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D
Laser Spectroscopy of
Conclensed Matter
INTRODUCTION
The advent of lasers has greatly facilitated research in materials
science. That a laser beam has the characteristics of high intensity,
strong directionality, and extreme monochromaticity, and can appear
in ultrashort pulse form, makes it a unique tool for materials studies. In
the past two decades, a large number of laser techniques have been
invented for the investigation of matter in all its phases. They have
opened the possibility of research in many hitherto unexplored areas of
materials science.
Generally speaking, lasers can be applied to two types of problem:
(1) to probe a material and (2) to modify or process a material. The past
decade has witnessed great advances along both lines. For example,
laser light scattering has become a conventional technique for studying
excitations in condensed matter; nonlinear optical spectroscopy allows
the study of forbidden transitions in a medium and the study of
homogeneous broadening of spectral lines as narrow as PI kHz;
transient optical spectroscopy can probe dynamic properties of a
medium on a time scale as short as a subpicosecond (i.e., <10-''
second); optical mixing is useful for monitoring and studying molecular
adsorbates on surfaces; and laser heating is promising as a new method
for annealing crystalline films or for growing various types of amor
258
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APPENDIX D 259
phous, crystalline, and alloy layers. As laser techniques become
increasingly more sophisticated, we anticipate in the coming years an
exciting period for laser-related materials research, from both the
practical and scientific points of view.
ACCOMPLISHMENTS OF THE PAST DECADE
Spectroscopic study is essential for the microscopic characterization
of a medium. Laser spectroscopy has brought new life to optical
spectroscopy of condensed matter. In this section we survey the
accomplishments of the past decade.
Nonlinear Optical Spectroscopy
This type of spectroscopy flourishes only because tunable lasers
have become easily available. It comes in many different forms,
depending on the nonlinear optical process involved. A few of them are
considered here.
TWO-PHOTON SPECTROSCOPY
Two-photon spectroscopy is commonly used to study transitions
between states of the same parity. In research on excitor polaritons in
a semiconductor, for example, since two-photon spectroscopy is
unaffected by the reststrahlen band it can yield detailed information
about the damping and dispersion curve of the excitor polaritons. The
technique has also been applied to the study of excitonic molecules in
solids, which is a subject of immense theoretical interest. More
recently, two-photon spectroscopy has been used to measure the (din
(off transitions of rare-earth ions in solids not observable in
one-photon absorption owing to the presence of the (din ~ In-
transitions. This is most interesting because through such an investi-
gation one can expect a much better understanding of rare-earth ions
and their interaction with the lattice in a solid.
HOLE BURNING IN INHOMOGENEOUSLY BROADENED SPECTRA
A laser beam can be intense enough to saturate a transition in a
medium. If the laser linewidth is much narrower than the inhomogene-
ous broadening of the line then, with the laser frequency fixed, only a
small group of ions or molecules can be resonantly excited by the laser.
By saturating the transition of this group of ions or molecules, a hole is
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260 APPENDIX D
created in the inhomogeneously broadened spectrum. The hole width is
generally limited by homogeneous broadening, assuming that the laser
linewidth is negligible. This hole-burning effect makes high-resolution
spectroscopy of ions or molecules in condensed matter a reality. Both
absorption or fluorescence can be employed to probe the holes. Lines
as narrow as a few megahertz have been observed. This saturation-
spectroscopy technique can be used to study hyperfine and superhy-
perfine interactions of rare-earth ions in solids and fine structure of
organic molecules in solid matrices. At sufficiently low temperatures,
the laser-induced holes can last almost indefinitely. As many as ~105
holes can be burned on an inhomogeneously broadened line 10 cm-
wide. They can therefore be used for making optical memory devices
with high densities of data bits. Such a possibility is currently being
pursued vigorously by several industrial laboratories.
OTHER NONLINEAR SPECTROSCOPY TECHNIQUES
Four-wave mixing and coherent Raman spectroscopy allow us to
study excitations in both the ultraviolet and the infrared ranges.
Because their sensitivity is high, they can be employed to detect
impurities in condensed matter. Their spectral resolution is limited
only by the laser linewidth. Because the output is coherent and
directional, spatial filtering can be adopted to suppress the lumines-
cence background. Thus, these techniques can be used to study
excitations that are normally masked by luminescence. Applications of
these techniques to condensed-matter physics have attracted only
limited attention in the past; but more recently, with the advance of
laser technology, they have begun to receive increasing recognition in
the community.
