AN10175
MICROCAVITIES PROBE FUNDAMENTAL OPTICAL PROCESS IN NANOMETER SIZED
SILICON
Silicon, the flesh and blood of the microprocessor, suffers from a
major drawback when it comes to photonic functionality: it cannot emit
light. Efficient light emission from silicon will enable functionality
hitherto unobtainable in semiconductor chips and will forever change
the face of electronic industry as we know it. In 1990, it was
observed in several laboratories across the world, that silicon does
emit light when it is diced up into nanometer sized chunks - popularly
known as nanocrystals. In spite of these advances, the dream of a
silicon laser has remained elusive for nearly two decades. Is it even
possible in theory? This is the question we seek to answer in our paper.
In doing so, we have demonstrated the use of optical microcavities to
probe fundamental optical losses in silicon nanocrystal doped devices.
Microcavities are not only an essential component of any laser but can
also serve as excellent diagnostic tools for optical processes. This
analysis has enabled us to fabricate state-of-the-art resonators
having quality factors exceeding 1,400 -- nearly 4-5 times larger than
reported previously. More importantly, we have been able to estimate
the limiting performance silicon nanocrystal-doped cavities. Our study
indicates that, even at low temperatures, it will be extremely
difficult to overcome the optical losses in a continuous-wave
operation. Owing to the prevalence of fast optical loss processes, our
only hope may be to resort to pulsed operation.
Over the past few years, microcavities have been at the center of
attention in the photonics community. Our work has highlighted their
use as extremely sensitive diagnostic tools for exploring fundamental
optical processes at the materials and device levels.
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LL11012ER
Hot Transformers: Chemical Oscillations Found in Collapsing Bubbles
The mysterious slow modulations of an air bubble in water
oscillating nonlinearly by high frequency ultrasound driving
are shown to have their reason in chemical oscillations.
Collapsing bubbles are known for a phenomenon called sonoluminescence:
A very short light flash is emitted at the time when its volume is
compressed a million times from its biggest size.
And this process was thought to be very stable. Until the team of
Thomas and Holt of Boston University measured
large periodical variations.
Nobody found a definite explanation by that time.
In this paper it has been shown for the first time
that a bubble may transform slowly
from a big and relatively cold nitrogen bubble
to a smaller but hotter argon bubble.
But this process may get unstable - the bubble gets too small
and colder and transforms back into a nitrogen containing sphere.
One single chemical oscillation usually takes 2 - 5 seconds to complete
until it repeats all over. And these oscillations are stable,
they only change when an external parameter is varied.
The chemistry happening during the brief moments of collapse
is normally of interest only for rocket scientists:
At 10,000 deg Kelvin under pressures
exceeding 10,000 bars the gases in the bubble are compressed nearly to
liquid density. The calculations in this paper help to explore
reactions in the same extreme environmental conditions,
only for a much smaller approx. 10 micrometer in diameter sized bubble.
***
ER10316
Hierarchical Structure in Branching Patterns
Branching objects appearing in nature have hierarchical structures.
Regarding a river network, for example, merging of two streams forms a broader stream and all streams finally flow into the main stream.
To measure the hierarchical structures quantitatively, a method of branch-numbering, named Horton-Strahler ordering, was introduced.
A stream originating from a source has order 1, joining of two streams of order 1 arises a stream of order 2, and so on.
And a simple scaling law (called Horton's law of stream numbers) has been confirmed empirically and analytically.
In this paper, we consider the simplest case in which every type of branching patterns is appeared randomly.
We introduce "hierarchical hanging method" and calculate the average of the number of streams of the same order.
This is expressed in the form of a recursive equation about the order of streams.
It is noted that for the streams of order 2, the exact solution is derived.
By using this recursive equation, Horton's law is proved numerically and analytically.
Furthermore, we also derive a relation for the k-th moment, which is regarded as the extended form of Horton's law.
***
BS10524
Exciton magnetic properties very strongly dependent on the kinetic energy
Excitons are quanta of electronic excitation traveling through condensed
matter and are central to a huge range of optical and energy transfer
phenomena in the physical and life sciences. In semiconductors, excitons
are of the Wannier type, in which an electron is bound to a hole by the
Coulomb interaction, the resulting energy states being analogous to those
of a hydrogen atom. Motion of excitons in bulk material can therefore be
thought of in terms of hydrogen-like particles or, alternatively, in terms
of waves. However, little has been known about how excitons behave when
they are actually moving, largely because there are very few experiments
which enable them to be ‘caught’ whilst under motion. In the present work,
we investigated how the magnetic properties of excitons depend on kinetic
energy in quantum wells which are very wide, but in which the exciton
energies (and hence those of the optical transitions) are nevertheless
quantized. By studying each transition individually, we find the magnetic
properties to be very strongly dependent on the kinetic energy. This
unusual behavior is ascribed to mixing between the 1S ground state and
higher-lying states. The mixing is induced by the exciton’s motion and
appears to be a universal feature for semiconductors.
***
AM10316
Interference of Subwavelength Light Beams Enhances the Light Energy
The wave nature of light causes the light waves passing through slits in
screen to interfere in the observation plane, creating an interference pattern
of bright and dark bands on the screen. Normally, the interference (addition)
of two or more waves causes enhancement or suppression of the light intensity,
but not the light energy. We show that the waves (beams) generated by
multiple, subwavelength-wide slits can have similar phases and can enhance the
light energy. If the spacing of the slits is smaller than the optical
wavelength, then the phases of the beams at the detector are nearly the same
and beams add coherently (the light energy increases as the number of
light-sources squared, regardless of periodicity). If the spacing is larger,
then the addition is not so efficient, but still leads to enhancements and
resonances versus wavelength in the total energy transmitted. The anomalies in
transmission spectra of gratings, a long standing problem in optics, follow
naturally from the interference properties of the model. The mechanism could
be interpreted as a non-quantum analog of the superradiance emission of a
subwavelength ensemble of atoms (the light energy scales as the number of
light-sources squared) predicted by the Dicke quantum model.
