Quantum cascade lasers are semiconductor lasers that emit in the mid- to
far-infrared portion of the electromagnetic spectrum and were first
demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori,
Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.
Unlike typical interband semiconductor lasers that emit electromagnetic
radiation through the recombination of electron–hole pairs across the material
band gap, QCLs are unipolar and laser emission is achieved through the use of
intersubband transitions in a repeated stack of semiconductor multiple quantum
well heterostructures, an idea first proposed in the paper "Possibility of
amplification of electromagnetic waves in a semiconductor with a superlattice"
by R.F. Kazarinov and R.A. Suris in 1971.
Intersubband vs. interband transitions Within a bulk semiconductor crystal,
electrons may occupy states in one of two continuous energy bands - the
valence band, which is heavily populated with low energy electrons and the
conduction band, which is sparsely populated with high energy electrons.
The two energy bands are separated by an energy band gap in which there are no
permitted states available for electrons to occupy. Conventional semiconductor
laser diodes generate light by a single photon being emitted when a high energy
electron in the conduction band recombines with a hole in the valence
band. The energy of the photon and hence the emission wavelength of laser diodes
is therefore determined by the band gap of the material system used.
A QCL however does not use bulk semiconductor materials in its optically
active region. Instead it consists of a periodic series of thin layers of
varying material composition forming a superlattice. The superlattice
introduces a varying electric potential across the length of the device, meaning
that there is a varying probability of electrons occupying different positions
over the length of the device. This is referred to as one-dimensional multiple
quantum well confinement and leads to the splitting of the band of permitted
energies into a number of discrete electronic subbands. By suitable design
of the layer thicknesses it is possible to engineer a population inversion
between two subbands in the system which is required in order to achieve laser
emission. Since the position of the energy levels in the system is primarily
determined by the layer thicknesses and not the material, it is possible to tune
the emission wavelength of QCLs over a wide range in the same material system.
Additionally, in semiconductor laser diodes, electrons and holes are
annihilated after recombining across the band gap and can play no further part in
photon generation. However, in a unipolar QCL, once an electron has
undergone an intersubband transition and emitted a photon in one period of the
superlattice, it can tunnel into the next period of the structure where
another photon can be emitted. This process of a single electron causing the
emission of multiple photons as it traverses through the QCL structure
gives rise to the name cascade and makes a quantum efficiency of greater than
unity possible which leads to higher output powers than semiconductor laser
diodes. Operating principles
= Rate equations= QCLs are typically based upon a
three-level system. Assuming the formation of the wavefunctions is a fast
process compared to the scattering between states, the time independent
solutions to the Schrödinger equation may be applied and the system can be
modelled using rate equations. Each subband contains a number of electrons
which scatter between levels with a lifetime , where and are the initial
and final subband indices. Assuming that no other subbands are populated, the
rate equations for the three level lasers are given by:
In the steady state, the time derivatives are equal to zero and . The
general rate equation for electrons in subband i of an N level system is
therefore: Under the assumption that absorption
processes can be ignored, the middle rate equation gives
Therefore, if then and a population inversion will exist. The population
ratio is defined as If all N steady-state rate equations are
summed, the right hand side becomes zero, meaning that the system is
underdetermined, and it is possible only to find the relative population of each
subband. If the total sheet density of carriers in the system is also known,
then the absolute population of carriers in each subband may be determined using:
As an approximation, it can be assumed that all the carriers in the system are
supplied by doping. If the dopant species has a negligible ionisation
energy then is approximately equal to the doping density.
= Active region designs= The scattering rates are tailored by
suitable design of the layer thicknesses in the superlattice which determine the
electron wave functions of the subbands. The scattering rate between two subbands
is heavily dependent upon the overlap of the wave functions and energy spacing
between the subbands. The figure shows the wave functions in a three quantum
well QCL active region and injector. In order to decrease , the overlap of
the upper and lower laser levels is reduced. This is often achieved through
designing the layer thicknesses such that the upper laser level is mostly
localised in the left-hand well of the 3QW active region, while the lower laser
level wave function is made to mostly reside in the central and right-hand
wells. This is known as a diagonal transition. A vertical transition is one
in which the upper laser level is localised in mainly the central and
right-hand wells. This increases the overlap and hence which reduces the
population inversion, but it increases the strength of the radiative transition
and therefore the gain. In order to increase , the lower laser
level and the ground level wave functions are designed such that they
have a good overlap and to increase further, the energy spacing between the
subbands is designed such that it is equal to the longitudinal optical phonon
energy so that resonant LO phonon-electron scattering can quickly
depopulate the lower laser level. Material systems
The first QCL was fabricated in the InGaAs/InAlAs material system
lattice-matched to an InP substrate. This particular material system has a
conduction band offset of 520 meV. These InP-based devices have reached very high
levels of performance across the mid-infrared spectral range, achieving
high power, above room-temperature, continuous wave emission.
In 1998 GaAs/AlGaAs QCLs were demonstrated by Sirtori et al. proving
that the QC concept is not restricted to one material system. This material
system has a varying quantum well depth depending on the aluminium fraction in
the barriers. Although GaAs-based QCLs have not matched the performance levels
of InP-based QCLs in the mid-infrared, they have proven to be very successful
in the terahertz region of the spectrum. The short wavelength limit of QCLs is
determined by the depth of the quantum well and recently QCLs have been
developed in material systems with very deep quantum wells in order to achieve
short wavelength emission. The InGaAs/AlAsSb material system has
quantum wells 1.6 eV deep and has been used to fabricate QCLs emitting at 3 μm.
