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Fachgebiet 3D-Nanostrukturierung, Institut für Physik
16.12.2016
Contact: [email protected]; [email protected]
Office: Heisenbergbau V 202, Unterpörlitzer Straße 38 (Tel:
3748) Meitnerbau 1.2.106, Gustav-Kirchhoff-Straße 5 (Tel: 4902)
http://www.tu-ilmenau.de/3dnanostrukturierung/
Prof. Yong Lei & Dr. Yang Xu
Laserphysik
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Laser physics • Definition, principle, characteristics of laser
(class 1)
• Laser emission line broadening (class 2)
• Necessary condition: population inversion (class 3)
• Population inversion in 2-, 3-, and 4-level systems
(class 4)
• Producing population inversion: direct and indirect
pumping (class 5)
• Laser systems: low- and high-density gain medium
(class 6)
• Sufficient condition: saturation intensity (class 7)
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• Definition, principle, and characteristics of laser (class
1)
• Laser emission line broadening (class 2)
• Necessary condition: population inversion (class 3)
• Population inversion in 2-, 3-, and 4-level systems (class
4)
• Producing population inversion: direct and indirect pumping
(class 5)
• Laser systems: low- and high-density gain medium (class 6)
• Sufficient condition: saturation intensity (class 7)
• (class 8) Pumping pathways
• (class 9) Lab tour
• (class 10) Cavity modes:
Longitudinal mode (plane mirrors)
Transverse mode (two plane or curved mirrors)
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Exam
• Oral exam
• Time: 20 minutes
• Date: the first two weeks in March
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Definition
A laser is a device that amplifies light and produces a high
directional, high-intensity beam that most often has a very pure
frequency or wavelength.
Light Amplification by Stimulated Emission of Radiation
3 mm
1 m
Radiating Concentrating
1,000,000 times as bright!
Amplifying medium/ gain medium
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Principle: simulated emission
The energy difference E2-E1 is delivered in the form of a wave
(e.m.) that adds to the incident one.
0 0
0
0
0
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Characteristics
• Monochromaticity
• Coherence
• Directionality
• Brightness
• Short time duration Spatial coherence
Temporal coherence
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Emission broadening and linewidth
Classical emission linewidth of a radiating electron: Lorentzian
distribution (Homogeneous broadening)
Classical linewidth
Natural emission linewidth:
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Additional emission-broadening processes
Collisional broadening
• Dephasing collisions (homogeneous broadening)
• Amorphous crystal broadening
• Doppler broadening in gases (Inhomogeneous broadening)
• Isotope shifts
When many atoms are concentrated in a small volume to produce a
high atom density, their interactions with each other can produce
significant broadening of the emission linewidth in addition to the
effect they have upon the decay time of the level (collision
decay).
Decreasing the decay time τu of the atoms residing in the
excited level u A homogeneous process, Lorentzian-shaped emission
spectrum.
Not affecting the lifetime but affecting the linewidth
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Doppler broadening in gases
Natural emission linewidth (Lorentzian profile) of many
individual atoms traveling in two different directions, thereby
producing an overall Doppler profile: Gaussian distribution
(inhomogeneous broadening).
Doppler width:
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Population inversion
More atoms are excited into higher quantum energy level than in
lower energy level in the gain medium.
Not only excited atoms, but more excited atoms.
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Intensity at a specific distance z into the medium for
homogeneous broadening
• ΔNul > 0, beam intensity will increase exponentially with
distance z, meaning more atoms in upper level than in lower level,
population inversion, net transition rate or net flow of atoms will
be downward. producing a net amplification.
• ΔNul < 0, the beam intensity will decrease exponentially,
meaning more atoms in lower energy level than in upper level, the
net transition rate or net flow of atoms will be upward, the
applied signal absorbed or attenuated.
Gain
Gain coefficient
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Population inversion in a 2-level system
A cell of dimension L containing atoms of total density N at
RT.
The cell is square-shaped with optical-quality walls so that a
beam could be transmitted through the cell without being distorted
by the cell walls.
Boltzmann distribution: Nl≈N
Total decay rate
Pump atoms from level l to level u by shining light of intensity
of I0 and of frequency of ν0 into the cell:
When Nl/N reaches 0.5, no more energy can be absorbed, then no
mechanism to increase the population in level u!
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Population inversion in a 3-level system
Decay from l is largely greater than
the decay from u!
Flux rate of pumping from l to i must be
much larger than decay rate from u to l.
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Population inversion in a 4-level system
A higher level or a group of higher levels
At thermal equilibrium, under the Boltzmann relation,
essentially all the atoms will be in the ground energy level.
