N. Devins – Encyclopedia of Physics Research

Автор: N. Devins
Название книги: Encyclopedia of Physics Research
Формат: PDF
Жанр: Физика
Страницы: 1029
Качество: Изначально компьютерное, E-book

This book presents current research in the field of physics, with a particular focus on
atomic molecular and optical physics, as well as cryogenics and photonic crystals. Topics
discussed include pulse structuring in laser-light dynamics; excitation of atoms and ions by
electron impact; Fiber Bragg Gratings and their applications as temperature and humidity
sensors; liquid oxygen magnetohydrodynamics; cryogenic treatment and fatigue resistance;
cryogenic grinding; fabrication and applications of polymer photonic crystals; physics of
photonic crystal couplers and their applications and a photonic band gap for quasicrystalrelated
structures.
*Chapter 1 – We revisit, with an in-depth and self-consistent approach, the small and the
strong-harmonic expansion methods that handle the dynamic properties of the well-known
integro-differential “Maxwell-Bloch” equations, which describe light-matter interactions
inside a single-mode inhomogeneously broadened [SMIB] laser. The chapter foremost reexamines
the well-established small-side-band routine, outlining the limits of its applicability
in the unstable regime of operation. These limits are overcome with the application of a
strong harmonic-expansion procedure that allows for the derivation of new analytical
information in the control-parameter space within which regular pulsing behavior takes place.
In particular, despite the complicated nature of the SMIB laser equations, owing to the
presence of an integral over the polarization variable, we show that an extension of the
analytical procedure, up to third order in field amplitude, is sufficient to obtain the oscillation
frequencies of the asymptotic solutions and an analytical expression that illustrates the
dynamic-gain contour during pulse build-up. Furthermore, the evaluation of the long-term
operating frequency is shown to bear fundamental importance in the construction of complete
analytical solutions, inside the control parameter space that exhibits periodic self-pulsing,
through simple iterative extraction of the high-order field amplitudes. Hopefully, these new
findings will allow for significant advance and understanding of the physics underneath selfpulsing
and time structuring in bad cavity configured lasers.
Chapter 2 – The present chapter extends the strong harmonic expansion method of the
preceding report to the much simpler Lorenz-Haken model that describes self-pulsing in
single-mode homogeneously broadened lasers. The study focuses on a typical pulsestructuring
hierarchy of periodic solutions, which include a period-doubling cascade that builds up with increasing excitation levels. Despite the uncomplicated formalism they
display, the Lorenz-Haken equations are shown to carry the same qualitative features as those
of the much more complex integro-differential system. Remarkable analogies are found
between a few typical solutions of both systems as direct one-to-one comparisons reveal. The
obvious advantage of a simpler set of equations lies in the fact that it renders the analytical
and numerical handling much easier to carry out. As a consequence, new and ampler
information with respect to pulse structuring are more easily extracted. In particular, the
iterative harmonic-expansion algorithm is carried out, up to the fifth-order in field amplitude,
giving persuasive and explicit credit to the strong-sideband ability to provide an accurate
description of pulse structuring in laser-light dynamics.
Chapter 3 – This chapter gives an update of the study of laser assisted collisions in dense
resonantly laser-excited In or Ga homo/heteronuclear vapours. In this way, Rydberg levels, as
well as autoionizing levels and collisional ionization, have been evidenced for each element
and the relative cross sections have been measured via laser induced fluorescence
spectroscopy (LIF). The presence of a large density of excited atoms favours the collision of
an atom in the fundamental state and another one in the first excited state to give a molecule
directly in an excited electronic state from which it radiates. At the same time, the presence of
a rather large density of ions can also favour the onset of molecular ions via collisions, which
can radiate after electron/ion recombination and/or eventually dissociate. All of these
processes produce molecular signals in the fluorescence spectrum following the resonant
atomic excitation. In this way, some unresolved molecular bands and excimer emission have
been detected. Special emphasis is dedicated to the description of the most recent LIF
experiments performed in a heteronuclear In-Ga vapour by the resonant excitation of either
element. Besides the evidence of homonuclear molecules, the presence of the GaIn molecules
is inferred from the time- resolved analysis of some fluorescence features. The experiments
are carried out in the 900–1100°C temperature range with the vapors confined in fused silica
cells. The permanent modifications induced in the silica optical properties by the migration of
the In/Ga atoms inside the silica matrix are also described. In this chapter, a section is
dedicated to the analysis of how the latter effect, added to all the processes mentioned
previously, affects the basic physical properties of laser-excited dense vapors as the radiation
trapping, which turns out to be greatly reduced, if not quenched.
