|
Items in cart [0] |
Questions? Call 888-624-8373 |
PAPERBACK PDF BOOK Free Resources |
||||||||||||||||
|
||||||||||||||||
|
|
|||||||||||||||
| ||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 110
6
Optical Physics
Optical physics encompasses the physics of electromagnetic radia-
tion and the interaction of matter and light. It includes the generation
and detection of light, linear and nonlinear optical processes, and
spectroscopy. The distinction between optical physics, applied phys-
ics, and optical engineering is blurred, for devices and applications are
close companions to basic research in this area of physics.
The first two sections of this chapter deal with lasers and laser
spectroscopy topics that have transformed optical science. The last
two sections are devoted to quantum optics and coherence, and to
femtosecond optics. Nonlinear optics, a major area of modern optics,
encompasses so many different streams of research and applications
that we have not attempted to describe it separately. Nonlinear optics
plays a role in most of the topics discussed in this chapter.
LASERS—THE REVOLUTION CONTINUES
From checkout scanners at supermarkets to laser disk recordings,
lasers have become commonplace, but the scientific revolution they
precipitated is continuing, propelled not only by the discovery of more
and more applications but by the steady development of new lasers and
new laser techniques.
The development of tunable lasers that can operate throughout the
visible and into the infrared and ultraviolet ranges has had a major
110
OCR for page 111
OPTICAL PHYSICS 111
impact on basic science during the past decade; instances of the
advances are scattered throughout this report. Dye lasers are the most
ubiquitous of these tunable light sources. Continuous-wave dye lasers
achieve a stability and resolution far exceeding those of traditional light
sources improvements by factors of hundreds to hundreds of thou-
sands are typical. (See Figure 6.1.) Pulsed dye lasers provide such
intense radiation that nonlinear processes such as frequency doubling
and multiphoton absorption are now widely employed. It is possible to
use several lasers in one experiment, providing innumerable new
strategies for studying atomic and molecular phenomena.
Many new laser sources have come into use during the past decade,
from color-center and semiconductor lasers in the infrared to excimer
lasers in the ultraviolet. The semiconductor diode laser is already a key
component of a major new industry- fiber-optic communications- as
discussed in Chapter 8. Notwithstanding these advances, other sources
are urgently needed. We possess no efficient optical lasers, and many
wavelength regions outside of the visible are difficult to achieve or are
inaccessible. There is wide interest in ultraviolet lasers, and the x-ray
region continues to be a tantalizing goal.
Major advances in lasers have come from research in atomic,
molecular, and optical (AMO) physics, and the level of activity and
excitement continues to be high. Current developments include
superstable optical lasers, hollow-cathode lasers that operate in the
ultraviolet using sputtered metal ions, and pulsed-gas ultraviolet lasers.
One of the most dramatic developments in laser technologies during
the past decade has been the construction of gigantic neodymium glass
lasers powerful enough to ignite thermonuclear fusion reactions. These
devices stand as triumphs of optical engineering: they have achieved
energy densities far greater than anything previously produced by man.
One final class of lasers must be mentioned the free-electron laser.
First demonstrated as an infrared laser, these devices are now being
engineered for wavelengths from the far infrared to the vacuum
ultraviolet. Free-electron lasers generate coherent radiation by stimu-
lated emission from relativistic electrons traveling through a periodi-
cally varying magnetic field. They are attractive because their wave-
length can be varied simply by changing the energy of the electrons and
because high power and high efficiency appear to be possible.
Synchrotron radiation provides an alternative to laser light as a
source for ultraviolet radiation. Because of their high brightness at
short-ultraviolet and x-ray wavelengths, these sources are being used
increasingly, particularly in condensed-matter physics and surface
science.
OCR for page 112
1 1 2 A TOMIC, MOLECULAR, AND OPTICA ~ PHYSICS
Shelf for Electronics A.
.