Transient Optical Spectroscopy
Transient coherent phenomena arising from the resonant interaction
of radiation with matter are among the most fascinating topics in
condensed-matter physics. They were studied extensively in magnetic
resonance before the laser era. With lasers, extension of these studies
to optical transitions becomes possible. The past decade has seen
increasing activity in this area of research. With the use of photon echo
and optical notation techniques, for example, homogeneous linewidths
of optical transitions of rare-earth ions in crystals as narrow as ~ 1 kHz
have been measured. Thus, detailed information about spin-spin inter-
actions between the ions and surrounding atoms can be obtained.
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APPEND/X D 261
Ultrafast Laser Spectroscopy
PICOSECOND LASER SPECTROSCOPY
In 1966, laser scientists discovered how to use the technique of mode
locking to produce optical pulses as short as ~5 ps in duration.
Stimulated by progress in laser technology, the past decade has seen
dramatic improvements in picosecond instrumentation and techniques,
with the result that picosecond laser spectroscopy is an area experi-
encing major advances at present. Several laser manufacturers now sell
reliable cw mode-locked dye lasers delivering ~3-picosecond-long
pulses at ~100-MHz repetition rates. These pulses may be selectively
amplified to energies in the millijoule regime (i.e., having peak powers
of ~108 watts), and pulse length measurements, including delay times,
are routinely carried out by the technique of autocorrelation by
. . .
non Inear mixing.
In general, by exciting a medium with a picosecond pulse, followed
by probing with another picosecond pulse, the dynamic properties of
the medium can be studied on a picosecond time scale. This is exciting
since it offers the opportunity to measure directly, for the first time, the
carrier relaxation time, the excitation lifetime, the phonon relaxation,
and other properties in a condensed medium. It opens a new, important
area of research that seldom has been explored in the past.
As an example, time-resolved photoemission spectroscopy (PES) of
semiconductor, metal, and insulator surfaces yields important data on
the transient behavior of selectively excited carriers. Using a picosec-
ond laser pulse to excite a narrow energy distribution of electrons, a
delayed picosecond pulse has been used to probe the relaxed distribu-
tion by PES. By energy analyzing the photoemitted electrons as a
function of time delay, fundamental information about the energy
relaxation processes affecting the electron distribution was obtained.
Furthermore, by using circularly polarized laser light and studying the
spin polarization of the photoemitted electrons, phase-destroying pro-
cesses may be studied. Thus momentum relaxation processes may be
distinguished from energy relaxation processes.
In a new approach to high-speed electronic instrumentation, known
as picosecond optical electronics, a short laser pulse illuminates a
high-speed photoconductor, thereby producing a fast switch for an
electrical signal. By combining two of these switches with a control-
lable time delay, a sampling system capable of time resolution of better
than 2 picoseconds has been demonstrated. Studies of propagation
delays between the drain and gate signals of GaAs ferroelectric
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262 APPENDIX D
transistors have been carried out. The response time of a high-speed
silicon photodiode has been measured. Electron transport in materials
with particular types of electronic defects has been characterized with
picosecond time-resolved photocurrent spectroscopy. The extension
of optical electronics to the subpicosecond regime calls for ways of
overcoming dispersive and capacitive effects in electronics compo-
nents. Efforts are being devoted to finding better ways of rectifying the
light pulses to produce short dc pulses (such pulses really look like
microwave radiation since they contain frequencies in the terahertz
range).
FEMTOSECOND LASER SPECTROSCOPY
In several research laboratories techniques were developed recently
to produce laser pulses as short as 70 femtoseconds (1 femtosecond =
10-'5 second). A method of pulse compression has recently been
developed that has resulted in the realization of pulses only 16
femtoseconds long. One of the results that has been achieved by the
use of such ultrashort laser pulses is described in what follows.
An area of great interest over the past several years has been laser
processing and laser annealing of materials. While many different
facets of this field have been explored by studying the results of laser
irradiation of materials, little has been done to time-resolve the actual
annealing process. Recently, such studies have been carried out with
femtosecond pulses, and the results support a model where the light is
absorbed and first creates electron-hole pairs, after which the irradi-
ated surface is converted from a crystalline structure within an
electron-hole plasma to a molten state. By such studies, one can
determine the dynamics of the energy absorption process responsible
for laser processing. This is an exciting new area of materials science.