Pumping rate Rp0 (atom/second)
Gas lasers: electron impact in a gas discharge Solid-state
lasers: intense incoherent light from a pulsed flash lamp or an arc
lamp
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Producing population inversion
• Direct pumping
• Pump and transfer (indirect pumping)
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Direct pumping • Optical pumping: solid-state and
organic dye lasers. Absorption of the pumping light within the
gain medium.
• Particle pumping: gas and semiconductor lasers.
Disadvantages:
• Might be no efficient direct route from the ground state 0 to
the laser state u.
• Might be a better route from 0 to l. • Might be no good source
of pumping
flux: insufficient intensity for optical pumping or insufficient
electron density for particle pumping.
Γ0u
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Indirect pumping Intermediate level q: always much more closely
located in energy to upper laser
level u than to the initial level 0.
Advantages:
• τq>> τu, Nq>>Nu, a reservoir of
population, q-u transfer is much easier
than 0-u transfer.
• Pumping probability (cross section): 0-
q>>0-u, significantly lower the
pumping requirements.
• q-u transfer can be highly selective.
• q can belong to different species.
Energy transfer to laser level by a
collisional or a radiative process from
one species to another.
• q-level capability: a very broad width
and accepting pumping flux over a
broad range of energies.
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Transfer from below
Ar ion laser Ar++ laser Se24+ soft X-ray laser
Gas lasers
Level q: long lifetime, accumulating energy, serving as a
storage state.
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Transfer across The conservation of energy and momentum allow
efficient transfer from one excited state to another state, but
only if those two states have equal or nearly equal energy.
He-Ne laser
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He-Se laser
CO2 laser
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Transfer from above
• Most widely used excitation process: especially effective in
producing population inversion.
• Pump energy can occur over a wide range of excitation
energies: pumping band can have a range of wavelength of tens of
nanometers in visible spectral region.
• Population in level q preferentially decays to level u as
opposed to level l.
• Energy moves to level u from level q “automatically” or
without additional stimulus of any kind at a very fast rate because
of the thermalization process.
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Ruby laser Nd:YAG laser
Dye laser
The energy band of n-type doping lies higher than that of p-type
doping.
Level q and u are both part of the same energy band.
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Saturation intensity
Gain:
A simple relationship that provides the intensity at which the
beam stops growing exponentially. The length z of the medium at
which that saturation effect occurs can be expressed as the
saturation length Lsat. The intensity achieved by the beam when
z=Lsat will be referred to as the saturation intensity Isat
The laser beam can no longer grow exponentially above Isat.
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Saturation length
Lsat donate the length at which the beam reaches the saturation
intensity.
More simply as:
This gain will be referred to as the threshold gain and will be
indicated by writing the gain coefficient as gth. Lsat/da ratio
ranging from about 10 to 1000
It would be useful to obtain a value of the gain or the
exponent, at which the beam would reach the saturation
intensity.
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The exponential gain coefficient:
Minimum threshold condition for making a laser:
σul: essentially fixed for a particular laser transition.
Nj: ground state in many cases. • Solid-state laser: dopant
concentration (number ions per cubic meter) or the number of
laser species per unit volume. • Organic dye laser: dye
concentration mixed into the solvent. • Gas laser: density of the
gas or a separate species that can accumulate and store
population for laser transfer to the upper laser level.
Γju: provided by e.m. waves or light (optical pumping) or
particles such as electrons (particle pumping)
Producing gain
Detrimental to produce gain
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Pumping pathway (class 8)
• Particle pumping
• Electron collisional pumping
• Heavy particle pumping
• Optical pumping (geometry)
• End pumping
• Transverse pumping
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Particle pumping: electron collisional pumping
• Produced by applying an electric field in gas media: free
electrons, 106-107 m/s
• Electrons gain energy by electric field but lose it by
collisions with atoms or molecules
• Low electron energy: elastic collisions, no excitation
• High electron energy: inelastic collisions, raising the bound
electron to an excited energy state (excitation) or freeing the
electron from the atom (ionization)
• Electrons are accelerated again by electric field until it
suffers another collision
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Particle pumping: heavy particle pumping
• Any particles other than electrons: particles in metastable
atomic levels, ions, nuclear reactor fragments, or high-energy ions
produced by accelerators.
• Pumping flux
Allowed transition has a reasonable radiative transition
probability according to the electric dipole selection rules.
Electron collisional pumping:
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Optical pumping: pumping geometries
Transverse pumping
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Transverse pumping Energy input per unit length is constant,
more length generally provides more laser energy. Non-uniformity in
the gain medium in the transverse direction to the laser beam axis.
