InstrumentationInstrumentation

Applied Spectroscopy

1) Sources

2) Wavelength Selectors

3) Detectors

Recommended Reading:

Spectrophysics, Thorne Chapters 11 and 12

Principles of Instrument Analysis,Skoog Holler and Nieman Chapter 7

Basic ComponentsBasic ComponentsFive basic elements in all spectroscopic instruments.

1) A stable source of radiation

2) A sample

3) A wavelength selector (spectrometer)

4)A radiation detector

5) A signal processor

Two main categories of spectrometer:

1) Dispersive spectrometers, use prisms and gratings to spread the wavelengths out spatially

2) Interferometric spectrometers, eg Michelson and Fabry-Perot.

Radiation SourcesRadiation Sources

Radiation SourcesRadiation SourcesRadiation (light) sources for spectroscopy must satisfy the following two requirements 1) sufficinetpower2) stabilityTwo types of Source1. Continuum Sources: give a broad featureless continuous distribution of radiation.UV region Ar, Xe, He discharge lampsVisible Tungsten Filament LampInfra Red Blackbody radiation from heated inert bodies, Nernst,

Globar2. Line Sources: produce relatively narrow bands at specific wavelengths generating structured emission spectrum

UV / Visible Hg and Na vapour lampsUv/Visible/IR Lasers

1+2) Line plus continuum sources contain lines superimposed oncontinuum background- medium pressure arc lamps, D2 lampSources may be continuous or pulsed in time

Continuum SourcesContinuum SourcesContinuum sources are preferred for spectroscopy because of their relatively flat radiance versus wavelength curves

Nernst Glower

Tungsten Filament

D2 Lamp Arc Lamp

Arc Lamp with Parabolic Reflector

Blackbody SourcesBlackbody SourcesA hot material, such as an electrically-heated filament in a light bulb, emits a continuum spectrum of light. The spectrum is approximated by Planck's radiation law for blackbody radiators:

( ) ⎟⎠

⎞⎜⎝

⎛−⎟

⎟⎠

⎞⎜⎜⎝

⎛=

1kThexp1

ch2P 2

3

νν

where h is Planck's constant, ν is frequency, c is the speed of light, k is the Boltzmann constant, and T is temperature in K

Blackbody SourcesBlackbody SourcesInfra RedGlobar : 1 - 40 µmSilicon Carbide (SiC) rod (50mm long 50 mm diameter) electrically heated to about 1400K

Nernst glower (ZrO2, YO2): 400 nm - 20 µmCylinder of rare earth oxide electrically heated to about 2000K.

- 1000-1500 K in air- λmax lies in IR- relatively fragile-low spectral radiance ( ~10-4 W·cm-2 ·nm-1 ·sr-1 )

Heated filaments (W incandescent lamp, QTH)

- 2000-3000 K in evacuated envelope

- greater radiance (U=σ·T4 ) Pλ ~ 10-2 W·cm-2·nm-1·sr -1

- greater UV-Vis output - λmax still in IR

- QTH heated up to 3600 K

Blackbody SourcesBlackbody Sources

( ) ( )( ) ( )

( ) ( ) 2W Hot

2

22

3600K

IsWgWI

gWI IgW

gW sW

+⎯⎯⎯ →⎯

⎯→⎯+

⎯⎯⎯ →⎯

Discharge LampsDischarge LampsDischarge lamps, such as neon signs, pass an electric current through a rare gas or metal vapor to produce light. The electrons collide with gas atoms, exciting them to higher energy levels which then decay to lower levels by emitting light.Low-pressure lamps have sharp line emission characteristic of the atoms in the lamp, and High-pressure lamps have broadened lines superimposed on a continuum.

Common discharge lamps and their wavelength ranges are:hydrogen or deuterium : 160 - 360 nmmercury : 253.7 nm, and weaker lines in the near-uv and visibleNe, Ar, Kr, Xe discharge lamps : many sharp lines throughout the near-uv to near-IRxenon arc : 300 - 1300 nm

The sharp lines of the mercury and rare gas discharge lamps are useful for wavelength calibration of optical instrumentation. Mercury and xenon arc lamps are used to excite fluorescence.

