845 research outputs found
Single Crystal Sapphire at milli-Kelvin Temperatures: Observation of Electromagnetically Induced Thermal Bistability in High Q-factor Whispering Gallery Modes
Resonance modes in single crystal sapphire (-AlO) exhibit
extremely high electrical and mechanical Q-factors ( at 4K),
which are important characteristics for electromechanical experiments at the
quantum limit. We report the first cooldown of a bulk sapphire sample below
superfluid liquid helium temperature (1.6K) to as low as 25mK. The
electromagnetic properties were characterised at microwave frequencies, and we
report the first observation of electromagnetically induced thermal bistability
in whispering gallery modes due to the material dependence on thermal
conductivity and the ultra-low dielectric loss tangent. We identify "magic
temperatures" between 80 to 2100 mK, the lowest ever measured, at which the
onset of bistability is suppressed and the frequency-temperature dependence is
annulled. These phenomena at low temperatures make sapphire suitable for
quantum metrology and ultra-stable clock applications, including the possible
realization of the first quantum limited sapphire clock.Comment: 5 pages, 4 figure
Discovery of Bragg confined hybrid modes with high Q-factor in a hollow dielectric resonator
The authors report on observation of Bragg confined mode in a hollow
cylindrical dielectric cavity. A resonance was observed at 13.4 with an
unloaded Q-factor of order , which is more than a factor of 6
above the dielectric loss limit. Previously such modes have only been realized
from pure Transverse Electric modes with no azimuthal variations and only the
component. From rigorous numeric simulations it is shown that the
mode is a hybrid mode with non-zero azimuthal variations and with dominant
and electric field components and magnetic field
component.Comment: Accepted to be published in Applied Physics Letter
Detrapping and retrapping of free carriers in nominally pure single crystal GaP, GaAs and 4H-SiC semiconductors under light illumination at cryogenic temperatures
We report on extremely sensitive measurements of changes in the microwave
properties of high purity non-intentionally-doped single-crystal semiconductor
samples of gallium phosphide, gallium arsenide and 4H-silicon carbide when
illuminated with light of different wavelengths at cryogenic temperatures.
Whispering gallery modes were excited in the semiconductors whilst they were
cooled on the coldfinger of a single-stage cryocooler and their frequencies and
Q-factors measured under light and dark conditions. With these materials, the
whispering gallery mode technique is able to resolve changes of a few parts per
million in the permittivity and the microwave losses as compared with those
measured in darkness. A phenomenological model is proposed to explain the
observed changes, which result not from direct valence to conduction band
transitions but from detrapping and retrapping of carriers from impurity/defect
sites with ionization energies that lay in the semiconductor band gap.
Detrapping and retrapping relaxation times have been evaluated from comparison
with measured data.Comment: 7 pages, 6 figure
Modified permittivity observed in bulk Gallium Arsenide and Gallium Phosphide samples at 50 K using the Whispering Gallery mode method
Whispering Gallery modes in bulk cylindrical Gallium Arsenide and Gallium
Phosphide samples have been examined both in darkness and under white light at
50 K. In both samples we observed change in permittivity under light and dark
conditions. This results from a change in the polarization state of the
semiconductor, which is consistent with a free electron-hole
creation/recombination process. The permittivity of the semiconductor is
modified by free photocarriers in the surface layers of the sample which is the
region sampled by Whispering Gallery modes.Comment: 8 pages, 3 figure
- …