Laser matter interaction

Transcription

1 Laser matter interaction PH413 Lasers & Photonics Lecture 26

2 Why study laser matter interaction? Fundamental physics Chemical analysis Material processing Biomedical applications Deposition of novel structures 2

3 What is a Plasma? A quasi-neutral gas of charged and neutral particles which exhibits collective behavior (F. F. Chen) Fourth state of matter 99% Universe 3

4 Debye length ε k T λ = ( ne D 0 B e ) 1/ 2 2 ε o = permittivity of free space k B = Boltzmann constant T e = electron temperature n = electron number density e = electric charge High density Q Low density Debye sphere Q 4

5 Criteria for plasma 1. λ D << L 2. N D >> 1 3. ωτ n > 1 5

6 Plasma Production Low pressure cold cathode discharge Thermionic arc discharge RF produced plasma Solar plasma Laser-produced plasma 6

7 Laser-ablated plasma Laser Target Plasma Converging lens Characteristics 1. High temperature ( KeV) 2. High density ( cm -3 ) 3. High velocity ( 10 7 cm s -1 ) 7

8 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. 8

9 Various processes in plasma Collisional excitation/de-excitation/ionization Photo-excitation/ionization Bremsstrahlung (recombination) Inverse-bremsstrahlung (absorption of photon) 9

10 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) 10

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13 Adiabatic regime 13

14 Measurement Techniques for Plasma Parameters Diagnostics Optical Emission Spectroscopy Plasma parameters Electron temperature, electron density Absorption Spectroscopy Ground state electron density Fast Photography & Imaging Plume front velocity, vapour pressure, vapour temperature 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 14

15 Experimental setup for laser ablation and deposition 15

16 Formation of CN band: Temporal & spatial dependence C + N 2 CN + N - 2 ev Fluence: 20 Jcm -2 (violet B2 Σ+ X2 Σ+ band system): (0-1) at nm, (1-2) at nm, (2-3) at nm, (3-4) at nm (4-5) at nm, and (5-6) at nm. 16

17 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 obtain/discuss using other techniques (plume splitting, instability). 17

18 ICCD images of expanding Al plasma in N 2 at 88 mj 0 ns 20 ns 40 ns 60 ns 80 ns Distance R (mm) 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) Delay time (ns) Distance R (mm) Delay time (ns) Distance R (mm) (b) R(t) = 1.5t Torr (c) R(t) = 3.2(1-e -0.04t ) 10 Torr R(t) = 0.87t Torr (d) Delay time (ns) 100 ns Shock wave model Vac 0.01 Torr 0.1 Torr 120 ns 140 ns 160 ns R = ξ o E ρo Drag Model 1/5 2 /5 R = R max (1-e -βt ) t 18

19 Distribution of various atomic species within the plasma Shifted Maxwellian velocity distribution D Alessio et al, Appl. Surf. Sci , 113 (2003) 19

20 Lateral dimensions of plasma using nano- and picosecond pulses (a) (b) 0.1 Torr nitrogen ambient 1 Torr nitrogen ambient 20

21 Nano-, pico- and femto-second laser ablation 80 ps, 3.7 J cm ns, 4.2 J cm fs, 0.5 J cm µm thick steel foil Chichkov et al, Appl. Phys. A 63, 109 (1996) 21

22 Thermal diffusion length L = ( 2 τ p th D th 1/ 2 ) Al Diffusion length Pulse width 1.6 µm 8 ns 100 nm 35 ps 6 nm 100 fs Energy absorbed in diffusion length F = ( 1 R) Iτ Reflectivity Wavelength p Si µm nm 22

23 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. 23

24 Drawbacks of PLD Formation of droplets. Impurities in the target material. Crystallographic defects in the films caused by bombardment by high kinetic energy ablation particles. Inhomogeneous flux and angular energy distributions within the ablated plume. Droplets/splashing Flux/distribution 24

25 What structures can be grown by PLD? Nanoparticles Quantum well Nano-rods /wires Heterostructures, p-n junction Superlattices 25

26 Nanorods/wires ZnO nanorods on ZnO/Si film Jie et al, Appl. Phys. Lett. 86, (2005) Various shapes of ZnO nanostructures Kwok et al, Appl. Phys. Lett. 87, (2005) 26