lecture 11 - concordia universityusers.encs.concordia.ca/~nrskumar/index_files/mech6491...lecture 11...

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MECH 6491 Engineering Metrology

and Measurement Systems

Lecture 11

Instructor: N R Sivakumar

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Spectroscopy

Introduction

Different Spectroscopy Methods

Fiber Optics

Introduction

Light Propagation

Fiber Optic Sensors

Fiber Bragg Sensors

Outline

Confocal Microscopy

Ordinary light microscope produces 2-D superposition of in-focus and

out-of-focus regions

Tracing only focused parts of photographs at different focus settings

difficult for specimens lacking clear borderlines

Slicing specimen into large number of thin sections is time-consuming

and may deform the specimen; cannot study living specimens this

way and alignment of images of different slices requires considerable

computer processing

In Confocal microscopy, a Laser beam illuminates spots on specimen

Computer compiles images created from each point to generate a 3-

dimensional image

Optical Principles

Specimen illuminated one point at a

time

Detector only registers light from

illuminated point

Resolution limit is improved

Very pronounced depth discrimination

is obtained

Can study different depth layers much

more clearly (no out-of-focus

information in image) and can study

surface structures

Why Confocal Microscopy?

3-D MICROSCOPY!

Scan a number of confocal images, refocusing

between successive images

Stack resulting images to produce 3-D structure

No need for alignment processing

Can record sections of live specimens

Can make projections of image to view from

different angles

Feasibility Studies

To successfully perform confocal

microscopy, specimen must be reasonably

transparent to allow light to penetrate to

regions below the surface of the specimen

Carlsson and Åslund selected three

different applications for their paper: two

physiological and one botanical

Feasibility Studies

Studied various neurons in spinal cord of lamprey

Used fluorescent confocal microscopy by staining cells with a dye

Obtained good understanding of 3-D structure of neurons by displaying projections through recorded volume in rapid succession

One difficulty in studying neurons is the specimen thickness (100s of μm): makes it impossible to scan deepest part of specimen using objective with large N.A.

Feasibility Studies

Studied ovule of Neuwiedia (an orchid)

Fragility of ovules limits physical slicing to 8μm; confocal microscopy allows specimen to remain intact and take ~1μm slices

Staining unnecessary due to autofluorescence from ovule wall and nuclei

Viewing sections more useful in this case

Digitization of data facilitates measurements of 3-D positions of nuclei, size of ovule, etc.

Feasibility Studies

Studied lung tissue from rat

Were able to avoid physical sectioning of specimen through confocal microscopy

Used data from individual sections directly for evaluation

Staining tissue before embedding in epoxy makes registering sections with high contrast possible

Stack of such sections allows measurement of interesting parameters concerning 3-D structure of lung tissue

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A method used to identify and/or quantify

elements, atoms, molecules, matter, molecular structure

by observing absorption, emission or scattering interactions

with electromagnetic radiation (“light”)

generally involves measuring the absorption or the

emission of light by sample

What is Spectroscopy

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From http://www.princeton.edu/~ehs/laserguide/section_1.htm

Electromagnetic radiation

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Equation: C = v l

Speed of light, C = 3 x 1010 cm/sec

Frequency, v = cycles per second in Hz

Wavelength, l = distance between adjacent waves cm

Wave number, W = inverse of Wavelength in cm-1

Electromagnetic radiation

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Absorption

“light” goes in, doesn’t come out

Emission

“light” is given off

Scattering

path of “light” is different than what went in,

frequency may be different too

Typical interactions

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Absorption of light

Visible/UV (most often quantitative)

IR (more qualitative)

ESR (microwave)

NMR (radio frequency)

Emission of light

Fluorescence

Chemiluminescence

Other

Light scattering

X-ray diffraction

ORD (optical rotatory dispersion)

Typical interactions

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What is a spectrum?

