Transcript Laser Molecular Spectroscopy CHE466 Fall 2007

Laser Molecular Spectroscopy
CHE466
Fall 2009
David L. Cedeño, Ph.D.
Illinois State University
Department of Chemistry
Lasers
LASERS
Definition
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The first laser
was designed and made by Theodore Maiman in 1960.
Lasers work on the basis of stimulated emission, in which excited molecules are stimulated to emit
by a photon with frequency equivalent to the energy difference between the excited state and a
lower energy state (not necessarily the ground state).
LASERS
Stimulated Emission and Light Amplification
For a laser to work efficiently, the number of excited molecules (or atoms) must exceed the number
of molecules in the lower energy state. This is a process called population inversion and is not
spontaneous. Population inversion condition is required for a laser to work, because otherwise the
incident photon would be absorbed by the sample (recall that the rate of photon absorption is equal
to the rate of stimulated emission, B0n = Bn0). Population inversion requires external work (i.e.
energy input) via a process called pumping.
Incident photons
Pumping
Emitted photons
Population Inversion
Stimulated Emission
LASERS
Population Inversion Schemes
The efficiency of a laser is related to its ability to sustain a population inversion and regenerate a
population in the low energy state to initiate a new pumping cycle. As you may see a 2 level laser
would be very inefficient, because a population inversion may not be acquired as a result of
population equilibration. An additional level is then necessary to quickly move the pumped
molecules to another level in which population inversion is achieved. Stimulated emission then
occurs from this level. A four-level system is also common, and much more efficient because it
allows for fast replenishing of molecules in the lowest energy level for efficient pumping.
3 level laser system
4 level laser system
LASERS
Light Amplification and Optical Cavities
The other process necessary for a laser is the effect of amplifying the stimulated emission process.
For such a process the lasing medium must be enclosed in an accurately aligned optical cavity, in
which the emitted photons are confined to bounce back to the sample to stimulate further emission
(optical feedback). An optical cavity consists of a closed containment ended on polished surfaces
or mirrors that allow photon feedback and output. Mirrors could be planar or spherical.
Lasing medium
Output mirror (R2 < R1)
Mirror
(R1)
d
Optical cavities are also called resonators because the constructive interference of the travelling
photons will create standing photon waves that resonate at a frequency (n) related to the length of
the cavity (d):
n
nc
2d
Where n is the number of half-wavelengths that are built within the cavity and c is the speed of
light.
LASERS
Light Amplification and Optical Cavities
Example: Calculate the length of the optical cavity required to have one million half wavelengths
of photons with a wavelength of 337.1 nm.
nc n 106  337.1 nm
d


 16.855 cm
2n
2
2
Laser Cavity Modes
The cavity itself has different modes of oscillation of two types transverse (normal to photon
propagation) and longitudinal (along photon propagation). Cavity modes perturb the propagation of
the photons creating an interference pattern that is reflected in the output beam. The transverse
modes (also called TEM) affect the intensity distribution of the laser beam along the plane normal
to its propagation direction.
LASERS
Laser Cavity Modes
Laser cavities are designed to obtain a TEM00, in which there are no nodes and a Gaussian intensity
distribution.
Images from Photon Inc., www.photon-inc.com
Axial modes are usually very important in the design of a laser. Lasers operate only if n takes a
value in which the resonant frequency n of the cavity corresponds to the line width of the lasing
transition. The axial cavity modes interfere with the photons and “split” (modulate) them into
packets that are temporally separated with a frequency:
n 
c
2d
LASERS
n
Laser cavity
modes
Transition
band width
Modulated
laser output
Intensity
Laser Output and
Longitudinal Modes:
A laser usually operates in a
multimodal way. Note, however
that it is possible to reduce the
bandwidth of a laser transition
by selecting a single mode
operation. This requires a
shortening of the cavity at the
expense of reducing the
intensity of the output. Single
mode operation is the preferred
way for high resolution
spectroscopy when laser band
widths of 0.001 cm-1 are
needed.
n (Hz)
LASERS
Laser Threshold:
This term refers to the required pumping ability to maintain lasing action. Since resonance
inside a cavity decreases the potential output of a laser (call cavity loses), an additional
amount of energy is required to overcome such losses. Pumping is performed using
electrical power (electrical discharges), or photon energy (using flashlamps, arc lamps, or
another laser). The threshold is quantified in terms of a gain threshold (gt):
gt  
1
ln( R1R2 )
2d
The expression above is in terms of cavity design parameters, namely the reflectances of the
mirrors (R1 and R2) and the length of the lasing medium (d).
It is also related to the characteristics of the transition and the population inversion excess
(Nn – N0):
2 A0 n
gt  N n  N 0 
4n bw
 is the wavelength of the transition, nbw is the bandwidth of the transition and A0n is the
Einstein coefficient for spontaneous emission.
Pumping of a laser could be pulsed or continuous, which then determines the mode of
operation of the laser: pulsed or CW (continuous wave).
Laser Power and Flux
LASERS
The spatial coherence and directionality of a laser makes it advantageous over other light
sources, because it allows for large energy (or power) flux, i.e. the concentration of many
photons in a very small area.
The intensity of a laser is defined by the frequency and amount of photons leaving the
optical cavity. The net power or energy can be measured using special detectors. The power
(P) of a laser is defined as:
P = E/t
For a pulsed laser, E is the energy output measured using a Joule meter and t is the time
duration of the pulse. In a CW laser, the power is measured directly with a power meter.
The power flux is the power per unit area:
F = P/A
Example: Find the power and power flux of a pulsed Nd-YAG laser beam (1064 nm, 350
mJ/pulse, 10 ns pulse width) with a 5 mm beam diameter.
E 350 10 3 J
7
P 