End pumping
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Optical pumping: transverse pumping in elliptical cavity
Weakly absorbing gain medium Strongly absorbing gain medium
Symmetric Asymmetric
Symmetric Double elliptical cavity
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Optical pumping: end pumping
Blue laser: emits EM-radiation with a wavelength of 360-480 nm,
human eye sees as blue or violet. Blue beams are produced by He-Cd
gas lasers at 441.6 nm, and Ar-ion lasers at 458 and 488 nm.
Semiconductor lasers with blue beams are typically based on GaN
(violet) or InGaN (often true blue, but also able to produce other
colors).
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Shuji Nakamura a Japanese engineer and inventor specializing in
semiconductor technology, professor at Uni-California, Santa
Barbara, the inventor of blue LED, a major breakthrough in lighting
technology. Together with I. Akasaki and H. Amano, he is one of the
3 recipients of 2014 Nobel Prize for Physics "for the invention of
efficient blue LED, which has enabled bright and energy-saving
white light sources". In 1992 he invented the first efficient blue
LED, and 4 years later, the first blue laser. Nakamura used the
material deposited on sapphire substrate, although the number of
defects remained too high (106–1010/cm2) to easily build a
high-power laser. By prior development of many groups, including
Professor Akasaki's group, and Shuji Nakamura made a series of
inventions and developed commercially viable blue and violet
semiconductor lasers. From: en.wikipedia.org
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Optical pumping: transverse pumping in elliptical cavity
Weakly absorbing gain medium Strongly absorbing gain medium
Symmetric Asymmetric
Symmetric Double elliptical cavity
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Porous Anodic Aluminum Oxide (AAO) Templates
Interesting and useful features:
• Ordered pore arrays + large area
• Nanometer-sized pores
• High aspect ratio
• Controllable diameter (10 – 400 nm)
• Length 100 μm Configuration diagram of the PAMs
Template-based techniques to prepare functional
nanostructures
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26.01.2017 Page 37
(a) (b)
Regular arrays of short (a) and long Ni nanowires (b) after the
removal of PAM, the
diameter is about 90 nm, the length is about 800-1000 nm (a) and
3-4 μm (b), respectively.
thus the aspect ratio of the nanowires are about 10 (a) and 40
(b), respectively.
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Schematic of
addressing system
(only shows an
array of 3 × 3)
Addressing System for 3-D surface nanostructures
with nano-scale resolution
3D Surface Nano-Patterning: Addressing
nanowire ‘1A’
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26.01.2017 www.tu-ilmenau.de/nanostruk Page 39
Addressing system
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1. The short-range pore regularity of UTAMs or AAO templates
→ Template with large-scale perfect pore arrays (to mm2 or even
to cm2)
3D Surface Nano-Patterning: nano-templates with large-scale
(up to 1 mm2) perfect pore arrays without defects
2. Hexagonal pore arrangement
→ Rectangular pore arrangement
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Templates with large-scale (1 mm2) perfect rectangular pore
arrays without defect
2010
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Templates with large-scale (1 mm2) perfect rectangle pore arrays
without defect
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TiO2 nanotubes grown in the template
(Before removing template)
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Addressing system
We success realizing perfect match between the
top of nanowire and the electrode lines !
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Addressing system
We success realizing good matching between
the top and bottom electrode lines !!!
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The pores of AAO templates and the prepared nanostructures are
identical, or
‘iso-pores’, Both for hexagonal and rectangular pore
arrangement.
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26.01.2017 www.tu-ilmenau.de/nanostruk Page 49
A B
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26.01.2017 www.tu-ilmenau.de/nanostruk Page 50
Ni L
Cd LS K
Ti K
Ag LNi L
o
C P
Al
Ag
Ag
Ni
200 nm200 nm
200 nm 200 nm
200 nm
200 nm 200 nm
200 nm
(a)
(b)
(c)
(d)
(e)
(f)
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Binary nanowire arrays realized by electrodeposition via
template
TiO2/Au TiO2/Ag TiO2/Ni
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Au nanodot array Au-Ag binary nanodot array
Ar ion milling and Ag evaporation
Binary nanodot array via ultra-thin pre-patterned alumina
template
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Ordered three pore arrays on a single domain
Three pore arrays
Extension of the technique for developing multicomponent
template
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Wen L.Y., Xu R., Mi Y., Lei Y.*, ‘Multiple nanostructures based
on
anodized aluminium oxide templates’, Nature Nanotechnology
(Impact
factor 35.267), in press, 2016.
(www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2016.257.html)
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Aurel Laser system for scribing, drilling and cutting
Laser structuring technology in ZMN (Lab tour: Meitnerbau)
LPKF Multifunction Laser Micro Machining Ststem
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Thank you!
