ME 645: MEMS: ME 645: MEMS: Design Fabrication Design Fabrication - - PDF document

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ME 645: MEMS: ME 645: MEMS: Design Fabrication Design Fabrication

Lecture 17: Laser Fundamentals 1

Design, Fabrication Design, Fabrication and Characterization and Characterization

P.S. Gandhi P.S. Gandhi Mechanical Engineering Mechanical Engineering IIT Bombay IIT Bombay

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Acknoledgments: Prof S S Joshi, Mukul Tikekar

Motivation Motivation

Why study lasers?? What is their Why study lasers?? What is their importance?? importance?? MEMS context?? MEMS context?? Other micromachining techniques: Other micromachining techniques:

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compared with laser. compared with laser.

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Technology / Feature G t Minimum Feature Si /F t Feature Positional T l Material Removal Rate Materials

Capabilities of Various Capabilities of Various Micro Micro-

• machining

machining Technologies Technologies

Geometry Size/Feature Tolerance Tolerance FIB 2D/3D 200 nm / 20 nm 100 nm 5 μm3/s Any Micro- milling/Micro- turning 2D/3D 25 μm / 2 μm 3 μm 10,400 μm3/s PMMA, Al, Brass, mild steel Excimer laser 2D/3D 6μm /submicron submicron 40, 000μm3/s Polymers, ceramics, metals to a l d

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lesser degree Femtosecond laser/ 2D/3D 1 μm / submicron submicron 13,000 μm3/s Any Micro-EDM (Sinker or wire) 2D / 3D 25 μm / 3 μm 3 μm 25 million μm3/s Conductive materials LIGA / 2D Sub micron / 0.02μm ~ 0.5 μm ~0.3 μm NA Cu, Ni, polymers, ceramics

Light Light Basics Basics

E

1 mm 50 µm 3 µm IR - C IR - B (far - IR) (mid - IR) infrared (IR) electric field strength

x

1.4 µm 780 nm 380 nm 315 nm IR B IR - A UV - A (near - IR) (near - UV) ( d ) red green blue visible (VIS) wavelength λ

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H

280 nm 200 nm 100 nm 30 nm UV - B UV - C (mid - UV) (far - UV = FUV) (vacuum UV = VUV) (extreme UV = EUV) ultraviolet (UV)

• ptical

spectrum

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Light Basics Light Basics

1.2 MeV 1.2 keV 1.2 eV 1.2 meV 1.2 µeV 1.2 neV The Optical Range Photon 300 FHz 300 PHz 300 THz 300 GHz 300 MHz 300 kHz γ rays x rays infrared microwaves radiowaves ultra-violet visible energy Frequency

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h=4.14x10-15 eVs

1 pm 1 nm 1 µm 1 mm 1 m 1 km Wavelength 400 500 600 700 nm 3.3 eV 1.7 eV Violet Indigo Blue Green Yellow Orange Red Visible Spectrum

• Light is considered to

Light is considered to be consisting of QUANTA be consisting of QUANTA of energy,

• f energy,

known as PHOTONS. Each photon carries with it known as PHOTONS. Each photon carries with it amount amount of

• f

energy which depends upon its wavelength or frequency energy which depends upon its wavelength or frequency – – W = hf hf = hc hc/ /λ

Emission Basics Emission Basics

Wph

ph =

= hf hf = = hc hc/ /λ where h is Planck's constant = 663x10 where h is Planck's constant = 663x10-36

36 J s and c is the velocity

J s and c is the velocity

• f propagation of the
• f propagation of the light in

light in free space = 300x10 free space = 300x106 m/s m/s

• Absorption and emission of light between atomic levels: The

Absorption and emission of light between atomic levels: The excited atom can SPONTANEOUSLY (randomly) de excited atom can SPONTANEOUSLY (randomly) de-

• excite to a

excite to a lower level if a vacant site permits. The lower level if a vacant site permits. The average average length of time length of time an atom stays in excited state is tens of nanoseconds an atom stays in excited state is tens of nanoseconds

Other ways of excitation possible

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Wu Wl ∆W ∆W=hc/λ

Absorption Spontaneous emission

Emitted light Absorbed Photon Other ways of excitation possible

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BOLTZMANN BOLTZMANN Distribution Distribution

Emission Basics Emission Basics

• Wph

ph =

= ΔW = |W W = |Wu

u - W

Wl

l|

PHOTON PHOTON ENERGY ENERGY

• The population density of atoms, N

The population density of atoms, Nu, in , in an excited state, W an excited state, Wu, in relation to , in relation to those, those, N Nl

l, in a lower energy state

, in a lower energy state W Wl

l is

is given by the Boltzmann relationship, given by the Boltzmann relationship, as as