Chapter 4 – Electron-ion collision processes play an important role in the understanding
of energy balance in various types of astrophysical plasmas and in controlled thermonuclear
fusion plasmas. Electron collisional excitation rates and transition probabilities are important
for computing electron temperatures and densities, ionization equilibria and for deriving
elemental abundances from emission lines formed in the collisional and photoionized
plasmas. Over the last several years our research work has been focused on accurate
calculations of electron-ion and atom excitation processes using the close-coupling methods
for application to astrophysical and laboratory plasmas. Our effort has been to benchmark
theory against experiment for the electron impact excitation of selected atomic systems for
transitions of diagnostic importance. Accurate representation of target wave functions that
properly account for the important correlation and relaxation effects and inclusion of coupling
effects including coupling to the continuum are essential components of a reliable collision
calculation. The resonant excitation processes are important in the low energy region. This
chapter presents a discussion of the state of our knowledge of the electron impact excitation processes together with some recent results for neutral atoms and singly and multiply charged
ions.
Chapter 5 – Since the discovery of photosensitivity in optical fibers by Hill et al. in 1978
[1], Bragg gratings fabricated in optical fibers and planar waveguides have been extensively
investigated over the last three decades and have been widely used in optical communication
and sensor applications. In this chapter, the properties of birefringence in optical fibers and
planar waveguides with Bragg grating structures are investigated experimentally, and the
birefringence-induced impairments in communication systems with Bragg-grating–based
components are evaluated by simulation.
Chapter 6 – As an important waveguiding medium, optical fiber plays significant roles in
optical communications, optoelectronics, and sensors. A new type of microstructure inscribed
in the optical fibers, i.e., fiber Bragg gratings (FBGs), has received considerable attention in
recent years. A FBG is a type of distributed Bragg reflector constructed in a short segment of
optical fiber that reflects specific wavelengths of light and transmits all the other components.
In this chapter, optical properties of FBGs will be reviewed first with the underlying physical
mechanisms. Different techniques to fabricate FBGs will be illustrated with the comparison
of their advantages and drawbacks. For their important sensing applications, FBGs as
temperature and humidity sensors will be discussed. The FBG sensors exceed other
conventional electric sensors in many aspects, for instance, immunity to electromagnetic
interference, compact size, light weight, flexibility, stability, high temperature tolerance, and
resistive to harsh environment. A novel approach to realize separate temperature or humidity
measurement, and their simultaneous measurement will be demonstrated by the use of FBGs
coated with different polymers. The polymer-coated FBGs indicate linear shifts in the Bragg
resonance wavelengths of the gratings with the temperature changes. A polyimide-coated
FBG is sensitive to humidity due to the unique hygroscopic properties of polyimide while an
acrylate-coated FBG shows insensitivity to humidity. The experimental results are in good
agreement with the theoretical analysis.
Chapter 7 – Spatial optical techniques have shown great potential in the field of
information security to encode high-security images. Among them, the dual random phase
encoding method has received much attention since it was proposed by Réfrégier and Javidi
in the middle 1990s. Since then, a number of works in the field were proposed introducing
different variations of this technique. On the other hand, the space-time duality refers to the
close mathematical analogy that exists between the equations describing the paraxial
diffraction of beams in space and the first-order temporal dispersion of optical pulses in
dielectric media. It is generally used for extending to the temporal domain well-known
properties of spatial optical configurations. In this work a new approach is developed for the
secure data transmission problem in fiber optic links. We propose the encoding of timevarying
optical signals, mainly for short-haul applications, with encryption methods that can
be considered the time domain counterparts of the dual random phase encoding process and
two of its more frequent variations: the fractional Fourier transform dual random phase
encoding and the Fresnel transform dual random phase encoding. Further, as performance is a
very relevant subject in fiber optic links, we will analyze mechanisms to produce time
limited, as well as bandwidth limited, encoded signals. To this end, the different signal
broadenings produced by each stage of the encoding process, in both time and frequency
domains, are analyzed by using the Wigner distribution function formalism, and general
expressions for the time width, as well as the bandwidth, in every encryption stage is obtained. The numerical simulations show good system performances, and a comparison
between the different encryption processes is made. Furthermore, the robustness of the
proposed methods is analyzed against the variation of typical parameters of the encryptiondecryption
setup. Finally, the implementation of this proposal with current photonic
technology is discussed.