3
.
5

10
W  35 MW
t
10 10-9 s
P
35 MW
MW
F 

178
A  (0.25) 2 cm 2
cm 2
LASERS
Laser Power and Flux
The spatial coherence and directionality of a laser makes it advantageous over other light
sources, because it allows for large energy (or power) flux, i.e. the concentration of many
photons in a very small area.
The intensity of a laser is defined by the frequency and amount of photons leaving the
optical cavity. The net power or energy can be measured using special detectors. The power
(P) of a laser is defined as:
P = E/t
For a pulsed laser, E is the energy output measured using a Joule meter and t is the time
duration of the pulse. In a CW laser, the power is measured directly with a power meter.
The power flux is the power per unit area:
F = P/A
Example: Find the power and power flux of a pulsed Nd-YAG laser beam (1064 nm, 350
mJ/pulse, 10 ns pulse width) with a 5 mm beam diameter.
E 350 10 3 J
7
P 

3
.
5

10
W  35 MW
t
10 10-9 s
P
35 MW
MW
F 

178
A  (0.25) 2 cm 2
cm 2
LASERS
Pulse Width Control
One of the main advantages of a laser beam as an spectroscopic tool is the control over pulse
width, in other words the possibility of tracking down ultrafast events which may be
photoinitiated. The pulse width of some commercially available lasers could be below 100 x
10-15 s, which could be compressed even lower into the subfemtosecond regime.
Q-Switching
The quality factor of an optical cavity (Q) is defined as the ratio of the photon frequency to
the width of the laser.
Q = n/n
Q is also related to the amount of energy stored in the cavity (Ec) and the amount of energy
allowed to leak out (Et) in a given amount of time t:
Q
2nEc t
Et
Q-Switching is an optical procedure that allows the compression of a pulse by
decreasing/increasing the cavity quality for a very short time. Since the energy stored in the
cavity is released in a shorter time, the power of the output pulse increases.
LASERS
Q-Switching
The switching is usually made by using an fast electrooptical device. The most common is a
crystalline material with a voltage dependent birefringence (i.e. doubly refracting).
Without Q-switching
Polarizer
M1
Laser medium
–V
+V
M2
Q-switched output
Pockels Cell
Q-switching produces pulse as short as 10 ns, only limited by the speed of the switching
process.
Mode Locking
LASERS
In order to obtain pico or femtosecond pulses, mode-locking is commonly used. This
consists of modulating the photons to a particular longitudinal cavity mode of an optical
cavity. The modulation guarantees phase and amplitude synchronization in which the
amplitude of the edge modes is increased at expenses of the internal modes. The modulation
is achieved by an acousto-optical device (a fancy light chopper) at a frequency equivalent to
one complete round-trip of light inside the cavity:
tr = 2d/c
The pulse width is dependent on the number of modes excited (2N+1) and their frequency
separation (n):
2
t 
(2 N  1)n
LASERS
Mode Locking
The figures below show the concept of mode locking (Figures courtesy of D. T. Moore,
www.unc.edu/~dtmoore)
The output of a cavity contains a random
distribution of longitudinal modes (red).
Amplitude and phase modulation to one
specific mode creates a mode locked output
(blue)
The dependence of the number of modes
locked is shown here. The red trace shows
wider pulses as a result of locking a smaller
amount of modes.
LASERS
