Transcript Periodic Structures via Laser Matter Interaction

Periodic Structures via Laser
Matter Interaction
Alika Khare
Physics Department
Indian Institute of Technology Guwahati
Layout
• Introduction
• Lithography using high power laser
Interferometry
• Manipulation of atomic trajectories
via dipole force
• Future Scope
• Conclusion
• Acknowledgement
Introduction
Need for small sized periodic structures?
Example:
Information technology
Demand:
High Speed>>10Gb/s
Small storage space
Limitations:
1. Material Limitation
Bulk material:
2. Fabrication Limitation
VLSI
Slow response
80nm
Remedy
• Optical technologies
• Information carriers:
Photons
Photonics devices
Fast Optical devices
Tunability over wide spectrum of parameters
Temporal response
<10-12 s
Size
nanometer
What is to be done?
•  Synthesis of new Materials
• Materials having periodic structures
dimensions <100nm
• Quantum confinement effect
Drastic changes in optical, electrical mechanical,
thermal and magnetic characteristics of the
materials from its bulk behaviour and offers the
tremendous
scope
in
microelectronics,
optoelectronics and Photonics industries.
Manipulation of the materials via Laser
Matter Interaction
• Modification into surface morphology via
selective laser ablation of thin films
• Modification into the trajectories of atom
via dipole force
Lithography using High Power Laser
Interferometry
• Selective ablation of Thin films

Modification into the surface morphology
of the orders of tens of nm.
Two step process
• a. Deposition of thin films
b. Selective ablation
Single step
Simple set-up
a. Deposition of thin film
• Techniques used
• Thermal Evaporation technique
• Pulsed Laser Deposition Technique
Thin films used
• Thermal evaporation
Indium and chromium thin films
Pulsed Laser deposition technique for thin
films
• Schematic of experimental set-up
Experimental set-up
Advantages of PLD
• Applicable to any material
• Applicable to any form of the target
material:
Solid
Liquid
Gas
• By controlling the environment any
composition can be deposited
Pulsed Laser deposition
• Laser:
• 2nd harmonic of Q switched Nd: YAG laser
10ns, 10pps, 400mJ in fundamental
• Deposition time 5 minutes to 30 sec.
• Vacum 10-5-10 –6 Torr
• Deposition thickness 200nm-2m
Target Material
• Copper
• Silicon wafer
• Zinc Oxide
b. Selective Ablation of thin
films by High Power Laser
Interferometry
• The thin films can be selectively ablated by
illumination with the interference pattern
form by High power laser Interferometer.
Experimental Set-up
Interference Pattern
Intensity Distribution
Selective Ablation
• Part of the thin film illuminated by the
bright fringe will be ablated
• The dark fringe region will remain un
effected
• Thus selective ablation results into the
series of periodic lines of the materials
• (grating Structure)
AFM image of selectively
ablated Cu film
Three dimensional view
Periodicity 50 m
Minimum Line width ~5
m
Scale in m
Two dimensional view
Ref: AKhare et.al, Rad Phys and chem, 70,553-558 (2004)
Micrograph of selectively ablated
ZnO thin film
Periodicity
~20m
AFM image with (improved laser
mode structure)
Indium thin film in air
Scale nm
Further reduction into size
• By focusing the interference pattern on to
the thin film
L
Micrograph of selectively
ablated film via focusing of
interference pattern
Line Thickness< 1 m
Formation of two dimensional
arrays
• Two interferometer in tandem
• Out put of one interferometer illuminates the
second stage of interferometer
Four beam interference

Square arrays of two dimensional light spot
Four beam interferometer
Recorded CCD image of tiny arrays of
light spot from four beam interferometer
On illumination with such patterns, in the region of maximum
intensity tiny holes will be drilled
Ref: A S Patra and Alika Khare, Optics and Laser technology, (in press)
Four beam Interferometric setup
used for selective ablation
Square matrix of tiny holes
Sample:
Indium thin film placed in air for
selective ablation via four beam
interference
Micrograph after selective ablation
Scale 20 mX20 m
Results when the films were
placed under vacuum
After illumination with
the interfernce pattren
directly, beam
energy~20mJ
Scale in nm
Results when the films were
placed under vacuum
Enlarged image
Scale nm
Advantage of the technique
• Applicable to any material
• Complete writing in
Single step,
Single shot
• Structure size
tens of nanometer
• Relatively simple
Limitation of the selective ablation via
high power laser interferometer
• Periodicity
~
What is to be done to reduce
the periodicity?
• Manipulation of Atomic
Trajectories using Dipole force
Origin of dipole force
Interaction of induced dipole moment
with non-uniform near resonant light
distribution.
Dipole force
• Classically an atom placed in an
electromagnetic field is equivalent to a
dipole of dipole moment
p E (electric field)
Results into a force
F= -(p.E)
Hence
F  I (intensity of the field)
Dipole force
• Using Semi classical approach, expression
for the dipole force:

   ( I / 2 I )
F
2 ( I / 2 I )  4   
Where,
2

sat
2
2
sat
  det uning
  Natural linewidth
I  Intensity
I  Saturation intensity
sat
2
Configuration details

Mono energetic Collimated and diverging
Atomic Beams both
Laser field
Standing wave
Gaussain Beam
Atomic Beam
Single beam
Arrays of Beams
Simulated results
• Example Rubidium
• Energy level diagram
Standing Wave configuration
Atomic Beam
Standing wave
Simulated Results for Standing wave
• One Dimensional focused pattern of atoms
First Focus
Ref. A Khare, et al, Radiation Physics and Chemistry, 70, 553 (2004)
Multiple focus
Limitation
• Periodicity
/2
Arrays of micro-oven
•
•
•
•
New scheme:
Laser produced neutral atomic beams
Arrays of Micro-ovens in square geometry
Technique: Selective ablation of thin films via
four beam interferometer
Illumination from the rear side
•  Large number of atomic beams in square
geometry
Production of Arrays of Atomic beam
Cross-section of arrays of
Atomic beams
• Location of atoms in the launching plane
Focused pattern of arrays of
collimated atomic beams
Pattern for the divergent
atomic beam beam
Future Scope
Selective ablation technique is very general
and can be applied on any material with any
high power laser
 Formation of
• Tiny Arrays of laser
• Wave guide
• Optocoupler
• Photonic band gap material
Future scope
• The manipulation of atoms via dipole force
is a coming up field where the process has
to be understood fully, involving the atom
laser interaction. The concept of series of
micro-oven is yet to be perfected
experimentally. Periodicity and line width
both can be reduced by appropriate choice
of atomic beam system and laser field.
Conclusion
• Two schemes based on Laser matter
interaction for the generation of periodic
small structures
• Selective ablation via high power laser
interferometer
periodicity  1m
• Simulated pattern for the dipole force using
multiple atomic beam
• Periodicity as well as spot size ~tens of nm
Acknowledgement
• 1. Research Scholars
• Mr AS Patra and Mr Kamlesh Alti
• 2. Partial financial assistance from
• i. CSIR, New Delhi, India,
Scheme No. 03(831)/98/EMR-II
• ii. MHRD, New Delhi, India
Scheme No. F.26-1/2000/TSV
Thank you