Discharge LampsDischarge Lamps

Hydrogen Discharge Lamp

Hollow Cathode Lamps (HCL)Hollow Cathode Lamps (HCL)Hollow-cathode lamps are a type of discharge lamp that produce narrow emission from atomic species. They get their name from the cup-shaped cathode, which is made from the element of interest. The electric discharge ionizes rare gas atoms, which are accelerated into the cathode and sputter metal atoms into the gas phase. Collisions with gas atoms or electrons excite the metal atoms to higher energy levels, which decay to lower levels by emitting light.

Hollow-cathode lamps have become the most common light source for atomic absorption (AA) spectroscopy. They are also sometimes used as an excitation source for atomic-fluorescence spectroscopy (AFS).

LasersLasersA laser is a coherent and highly directional radiation source. LASER stands for Light Amplification by Stimulated Emission of Radiation.A laser consists of at least three components:

1. a gain medium that can amplify light that passes through it2. an energy pump source tocreate a population inversion in the gain medium. Requires at least a three state medium3. two mirrors that form a

resonator cavity

Pump

0

1

2Fast Decay

Lasing

Pump so that N1 > N0 (population inversion)Pumping Methods:Optical - flashlamp, laserElectrical - capacitive electrical dischargeChemical - reaction leaving product in excited state

Gas LasersGas LasersGas lasers are typically excited by an electrical discharge.

Some gas lasers and their dominant lasing wavelength(s): nitrogen : 337 nm (pulsed) He-Ne : 632.8 nm (cw) Ar ion : 488, 541 nm (cw) CO2 : 10.6 µm (cw or pulsed)

excimer: ArF* - 248 nm, XeCl* - 308 nm (pulsed)

The gain medium in a dye laser is an organic dye molecule that is dissolved in a solvent. The dye and solvent are circulated through a cell or a jet, and the dye molecules are excited by flashlamps or other lasers. Pulsed dye lasers use a cell and cw dye lasers typically use a jet. The organic dye molecules have broad fluorescence bands and dye lasers are typically tunable over 30 to 80 nm. Dyes exist to cover the near-uv to near-infrared spectral region: 330 - 1020 nm.

Dye LasersDye Lasers

Semiconductor lasers are light-emitting diodes within a resonator cavity that is formed either on the surfaces of the diode or externally. An electric current passing through the diode produces light emission when electrons and holes recombine at the p-n junction. These lasers are used in optical-fiber communications, CD players, and in high-resolution molecular spectroscopy in the near-infrared. Diode laser arrays can replace flashlampsto efficiently pump solid-state lasers.

Semiconductor LasersSemiconductor Lasers

Diode lasers are tunable over a narrow range and different semi-conductor materials are used to make lasers at 680, 800, 1300, and 1500 nm.

Solid State LasersSolid State LasersThe gain medium in a solid-state laser is an impurity center in a crystal or glass. Solid-state lasers made from semiconductors are described below. The first laser was a ruby crystal (Cr3+ in Al2O3) that lased at 694 nm when pumped by a flashlamp. The most commonly used solid-state laser is one with Nd3+ in a Y3Al5O8 (YAG) or YLiF4(YLF) crystal or in a glass. These Nd3+ lasers operate either pulsed or cw and lase at approximately 1064 nm. The high energies of pulsed Nd3+:YAG lasers allow efficient frequency doubling (532 nm), tripling (355 nm), or quadrupling (266 nm), and the 532 nm and 355 nm beams are commonly used to pump tunable dye lasers.

Wavelength SelectorsWavelength Selectors

Resolving PowerResolving PowerA more fundamental concept than dispersion.

I0

I0/2Full Width at Half Maximum FWHM

Even an ‘ideal’ spectrometer, illuminated by an ‘ideal’ source of monochromatic light still has a finite width set by diffraction limits of the optics ⇒ instrument function.If λ and λ +δλ are the wavelengths of two monochromatic lines that can be just separated by a spectrometer, then the resolving power of the spectrometer is defined as

= Rayleigh Criterion

R δλλ

=

Note that R is dimensionless.

