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October, 2008
J. Brnjas-Kraljević
Light amplification by stimulated
emission of radiation
laser functioning is based on quantum processes
 energy states of atom are quantized
 atoms are emitting or absorbing visible light due to transition of
electron between quantum states of atom
 electron's transition is connected with emission or absorption of
quantum of electromagnetic radiation and its energy is equal to energy
difference of two states
 1917 year Einstein had postulated that the probabilities for
spontaneous emission and stimulated emission are connected
 light photon which excites atom and induces the transition of electron
in higher state with equal probability induces the relaxation of
excited atom to ground state
absorption of photon
– transition to higher state
stimulated emission
spontaneous emission
 light photons do not posses enough energy to induce the transitions
of electrons to excited state, the process which results in inversion
of population.
 laser mechanism is not possible in system with only two energy
states; at least three states are needed for such process
Stimulated emission
 if the photon with energy equal to the energy
difference of excited and ground state interacts with
electron in excited state, there is probability that
photon will induce the transition of electron into ground
state and simultaneously the emission of another
photon of equal energy
 in the condition of population inversion, significantly
higher number of electrons are in excited state and it
enables the stimulated emission of more photons
 the light amplification – emitted photons are correlated
in phase and time - the emitted light is coherent
 in processes of absorption, emission and stimulated
emission, photon must have energy exactly equal to
energy difference of two states
Inversion of population
 the inversion of population in favor of higher state is
requirement for laser transition
the inversion of population is the result of subsequent
absorptions of photons and a large number of transitions
of electrons to higher state
inversion could not be achieved in system with one
excited state, because it is short-lived, only 10-8 s, and
atom relaxes to ground state by spontaneous or
stimulated emission
the appropriate system should have two excited states,
one of them is long-lived
this state is metastable one, with different spin
orientation of electron, and the transition from this
state to ground state is of very low probability forbidden transition
He-Ne laser is the example of such three state system
He-Ne laser
 He-Ne laser is most frequent and
rather low cost gas type laser,
which emits red light of 632.8 nm.
 it can also produce the beam of
green light, 543.5 nm and
infrared, 1523 nm.
 the excitation energy of He is
20.61 eV, very close to
metastable state of Ne - 20.66 eV
 in collisions of atoms, energy is
transferred from He to Ne and it
is switched to excited state
 laser can be dangerous, it is out of
focus, power is 1 mW - Sun
brightness (0.1 watt/cm2)
generation of laser beam
He-Ne laser
it is “pumped” to
higher level by
energy of electric
Energy is
transferred by
collisions from
He to Ne and
raises it to
emission decreases
the population of
lower excited level
and keeps up
population inversion
Properties of laser radiation
 1. coherence. Different parts of laser beam are in phase.
This phase state is long enough for detection and
measurement of interference effects
 2. monochromatic. Laser beam has only one wavelength,
because the photons are generated by stimulated
emission from one atomic state.
 3. narrow beam. Inside laser cavity, photons are
reflected many times from end mirrors. Photons are
perpendicular to mirrors, so the laser beam is narrow and
almost not diverging
Coherent light
 coherence is special property of laser beam
 it is the consequence of stimulated emission – process
which enables the light amplification
 emitted photons are propagating together
 coherence is present in time and space domain
 natural sources do not produce the coherent light
Parallel beam
the emitted beam is extremely narrow
parallel mirrors – the front one transmits 99% of light
and the back one reflects 100% of light.