Energy exp(- ∆W/kT) Δ W4 Δ W3 Δ W2 Thermal equilibrium

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as, as, Nu/N Nl

l = exp[

= exp[-

• (W

(Wu-W Wl

l)/

)/kT kT] ] = = exp exp[ [-

• ΔW/

W/kT kT] ]

• Equilibrium between Absorption

Equilibrium between Absorption vs vs spontaneous emission spontaneous emission

Nu/Nl at a given temperature T Δ W2 Δ W1

Principles of Laser Principles of Laser

Thermal equilibrium

Thermal equilibrium balance balance

W2 Incident photon W = hf Stimulated photon W = hf

q

• f Absorption
• f Absorption vs

vs spontaneous spontaneous emission emission

A third mechanism also exists:

A third mechanism also exists: crucial to the formation of crucial to the formation of LASER action this process is LASER action this process is known as known as STIMULATED STIMULATED

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W1

Stimulated emission

EMISSION EMISSION

Not a dominant process in

Not a dominant process in thermal systems at room thermal systems at room temperatures. temperatures.

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Principles of Laser Principles of Laser

S i h i h lifi i b lifi i b S S i l d i l d

• LASER:

LASER: Light ight Amplification by mplification by S Stimulated timulated E Emission of mission of Radiation, has become adiation, has become synonymous with everything that is synonymous with everything that is high high-

• tech and futuristic

tech and futuristic

• Lasers work on the basic principle of

Lasers work on the basic principle of

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• Lasers work on the basic principle of

Lasers work on the basic principle of converting electrical energy into a high converting electrical energy into a high energy density beam of light through energy density beam of light through “stimulated emission” “stimulated emission” leading to leading to amplification amplification

Principles of Laser Principles of Laser

Stimulated emission:

Stimulated emission: atoms in an upper energy level atoms in an upper energy level can be triggered or stimulated can be triggered or stimulated by incident photon by incident photon

Incident photon must have an energy corresponding

Incident photon must have an energy corresponding to the energy difference between the upper and to the energy difference between the upper and lower lower states states

The

The incident photon is not absorbed by the atom incident photon is not absorbed by the atom. .

electron atomic E4 ionization y E

energy-level diagram of an atom atomic model (Bohr)

10 10 nucleus electron shell E2 E1 hν = E2 – E1

absorption

E2 E1 hν = E2 – E1

spontaneous emission

E2 E1 hν = E2 – E1

stimulated emission

2hν = 2(E2 – E1) E2 E3 E4 E1 excited states ground state energy A A A B B B

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The stimulated photon has unique properties:

The stimulated photon has unique properties:

Principles of Laser Principles of Laser

The stimulated photon has unique properties:

The stimulated photon has unique properties:

It is in phase with the incident photon

It is in phase with the incident photon

It has the same wavelength as the incident photon

It has the same wavelength as the incident photon

Travels in same direction as incident photon

Travels in same direction as incident photon

It is possible that the rate at which atoms are PUMPED into

It is possible that the rate at which atoms are PUMPED into

• ne of these states exceed the rate at which they leave. A
• ne of these states exceed the rate at which they leave. A

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large number of atoms can be excited into, and held in, the large number of atoms can be excited into, and held in, the upper state upper state

Atoms can stay in this

Atoms can stay in this metastable metastable state (de state (de-

• exciting only on

exciting only on stimulation by another photon) while the population is being stimulation by another photon) while the population is being built up this is known as a POPULATION INVERSION built up this is known as a POPULATION INVERSION

Principles of Laser Principles of Laser

Q: Laser corresponding to what wavelength?? Q: Laser corresponding to what wavelength?? Typical scenario

Energy exp(- ∆W/kT) W4 W3 De- excitation Population inversion

Q p g g Q p g g

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Population of atoms W2 W1 LASER ACTION

Pumping action

Population inversion Between W1 and W2

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Principles of Laser Principles of Laser

Another way this behavior is Another way this behavior is

• btained is with a four
• btained is with a four-
• level

level structure where the laser structure where the laser transition takes place between transition takes place between the third and second excited the third and second excited states states

Wp Wu W

Fast relaxation Laser transition Pump level Upper laser level energy

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we need depopulation of the we need depopulation of the lower laser level to be rapid to lower laser level to be rapid to ensure that the upper level is ensure that the upper level is always full and the lower level always full and the lower level always empty always empty

