, IR4-level systemTunable 20~50 nmBand width : ~ 1/100 nm

Tuning : replace non-transmitting mirrorwith monochromator with reflection grating

Wavelength selection --- rotate grating

7B-3. Laser Sources

• Semiconductor diode lasers :

Egfor semiconductor

band gap

new type, nearly monochromatic

7B-3. Laser Sources

For conductors: band-gap energy is so small that electrons easilyPromoted to conduction band

- Types of Semiconductor diode lasers

: GaAs pn-junction diode975 nm (IR)band width 10-5nm withgrating integrated in resonant cavity

: Light sources for CD player, bar coded scannerbut only around red (IR region)can be overcome by frequency doubling

GaAlAs(900nm), GaP(550nm), GaN(465nm)too low energy for spectroscopy

(distributed-Bragg-reflector) laser diode

7B-3. Laser Sources

A frequency-doubled system for converting 975-nm laserOutput to 490 nm.

7C. Wavelength Selectors

For spectroscopic analysis, narrow band width needed

7C-1. Filters

interference filters, absorption filters (VIS only)

7C-2. Monochromators

a) Czerney-Turner gratingb) Bunsen prism type

Dispersion by prism. a) quartz Cornu types b) Littrow type

• Grating UV, VIS, IR

UV, VIS : 300~2000 grooves/nm(1200~1400)

IR : 10~ 200 (100 common) grooves/nm

From a master – casting to make replica gratingSurface coated with Al, Au, or Pt

Echellette-type

(CB + BD) = nCB = d Sin iBD = d Sin r

n = d (Sin i + Sin r)

n=1,2,3…

Concave grating - focusing function added

• Resolving power of monochromator

103 ~ 104 for typical UV-VIS monochromator

n: diffraction orderN: number of

grating blazes

• Effect of bandwidth 0.5nm bandwidth 1.0nm bandwidth

2.0nm bandwidth

7D. Sample Containers

Cells, cuvettes

Fused silica : UV below 350 nmSilicate glass : 350~2000 nmPlastic : VISNaCl : IR

7E. Random Transducers

7E-1. Introductionphotographic film transducers

Convert radiant E to V or I• Ideal transducers- high sensitivity- high S/N- constant response vs. wide range l- fast response time- zero output signal at no illumination

(dark current) S = k P + kdP: radiant powerS: electrical responsekd: dark current

• types of transducers

Photon – photoelectric UV, VIS, near-IR : poor cons. responseHeat – IR (lower sensitivity) vs. wide

7E-2. Photon Transducers

• Photovoltaic or Barrier-Layer cells

E I (10~100A) at the interface of semiconductor & metal

Max sensitivity at 550nm10% of max at 350, 750nm

VIS region

Photon hits semiconductor -- covalent bond broken--- e-holes--- e moves toward metallic film

current generated

• Vacuum phototubes

Photon

Electrons from cathode(photoemissive)

photocurrent

Photoemissive surface: Na, K, Cs, SbIndividual or multi-alloy

• Photomultiplier tubes (PMT)

106~107 electrons/photondynode

Features1. Sensitive to UV, VIS2. Fast response3. Dark current reduced

by cooling (-30oC)4. Care for exposure to

daylight

7E-3. Multi-channel photon transducers

# of transducer elements64~4096, 1024 (common)Storage cap: 10 pF for each diode

~10 to 100 photonsBased in silicon diode

7E-4 Photoconductivity transducers

Crystalline semiconductors(sulfides or selenides of : Pb, Cd, Ga, In)ie. PbS - 0.8~3m

Near-IR (0.75~3m)Change in electronic conductivity- Resistance decrease at absorption

7E-5. Thermal transducers

: Small blackbody :

: problem – thermal noiseto reduce, vacuum & beam chopping

Minute temp rise upon small radiant power(~1/1000 K) (10-7 ~ 10-9)

• thermocouples

Copper fused to constantan (alloy)

Potential between two junctions

T: 10-6K ~ 6~8 V/W

• Bolometers

Resistance thermometer (strips of metal Pt, Ni)Semiconductor – thermistorsLarge change in resistance, mid-IR

• Pyroelectric transducers

: triglycerin sulfate: (NH2CH2COOH)3H2SO4: fast response time --- most FT-IR

When electric field applied across dielectric material

Temperature dependent polarizer

Single crystalline wafers of pyroelectric materials- insullators (dielectric material) with

special thermal & electrical properties.

electric polarization induceddepending on temperature

7F. Signal processors & readouts

Signal processor:

Readout: d’Arsonval meterDigital meterPotentiometerRecordersCathode-ray tubes

• photoncounting

Output from PMT -- a pulse of electrons for each photonRadiant power is read by the number of pulses / unit time

(rather than average current or potential : analog type)

amplifier of electrical signalperform math. operations - integration, differentiation

7G. Fiber Optics

Late 1960s

• properties of optical fibers

Glass or plastic fibers (0.05 m id ~ 0.6cm): transmitting radiation (UV, VIS, IR)

Fiber, coating, medium

7I. Principles of FT-Optical measurements

FT-spectroscopy (1950s)

7I-1. Advantage of Fourier Transform

-Throughput adv. (Jaquinot) high S/Nlarge power to detector (than in dispersive instr.)

-High resolving power (/)thus, wavelength reproducibility

-Fast analysis, all signals reach detector at oncei.e. scan IR region 500~5000cm-1

if = 3cm-1 m=15000.5 s 750s or 12.5m

for each meas.

Decrease in width induce decrease in S/N(weaker source signal)

But detector noise does not increase

xAverage signal

Average noise

Increase S/N to 2, require N=4 4 spectra4x750s=50m

In FT method: measure all at oncedecrease multiple measurement time

7I-2. Time-Domain Spectroscopy

Conv.method ---- frequency domain spectroscopy f()f(t) change in radiant power with time

)2cos()2cos()( 21 tktktP

7I-3. Michelson Interferometer

Signal modulation : split beam into two beams & recombine

: measure intensity variationas a function of lengthof the path of two beams

difference in the path lengths

retardation: 2(M-F) =

path “a”

moving mirror

fixed mirror

detector

50% beamsplitter

sourcesample

FTIR apparatus

j- stop

Mirror travel

Frequency, (cm-1)

4004000

Interferogram:

Single beamspectrum of air:

H2O H2OCO2

Interferrogram: a plot of output power vs. ; time for mirror to move /2 cm

M M: moving velocity of mirror

: frequency of radiationc=3x1010cm/s

ftPP 2cos)(2

amplitude of theinterferrometer signal

radiant powerof beam

f: frequency of signalat detector

if v =1.5cm/s

2211 2cos)(2cos)()( BBP

2cos)()(

For continuum source

ftBP 2cos)()(

2cos)()( BP

tBP M2cos)()(

He-Ne lasertop of

beamsplitter

Perkin Elmer Galaxy 2000 FTIR

optical methods typical components - 연세대학교...

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