parallel beam can be dangerous, but also very useful
it is not allowed to look directly in laser beam, the
damage to eye retina can be immediate and serious
Monochromatic light
 the result of one atomic transition – one wavelength
 it is not totally monochromatic - Doppler effect of
moving atoms and molecules in laser
 the wavelength is small in comparison with laser cavity–
a lot of resonant modules
Basic data for laser
 Cw - continuous wave - emission is constant
 pulse laser
Parameters :
 wavelength: 488 nm – 1000 nm
 pulse duration: 100 ns – 10 fs
 energy density: 1 Jcm-2 – 1000 Jcm-2
Types of lasers
CO2 laser
 cw mode - power over 10 kW
 pulse mode – high power
 emission in infrared range
 pumping by energy of electric field, using nitrogen to excite
 efficiency is high - 30% (more than ordinary bulb, where 90%
of radiation is in the form of heat)
Argon Laser
 25 wavelengths in visible range 408.9 - 686.1 nm, best for green
light 488 nm and 514.5 nm
 power 30 - 100 W
 cw mode 9 - 12 kW
Neodium-YAG Laser
solid-state laser, crystal of itrium-aluminium-garnet (YAG) doped with Nd3+ ion
in cw mode the power is over 1 kW at 1065 nm
in pulse mode -very short pulses are produced
with resolution of 1 fs
Ruby Laser
 the fist one 1960g - T. H. Maiman. pulse mode at 694.3 nm.
Diode laser
 p-n semiconductor - galium doped with arsenic
 5 mW at 840 nm; 50 mW at 760nm; 20 mW at 1300 nm
Eximer Lasers
 Eximer means "excited dimer", laser system is excited twoatomic
emission in ultraviolet range
gasses of lantanides and halogen elements excited by electrons
molecules as XeF are stable in excited state and rapidly are
dissociated by transition in ground state. It is possible to achieve the
population inversion because the ground state becomes less populated
due to dissociation
excited states are not long-lived so the pumping must be very quick
eximer lasers emit pulses of high power in blue and ultraviolet range
Dye Lasers
they emit almost continuous spectrum
molecules of organic dyes – large number of spectral lines, each with
characteristical frequency distribution – overlapping of lines enables
laser effect
Laser – Tissue
Application in Medicine
Interaction with tissues
visible radiation 
excitation of electrons –
strongly absorbed, specially
higher frequencies –
induces heating - not ionization
strongly absorbed in surface
skin layer
higher energies, equal to
ionization energies of molecules
- photoionization
To determine possible interactions
we need to know:
 Optical properties of tissue (coefficients of reflection,
absorption and scattering)
 Thermal properties of tissue (specific heat capacity,
heat conductivity)
 Properties of laser radiation (wavelength, exposure
time, applied energy, focal spot size, energy density and
power density)
Optical properties of tissues
 parameters: intensity of transmitted, reflected and scattered radiation
 measured by spectroscopy
 parameters depend on temperature – it is necessary to measure simultaneously
incident radiation
detector Tc
transmitted and front
scattered radiation
detector Rd
detector Td
reflected and back
scattered radiation
Interaction types:
Photochemical interactions
Thermal interactions
Mechanical interactions
 Plasma induced ablation
 Photodisruption
All these different interaction types share a single common
datum: the characteristic energy density ranges from
approximately 1 J/cm2 – 1000 J/cm2, and the power
density varies over 1015 orders of magnitude, thus the
duration of laser exposure, distinguishes and controls all
these processes.
< 1 ns
From 1 ns – 1 ms
From 1 ms – 1 min
Continuous wave
The duration of laser exposure is mainly identical with
the interaction time itself.
Depending on laser power different effects on tissue are
 Low to medium power will induce certain chemical and
metabolic reactions which are called biostimulations.
 The enhancement of power results in thermal effects
which cause coagulation, cutting and melting.
 Wavelengths for which the main interaction process is
transmittion are used for biostimulation.
 For cutting, coagulation and treatment of defects one have
to choose the wavelengths for which absorption is dominant.
Photochemical Interactions
 Chemical interactions induced by light.
 Take place at very low power density (1 W/cm2), and long
exposure times (10 min)
 Wavelengths in the visible range (Rhodamine dye lasers
630 nm) are used because of their efficiency and their
high optical penetration depth.
 Specially adapted chromophores which are capable of
causing light-induced reactions are injected into the body.
After resonant excitation by laser irradiation these
molecules perform a series of chemical reactions resulting
in release of highly cytotoxic reactants which cause
irreversible oxidation of essential cell structure.