Also

νδν

δδνν

δλλ

~~

EE R ====

Wavelength SelectorsWavelength SelectorsIdeally the output from a wavelength selector should be radiation of a single wavelength or frequency (monochromatic). No real wavelength selector approaches this ideal. Instead a band of wavelengths is obtained. The effective bandwidth is an inverse measure of the performance of a wavelength selector.

Narrow bandwidth ⇒ better performanceTwo types of wavelength selectors usually encountered:

1) Filters

Interference Filters

Interference Wedges

Absorption Filters

2) monochromators

Prism type

Grating type

CharacterisationCharacterisation of Filtersof FiltersInterference and Absorption filters are characterized by three parameters:1) The centre wavelength of the transmitted radiation λo

2) Percentage of λotransmitted by the filter = % transmittance3) Effective Bandwidthi.e. The Full Width at Half Maximum (FWHM) of the transmitted line shape.

Range from UV → visible → IRBandwidths (Δλ / λ) ~ 1.5% with ~ 90% transmittance but can also get (Δλ / λ) ~ 0.15% with ~ 10% transmittance

Interference FiltersInterference FiltersWhite Radiation

Narrow band of radiation

Glass PlateMetal film

Dielectric Layer,eg MgF, CaF

θ

θ t

λ

λ’

λ

nλ’ = 2t / cos(θ)

for θ small then cos(θ) ≈1

nλ’ ≈ 2t

refractive index of medium = η

λ = ηλ’ ⇒

nηt2λ =

Interference WedgesInterference Wedges

continuous distribution of wavelengths

n

2tηλ(t) =

wavelength is now a function of position along the wedge

Absorption FiltersAbsorption FiltersTransmit a narrow band of radiation. Used in Visible Region only. usually made of 1) Colored Glass or 2) Dye suspended in gelatin

%

tran

smitt

ance

wavelength

Cutoff Filters

Performance of absorption filters is inferior to interference filters

Filters only give a fixed band of wavelengths but usually need to SCAN the spectrum, e.g. to measure absorption (or reflection) as a function of wavelength.

MonochromatorsMonochromatorsMonochromators - instruments designed for spectral scanning. Separate EM radiation into individual wavelength components. Monochromators for the UV, vis, IR similar in construction but different materials used.

All monochromatorshave the following common elements

1) Entrance Slits2) Collimating lens or Mirror3) Dispersing element (Prism or Grating)4) Focusing element5) Exit slit

MonochromatorMonochromator ComponentsComponents

Prism or

Grating

Entrance Slit

S

Collimating Lens L1

Exit Slit or

Photographic Plate

Disperser

Focusing Lens L2

f1 f2

λ2

λ1

If rays of wavelength λ and λ + dλemerge from the disperser at angles θ and θ + dθ, then the angular dispersion Da, of the spectrometer is defined as

DispersionDispersionDispersive spectrometers separate different wavelengths by spreading them out spatially. Dispersion is a measure of this spreadingHow do we quantify the spatial separation of wavelengths on the exit focal plane?

θ λ

λ+dλ

y1

θ +dθ

Dispersive Element(Prism, grating…)

y+dy

f

ddD a λθ

= Units: rad.nm-1.

The linear dispersion D, is a measure of the linear separation of the two wavelengths in the focal plane f of the lens or concave grating,

ddyD λ

=Units: mm.nm-1.

Reciprocal DispersionReciprocal DispersionA relationship between angular and linear dispersion can be obtained from the fact that for small angles dy = f dθ, where f is the focal length of the instrument.

Then D = f Da.

dd

f1

dydD 1

θλλ

==−

It is more usual to use reciprocal dispersion D-1 = dλ / dy

Units: nm.mm-1.

Explicit quantities for these expressions depend on the type of dispersive instrument used, e.g. prism or grating and will be derived below.