InAs/AlSb QCLs have quantum wells 2.1 eV deep and electroluminescence at
wavelengths as short as 2.5 μm has been observed.
QCLs may also allow laser operation in materials traditionally considered to
have poor optical properties. Indirect bandgap materials such as silicon have
minimum electron and hole energies at different momentum values. For interband
optical transitions, carriers change momentum through a slow, intermediate
scattering process, dramatically reducing the optical emission intensity.
Intersubband optical transitions however, are independent of the relative
momentum of conduction band and valence band minima and theoretical proposals
for Si/SiGe quantum cascade emitters have been made.
Emission wavelengths QCLs currently cover the wavelength
range from 2.63 μm to 250 μm Optical waveguides
The first step in processing quantum cascade gain material to make a useful
light-emitting device is to confine the gain medium in an optical waveguide.
This makes it possible to direct the emitted light into a collimated beam,
and allows a laser resonator to be built such that light can be coupled back into
the gain medium. Two types of optical waveguides are in
common use. A ridge waveguide is created by etching parallel trenches in the
quantum cascade gain material to create an isolated stripe of QC material,
typically ~10 um wide, and several mm long. A dielectric material is typically
deposited in the trenches to guide injected current into the ridge, then
the entire ridge is typically coated with gold to provide electrical contact
and to help remove heat from the ridge when it is producing light. Light is
emitted from the cleaved ends of the waveguide, with an active area that is
typically only a few micrometers in dimension.
The second waveguide type is a buried heterostructure. Here, the QC material
is also etched to produce an isolated ridge. Now, however, new semiconductor
material is grown over the ridge. The change in index of refraction between
the QC material and the overgrown material is sufficient to create a
waveguide. Dielectric material is also deposited on the overgrown material
around QC ridge to guide the injected current into the QC gain medium. Buried
heterostructure waveguides are efficient at removing heat from the QC active area
when light is being produced. Laser types
Although the quantum cascade gain medium can be used to produce incoherent light
in a superluminescent configuration, it is most commonly used in combination
with an optical cavity to form a laser. = Fabry–Perot lasers=
This is the simplest of the quantum cascade lasers. An optical waveguide is
first fabricated out of the quantum cascade material to form the gain
medium. The ends of the crystalline semiconductor device are then cleaved to
form two parallel mirrors on either end of the waveguide, thus forming a
Fabry–Pérot resonator. The residual reflectivity on the cleaved facets from
the semiconductor-to-air interface is sufficient to create a resonator.
Fabry–Pérot quantum cascade lasers are capable of producing high powers, but
are typically multi-mode at higher operating currents. The wavelength can
be changed chiefly by changing the temperature of the QC device.
= Distributed feedback lasers= A distributed feedback quantum cascade
laser is similar to a Fabry–Pérot laser, except for a distributed Bragg reflector
built on top of the waveguide to prevent it from emitting at other than the
desired wavelength. This forces single mode operation of the laser, even at
higher operating currents. DFB lasers can be tuned chiefly by changing the
temperature, although an interesting variant on tuning can be obtained by
pulsing a DFB laser. In this mode, the wavelength of the laser is rapidly
“chirped” during the course of the pulse, allowing rapid scanning of a
spectral region. = External cavity lasers=
In an external cavity quantum cascade laser, the quantum cascade device serves
as the laser gain medium. One, or both, of the waveguide facets has an
anti-reflection coating that defeats the optical cavity action of the cleaved
facets. Mirrors are then arranged in a configuration external to the QC device
to create the optical cavity. If a frequency-selective element is
included in the external cavity, it is possible to reduce the laser emission to
a single wavelength, and even tune the radiation. For example, diffraction
gratings have been used to create a tunable laser that can tune over 15% of
its center wavelength. Growth
The alternating layers of the two different semiconductors which form the
quantum heterostructure may be grown on to a substrate using a variety of
methods such as molecular beam epitaxy or metalorganic vapour phase epitaxy,
also known as metalorganic chemical vapor deposition.
Applications Distributed feedback quantum cascade
lasers were first commercialized in 2004, and broadly-tunable external
cavity quantum cascade lasers first commercialized in 2006. The high optical
power output, tuning range and room temperature operation make QCLs useful
for spectroscopic applications such as remote sensing of environmental gases
and pollutants in the atmosphere and homeland security. They may eventually
be used for vehicular cruise control in conditions of poor visibility, collision
avoidance radar, industrial process control, and medical diagnostics such as
breath analyzers. QCLs are also used to study plasma chemistry.
Their large dynamic range, excellent sensitivity, and failsafe
operationcombined with the solid-state reliability should easily overcome many
of the technological hurdles} that impede existing technology in these
markets. When used in multiple-laser systems, intrapulse QCL spectroscopy
offers broadband spectral coverage that can potentially be used to identify and
quantify complex heavy molecules such as those in toxic chemicals, explosives,
and drugs. Unguided QCL emission in the 3–5 μm
atmospheric window could be used as a cheaper alternative to optical fibres
for high-speed Internet access in built up areas.
In fiction The upcoming video game Star Citizen
imagines external-cavity quantum cascade lasers as high-power weapons.
References External links
Bell Labs summary Bell Labs technical information
neoplas control overview of some QCL applications
Alpes Lasers summary of QCL technology Block Engineering overview of QCL
technology Daylight Solutions External Cavity
Quantum Cascade Laser Technology Page Optipedia: Quantum Cascade Laser
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