5,163 research outputs found
Dark state lasers
We propose a new type of laser resonator based on imaginary "energy-level
splitting" (imaginary coupling, or quality factor Q splitting) in a pair of
coupled microcavities. A particularly advantageous arrangement involves two
microring cavities with different free-spectral ranges (FSRs) in a
configuration wherein they are coupled by "far-field" interference in a shared
radiation channel. A novel Vernier-like effect for laser resonators is designed
where only one longitudinal resonant mode has a lower loss than the small
signal gain and can achieve lasing while all other modes are suppressed. This
configuration enables ultra-widely tunable single-frequency lasers based on
either homogeneously or inhomogeneously broadened gain media. The concept is an
alternative to the common external cavity configurations for achieving tunable
single-mode operation in a laser. The proposed laser concept builds on a high-Q
"dark state" that is established by radiative interference coupling and bears a
direct analogy to parity-time (PT) symmetric Hamiltonians in optical systems.
Variants of this concept should be extendable to parametric-gain based
oscillators, enabling use of ultrabroadband parametric gain for widely tunable
single-frequency light sources
A Stochastic Compartmental Model for Fast Axonal Transport
In this paper we develop a probabilistic micro-scale compartmental model and
use it to study macro-scale properties of axonal transport, the process by
which intracellular cargo is moved in the axons of neurons. By directly
modeling the smallest scale interactions, we can use recent microscopic
experimental observations to infer all the parameters of the model. Then, using
techniques from probability theory, we compute asymptotic limits of the
stochastic behavior of individual motor-cargo complexes, while also
characterizing both equilibrium and non-equilibrium ensemble behavior. We use
these results in order to investigate three important biological questions: (1)
How homogeneous are axons at stochastic equilibrium? (2) How quickly can axons
return to stochastic equilibrium after large local perturbations? (3) How is
our understanding of delivery time to a depleted target region changed by
taking the whole cell point-of-view
Tunable coupled-mode dispersion compensation and its application to on-chip resonant four-wave mixing
We propose and demonstrate localized mode coupling as a viable dispersion
engineering technique for phase-matched resonant four-wave mixing (FWM). We
demonstrate a dual-cavity resonant structure that employs coupling-induced
frequency splitting at one of three resonances to compensate for cavity
dispersion, enabling phase-matching. Coupling strength is controlled by thermal
tuning of one cavity enabling active control of the resonant
frequency-matching. In a fabricated silicon microresonator, we show an 8 dB
enhancement of seeded FWM efficiency over the non-compensated state. The
measured four-wave mixing has a peak wavelength conversion efficiency of -37.9
dB across a free spectral range (FSR) of 3.334 THz (27 nm). Enabled by
strong counteraction of dispersion, this FSR is, to our knowledge, the largest
in silicon to demonstrate FWM to date. This form of mode-coupling-based, active
dispersion compensation can be beneficial for many FWM-based devices including
wavelength converters, parametric amplifiers, and widely detuned correlated
photon-pair sources. Apart from compensating intrinsic dispersion, the proposed
mechanism can alternatively be utilized in an otherwise dispersionless
resonator to counteract the detuning effect of self- and cross-phase modulation
on the pump resonance during FWM, thereby addressing a fundamental issue in the
performance of light sources such as broadband optical frequency combs
Ultra-low-loss CMOS-Compatible Waveguide Crossing Arrays Based on Multimode Bloch Waves and Imaginary Coupling
We experimentally demonstrate broadband waveguide crossing arrays showing
ultra low loss down to dB/crossing (), matching theory, and
crosstalk suppression over dB, in a CMOS-compatible geometry. The
principle of operation is the tailored excitation of a low-loss spatial Bloch
wave formed by matching the periodicity of the crossing array to the difference
in propagation constants of the 1- and 3-order TE-like
modes of a multimode silicon waveguide. Radiative scattering at the crossing
points acts like a periodic imaginary-permittivity perturbation that couples
two supermodes, which results in imaginary (radiative) propagation-constant
splitting and gives rise to a low-loss, unidirectional breathing Bloch wave.
This type of crossing array provides a robust implementation of a key component
enabling dense photonic integration
The Distribution of Minimum of Ratios of Two Random Variables and Its Application in Analysis of Multi-hop Systems
The distributions of random variables are of interest in many areas of science. In this paper, ascertaining on the importance of multi-hop transmission in contemporary wireless communications systems operating over fading channels in the presence of cochannel interference, the probability density functions (PDFs) of minimum of arbitrary number of ratios of Rayleigh, Rician, Nakagami-m, Weibull and α-µ random variables are derived. These expressions can be used to study the outage probability as an important multi-hop system performance measure. Various numerical results complement the proposed mathematical analysis
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