D -1 is typically around 0.01 – 2 nm.mm-1 in UV/Visible.

Prism InstrumentsPrism Instruments

λ2

λ1

L1

L2

SPrism

Prism can serve several different purposes in a spectrome

- change the direction of a beam- change the polarization of a beam- split a beam into two- disperse the beam

A variety of shapes and materials are available to perforfunctions.

Deviation and DispersionDeviation and Dispersion

αθα

θ

−+=⇒+=

−=−=

+=

2121222111

21

ii rrrid and rid

d d

If prism is at or close to position of minimum deviation then

r2α andi2

rr and ii

12121

=

−=⇒

==

αθ

Combine these relationships with Snell’s Law of refraction, Sin(i) = n.Sin(r) for refractive index n gives

( ) ⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛ +

=2

sin.n 2

sinisin ααθthe change in deviation θwith wavelength determines the angular dispersion of the prism

λnθ

λθ

dd

dnd

dd

⋅= and from Snell’s law( )

( ){ } 2122 2sinn1

2sin2dd

α

αnθ

−=

b

r

1

r

2

i

2

i

1

α

α

θd1 d2

( )

( ){ } λα

αλn

θθddn

2sinn1

2sin2ddn

dd

λdd

2122−==therefore

λddn

for a 60° prism sin(α/2) = 1/2 and then

( ){ } λλnθ

λθ

ddn

2n1

1ddn

dd

dd

212−==

λλθ

ddnn

ddDA ==

for values of n in the range 1.4 to 1.6 the first term on the RHS is approximately n to with in 4%, then

angular dispersion

Resolving Powerλd

dnbR = where b = length of the base of the prism

there will be no dispersion if n(λ) is constant

- dispersion in prism occurs because of the change in refractive index of the prism material as a function of wavelength

- if prism material exhibits normal dispersion, higher frequency (shorter wavelength) light experiences a higher refractive index than lower frequency (longer wavelength) light

λλθ

ddnn

dd

=because

(glass@357 nm) =1.94 x 10 -4 nm-1

(glass@825 nm) =1.78 x 10 -1 nm-1

λddn

λddn

Prisms not often used as dispersion elements because of non-constant DA with wavelength- produces non-constant bandwidth- means range of λ's projected onto exit slit varies with λ

LittrowLittrow MountingMounting

Wadsworth MountingWadsworth Mounting

90°

Prism Mirror

This is a constant deviation mounting. The Mirror rotates with the prism such that the deviation of the beam is always 90 degrees.

P

Reflecting back surface on prism

S

R

L

Diffraction GratingsDiffraction GratingsTwo types (1) Transmission Grating, (2) Reflection Grating

d

α β

βα

d

βα

for light of wavelength λ the condition for constructive interference is ( )β+α= sinsindmλ

m is an integer, the diffraction order

β is positive if it is on the same side of the normal as α, otherwise it is negative

Zero order (m = 0) means straight through transmission or specular reflection

Grating Equation

Definition of Blaze Angle Definition of Blaze Angle γγ

the angle γ between the groove facet and the horizontal is called the blaze angle of the grating

- diffraction angle depends on d

- longer λ's diffracted more than shorter ones (β 600 nm > β 500 nm )

- When m = 0 (zero order), sinα = -sin β or α = - β. In this case, all λ's are diffracted at the same angle

If blaze was parallel to the grating plane (γ = 0°), the zero order beam would also appear in the speculardirection (most of the reflected light not dispersed) (see diagram on next page)

If blaze angle ≠ 0°, specular and zero-order angles do not correspond and majority of the light is dispersed

In the special case when incident beam is along the surface normal, α = 0 and first-order beam is in specular direction

- in this case, β is twice the blaze angle, γ . The wavelength at this angle is called the blaze wavelength. (see diagram on next page)

Important points about Diffraction Important points about Diffraction GratingsGratings

( )

γsind sind

sinsindm

blaze

blaze

2βλ

βαλ

=

=

+=In the special case when incident beam is along the surface normal, α = 0 and first-order beam is in specular direction

