ph413 lasers & photonics · advantages of pulsed laser deposition (pld) • any material can be...

Laser matter interaction

PH413 Lasers & PhotonicsPH413 Lasers & PhotonicsLecture 26

A.K.Sharma

13.10.11

Department of Physics, IIT Guwahati

Why study laser matter interaction?

• Fundamental physics

• Chemical analysis

• Material processing

• Biomedical applications

• Deposition of novel structures• Deposition of novel structures

A.K.Sharma

13.10.11

2Department of Physics, IIT Guwahati

What is a Plasma?

A quasi-neutral gas of charged and neutral particles

which exhibits collective behavior

(F. F. Chen)(F. F. Chen)

• Fourth state of matter

• 99% Universe

A.K.Sharma

13.10.11


Debye length

• High density

Q

2/12

0 )(ne

Tk eBD

ελ =

εεεεo = permittivity of free space

k = Boltzmann constant

• Low density Debye sphere

Q

kB = Boltzmann constant

Te = electron temperature

n = electron number density

e = electric charge

A.K.Sharma

13.10.11


Criteria for plasma

1. λλλλD << L

2. ND >> 1

3. ωτωτωτωτn > 1

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Plasma Production

• Low pressure cold cathode discharge

• Thermionic arc discharge

• RF produced plasma

• Solar plasma

• Laser-produced plasma

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Laser-ablated plasma

TargetPlasma Converging

Laser

Characteristics

1. High temperature (∼∼∼∼ KeV)

2. High density (∼∼∼∼ 1021 cm-3)

3. High velocity (∼∼∼∼ 107 cm s-1)

Plasma Converging

lens

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What is Laser Ablation ?

When a short-pulsed, high-peak-power laser beam is focused onto any solid target,

a portion of the material instantaneously explodes into vapor. The name "laser

ablation" is used generally to describe the explosive laser-material interaction, a

more appropriate definition that does not imply a mechanism. Laser-material

interactions involve coupling of optical energy into a solid, resulting in

vaporization; ejection of atoms, ions, molecular species, and fragments; shock

waves; plasma initiation and expansion; and a hybrid of these and other processes.

A.K.Sharma

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Various processes in plasma

• Collisional excitation/de-excitation/ionization

• Photo-excitation/ionization

• Bremsstrahlung (recombination)

• Inverse-bremsstrahlung (absorption of photon)

A.K.Sharma

13.10.11


Laser-matter interaction regime

1. Evaporation regime (Laser-target interaction)

2. Isothermal regime (Laser-plasma interaction)

3. Adiabatic regime (Plasma expansion after the termination of the

laser pulse)

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A.K.Sharma

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Adiabatic regime

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13.10.11


Measurement Techniques for Plasma Parameters

Diagnostics Plasma parameters

Optical Emission Spectroscopy

Absorption Spectroscopy

Fast Photography & Imaging

Ion Probe Diagnostics

Electron temperature, electron density

Ground state electron density

Plume front velocity, vapour pressure,

vapour temperature

Electron temperature, electron density Ion Probe Diagnostics

Laser-Induced Fluorescence

Time-of-Flight Mass Spectroscopy

Interferometry

Laser Beam Deflection Method

Electron temperature, electron density

Ground state electron density

Velocity of species, states of ionization

Electron density

Density gradient, ablation threshold

A.K.Sharma

13.10.11


Experimental setup for laser ablation and deposition

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13.10.11


Formation of CN band: Temporal & spatial dependence

C + N2 ⇔⇔⇔⇔ CN + N - 2 eV

Fluence: 20 Jcm-2

(violet B2 ΣΣΣΣ+ −−−− X2 ΣΣΣΣ+ band system):

(0-1) at 421.6 nm, (1-2) at 419.7 nm,

(2-3) at 418.1 nm, (3-4) at 416.8 nm

(4-5) at 415.6 nm, and (5-6) at 415.2

nm.