Graphical representation of what happens to electromagnetic

radiation

Ex. Infrared Spectrum (Aniline)

X: wavenumbers, y: transmittance

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Some common techniques

Mass Spectrometry (1940s)

Raman (1930s/1960s)

UV/Vis (1941)

IR (1951)

Nuclear Magnetic Resonance (1952)

Electron Spin Resonance

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Spectroscopy involves

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Sources Provide some incident light or energy to be absorbed

by the sample that results the emission or transmission

As simple as a heated rod (glowbar)

A basic light bulb with a tungsten filament

Hollow cathode lamps

Lasers

Can have:

a broad range of emitted light

white light

a narrow range of emitted light

laser sources

adjustable intensity (like a dimmer on a lamp)

fixed intensity (like a simple light switch)

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A typical UV-VIS Spectrophotometer has a Deuterium Gas Filled Lamp

for the UV-Range (200-350 nm) and a Tungsten Filament Lamp for the

visible to near IR range (350-1100 nm)

Some mechanism exists to switch from one lamp to the other when you

approach the threshold

Sources

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Acronym:

ROY G BIV

Monochromator =

Dispersion element

plus slit system

MonoChromatorsEvery Good Spectrometer Needs a Monochromator

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Monochromator = Wavelength Selector

Exit slit

MonoChromatorsEvery Good Spectrometer Needs a Monochromator

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MonoChromators

Used to select wavelength (or range) of light that impinges

on the sample (absorbance, fluorescence)

This selected wavelength then strikes the detector

the ability to select the wavelength helps you to discriminate

between phenomena caused by sample

and that caused by interfering or

non-relevant species

Various types

based on filters

based on prisms

based on gratings

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Filters Two types:

Interference filters depend on

destructive interference of the

impinging light to allow a

limited range of wavelengths

to pass through (expensive)

Absorption filters absorb

specific wavelength ranges of

light (cheaper)

Cutoff filters absorb light in a

specific range of wavelengths

Bandpass filters absorb light

outside of a specific range

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Prisms

First type of widely used, wavelength selection devices

Often made of salts such as sodium chloride, etc.

VERY delicate. Often subject to damage in humidity

Not widely used today in spectroscopy equipment

The spectrometer resolving power, R, is defined as the wavelength

over the smallest resolvable wavelength change

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Gratings Widely used in instruments today.

Light reflected off a surface is used for

selection of wavelengths

High resolution (<0.01 nm) if needed

Most expensive optical part of an

instrument

Resolving Power

Where m is diffraction

order and

Higher resolving power means a better

ability to separate light

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Monochromator - Configuration

Czerny-Turner

(most common)

Prism

(Old School)

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Slits Control the entrance of light into and out from the monochromator

Entrance slits control the light intensity entering the

monochromator and help control the range of wavelengths of light

that strike the grating

Less important than exit slits

Exit slits help select the range of wavelengths that exit the

monochromator and strike the detector

More important than entrance slits

Can be:

Fixed (just a slot)

Adjustable (effective bandwidth and intensity)

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No absorption

Direction of light altered

Reflection; Scattering

Refraction (as a prism) due to different refractive index

Diffraction – dispersion into wavelengths (diffraction grating)

Absorption by molecule of interest

Energy is transferred to molecules absorbing light. I is the

intensity of light (# photons/sec).

When Light hits the Sample …

Io (Incident light) I (Emergent light)

reflection

light path

scatter

Transmission

Absorption

Io (Incident light) I (Emergent light)

reflection

light path

scatter

Transmission

Absorption

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Absorbance

log scale

0 1 20 40 60 80 100

%T

012

linear scale

Absorbance vs. Transmittance

T = I1/I0 A = -logT = log (I0/I1) Or, log (1/T)

Beer-Lambert Law

where = molar extinction coefficient (M-1cm-1)

b = path length (cm); c = concentration (mole/liter)

Therefore, with this relationship you can see that absorbance (A)

is directly proportional to the pathlength (b) and the sample

concentration (c).