Laser Frequency Changes: Non-linear effects and harmonic generation
Many spectroscopical applications require the acquisition of an spectrum in a wavelength
range. Unfortunately, most lasers have near monochromatic output which limits the user’s
choice of wavelength ranges.
Highly polarizable materials allow us to obtain photons of different wavelengths via the
interaction of the electric fields of the incident photon and the hyperpolarizability terms
(non-linear terms of the induced dipole) of the material. The induced dipole is:
m = aE + ½ bE·E + 1/6 gE·E·E + …
With E = Asin(2nt)
E2 = ½ A2(1-cos(2(2n)t)
E3 = A3 (3/4 sin(2nt) – 1/4sin(2(3n)t)
Thus second order scattered radiation form the material has a frequency that is doubled the
incident frequency. This process is called harmonic generation and allows the doubling,
tripling and quadrupling of laser radiation.
Lasers
Some typical lasers and their spectral output
Lasers
The Helium Neon Laser (Gas)
2p55s1
21S
3390 nm
2p54p1
23S
632.8 nm
2p54s1
1150 nm
Electrical pumping
He/Ne mixture (10:1 typical at low P)
Bandwidth: 1.5 GHz (at 633 nm)
CW operation
2p53p1
2p53s1
11S
More info at Olympus
He
Ne
2p6
Lasers
The Nd-YAG Laser (solid)
4F
5/2
Nd3+ is the lasing component. Ion is
embedded in an Yttrium-Aluminum-Garnet
matrix.
Pulsed operation, Q-Switched in most
applications (10 ns pulses), high output allow
frequency doubling, tripling and quadrupling
(532, 355, 266 nm)
Optical pumping
4F
4I
9/2
3/2
1064 nm
4I
11/2
Lasers
The Nitrogen Laser (gas)
C3Pu
First UV laser made, it operated with one
mirror.
Pulsed operation, with electrical pumping
(spark gap or thrystor switching) does not
require Q-switching, operates at ~ 1-10 ns
widths.
Used to pump dye lasers, and in MALDI-TOF
Mass spectrometry.
Electrical pumping
337.1 nm
X1Sg+
B3Pg
Lasers
An excimer is a metastable excited state of a
dimer molecule. In general the excited state
is more stable than the ground state.
Consider the Xe2 molecule. According to MO
theory, it will not be stable. However, the
excited state has better probability for living.
Pulsed operation, with electrical pumping
(thrystor switching) does not require Qswitching, operates at 10 ns widths and high
frequencies.
Common Excimer lasers:
ArF: 193 nm
KrF: 248 nm
XeF: 351 nm
KrCl: 222 nm
XeCl: 308 nm
XeBr: 282 nm
Used in eye surgical procedures
Electrical pumping
The Excimer Laser (gas)
Lasing
X1S+
Lasers
The Carbon Dioxide Laser (gas)
301
v=1
10.6 mm
9.6 mm
Electrical pumping
Near IR laser capable of delivering high
power output. Can be operated pulsed or
CW (1 kW). Pumping is done via collisional
excitation with N2.
101
201
v=0
N2
CO2
202
Lasers
The Dye Laser (solution phase)
The best tunable laser. Requires optical
pumping and works either CW or pulsed.
Power is usually 10% (or less) of pump
power at maximum of emission band.
Highly fluorescent dyes are commonly
used at small concentrations (mM).
Pulses are usually 1-1000 ns depending
on pump width.
optical pumping
S1
v’ = 0
XS0
v” = 0
For a list of commercially available dye lasers:
http://www.exciton.com/
Lasers
The Diode Laser (solid, semiconductor)
The cheapest lasers available. Also the
smallest ones. Based on n-p type
juctions
+
p
n
–
E’F
Conduction band
Impurity level closes the gap
EF
voltage
E”F
Valence band
Semiconductor bands
n-type: EF close to conduction
p-type: EF close to valence
Find more info at Molecular Expressions
n
j
p