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13.10.11


Fast photography of expanding plasma

• Plume dynamics of the plasma.

• Conservation of mass, momentum, and energy equations to

estimate physical parameters of interest.

• Plume length (optimized distance for thin film deposition).

• Interesting features which otherwise are not possible to• Interesting features which otherwise are not possible to

obtain/discuss using other techniques (plume splitting, instability).

A.K.Sharma

13.10.11


ICCD images of expanding Al plasma in N2 at 88 mJ

0 ns

20 ns

40 ns

60 ns

0 100 200 300 400 5000

1

2

3

4

5

6

7

R(t) = 3.2(1-e-0.04t

)

10 Torr

R(t) = 0.87t0.33

1 Torr

R(t) = 1.5t0.3

0.01 Torr

(d)

(c)

Dis

tan

ce R

(m

m)

Delay time (ns)

(b)

0 20 40 60 800.0

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 40 50

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Dis

tan

ce R

(m

m)

Delay time (ns)

Dis

tance R

(m

m)

Al in vacuum (< 10-5 Torr)

λ = 1064 nm E = 88 mJ

R(t) = 0.035t

Inset R(t) = 0.065t

(Ablation at early times)

80 ns

100 ns

120 ns

140 ns

160 ns

0.01 Torr 0.1 Torr

Vac

Delay time (ns)Delay time (ns)

Drag Model

R = Rmax(1-e-βt)

5/2

5/1

o

o tE

R

ρξ=

Shock wave model

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Distribution of various atomic species within the plasma

D’Alessio et al, Appl. Surf. Sci. 208-209, 113 (2003)

Shifted Maxwellian velocity distribution

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Lateral dimensions of plasma using nano- and picosecond

pulses

(a) 0.1 Torr nitrogen ambient

(b) 1 Torr nitrogen ambient

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13.10.11


Nano-, pico- and femto-second laser ablation

100 µm thick steel foil

80 ps, 3.7 J cm-2 3.3 ns, 4.2 J cm-2 200 fs, 0.5 J cm-2

Chichkov et al, Appl. Phys. A 63, 109 (1996)

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Thermal diffusion length

2/1)2( pthth DL τ=Diffusion length Pulse width

1.6 µm 8 ns

100 nm 35 ps

6 nm 100 fs

Al

Energy absorbed in diffusion length

pIRF τ)1( −= Reflectivity Wavelength

0.99 1.064 µm

0.3 248 nm

Si

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13.10.11


Advantages of Pulsed Laser Deposition (PLD)

• Any material can be ablated.

• Pulsed nature of PLD means that film growth can be controlled to any

desired amount.

• Laser is outside the vacuum chamber and therefore provides greater

flexibility in geometrical arrangements.

• Compositional fidelity is often retained between the target material and

the deposited film and hence is attractive for fabricating stoichiometric

multicomponent films.

• Amount of evaporated source material is localized to the area on which

the laser is focused.

• Kinetic energies of the ablated species lie in a range that promotes

surface mobility and avoid bulk displacement.

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Drawbacks of PLD

• Formation of droplets.

• Impurities in the target

material.

• Crystallographic defects in the

films caused by bombardment Droplets/splashing

by high kinetic energy ablation

particles.

• Inhomogeneous flux and

angular energy distributions

within the ablated plume.

Flux/distribution

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What structures can be grown by PLD ?

• Nanoparticles

• Quantum well

• Nano-rods /wires

• Heterostructures, p-n junction

• Superlattices

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Nanorods/wires

Kwok et al, Appl. Phys. Lett. 87,

223111 (2005)

Jie et al, Appl. Phys. Lett. 86, 031909 (2005)

ZnO nanorods on ZnO/Si film

Various shapes of ZnO

nanostructures

A.K.Sharma

13.10.11


ph413 lasers & photonics · advantages of pulsed laser deposition (pld) • any material can be...

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