A = b c

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lmax

Abso

rban

ce

Concentration (M)

Abso

rban

ce

Concentration (M)

UV/Vis Spectroscopy

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Fluorescence Spectroscopy

Fluorescence is an emission phenomenon

The incident energy (light) kicks an electron of an atom from a

lower energy state into an excited state

Then, the electron releases energy in the form of fluorescent light

when it falls back to the ground (lower energy) state

The energy transition from a higher to lower state within the

molecule concerned is measured by the detection of this emitted

radiation rather than the absorption

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Fluorescence Spectroscopy

Fluorescent light (emission)

always has a longer

wavelength than the exciting

light

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Introduction

• Ellipsometry is an optical technique for determining properties of surfaces and thin films.

• The instrument analyses polarized light. (A change in the polarization upon reflection from a surface).

• It does not measure a film thickness, or a surface coverage, or a refractive index.

Optical methods are, for this purposes, indirect methods:

Information about the samples is not directly obtained,but requires calculations based on idealized models.

Film Thickness - Ellipsometry

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We are interested in...

• film thickness

• surface coverage

• refractive indexof the film


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The phenomenon of polarization

Malus’ law

2

0 cosII

Malus’ law describes the angular dependence of the intensity of light emitted from two polarizers in series. The angle is between the two polarization planes. I0 is the maximum intensity.

1cos00

0cos900

Maximum intensity

Nil intensity


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Polarized light

Superposition of two

linear polarized waves

Linear polarized light

No phase shift

90° phase shift, equal amplitudes


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The polarization can be described by:

• ratio of amplitudes

• phase shift

Elliptically polarized light

• 90° phase shift, unequal amplitudes• Phase shift other than 0° or 90°

elliptically polarized light


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Elliptically polarized light

Upon reflection the polarization will change

Upon reflection from the sample surface:

The amplitude ratio and/or the phase shift change

D (Del) and Y (Psi) quantify these changes

tanY is connected to the ‘amplitude ratio’,

and D is connected to the ‘phase shift’.

Ellipsometers measure two angles, called D (Del) and Y (Psi)


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When the light beam reaches the surface, some of the light is reflected and some passes into the material

The law of reflection says that the angle of the incidence is equal to the angle of reflection

Interaction of light with material

Snell’s law

N1 sin1 N2 sin2

for dielectric materials k 0

n1 sin1 n2 sin2


iknN

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The incident beam and the reflected beam define the

plane of incidence. Reflectivity.

Calculations are done with Fresnel’s formulas.

Reflectivity is calculated for linear polarized light

(p- polarized or s-polarized), yielding Rp and Rs.

P = parallel to the plane of incidence

S = perpendicular to the plane of incidence

Total reflection coefficients

Ep

Es

R – the ratio of the amplitude of the outgoing wave compared to that of the incoming wave


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medium1

medium2

p - WAVE

Hr

Er

Hi

Ei

Ht

Et

E - electric vector

H - magnetic vector

i - incident

r - reflected

t - transmitted


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s - WAVE

HrEr

Ei

Ht

Et

Hi

medium1

medium2

E - electric vector

H - magnetic vector

i - incident

r - reflected

t - transmitted


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The fundamental equation of ellipsometry

(precisely measured) (calculated for a given model)

tanYe(iD) = Rp / Rs

These total reflection coefficients (Rp and Rs) contain

complete information on the change in polarization.


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2112

2112

2211

2211

coscos

coscos

coscos

coscos

NN

NNR

NN

NNR

p

s

The fundamental equation of ellipsometry

iknN n=index of refraction

k=extinction coefficient

Optical constants n and k describe how light interacts with the material

v

cn

Speed of light in a vacuum

Speed of light in a material

k measures how much light is absorbed in the material and is dependent on l

1

2The incident angle

The refracted angle

N The complex index of refraction

1N

2N

The complex index of refraction of material 1The complex index of refraction of material 2

tanY e(iD) = Rp / Rs


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Quarter wave plate(compensator)

Light source Polarizer

Sample

Analyzer

Light detector

Apparatus Basics – nulling ellipsometer


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Ellipsometer


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Reflections with films

Multiple interfaces If more than one interface is present, the resultant reflected beam is made up of the initially reflected beam and the infinite series of beams which are transmitted from medium 2 back into medium 1

Rp and Rs in this case are function of b, where: 22 cos2 l

b Nd

l – the wavelength of the operation.