,'Tunoble Cavity
,=_ Fost Servo Electronics
~4. 0 `,Ring Dye Loser ~~J ~¢ isolated Reference Covity
In I ntens ty ~ :
flocking ~ IStobls
Amplifier and Coo - LOO Chopper '
Erg\ ~ Lost L' To O3 ~ c 1~
,__Beom Splitter
for Opticol
Heterodyne
10detector
Loser \~ ~ MEL e
\~ ~i ~ jPhot'
~Spotiol Filter
For Lombdomeler
-
I, i; ~~ ~~ -Gil ...~_
~: ~ '"~3_
FIGURE 6.1 Superstable Tunable Lasers. This tableful of optical components is a
tunable-dye-laser system that is so stable that the frequency of the light can be adjusted
much like the signal of a radio-frequency or microwave generator. The jitter in the
frequency of the laser is only 100 Hz. (The frequency of light is about 5 x 10'4 Hz.)
Highly stabilized lasers can be used to create and study new types of atomic and
molecular species and to carry out ultrahigh-precision spectroscopy. They are also useful
for applications such as stopping atoms and studying relativity and for gravity-wave
detection.
Although this highly stabilized system is at the benchtop stage in a research
laboratory, industry has been very effective in making advanced laser and optical
technologies available rapidly. (Courtesy of the Joint Institute for Laboratory
Astrophysics.)
OCR for page 113
OPTICAL PHYSICS 1 13
This summary is by no means complete one could mention numer-
ous new solid-state and gas lasers, advances in the design of optical
resonators, and in new pumping methods—but it should give some idea
of the level of activity and the rapid progress in laser design.
Laser-based ultraviolet and x-ray sources are also being developed.
For instance, intense, highly monochromatic tunable vacuum ultravi-
olet radiation has recently been generated by scattering laser light from
metastable excited atoms. Another method uses an infrared pulse of a
few Joules of energy from a neodymium-glass laser. When the light is
focused on a heavy-metal target it creates a highly ionized plasma that
emits a substantial fraction of the incoming laser energy as an intense
short burst of soft x-ray radiation, emerging from a pointlike origin and
covering a continuous spectrum. The method is disarmingly simple.
(See Figure 1.3.)
Every one of these lasers and light sources has an interesting
scientific and technological history. To illustrate the role of AMO
physics in the development of lasers, we have chosen one device the
excimer laser to describe in some detail.
Excimers and Excimer Lasers
Excimers are diatomic molecular systems for which the electroni-
cally excited state is tightly bound but the ground state is a very loosely
bound, essentially unbound, van der Waals molecule. The emission
spectrum for excimers is characteristic of transitions from bound
molecules to free atoms; such molecular transitions are ideal for
high-power gas lasers. Most excimer systems involve a rare-gas halide
molecule (a molecule composed of a rare-gas atom and a halide atom,
for instance, xenon-fluoride). The application of rare-gas halide
excimer molecules to efficient, high-power lasers is a success story for
high technology that has its roots in fundamental AMO physics.
Rare-gas halide lasers employ an electron beam or electrical dis-
charge to deposit energy into rare-gas mixtures with a halogen-
containing fuel. The electronically excited state of the rare-gas halide
molecules is formed efficiently owing to the unique properties of
rare-gas atoms and molecules. The stored energy in the rare-gas halide
excimer molecules is then extracted by laser action.
Within a S-year period following the discovery of the rare-gas halide
emission spectra, small commercial lasers were available for labora-
tory use and large devices were under construction for military and
national energy-related goals. This rapid development required wide
collaboration within the AMO community, including specialists in the
OCR for page 114
1 1 4 A TOMI C, MOLEC ULAR, A ND OPTI CA ~ PH YSI CS
interactions of electrons, ions, ground and excited state atoms and
molecules; the optical properties of the laser medium; and the hard-
ware associated with electrical deposition of energy into high pressures
of rare gases. The rapid development of rare-gas excimer lasers
illustrates the value of maintaining a reservoir of trained personnel in
AMO physics.