Chapter 8 – In this chapter, we report electronic and magnetic structure of pure and (As-)
doped manganese clusters from density functional theory using generalized gradient
approximation for the exchange-correlation energy. Ferromagnetic to ferrimagnetic transition
takes place at n = 5 for pure manganese clusters, Mnn, and remarkable lowering of magnetic
moment is found for Mn13 and Mn19 due to their closed icosahedral growth pattern and
results show excellent agreement with experiment. On the other hand, for As-doped
manganese clusters, MnnAs, ferromagnetic coupling is found only in Mn2As and Mn4As
and inclusion of a single As stabilizes manganese clusters. Exchange coupling in the MnnAs
clusters are anomalous and behave quite differently from the Ruderman-Kittel-Kasuya-
Yosida like predictions. Finally, possible relevance of the observed magnetic behaviour is
discussed in the context of Mn-doped GaAs semiconductor ferromagnetism.
Chapter 9 – The investigation of the optical properties of porous silica samples is
presented. Optical spectroscopy measurements, including Raman scattering, steady state and
time resolved photoluminescence, optical absorption and excitation of photoluminescence are
reported. The chapter reviews the results of the research we carried out upon the emission
features of porous silica in the ultraviolet and visible wavelength range and the
characterization of the emission properties of dye-doped sol-gel synthesized silica samples. In
particular, the study of the emission band recorded at about 3.7 eV and its correlation with the
chemical and physical conditions of the surface is discussed. As regards dye-doped silica
samples, the analysis of the spectroscopic features of pre- and post-doped hybrid samples is
presented and their potential feasibility as solid state dye laser is proposed.
Chapter 10 – Noise figure analysis is one of the key topics in optical amplifiers design
and analysis. The pump to signal RIN transfer is one of the major causes for the noise in
optical fiber amplifiers. If there is intensity modulation to the pump power, the relative
intense noise (RIN) will transfer from the pump to the signal wavelength and degrade the
system performance. There have been corresponding papers published on this issue with
analytical expressions, The existing analysis gives deep insight into the problem; however,
the analysis mentioned above focused on Raman amplifiers or Brillouin fiber lasers with
single pump and single signal channel, which are not the most general case. For the case of
multiple pumps case, there has been no model for the pump to signal RIN transfer.
Moreover, the current analyses are based on the temporal model, which is very timeconsuming
and does not give a clear picture of the frequency response. In this chapter, we
will propose a novel frequency model to evaluate the pump to signal RIN transfer in optical
amplifiers and lasers with arbitrary pumps. Analytical expressions could be derived for one
pump specific case based on the model.
Chapter 11 – Among various candidates considered to implement quantum information
processing (QIP), cavity quantum electrodynamics (QED) has attracted much attention over
past years due to the availability to demonstrate few-qubit quantum gates experimentally and
the possibility to construct future quantum network. We review recent work of QIP using
cavity QED in weak dissipation. The concrete work we review includes W-state preparation,
Toffoli gating, Grover search implementation, and QIP by geometric phase. Under the idea of quantum trajectory, we could present analytical expressions to show the detrimental influence
of cavity decay in QIP.
Chapter 12 – The response of gas atoms or molecules to strong laser fields depends on
their internal electronic state. This fact can be exploited to gain insight into bound electron
structure, nuclear dynamics, and even electronic dynamics by measuring the emitted photons
created by the process of high harmonic generation (HHG). HHG is customarily explained by
recombination of a virtually detached electron upon returning to its initial bound state. In this
chapter we investigate the emission of high harmonic radiation from molecular systems in
excited electronic states. Specifically we report on numerical results obtained for systems in
two types of electronic excited states: (i) states with pronounced net internal angular
momentum and (ii) states which are excited by nonlinear plasmon oscillations in highly
polarizable molecules by a strong laser field. Our results can be summarized as follows. If the
involved state exhibits pronounced angular momentum both ionization and recombination are
influenced and the symmetry of the three-stage process is broken. We show that this can be
used to gain access to the phase of the bound state and that recombination to such a bound
state leads to creation of circularly polarized, spatially coherent attosecond X-ray pulses. By
solving the time-dependent Schr¨odinger equation for a model system containing 4 active
electrons using the multi-configuration time-dependent Hartree-Fock approximation, we
show that the harmonic harmonic spectrum exhibits two cut-offs. The first cut-off is in
agreement with the wellestablished, single active electron cut-off law. The second cut-off
presents a signature of multi-electron dynamics. Electrons that are ionized from an excited
multi-plasmon state and recombine to the ground state gain additional energy, thereby
creating the second plateau.