Dispersion and Resolving Power of a GratingDispersion and Resolving Power of a Grating

Angular dispersion can be found by differentiation the grating equation

( ) ( )βλβα

βλβα

βλβ

cossinsin

cosdsinsind

cosdm

ddDA

+=

+===

Near the grating normal, cos(β) ≈ 1, the dispersion has an almost constant value of m/d giving an almost linear wavelength scale

dWmmN R ==

Resolving Power R = order × number of grooves on the grating

number of grooves = width of grating / distance between grooves

gratings for the visible and UV typically have between 600 and 1200 lines/mm.

a 600 lines/mm grating used in first orderhas an angular dispersion of 6 × 10-4 rad.nm-1 which gives a reciprocal dispersion of 1.6 nm.mm -1 with a 1m focal length spectrometer.

ExampleExample

a 1200 lines/mm grating used in second order in the same spectrometerhas an angular dispersion of 24 × 10-4 rad.nm-1 which gives a reciprocal dispersion of 0.4 nm.mm -1 with a 1m focal length spectrometer.

The same 1200 grating in a 3m spectrometergives a reciprocal dispersion of 0.13 nm.mm -1.

For fixed values of α and β, nλ is constant.Example: nλ = 600.0 nm n: 1 2 3 4

λ (nm): 600 300 200 150

Comprised of- dispersive element- image transfer system (mirrors, lenses and adjustable slits)an image of the entrance slit is transferred to the exit slit after dispersion.

MonochromatorsMonochromators

One of the most common arrangements is the Czerny-Turnermonochromator:

S

R

M

G

D

LittrowMount

Other Grating MountsOther Grating Mounts

MG

S1

S2

Ebert Mount

Wavelength SelectionWavelength SelectionWavelength selection is accomplished by rotating the grating

Since angle between the entrance slit, grating and exit slitis fixed(2φ ), grating formula can be expressed in terms of the gratingrotation angle θ (between grating normal and optical axis)

Since α = θ - φ and β = θ + φ , ⇒ mλ = d [sin(θ - φ ) + sin(θ + φ )]= 2d.sinθ.cosφ(the trigonometric identity 1/2(sin(A+B)+sin(A-B)) = sinA·cosB)

Grating formula now in experimental variables: θ (the gratingrotation angle) and φ (half-angle between the entrance, grating andexit and slit).

Dispersive Characteristics in the Focal PlaneDispersive Characteristics in the Focal Plane

for monochromator operation we are much more interested in dispersion at focal plane (exit slit), defined by the linear dispersion, Dl = dx/dλ

For a Czerny-Turner arrangement, the linear dispersion is:

Dl = f × DAwhere f is the focal length of the focusing (exit) optic

inverse linear dispersion ( ) ( )βαβλsinsinf

cosD f DR 1A

1-D +

=== −

Grating: dλ/dyis constant

Prismdλ/dyvarieswith wavelength.

Dispersion: Grating vs. PrismDispersion: Grating vs. Prism

Spectral Spectral BandpassBandpass and Slit and Slit FunctionFunction

The spectral bandpass (nm) is the half-width of the range of wavelengths passing through the exit slit.

The geometric spectral bandpass

WD S 1g

−=where

D-1 is the inverse linear dispersionW is slit width

In a monochromator, an image of entrance slit is focused at the exit slit:

- when input is polychromatic, a monochromatedversion of the image appears at the exit slit

- when input is monochromatic image, rotating the grating angleθ will sweep monochromatic image across the exit slit

Slit FunctionSlit Function

The total intensity t(λ) measured at the exit slit as image is translated is called the slit function

- for equal entrance and exit slits, shape istriangular

- for unequal entrance and exit slits, shape istrapezoidal with a base of s and half-width of Sg

Mathematically, the slit function is

Slit FunctionSlit Function

( )