The development of rare-gas halide excimer lasers provides an
excellent example of a situation where detailed kinetic data at the
state-to-state level were vital. Such information will also be vital to the
design of other energy-storage gaseous systems. Thus there is an
urgent need for the knowledge needed to develop kinetic models at the
state-to-state level for reactive systems involving atoms and small
molecules.
A final point about molecular excimers is their role in stimulating
theoretical work in bound-free emission spectroscopy. Models have
been developed that permit bound-free spectra to be accurately simu-
lated for several rare-gas halide excimers. Further advances would be
of value not only because of the intrinsic interest of these molecules but
because of the possibility of discovering new excimer systems.
LASER SPECTROSCOPY
Much of what we know about the structure of matter comes from
spectroscopy. During the past decade both the techniques and uses of
spectroscopy have advanced so rapidly that the term has acquired new
meaning. Spectroscopy has truly undergone a revolution.
New spectroscopic techniques have achieved a precision and sensi-
tivity enormously greater than the classical techniques of absorption
and fluorescence; they have opened new areas in atomic and molecular
physics. Unusual species such as Rydberg atoms and molecules can be
created routinely, and familiar species can be viewed from new
perspectives. For example, the ability to excite a single electronic
molecular state with known quantum numbers has had a large impact
on molecular physics, as described in Chapter 5 in the section on The
New Spectroscopy. Beyond this, a new optical technology has
emerged, combining atomic, molecular, and optical science and leading
to innovations such as optical frequency standards, new light conjuga-
tors, four-wave mixers, and far-infrared detectors. The collective
enterprise has come to bee called laser spectroscopy. This term,
however, is something of a misnomer, for laser spectroscopy extends
far beyond the conventional idea of spectroscopy.
OCR for page 115
OPTICA ~ PHYSICS 1 15
Ultraprecise Laser Spectroscopy
A unique feature of laser light is its spectral purity. Conventional
monochromatic light sources typically achieve a spread in frequencies
of 1 part in 105. Commercial dye lasers now routely achieve 1 part in 108.
In advanced laboratories, dye lasers have been operated with a
stability and spectral purity greater than 1 part in 10~2.
The art of wavelength measurement has also made impressive
advances. For instance, automated digital wavemeters make it possible
to determine wavelengths to 1 part in 107 or 108 in a split second.
Photodiodes have been developed that can observe beats in the signals
of two different lasers at frequencies as high as several terahertz (1
terahertz = 10'2 cycles per second). As a result, lasers operating at
quite different frequencies can be compared with high precision. In
fact, the possibility of directly measuring optical frequencies in terms
of the cesium microwave frequency standard has recently been dem-
onstrated. Essentially, this creates a new optical technology in which
the frequency of light is directly measured, much as is done with
radio-frequency and microwave signals.
Ultrasensitive Spectroscopy
Lasers make it possible to observe extraordinarily weak absorption
of light using a variety of simple techniques. For instance, by placing
a small sample of a gas such as ordinary air inside the resonator of a
dye laser, strong yellow absorption bands of water vapor and molec-
ular oxygen appear in the laser's light. These bands are barely
perceptible by classical techniques, even with absorption paths of
many miles.
Photoacoustic detection is an ultrasensitive technique for observing
the absorption of light in gases, liquids, or solids. A laser beam is
modulated at an audiofrequency, and a microphone detects sound
waves generated by the periodic small heating of the sample. Another
ultrasensitive method is optogalvanic spectroscopy. Modulated laser
light enters a gas discharge; when the laser frequency is tuned to a
resonance between two energy levels, the discharge current or voltage
displays a modulated signal, even when both levels correspond to
excited states. The technique requires only a discharge tube, a laser,
and an oscilloscope. Among its many applications, optogalvanic spec-
troscopy permits studies of sputtered metal atoms and transient
species, such as ions and molecular radicals.
