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Microcavity Research
Whispering Gallery Mode Resonators
Whispering gallery modes (morphology-dependent resonances) play a
significant role in modern nonlinear optics. High quality factors
associated with whispering gallery modes in dielectric resonators,
along with small mode volumes, result in resonant enhancement of
nonlinear interactions of various kinds. Those features provide
the opportunity to achieve a high nonlinear response with weak
electromagnetic fields, even if the cavity is fabricated from a
material with low nonlinearity, which is usually the case for
optically transparent materials. This property makes the
whispering gallery modes very attractive for fundamental studies
as well as applications such as all-optical switching devices,
microlasers, and optical sensors.
High-Q WGMs were first observed in liquid droplets, as well as in
solidified droplets of fused amorphous materials, such as fused
silica. Although those materials are characterized by small
optical attenuation, the highest quality factor of WGMs
(approximately ten billions) has remained limited by Rayleigh
scattering of residual surface roughness. Liquids and amorphous
materials form only a small part of high quality optical materials
suitable for fabrication of WGM resonators. For instance, some
crystals are transparent enough to sustain high-Q WGMs, on one
hand; and are nonlinear enough, to allow us to manipulate
continuously by the WGMs' characteristics, on the other.
We found experimentally that it is possible to obtain crystalline
WGM optical resonators with very high Q-factors (more than a
billion), similar to that of surface-tension-formed resonators, by
adopting simple polishing techniques. With this approach, the
original crystal structure and composition is preserved, and the
unique linear and nonlinear crystal properties are enhanced with
the small volume of the high-Q resonator. Total internal
reflection at the walls of the WGM resonators provides the effect
of an ultra-broad band mirror, allowing very high Q-factors across
the whole material transparency range. This property makes
crystalline WGM resonators a unique tool for optical material
studies. With our fabrication process, we have achieved Q-factor
limited in value only by the absorption of the material.
We design and master various efficient optical and photonic
devices and develop new technologies with fused silica
microresonators as well as crystalline resonators. For instance,
we succeeded in fabrication of the following devices during last
couple of years:
OPTICAL AND PHOTONIC FILTERS:
We have demonstrated novel techniques to manipulate spectral
properties of high-Q WGMs in optical dielectric microresonators.
These include permanent frequency trimming of WGM frequencies by
means of UV photosensitivity of germanium doped silica resonators;
electro-optical tuning of WGM in lithium niobate resonators, and
cascading of microresonators for obtaining second-order filtering
function. We have presented theoretical interpretation of
experimental results, and examples of applications of these
techniques for photonic microwave filtering.
We have fabricated a third-order tunable optical filter using
three metal-coated LiNbO3 disc WGM resonators.
The filter has 29 MHz bandwidth and can be tuned in range of
±12 GHz by applying DC voltage in range of
±150 V to the metal coating.
Because free spectral range of the resonators is
approximately 13.3 GHz, the filter may be tuned practically at any
optical frequency in the transparency range of lithium niobate.
Large optical tuning was realized due to small thickness 50 microns
of the discs.
ELECTRO-OPTIC MODULATORS AND PHOTONIC RECEIVERS:
We have implemented a low noise resonant electro-optic modulator
based on all-resonant three-wave mixing using high-Q WGM
millimeter-size toroidal cavity fabricated from LiNbO3.
We observe an efficient modulation of light with coherent microwave
pumping at frequency 9 GHz with applied power of about 10 mW.
Used as a receiver, the modulator allows us to detect nW microwave
radiation. Preliminary results with a 33 GHz modulator prototype
are also reported.
PARAMETRIC FREQUENCY CONVERTERS:
We have demonstrated parametric frequency upconversion with
periodically poled lithium niobate disc WGM resonator. Due to high
Q-factor, we realized frequency doubling at 1550 nm with almost
50% efficiency at 25 mW pump power. The efficiency was
restricted due to not optimal structure of the periodical poling
of the resonator material, though the method allows to obtain much
higher efficiency with proper poling. The follow-up studies of the
parametric processes in WGM PPLN resonators are also important
because it has been predicted that an optical parametric
oscillator (OPO) based on a WGM PPLN resonator might have power
threshold below a microwatt — orders of magnitude less than that
of the state-of-the-art OPOs, typically at 0.5 mW.
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