Chapter 13 – A unified theory is given of dynamically modified decay and decoherence of
field-driven multipartite systems. When this universal framework is applied to two-level
systems (TLS) or qubits experiencing either amplitude or phase noise (AN or PN) due to their
coupling to a thermal bath, it results in completely analogous formulae for the modified
decoherence rates in both cases. The spectral representation of the modified decoherence rates
underscores the main insight of this approach, namely, the decoherence rate is the spectral
overlap of the noise and modulation spectra. This allows us to come up with general recipes
for modulation schemes for the optimal reduction of decoherence under realistic constraints.
An extension of the treatment to multilevel and multipartite systems exploits intra-system
symmetries to dynamically protect multipartite entangled states. Another corollary of this
treatmentis that entanglement, which is very susceptible to noise and can die, i.e., vanish at
finite times, can be resuscitated by appropriate modulations prescribed by our universal
formalism. This dynamical decoherence control is also shown to be advantageous in quantum
computation setups, where control fields are applied concurrently with the gate operations to
increase the gate fidelity.
†Chapter 14 – Cooling of surfaces of various objects is widely used in science and
engineering. This process assists in formation of better characteristics of various devices,
tools, and instruments by increasing the quality of their operation and reliability. On the other
hand, surface cooling and supercooling can generate severe problems in operation of vehicles sometimes leading to catastrophic situations. Therefore, despite the long story of studying the
cooling processes, it is still important because of numerous applications.
Chapter 15 – In the cryogenic realm, liquid oxygen (LOX) possesses a natural
paramagnetic susceptibility and does not require a colloidal suspension of particles for
practical application as a magnetic working fluid. Commercial ferrofluids have performed
well in industrial applications, but expanding their workable range to low temperatures
requires a suitable selection of the carrier fluid, such as LOX. In this chapter, the equation of
motion for the pure fluid is derived and applied to a slug of LOX being displaced by a pulsed
magnetic field. Its theoretical performance is compared to actual experimental data with
discussion on empirical parameters, sensitivity to measurement uncertainty, and geometric
similarity. The 1.1 T pulse of magnetic flux density produced oscillations in the slug of 6-8
Hz, generating up to 1.4 kPa of pressure change in a closed section when the slug acted like a
liquid piston. The experiments and theoretical model demonstrate that LOX could be used as
a magnetic working fluid in certain applications.
Chapter 16 – This chapter mainly focuses on the effects of low temperature (subzero)
treatments on microstructure and mechanical properties of aluminum and magnesium alloys.
Deep cryogenic treatment on A319 aluminum alloy showed that the abrasion resistance of the
alloy was improved after the treatment. This improvement was attributed to the strengthening
of the α-aluminum matrix which slows down the propagation of the existing defects. The
execution of deep cryogenic treatment on AZ91 magnesium alloy changed the distribution of
β precipitates. The tiny laminar β particles almost dissolved in the microstructure and the
coarse divorced eutectic β phase penetrated into the matrix. This microstructural modification
resulted in a significant improvement on mechanical properties of the alloy. The steady state
creep rates were measured and it was found that the creep behavior of the alloy, which is
dependent on the stability of the near grain boundary microstructure, was improved by the
deep cryogenic treatment. After the deep cryogenic treatment, the sliding of grain boundaries
was greatly suppressed due to morphological changes. As a result, the grain boundaries are
less susceptible for grain boundary sliding at high temperatures. After dry sliding wear tests
were performed, the wear resistance of the alloy improved remarkably after deep cryogenic
treatment. Furthermore due to interest in the subzero treatments of steels in the past few
decades, AISI H13 tool steel was chosen and cryogenic treatment at -72ºC and deep
cryogenic treatment at -196ºC were applied and it was found that the execution of low
temperature treatments on samples affected the microstructure of the H13 tool alloy to a great
extent. By applying the subzero treatments, the retained austenite was transformed to
martensite due to the completion of martensite transformation. The cryogenic treatment at a
very low temperature and holding the samples for a long time, also lead to precipitation of
more uniform and very fine carbide particles. This microstructural modification resulted in a
significant improvement on mechanical properties and wear resistance of the alloy.
Chapter 17 – Among the various applications of cryogenics, the implementation of cold
thermal processes with the aim of enhancing mechanical properties of materials is the most
attractive from the perspective of structural component engineering design. In particular,
considering the key-role played by the fatigue behavior of materials in this discipline, the
development of methodologies that allow to achieve longer service life is an evergreen topic,
which concerns many fields of application such as energy production and transportation… e.t.c.