OCR for page 116
1 16 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
The ultimate in sensitivity can be reached with a related technique:
resonant photoionization by intense laser light. A single gaseous atom
of almost any element or isotope can, in principle, be selectively
excited and detected, even in the presence of large numbers of atoms
of different species. Potential applications range from trace analysis
and the detection of impurities in semiconductor materials to the
search for rare unstable isotopes. (The presence of these isotopes in
mineral deposits has been proposed as a telltale indicator for solar
neutrino reactions or of the double beta-decay process that would
occur if the neutrino had a rest mass.)
Doppler-Free Laser Spectroscopy
In the past, spectral resolution in a gas was limited by the Doppler
effect, the frequency broadening due to the motion of the atoms. Laser
spectroscopy provides several methods for eliminating the first-order
Doppler broadening, permitting observation of the much narrower
natural width of the spectral line. Some of these methods are relatively
simple, well suited to observing rare or short-lived species. Others
have played roles in stabilizing the frequency of lasers to atomic or
molecular transitions.
One method, saturation spectroscopy, employs a strong laser beam
to label a group of atoms and a counterpropagating beam to pick up a
signal from those atoms that have no Doppler shift. Saturation spec-
troscopy has been applied to problems ranging from the finest details of
molecular structure to collisional effects and precise measurements of
fundamental wavelengths in atomic hydrogen. Another method of
Doppler-free spectroscopy employs two counterpropagating laser
beams to induce a two-photon transition: the first-order Doppler shift
essentially disappears for all the atoms or molecules. Using this
method, the "forbidden" l S-2S transition in hydrogen has been
observed. This measurement represents an important spectroscopic
advance because its intrinsic linewidth is more than a million times
narrower than for normal optical transitions. It provides the opportu-
nity to measure the Lamb shift in the ground state, and the wavelength
can be directly related to the Rydberg constant and the electron-proton
mass ratio. The l S-2S transition in positronium has also been measured
by two-photon spectroscopy. As discussed in Chapter 4 in the section
on Elementary Atomic Physics, laser spectroscopy of positronium
opens a most useful new line of research in the physics of elementary
systems.
OCR for page 117
OPTICAL PHYSICS 1 17
Laser Cooling
Although Doppler-free methods remove the first-order Doppler shift,
the second-order Doppler shift remains. The effect is small the
frequency is typically shifted by 1 part in 10~- but it can result in
serious errors in high-precision measurements. The second-order Dop-
pler shift is proportional to the kinetic energy of the particles, and the
only way to reduce it is to reduce the particles' motion. This has been
accomplished by using the momentum of laser light in various inge-
nious experiments to "cool" atoms or ions, that is, to slow them or
even bring them to rest. Ions held in an electromagnetic trap have been
cooled to the millikelvin range by absorbing laser light that is tuned
slightly below resonance. (See Figure 6.2.) Recently an atomic beam of
sodium was cooled, actually brought to rest, by laser light.
A prime motivation for laser cooling is to create better frequency
standards, either at microwave frequencies or at optical frequencies.
Optical frequency standards have been proposed as candidates for the
next generation of atomic clocks.
Coherent Optical Transients
One area of laser spectroscopy, coherent optical transients, exploits
the temporal coherence of laser light. Gaseous and solid atomic
systems can be coherently excited, producing a new and unusual class
of nonlinear optical phenomena. Effects such as optical free-induction
decay the coherent emission from atoms excited by a single-laser
pulse and photon echoes the delayed burst of coherent radiation
following excitation by two successive laser pulses can be applied to
study dynamic interactions of atoms in their local environment. Optical
free induction provides new ways to study elastic collisions of atoms or
molecules that are not in a single eigenstate as in traditional scattering
experiments but in a superposition of ground and excited states. Cross
sections and other parameters can be determined from measurements
of the decay. The close impacts with a perturber that annihilates the
superposition state can be visualized classically in terms of separate
scattering trajectories, one for each state, resembling state selection in
a Stern-Gerlach experiment. Distant impacts or small-angle diffractive
scattering where the superposition is largely preserved require a
quantum description.