From Rp/Rs we find the thickness of the film.


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Examples

The delta/psi domain

The lower left quadrant is where the D /Y points for a free-film material (a substrate) will be located.The film-free values for several dielectrics, metals and semiconductors are shown

D


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Transparent filmExamples

The delta/psi trajectory for a transparent film of SiO2

(n=1.46, k=0) on a siliconsubstrate.The thickness where the trajectory closes on itself is a function of the angle of incidence, a wavelength and nair and nfilm.For measured D and Y there is a specific thickness of the film.


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ExamplesTransparent film

The D /Y trajectories for transparent films with different indices of refraction on silicon.Note that there are regions where the curves are not well separated. In these regions it is hard to determine the exact index of refraction.


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Absorbing film

Examples

The D /Y trajectory for a growing film of Ta on a silicon substrate.For a very thick film of an absorbing material, the delta/psi point will be characteristic of a substrate of the film material.The film-growth trajectory, therefore, is the movement from the silicon point to the tantalum point.


http://www.ccn.yamanashi.ac.jp/~kondoh/el

lips_e.html (ellips 2.xls)

http://www.ccn.yamanashi.ac.jp/~kondoh/ellips_e.html

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Fibre Optic Metrology

With a carrier frequency of some 1014 Hz, light has the potential of

being modulated at much higher frequencies than radio waves

Since the mid-1960s the idea of communication through optical fibers

has developed into a vital branch of electro-optics

Great progress has been made in many communication systems

From the viewpoint of optical metrology, optical fibers are an

attractive alternative for guiding light

An even more important reason for studying optical fibers is their

potential for making new types of sensors

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Light Propagation

Optical fiber consists of a central cylindrical core with refractive index n1,

surrounded by a layer of material called the cladding with a lower refractive

index n2

If < light will undergo total internal reflection between the core and the

cladding and propagate along the fiber, ideally with no loss

If > light will be lost. is an important parameter in fiber coupling

usually given by the NA

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Coupling of the light can be

done with the help of a lens,

Light Propagation

or by putting the fibre in close proximity to the light source and linking

them with an index-matching liquid to reduce reflection losses

demo-31_220K.rm

demo-31_220K.rm

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Sensors using optical fibers have the potential for sensing

acoustic pressure, magnetic fields, temperature, acceleration and

rate of rotation

Sensors for measuring current and voltage based on polarization

rotation induced by the magnetic field around conductors due to

the Faraday effect in optical fibers have been developed

Lot of standard optical equipment (Laser Doppler velocimeter)

has been redesigned using optical fibers

Fibre optic Sensors

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Temperature Sensor - Thin semiconductor chip is sandwiched between

two ends of fibers inside a steel pipe

Light coming through the fiber is partly absorbed by the semiconductor

This absorption is temperature-dependent and the amount of light detected

at the end of the fiber to the right is therefore proportional to the

temperature

Fibre optic Sensors

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Pressure Sensor

The optical fiber is placed between two corrugated plates

When pressure is applied to the plates, the light intensity transmitted by the

fiber changes, owing to micro-bending loss

Such systems have also been applied as hydrophones and accelerometers

Fibre optic Sensors

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Fiber-optic sensors based on interferometry - The fibers A and B can be

regarded as either arm in a Mach-Zehnder interferometer where the detector

will record an intensity which is dependent on the optical path length

difference through A and B

When fiber A is exposed to loads such as tension, pressure, etc., the optical

path length of A will change and one gets a signal from the detector

proportional to the load

Fibre optic Sensors

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Special application of optical fibers - two fibre bundles, A and B, are mixed

together in a bundle C in such a way that every second fiber in the cross-

section of C comes from, say, bundle A.