( ) else everywhere 0t

ss when s

1t g0g0g

0

=

+≤≤−⎟⎟⎠

⎞⎜⎜⎝

⎛ −−=

λ

λλλλλλ

where

λ is the incident (monochromatic) wavelength at entrance slit

λ0 is the wavelength setting of the monochromator(the wavelength directed to the center of the exit slit)

ResolutionResolutionResolution quantifies how well separated two features are at the exit slit

Resolution is related to - linear dispersion (Dl)(or angular dispersion (DA), and physical dimensions of themonochromator, through the focal length f) and

- the slit width W

If the width of a single peak base is S (= 2sg ), then two features will just be completely separated when the wavelength difference between them is S

WR2WD2s2SλΔ D1

gs ==== −

Alternatively, we may adjust slit width to obtain resolution of two features separated by Δλs

DR2W sΔλ

=

Effect of Slit Width on Effect of Slit Width on SpectraSpectra

2.0 nm Bandwidth Δλs 0.5 nm Bandwidth

Abs

orba

nce

Abs

orba

nce

Radiation DetectorsRadiation Detectors

Transducer: Devices to convert radiant energy (electromagnetic radiation) into an electrical signal.

Detectors /Radiation TransducersDetectors /Radiation Transducers

Ideal properties

1. High Sensitivity

2. High signal to noise ratio (S/N)

3. Constant response over a wide wavelength range

4. Fast response time

5. Zero output in the absence of radiation

6. Electrical signal, S, should be directly proportional to incident radiant power P ⇒ S = kP

Detectors /Radiation TransducersDetectors /Radiation TransducersTwo general types of radiation transducer

1) Photon transducersUsed in visible and UV spectroscopy

- respond to incident photon rate- highly variable spectral response (determined by photosensitive material)- respond quickly (microseconds or faster)- single or multichannel (1-D or 2-D)

2) Thermal transducersUsed for IR spectroscopy

- respond to incident energy rate- relatively flat spectral response curves (determined by window and coating)- generally slow (milliseconds or slower)- usually single channel

ResponsivityResponsivity R(R(λλ) and Sensitivity ) and Sensitivity Q(Q(λλ):):

( ) ( )( ) λ ΦλXλR = ( ) ( )

( ) λ dΦλdXλQ =

whereX(λ) is output signal (voltage, current, charge)Φ(λ) is incident flux (W)

Plot of R(l) or Q(l) versus l is called the spectral response

Photon detectors are based on- photoconductive materials (MCT transducer)- photovoltaic cells (Si, Se photocell)- photoemissive materials (PMT's, phototubes)- semiconductor pn junctions (photodiodes)

Photo Detector Photo Detector CharacteristicsCharacteristics

TransducersTransducersTwo common types:1. Photoemissive:- Based on photoelectric effect:

electronphoton

electrons released only if hn > Emin ; number of electrons ∝ number of photons

2. Photoconductive-photons striking device cause an increase in electrical conductivity-e.g., photodiodes, semiconductors

Two classes of detector to consider:1. Single-Channel- monitor intensity of a single resolution element at a time.2. Multi-Channel

- monitors intensities of many resolution elementsat a time.

Photoconductive cell:

- semiconductor material (CdS, PbS, PbSe, InSb, InAs, HgCdTe, or PbSnTe) behaves like resistor

- in series with constant voltage source and load resistor

- voltage across load resistor used to measure the resistance of the semiconductor

- incident radiation causes band-gap excitation and lowers the resistance of the semiconductor

- most sensitive in near IR (PbS)

- sometimes cooling is necessary to reduce thermal band-gap excitation

Photoconductive CellPhotoconductive Cell

Photovoltaic Cell :

- thin layer of crystalline semiconductor (Se, Si, Cu2O, HgCdTe)sandwiched between two different metal electrodes.

- no bias but irradiation causes formation of electron hole-pair formation.

- electron migrates one way, holes migrate in opposite direction

- if resistance of external circuitry is small, microamps produced

- high sensitivity in near IR to UV (102 -106 V·W-1 )

- eg Fe-Se-Ag 300-700 nm R(λ) peaking near 550 nm.