OCR for page 118
1 18 A TOMIC, MOLECULAR, AND OPTICAL PHYSICS
lon~Clo~
I_
a_
in_
~ ..,~
^'#~.~"~
FIGURE 6.2 Trapped Ions. Ions can be trapped in high vacuum using static and
oscillating electric fields and viewed by laser light. The experiments can be so sensitive
that single ions can be observed under close to ideal conditions of isolation. In this
experiment barium ions are formed in the center of the donut-shaped electrode by
bombarding barium vapor with electrons. The ions are observed by their fluorescence
under laser light. The photograph at bottom left shows the laser light scattered by a small
cloud of trapped ions. In the blown-up photograph at bottom right, the light scattered by
one barium ion can be discerned in the circled region. Laser light can also be used to
OCR for page 119
OPTICA ~ PHYSICS 1 19
Ultranarrow Optical Transitions
Optical free-induction decay of impurity ions in certain solids (the
praseodymium ion in lanthanum triduoride is one example) can display
extremely narrow linewidths, 1 kilohertz or less. This is 106 times
narrower than typical linewidths in solids. These optical transitions are
analogs of the Mossbauer eject: the optically excited impurity ion
suffers no recoil effect because its momentum is transferred to the
lattice as a whole. Furthermore, at cryogenic temperatures there is
virtually no second-order Doppler broadening. These systems are
prime candidates for studying the interactions that broaden optical
transitions, and possibly for establishing secondary optical frequency
standards. The method, which is made possible by the use of a dye
laser with a 100-Hz linewidth, has been applied to study the optical
Bloch equations, the starting point for many theories in quantum
optics. It has been found that in intense laser fields the optical Bloch
equations must be modified because the radiation inhibits the line-
broadening ejects of nuclear magnetic interactions. The phenomenon
is now understood in terms of a microscopic theory of nuclear magnetic
interactions.
Coherent Raman Spectroscopy
The term coherent Raman spectroscopy describes a class of
nonlinear optical techniques that are used to study and measure
Raman-active modes of molecules. The major techniques are coherent
anti-Stokes Raman spectroscopy (CARS) and stimulated Raman spec-
troscopy (SRS).
In contrast to ordinary Raman light-scattering methods, the signals
from CARS and SRS come in the form of strong and highly directional
laser beams. As a result, these methods offer tremendous discrimina-
tion against undesirable background fluorescence and luminescence.
Coherent Raman spectroscopy works in environments where the
cool the ions, reducing the energy-level shifts due to the second-order Doppler effect.
Trapped-ion methods are being applied to ultrahigh-resolution optical spectroscopy and
to the creation of new types of atomic clocks. The methods are also employed to study
collisions and chemical reactions, including reactions at very low temperature, and to
study collective motion in charged plasmas. Further discussion is in Chapter 5 in the
section on Molecular Dynamics and in Chapter 8 in the section on Precision Measure-
ment Techniques. (Courtesy of the University of Hamburg, Federal Republic of
Germany.)
OCR for page 120
120 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
background light level or the need for high resolving power make
conventional methods impractical. In combustion diagnosis, the
method provides a means of nonintrusively mapping the temperature of
a gas and the concentration of its species. CARS studies have been
performed in hostile environments such as the combustion chamber of
internal combustion engines, in gas-turbine combustors, and even in jet
engine exhausts.
Other applications for the coherent Raman techniques include stud-
ies of the energy-level distributions that result from optical photodis-
sociation, trace detection of pollutants in gas and liquid phase, the
spectroscopy of biological molecules, plasma diagnostics, and applica-
tions in the study of hydrodynamic flow.