Wl Wg

Fast relaxation Lower laser level Ground state Pump In an optical resonant cavity only specific LONGITUDINAL

In an optical resonant cavity only specific LONGITUDINAL MODES OF OSCILLATION can be supported, Analogy: MODES OF OSCILLATION can be supported, Analogy:

Principles of Laser Principles of Laser

Resonant Amplification

standing waves on a stretched string standing waves on a stretched string

Only those modes corresponding to multiples of half a

Only those modes corresponding to multiples of half a wavelength can be supported and all other modes will die wavelength can be supported and all other modes will die away away

The

The longitunal longitunal cavity will have a finite width and will cavity will have a finite width and will support TRANSVERSE modes arising from waves traveling off support TRANSVERSE modes arising from waves traveling off-

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axis along the cavity; axis along the cavity;

These modes influence the spatial profile of the beam,

These modes influence the spatial profile of the beam, which are defined in terms of the Transverse which are defined in terms of the Transverse ElectroMagnetic ElectroMagnetic wave distribution across the cavity, TEM modes. wave distribution across the cavity, TEM modes.

The FUNDAMENTAL mode is the TEM

The FUNDAMENTAL mode is the TEM 00 00 mode: Gaussian mode: Gaussian

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In practice, photons need to be confined in the system to In practice, photons need to be confined in the system to allow the number of photons created by stimulated emission to allow the number of photons created by stimulated emission to exceed all other mechanisms. exceed all other mechanisms. This This can be achieved by bounding the laser medium between can be achieved by bounding the laser medium between two mirrors. this forms an OPTICAL RESONANT CAVITY one two mirrors. this forms an OPTICAL RESONANT CAVITY one

Principles of Laser Principles of Laser

mirror is totally reflecting and the other partially reflecting. mirror is totally reflecting and the other partially reflecting.

PUMPING SOURCE Excited atom Ground atom A LASER SYSTEM

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Random photons Rear mirror (Totally reflecting) Output mirror (Partially reflecting)

LASER OUTPUT

Stimulated photons

Th t d f ti f th it idth Th t d f ti f th it idth

Principles of Laser Principles of Laser

The transverse modes are a function of the cavity width The transverse modes are a function of the cavity width

Radiant

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Power Radius Beam diameter at 1/e2 points

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• Monochromaticity

Monochromaticity: Laser light is concentrated in a narrow : Laser light is concentrated in a narrow f l th l d th t ( t f l th l d th t ( t

Properties of Laser Properties of Laser Light Light

range of wavelengths lasers produce the purest (most range of wavelengths lasers produce the purest (most MONOCHROMATIC) light available. MONOCHROMATIC) light available.

Coherence: All the emitted photons bear a constant phase

Coherence: All the emitted photons bear a constant phase relationship with each other in both time and phase the relationship with each other in both time and phase the light is said to be COHERENT light is said to be COHERENT

A TRAIN OF COHERENT PHOTONS

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(a) Coherent Light (b) Incoherent Light

Coherence in space Coherence in time

Properties of Laser Properties of Laser Light Light

Beam divergence: All photons travel in the same direction

Beam divergence: All photons travel in the same direction the light is contained in a very narrow pencil almost the light is contained in a very narrow pencil almost COLLIMATED laser light is low in divergence (usually). COLLIMATED laser light is low in divergence (usually).

High irradiance: Radiance is the amount of power per unit

High irradiance: Radiance is the amount of power per unit area emitted by a light source for a given solid angle, (in watts area emitted by a light source for a given solid angle, (in watts per square meter per radian). The solid angle can be thought per square meter per radian). The solid angle can be thought

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• f as a cone through which the light passes. The lasers have
• f as a cone through which the light passes. The lasers have

high power outputs for the small areas which is used to emit high power outputs for the small areas which is used to emit their beam of light. Thus, because lasers have low divergence, their beam of light. Thus, because lasers have low divergence, it causes a transmission over a small angle, producing a high it causes a transmission over a small angle, producing a high radiance. radiance.

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Properties of Laser Properties of Laser Light Light

Unfocused Laser Beam Focusing Lens

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1 m

Light Bulb Power: 100 W Power Density at 1m distance: 8 x 10-4 W/cm2 Laser Beam Power: 100 W Power Density at 1m distance: 8 x 105 W/cm2

The basic requirements of any laser are similar, they all

The basic requirements of any laser are similar, they all comprise of: comprise of:

Practical Lasers Practical Lasers

An ACTIVE MEDIUM with a suitable set of energy levels to

An ACTIVE MEDIUM with a suitable set of energy levels to support laser action. support laser action.