Shows two neighboring fibers, A and B. Fiber A emits a conical light beam.

Fiber B will receive light inside a cone of the same magnitude

Fibre optic Sensors

If a plane surface is placed a distance l in

front of the fiber ends, light will be

scattered back and the amount of light

received by fiber B will be proportional to

the area of overlap between the two cones

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A curve describing the relation between the received light

intensity IB versus the distance l therefore will look like that

given in c

For a distance l so long that the whole cross-section of fiber

B is covered with light, IB will have its maximum

A further increase in l will decrease the value of IB

Fibre optic Sensors

When bundle C is placed close to

surface the detector will get signal

proportional to distance

If it is in the linear portion, the

resolution can be in nanometers

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Most important innovation of the 1990s in optical fibre sensing is the

development of fiber bragg grating (FBG) sensors

FBG is a periodic perturbation of the refractive index along the fiber length

which is formed by exposure of the core of doped silica single-mode fibers by

UV laser light using phase masks

or more efficiently, directly during the drawing process by an interference

pattern of short laser pulses.

The formation of permanent gratings in an optical fiber was first

demonstrated by Hill et al. in 1978

Fibre Bragg Sensors

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FBGs work is similar to a reflection hologram

http://www.youtube.com/watch?v=CJGYVw8WpuQ

When the object and reference waves are incident from opposite sides of the

thick hologram emulsion, layers of metallic silver of the developed hologram

are parallel to the hologram plane.

By reconstructing the hologram in white light, the reconstructed wave would

be single colored with a wavelength equal to 2d where d is the layer spacing.

The refractive index variations in the core along the fiber length (the Z axis)

can be described as

Where q = 2/d is the frequency; nc is RI of Core (1.46); Dn is the amplitude

of RI variations and d is the grating period

Fibre Bragg Sensors

http://www.youtube.com/watch?v=CJGYVw8WpuQ

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When a wave ui(li) is incident from the left into a fiber, light with a narrow

bandwidth is reflected from the refractive index variations, maximum

reflectance occurring at the Bragg wavelength (lB = 2d)

The unreflected light is transmitted

Spectral Width Dl

Fibre Bragg Sensors

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The unreflected light is transmitted and a typical intensity profile for

the transmitted light is shown

The intensity profile of the reflected light is found by turning the figure

upside down

Fibre Bragg Sensors

Experiments have shown that

using L= 10 mm and d = lB/2n

= 1.5446/(2 * 1.46) = 0.53 µm,

gives Dl = 1.12 Å, which

compares quite well with the

experimental value of 1.75 Å

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FBGs have been used in fibre optic communication, optical

switching, optical signal processing and optical storage

FBGs as sensors - any change in fibre properties, such as

strain, which varies the grating pitch, will change lB.

Shifts in the spectrum are independent of the light intensity

With careful selection of the lBs, FBG sensors can be coupled

in tandem without affecting the measurand of each other

Fibre Bragg Sensors

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The sensitivity is governed by the fiber elastic, elasto-optic properties and the

nature of the load or strain applied to the structure that the fiber is attached to.

The shift in DlB due to strain is given by

Where the principal strains 1 and t are along and traverse to the fiber axis

respectively. If the strain is homogenous then

P11 and P12 are pockels coeff and is the poisson ratio

Fibre Bragg Sensors

Typical sensitivity to axial strains are

1nm/millistrain at 1300nm and 0.64nm/

millistrain at 820nm

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The temperature sensitivity is mainly due to the thermo-optic effect.

Figure shows DlB as a function of temperature at lB = 1556 m

Response is almost linear sensitivity =

FBG sensors can also be used to measure acoustic signals

Fibre Bragg Sensors

However, the sensitivity is quite

low (10-10 Pa-1) because the

glass fiber is very stiff

The sensitivity is also low for

measuring electric and

magnetic fields

lecture 11 - concordia universityusers.encs.concordia.ca/~nrskumar/index_files/mech6491...lecture 11...

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