QUANTUM OPTICS AND COHERENCE
The concept of the photon grew out of Einstein's preoccupation with
the statistical nature of light, but it was not until the advent of the laser
that the statistics of electromagnetic radiation began to be studied
methodically. Light can be observed only in its interactions with
matter, however; and so the study of light inevitably encompasses the
dynamics of atoms in the radiation field. These subjects collectively
form the main body of research in quantum optics and coherence.
During the past decade these studies have provided new insights into
the statistical nature of radiation and the dynamics of lasers and other
quantum systems. In addition, they have opened the way to new
methods of measurement and to new quantum devices.
Photon Antibunching
One can observe the arrival of photons at two separate detectors and
study the probability that the photons arrive in coincidence. (The light
beam is split by a semireflecting mirror, and each half is detected by a
separate phototube. The correlations are found from measurements of
the coincidence rate.) These are called second-order correlation exper-
iments, since they are sensitive to the product of two intensities. The
famous Hanbury-Brown and Twiss experiment determined the diame-
ter of a star from measurements of the second-order correlations in its
light. Second-order corrections are usually positive; photons tend to
bunch together. It has now been discovered, however, that it is
possible to prepare light so that the photons, instead of coming in
clumps, are antibunched. More precisely, if a phototube receives a
OCR for page 121
OPTICAL PHYSICS 121
photon, for a short period thereafter it is less likely than otherwise that
it will detect a second one.
Antibunching can be observed in light coming from a single atom. (It
would not suffice to attenuate a conventional light source, or laser light,
for that matter, for that would change only the average arrival rate of
photons, not the statistics of the radiation field. Antibunching occurs
only when the photons are produced by a nonclassical sources This
has been achieved in practice by using as the light source a single atom
that is coherently excited and radiates spontaneously. Other processes,
such as harmonic generation or parametric amplification, should also
exhibit the antibunching. The novelty of antibunching, however, lies
not so much in the realization that one atom can radiate only a single
photon at a time but that the statistical properties of the light are
different from any that have previously been observed. Antibunching
provides an example of light that is fundamentally different from any
light previously studied.
Closely related to antibunching is the production of light for which
the photon number fluctuations are smaller than random. This has also
been observed: it offers the interesting possibility of allowing optical
communication with less noise than with a coherent laser beam.
Optical Bistability
An optically bistable system has two stable output states for a given
input level of light. Typically, it consists of a nonlinear medium within
an optical resonator. Optical bistability was first observed using
sodium vapor in a Fabry-Perot etalon, and now there is an expanding
class of optically bistable devices. Often the devices are constructed of
tiny semiconductor chips whose faces are polished to form a resonator.
Optical bistability provides a new arena for the study of nonlinear
systems. Many of the dynamical phenomena that have been studied in
lasers, for instance fluctuations and regenerative pulsations, can be
observed under far better controlled conditions using optically bistable
devices. The transition from ordered to chaotic motion is of particular
interest. Such transitions have been studied in hydrodynamic, acous-
tical, and electrical systems; optical bistability allows the research to
be carried out under highly controlled conditions at extremely high
speed. The ability to gather data at high speed is of particular value, for
it offers the opportunity to study turbulent motion in ways never before
possible.
Much of the interest in optical bistability is due to its potential
applications to optical computing. A bistable device can serve as a fast
OCR for page 122
122 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
memory element; it can be used as an "optical transistor" to amplify
small signals at high speeds; and it can be employed as a discriminator,
a pulse shaper, an oscillator, or a general logic element. A room-
temperature optically bistable device has been created, 5 Em thick and
10 lam in effective diameter, that turns on in a few picoseconds (I
picosecond = 10-'2 second). These devices can be integrated into
larger systems, with numerous applications to optical computing and
data processing.
Systematic interest in chaotic phenomena can be traced back to
Poincare's studies early in this century. There has been a great renewal
of interest in recent years, and the question of regular versus disor-
dered motion is now a central problem in physics, with important
ramifications in mathematics and engineering. Two discoveries, in
particular, have contributed to the present interest. One was that
attempts to predict long-range weather patterns were inherently limited
by the onset of turbulence. The problem of chaos thereby assumes
enormous economic significance. The second discovery was the real-
ization of the ubiquitous nature of period doubling in naturally oscil-
latory phenomena, revealing an important route from regular to chaotic
motion.