SOME DIRECTIONS FOR FUTURE RESEARCH
Now that laser technology has become more mature, one can
anticipate rapid growth in several areas of laser applications to
condensed-matter physics in the coming years.
The increased use of two-photon spectroscopy for the study of
forbidden transitions of rare-earth ions in solids is anticipated.
In studies of materials properties, high-resolution laser spectroscopy
and nonlinear optical spectroscopy are expected to become common
laboratory techniques.
The future of transient optical spectroscopy for the study of optical
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APPENDIX D 263
transitions of rare-earth ions in crystals is particularly bright because
these techniques can be extended to the study of transitions between
excited states. Many of the sophisticated techniques developed earlier
in magnetic resonance are yet to be transplanted to this field.
Another area of rapid growth in the laser spectroscopy of condensed
matter is laser studies of surfaces. Both laser perturbation and laser
probing of surfaces are exciting new areas of research that have hardly
been explored.
It is clear that we have entered a new era in making measurements
on subpicosecond time scales. Laser-based techniques will allow such
measurements to be made routinely in physics, chemistry, biology,
materials science, and device studies. This area of research is now
known as femtosecond science. Some of the interesting subjects for
study in this area are listed below.
By using a femtosecond laser pulse to create photoemitted electrons
with a small energy spread and by accelerating these electrons across
a large potential, a pulse of electrons is made available for time-
resolved electron-diffraction studies. For the first time, such structural
changes as melting and structural phase transitions may be studied on
the femtosecond scale.
The response of crystalline solids to a short electrical pulse, i.e., the
electric field of the laser pulse, may be studied by femtosecond
spectroscopy. For example, the Franz-Keldysh effect (the change of
the band gap of a semiconductor in an electric field) can be studied in
a time-resolved fashion.
When a femtosecond light pulse causes photoemission, the electron
pulse and the light pulse are synchronized. This makes it possible to
carry out studies where one pulse creates an excitation and the other
pulse probes this excitation. Thus the electron pulse may create a
change in matter than can be monitored optically by the light pulse. It
might be possible to create or disturb a solid-state plasma using the
short electron pulse and then probe it by time-resolved reflectivity
measurements.
In the area of femtosecond-pulse technology, research is aimed not
only at producing still shorter pulses but also at developing techniques
to extend the wavelength region of coherent-light generation toward
the ultraviolet and the infrared regions.
On the side of laser technology we can also expect to see advances
in several areas. A severe limitation in the progress of high-power laser
technology comes from optical breakdown. This is a subject of great
practical importance and has been pursued vigorously in the past two
decades. From the scientific point of view, this is also a subject of great
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264 APPENDIX D
interest. Optical breakdown in solids is a highly nonlinear process.
How a laser beam excites the carriers, generates the plasmas, and
eventually shatters or melts a solid is not a trivial matter to understand.
Breakdown mechanisms for excitations with different laser frequencies
and pulse widths could be different. Solution of the problem requires
the joint efforts of theorists and experimentalists. Progress in this area
is being made step by step. It is hoped that in the next decade, optical
materials of much improved quality will be produced as a result of the
continuing research effort.
Another important area of materials research related to laser tech-
nology is the search for better nonlinear optical crystals. Such crystals
are essential in the extension of coherent light sources to the ultraviolet
and infrared regions. Recent theoretical calculations are fairly success-
ful in predicting nonlinearities of certain crystals. Organic and inor-
ganic crystals of large nonlinearities have actually been grown follow-
ing the theoretical hints. This is encouraging. It is likely that new and
better nonlinear optical crystals suitable for efficient frequency con-
version over a broad range and for use in nonlinear optical devices for
data processing will emerge in the near future.
Optical fibers have grown, in the past decade, into an important
branch of the optical industry. They are the key element for future
optical communications and data processing. An exciting development
in the research into fiber materials has taken place within the past year.
It is found that crystals can also be pulled into thin optical fiber form.
Thus, fiber lasers and fibers for optical frequency conversion may soon
appear in laboratories. Their possible applications are numerous,
limited only by imagination. They are likely to revolutionize science
and technology in many disciplines. The prospect of this field is truly
great. Its progress in the next 10 years will be interesting to follow.
Representative terms from entire chapter:
nonlinear optical