Photodynamic Therapy
 Photosensitive drug (photosenzitizer, hematoporphyrin
derivate) is injected into a vein of the patient. Within
next few hours it is distributed among all soft tissue
except the brain. It remains inactive until irradiated.
After 48 – 72 hours most of it is cleared from healthy
tissue while the concentration in tumor cells remains
constant even after period of 7 – 10 days. After
photosenzitizer is first transferred to an excited state
and then by relaxation to ground state transfers energy
to neighbor oxygen which transfer to very reactive state
and oxidize all tissue they are in contact with.
 Application:
 oncology – treatment of esophageal tumors
 dermatology – skin cancer
 neurosurgery, ophthalmology, gynecology
 Treatment of viral lesions (HPV, herpes) and psoriasis
Thermal Interactions
 Thermal effects can be induced by either
continuous wave or pulsed laser radiation when the
power density is > 10 W/cm2
 Primary heating
 Heat transfer
 Tissue reaction
Mechanism of interaction
 At microscopic level, thermal effects have their origin in
bulk absorption occurring in molecular vibration-rotation
bands followed by a nonradiative decay.
 Absorption: A + hn
 Relaxation: A* + M(Ekin)
A +M(Ekin + DEkin)
 Absorption of a photon promotes the molecule to an
excited state. Inelastic collision with some partner M of
the surrounding medium lead to a deactivation and a
simultaneous increase in the kinetic energy of M.
Therefore the temperature rises.
 Water has high absorption at 3 mm, so used lasers are
Er:YAG(2,94 mm), Er:YLF (2,8 mm)
Primary heating
 The source is the transfer of laser light into heat.
 Which percent of radiation will penetrate the tissue
depends on tissue reflectivity. Scattering determines
the path of radiation through the tissue.
 For l > visible light reflectivity is low, absorption is low
and radiation penetrates deep.
 Most of organic molecules have high absorption in UV
region so radiation does not penetrate deeply.
Heat transfer
 Heat is transferred mostly by conduction while
convection and radiation can be neglected.
 Coefficient of thermal conductivity influence the
transfer of heat. That parameter as well as the
exposure time influence the depth of penetration.
 In 1 ms heat will penetrate in water up to 0,7 mm in
Tissue reaction
 Final result of heating is tissue necrosis.
 Depending on the duration and peak value of the tissue
temperature achieved, different effects can be
 Hyperthermia
 Coagulation
 Vaporization
 Carbonization
 Melting
 Is caused by dissociation at small l (190 – 300 nm);
molecular bonds are braking and fragments of tissue
are ejected
 Excitation: AB + hn (AB)*
 Dissociation: (AB)*
A + B + Ekin
 Acts on surface (few mm)
 Power density 107 – 1010 W/cm2
 Lasers: excimer; mostly ArF(193 nm)
 Pulse duration: 10 – 100 ns
 It is used for non bleeding tissue for example in
ophthalmology for refractive corneal surgery.
Mechanical interactions
 Plasma formation – usage of big power on small area
causes ionization of atoms and formation of plasma.
 There is a big difference in pressure at the boundary of
ionized area which causes shock wave generation.
Propagation of this shock waves causes tissue disruption.
 Lasers: Nd:YAG, Ti:Saphire
 Pulse duration 100 fs – 500 ps
 Power density 1011 – 1013 W/cm2
 Application in ophthalmology for braking the
membranes after the lens implantation, for refractive
surgery, in dentistry for caries therapy ….
Mechanical interactions
 Cavitations – occurs when in soft tissue, due to
mechanical and thermal interactions, an explosive
vaporization does not happen but the gas bubble is
formed. This gas bubble then implodes.
 Laser beam is focused inside the tissue and the tissue is
split by mechanical force.
 Lasers: solid state lasers; Nd:YAG, Nd:YLF
 Pulse duration 100 fs – 100 ns
 Power density 1011 – 1016 W/cm2
 Application: posterior capsulotomy of the lens after
the cataract surgery, laser-induced lithotripsy of
urinary calculi.