Optical bistable devices provide a way to study the transition from
regular to chaotic motion in reproducible experiments that can be
carried out at very high speed. The simplest bistable optical system
comprises two mirrors and a nonlinear medium that is operated in a
transient mode using pulsed lasers.
The onset to chaos often reveals precursors. In the case of optical
bistability these have been discovered to produce short pulses with 100
percent modulation, even in a high-finesse cavity. The technique holds
the possibility of new methods for short pulse generation and possibly
for optical processing.
Squeezed States
According to quantum mechanics, two canonical variables, such as
the position and the momentum of a particle, cannot both be known
with great precision; the product given by the uncertainty in one
multiplied by the uncertainty in the other must exceed half of Planck's
constant. It follows that one variable can be determined accurately
only at the expense of large fluctuations in the value of the other.
Squeezed states are quantum states that exploit this property. They
have become particularly important for the electromagnetic field, in
which two oscillatory quadrature (90°-of-phase) components of the
OCR for page 123
OPTICAL PHYSICS 123
field play the role of canonical variables. In a squeezed state one
component of the field can be relatively free from fluctuations while the
other fluctuates appreciably. This has potentially important applica-
tions for optical communications and high-precision measurements,
provided that it is possible to encode and decode information in just
one quadrature component of the light.
Much effort has been devoted to exploring theoretically the different
conditions under which squeezing can be produced. It has been found
that squeezing can occur in parametric processes, harmonic genera-
tion, phase conjugation, resonance fluorescence, the free-electron
laser, and many other circumstances. The practical problem of encod-
ing and decoding information in squeezed light is not without difficul-
ties, but if they can be overcome it would be possible to achieve
signal-to-noise ratios in an optical communication channel that go
beyond the quantum limit for coherent or laser light.
The problem of detecting gravitational waves with detectors operat-
ing close to the quantum limit, where the signal is hidden by quantum
noise, has also generated much interest in squeezing. Our ability to
make new kinds of astronomical observations may benefit eventually
from the use of squeezed states.
Rydberg Atoms and Cavity Quantum Electrodynamics
Any neutral atom in which one electron is in a high-lying energy level
is known as a Rydberg atom. These atoms have opened a new area in
the study of fundamental radiative processes cavity quantum electro-
dynamics.
The interaction between Rydberg atoms and the electromagnetic
radiation field scales as n4, where n is the principal quantum number.
For n = 30, for instance, the interaction is 106 times larger than for
~ ordinary atoms. As a result, the rates at which Rydberg atoms absorb
and emit radiation are anomalously large. Thermal radiation at room
temperature, usually ignored, is intense for these atoms: it shortens
their radiative lifetimes, redistributes the atoms among the various
quantum states, and can photoionize the atoms at measurable rates. In
addition, thermal radiation can shift the energy levels. The effect is
somewhat analogous to the Lamb shift, except that its origin is the real
energy of the radiation field, not the virtual energy of the vacuum. The
blackbody shift is small but measurable. It needs to be taken into
account in the design of the next generation of atomic clocks.
Rydberg atoms are so sensitive to radiation that they provide a
natural medium for detecting infrared, submillimeter, and microwave
OCR for page 124
124 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
radiation. A number of schemes have been proposed and realized in a
laboratory setting. Rydberg atoms can be used to count photons with
an efficiency that comes close to the ideal quantum limit. Since
absorption is inherently frequency selective, Rydberg atoms can also
serve as tuned receivers. In addition, they can be employed in maser
amplifiers. Unlike conventional masers' the number of atoms required
is small; maser action with only one atom has been achieved.