Photovoltaic CellPhotovoltaic Cell

two electrodes enclosed in glass or silica envelope

- bias (70-180 V) is applied between two electrodes

- cathode is a photoemissive material (Cs3Sb, NaO, AgOCs) -emits photoelectrons

- current collected by anode

- photoemission only if hν > surface workfunction (1-5eV)

PhototubePhototube

- High sensitivity (10-3 -10-1

A·W-1 )

- Dark currents (typically 10-12 -10-14 A) caused by

- thermionic emission- field ionization (high bias)- ohmic resistance

- similar to phototubes - photoemissive cathode andanode

- multiple secondary electron emissive dynodes (MgO, GaP)

- each dynode is biased ~100 V more positive than previous to accelerate electrons from dynode to dynode

- gain per dynode, g, is typically 2-5

- total gain m = g n is 10 6 -108

- charge pulse at anode is few ns wide

PhotomultiplerPhotomultipler Tubes (Tubes (PMTsPMTs))

PMT Spectral Response CurvesPMT Spectral Response Curves

- R(λ) is a function of photocathode material

- very high sensitivity (10-105 A·W-1)

- Alternatively, the rate of charge pulses can be counted, a technique called photon counting.

- Dark currents in PMT's result from similar processes to phototube

- thermionic emission associated with the photocathode can be significant (multiplied by dynodes) 10-11 -10-7 A

- cooling the PMT (0 to -60 °C) helps.

Photodiode DetectorsPhotodiode Detectorscontains a reverse-bias semiconductor pn junction

- p-type semiconductor has excess holes (eg B-doped Si)- n-type semiconductor has excess electrons (eg P-doped Si)

- under reverse bias, depletion layer formed (resistivity of depletion layer is very high)- under irradiation, electron-hole pairs created that move under bias(holes → p-type, electrons → n-type)- momentary current is produced -ns or sub-ns- spectral response of a typical photodiode depends on band-gap of semiconductors used (typically near IR into near UV)

- R(λ) less than PMT (no internal gain) but Q(λ) constant over 6-7 orders of magnitude- poor sensitivity (10-2 - 1 A·W-1 )- can be made very small, ideal for use in multichanneldevices

Monitors intensities of many resolution elementssimultaneously-similar to FT-interferometry (multiplexed measurement), but in the frequency domain.

MultiMulti--Channel DetectorsChannel Detectors

Examples:- photographic plates- photodiode arrays (PDA)-Charge Integrating Devices (CID) and -Charge Coupled Detectors (CCD)

Most limited to UV/Vis

- based on pn-photodiodes constructed by semiconductor chip techniques

- regions of p-type Si deposited onto n-type Si crystal

- distance between elements is typically 25 or 50 um (up to 4096 elements per array)

- usually operated in the depletion layer is formed around each p-type islands

- upon irradiation (integration time) bias is turned off and electrons and holes are created in depletion regions

- holes migrate to the p-type islands and accumulate (max ~106 )

PhotoPhoto--Diode ArraysDiode Arrays

- during read-out period, each p-type region is interrogated

- thermal excitation of electron-hole pairs creates difficulties with long integration times - array is often cooled

- dynamic range 2-4 orders of magnitude

- at detection limit thermal excitation dominates

- sensitivity can be increased by coupling diode array with microchannel plate (MCP)

PhotoPhoto--Diode ArraysDiode Arrays

Charge Transfer DevicesCharge Transfer Devices

- photodiode arrays are inferior to PMT's in respect to sensitivity, dynamic range and signal-to-noise ratio

- charge transfer devices approach the performance of PMT's

- each pixel is metal oxide semiconductor

- negative bias is applied to each electrode, a potential well collects photogenerated holes

- more than 10 6 holes bleed onto adjacent pixels

- charge accumulated during the integration time can be integrated in two ways:

- charge-injection device (CID)- charge-coupled device (CCD)