A source of PUMPING ENERGY in order to establish a

A source of PUMPING ENERGY in order to establish a population inversion. population inversion.

An Optical cavity to introduce optical feedback so as to

An Optical cavity to introduce optical feedback so as to maintain the gain of the system above all losses maintain the gain of the system above all losses.

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maintain the gain of the system above all losses maintain the gain of the system above all losses.

Lasers are usually classified in terms of their active (lasing)

Lasers are usually classified in terms of their active (lasing) medium such as medium such as – –

Solid state laser

Solid state laser

Gas Lasers

Gas Lasers

Semiconductor lasers.

Semiconductor lasers.

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The Helium The Helium-

• Neon (

Neon (HeNe HeNe) LASER ) LASER

• The active laser medium is a gaseous mixture of He & Ne atoms, in

The active laser medium is a gaseous mixture of He & Ne atoms, in a a roughly 10:1 proportion. roughly 10:1 proportion.

• The gas is enclosed in a cylindrical quartz DISCHARGE tube sealed

The gas is enclosed in a cylindrical quartz DISCHARGE tube sealed at at h d b f h l d b f h l

Gas Lasers Gas Lasers

each each end by a mirror to form the optical end by a mirror to form the optical cavity cavity

• Pumping is done via an electrical discharge (A GLOW

Pumping is done via an electrical discharge (A GLOW DISCHARGE) DISCHARGE) created created between the electrodes. between the electrodes.

• A

A pulse of about 10 kV is pulse of about 10 kV is applied across applied across the electrodes to start the the electrodes to start the discharge. discharge.

• An

An electric current is induced through the gas; a steady current of 3 electric current is induced through the gas; a steady current of 3 to to 10 10 mA mA (dc) is sufficient to keep the discharge established. (dc) is sufficient to keep the discharge established.

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Gas Lasers Gas Lasers

• The lighter He atoms are excited by collisions with

The lighter He atoms are excited by collisions with electrons in electrons in the discharge the discharge

The

The He atoms collide with the heavier Ne atoms and He atoms collide with the heavier Ne atoms and transfer transfer their their energy to them. Ne atoms are excited by the energy to them. Ne atoms are excited by the collisions into collisions into their their metastable metastable state where population state where population inversion builds inversion builds up up

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inversion builds inversion builds up up

Laser

Laser light is emitted has wavelength of light is emitted has wavelength of 633 633 nm (red), nm (red), power in the range power in the range 0 0. .5 5 to to 50 50 mW mW; beam divergence about ; beam divergence about 1 1 mrad mrad. .

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The Argon-ion Laser

• Unlike the HeNe laser the active medium in the argon laser is a plasma of

Gas Lasers Gas Lasers

Unlike the HeNe laser, the active medium in the argon laser is a plasma of excited IONS.

• An electric discharge is created in a narrow tube of gaseous argon. The

argon atoms are first ionized and then excited by multiple collisions with electrons into their upper energy levels.

• Due to the high energy required to ionize and excite the argon atoms very

high current densities are needed, of the order of 1 A mm-2 .

• Argon lasers emit around 1 to 20 W of flux distributed amongst all the

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lasing wavelengths. 5 or 6 W can be obtained at the most powerful of these wavelengths, the 514 nm line.

• Common uses of argon lasers are holography, eye surgery,

spectrochemistry, optical image processing, semiconductor processing and last, but not least in terms of numbers of lasers supplied, laser light shows. Solid State Lasers

• Solid state lasers are characterized by having as their active medium,a

solid rod or slab of crystalline insulator doped with a small amount of

Solid State Lasers Solid State Lasers

solid rod or slab of crystalline insulator doped with a small amount of impurity.

• To help avoid confusion in terminology with the SEMICONDUCTOR

laser, solid state lasers are sometimes now referred to as DOPED INSULATOR LASERS.

• It is the impurity constituent which provides the required energy

structure to produce laser action.

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The Ruby Laser

• The ruby laser takes its place in history by being the first working laser

to be demonstrated.

• Theodore Maiman, working at Hughes Labs. In the USA, showed the

first working laser to the world in 1960.

• The active medium is a cylindrical crystal of synthetic sapphire (Al2O3)

doped with roughly 0.05%, by weight, of chromium ions (Cr3+) RUBY.

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Thank You Thank You

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in