When an atom is placed in a tuned cavity its radiative behavior is
fundamentally altered: the spontaneous radiation rate is enhanced; the
lifetime shortened. Rydberg atoms have made it possible to study these
effects. If the losses in the cavity are made sufficiently small, a point is
reached where the atom no longer decays to the lowest state. The atom
and the cavity behave like a pair of coupled oscillators one atomic,
the other man-made. Such a device represents a new entry into the field
of macroscopic quantum electrodynamics and provides a unique
opportunity to study the transition from reversible to irreversible
behavior and the origins of noise.
Cavities not only enhance the spontaneous radiation rate, they can
also inhibit it. Simply put, an atom cannot radiate a long wave into a
short cavity. From another point of view, the cavity can be viewed as
modifying the spectrum of zero-point fluctuations that induce sponta-
neous emission. If the cavity is mistuned, the fluctuations are removed
and spontaneous emission is inhibited. This effect has been seen. It is
possible to "turn off'' spontaneous emission, leaving an atom in a new
type of excited state, a state devoid of radiative damping. The natural
linewidth is suppressed, and other radiative interactions such as the
Lamb shift are altered.
The ability to observe basic radiative processes with Rydberg atoms
offers a new arena for studying electrodynamic phenomena. Although
quantum electrodynamics is usually regarded as a highly developed
theory, the new experiments suggest that there is a wide body of
phenomena yet to be discovered. For instance, it has been found that
it is possible to measure the number of thermal photons in a cavity by
counting the number of atoms that the cavity can excite. The technique
provides, in principle, an absolute thermal radiometer. A scheme has
been proposed for cooling a radiation field below the temperatures of
its surroundings. Undoubtedly other surprises are in store.
FEMTOSECOND SPECTROSCOPY
A decade ago picosecond optics was in its infancy; today picosecond
spectroscopy is a mature field and femtosecond (10-'5 second) optics is
OCR for page 125
OPTICAL PHYSICS 125
in its infancy. Pulses as short as 9 femtoseconds have been generated;
such a pulse contains only 5 cycles of light.
Femtosecond spectroscopy can provide revolutionary insights into
the dynamics of molecules and solids, and into chemical reactions,
since femtosecond pulses are compared to the characteristic times for
all of these. For instance, a molecule typically requires 1o-~4 to 1o-~3
second to vibrate; the time for electrons in a semiconductor to
equilibrate after they have been excited can be as short as 1o-~4
second; and proton and electron transfer in molecules can be quicker
than 1o-~4 second. The state-to-state dynamical steps in many solid-
state and surface processes span an enormous range of frequencies; for
the first time Femtosecond pulses of light make it possible to observe
these phenomena.
Chemical reactions in solutions and in biological systems also take
place in the Femtosecond regime. Their study is especially important
for chemistry since most chemical reactions organic, inorganic, or
biochemical—occur in solutions. The use of Femtosecond spectroscopy
for the direct, real-time observation of ultrafast relaxations and reac-
tions in condensed-phase chemistry is expected to open new horizons
in chemical research. In complex biomolecules the number of energy-
transfer paths can be so large that transport and relaxation processes
occur on a subpicosecond time scale. Femtosecond spectroscopy can
provide a unique means of identifying and studying these primary
biophysical events.
Femtosecond optics can also have important applications to fast
electronic circuitry and high-speed instrumentation. Femtosecond
techniques enable optical pulses to reach a domain inaccessible by
electronic techniques. Optical pulses can be used to investigate semi-
conductor processes that determine the ultimate speed potential of
electronic circuitry. For example, high-speed photoconducting pulse
generators and sampling gates have been used to measure the elec-
tronic input response of gallium arsenide field-effect transistors. The
information obtained in such studies will become increasingly impor-
tant for the design of faster and smaller computers. Ultimately,
all-optical modulation and switching techniques, utilizing nonlinear
interactions between the ultrashort light pulses themselves, have the
potential to go beyond electronics and advance signal-processing
speeds into the Femtosecond domain.
Representative terms from entire chapter:
optical physics