(two electrode per pixel):(1) during integration, one electrode (B) more negative than theother (A) - all photogenerated holes are accumulated under B(2) voltage applied to A is removed and the surface charge measured on A.(3) potential on electrode B is switched to a positive potential,causing the holes to migrate to electrode A(4) charge under A is remeasured and the signal is the differencebetween the two measurements(5) positive voltage applied to electrode A to repel accumulatedholes and return system to initial state

Charge Injection Device (CID)Charge Injection Device (CID)

-three electrodes per pixel

substrate is p-type Si so electrons (not holes) are accumulated

- each pixel contains three electrodes

- following the integration period, a three-phase voltage transfers electrons in a step wise manner along a row

- readout process is destructive

Charge Coupled Device (CCD)Charge Coupled Device (CCD)

Thermal TransducersThermal Transducers

Cannot use photon transducers in IR region ⇒Thermal Transducers

Operation : Infrared radiation is absorbed by a blackbody and the resultant temperature rise is measured.Radiant power levels are small 10-7 – 10-9 WattsNeed to detect heat changes as small as 10-3 K⇒ need blackbody with low heat capacity and small size.

In general thermal detectors are very noisy ⇒ must chop signal and use Phase Sensitive Detection (PSD) Methods.

Thermal TransducersThermal Transducers

Thermal Transducers: ThermocoupleThermal Transducers: Thermocouple

- Thermocouple: based on thermoelectric potentialwhen two dissimilar metal wires e.g. Bi/Sb are in contact.

- junction attached to blackened disc of known area but small

heat capacity (0.8-40 mm).

- output is nV-mV range (limited sensitivity)

- Q(λ) constant over modest temperature range (10-10-10-7 W)

- moderate responsivity R(λ) 5-25 V·W-1

- junctions with different sensitivities are available

- response time limited by capacitance of wires to ms

- multiple junction thermocouples called thermopiles

Thermopile: Can detect changes as small as 10-6 K

Sensitivity ~ 10μV / μW

Thermistor bolometer:

Measure change in resistance as a function of temperature. Either a resistance thermometer made from metals (Pt or Ni) or semiconductor material (thermistor).

- blackened metal or semiconductor with narrow band-gap (0.8-40 meV)

- radiation excites electron-hole pairs which decrease resistance

- decrease in resistance is compared with unirradiated bolometer

- difference is amplified - Q(λ) constant 10-6 -10-1 W

- high responsivity R(λ) 1000 V·W-1

- response time 1-10 ms, Slow response no good for FTIR spectroscopy

- Example: Ge bolometer operated at 1.5 K ideal for 25 – 2500 μm range.

Thermal Transducers: BolometerThermal Transducers: Bolometer

Pyroelectric detector:

Pyroelectric Infrared Detectors (PIR) convert the changes in incoming infrared light to electric signals. Pyroelectric materials are characterized by having spontaneous electric polarization, which is altered by temperature changes as infrared light illuminates the elements.

- based on a piezoelectric material - eg TriglycineSulphate, DTGS

- non-centrosymmetric crystal has permanent dipole moment across unit cell - acts like a capacitor

- when irradiated crystal expands slightly, capacitance decreases, current flows

- high responsivity R(λ) up to 104 V·W-1

- Q(λ) constant 10-6 -10-1 W

- fast response time <10 ms, good for FTIR spectroscopy.

Thermal Transducers: Thermal Transducers: PyroelectricPyroelectric

COBE The COBE dewar was a 660 literliquid helium cryostat. It

provided a stable 1.4 Kelvin environment for the two cold instruments, the Far Infrared Absolute Spectrophotometer

(FIRAS) and the Diffuse Infrared Background Experiment

(DIRBE).

The first phase of the COBE science mission came to an end on Friday, September 21, 1990,

after 306 days of cryogenic operations as the last of the superfluid helium contained

within the dewar was consumed.

The colors represent temperature variations

with red indicating regions that are a

hundredth of a percent warmer and blue

indicating regions that are a hundredth of a

percent cooler than the average temperature of 2.7 degrees above

absolute zero.

COBE

Map of the sky