arXiv subjects

H. Ahmad

Publications and source records attributed to H. Ahmad.

4 recordsLinked to original sources

arXiv2017

Dual-wavelength generation and tuning by controlling the apodized grating depth in microring resonators

Here we show a photonic design for tunable dual-wavelength generation deploying optical nonlinear mode coupling of two coupled III-V semiconductor microring resonators (MRRs) connected to a pump and drop waveguide buses. Here one of the two rings contains a grating, while the other has a planar surface. The underling mechanism for the dual wavelength generation originates from the resonance-detuning of the spectra resulting in non-linear mode mixing. Tunability of the wavelengths is achieved by altering the grating depth of the MRR and the power coupling coefficients. For the grating design of the MRR we select a trapezoidal-profiled apodized grating to gain low reflectivity at sidelobes. A time-domain travelling wave (TDTW) analysis gives a InGaAsP core refractive index of 3.3 surrounded by a grating InP cladding with n=3.2. We further confirm that the propagation of a Gaussian pulse input with 10 mW power and bandwidth of 0.76 ps is well confined within the mode propagation of the system. Taken together our results show a 2:1 fan-out of two spectrally separate signals for compact and high functional sources on chip.

physics.optics↗

arXiv2013

Transverse localization of light in 1D disordered waveguide lattices with backbone photonic bandgap

The role of a prominent photonic bandgap (PBG) on the phenomenon of transverse localization of light in a semi-infinite lossless waveguide lattice consisting of evanescently coupled disordered one-dimensional optical waveguides has been investigated numerically. The interplay between the underlying photonic bandgap due to inherent periodicity of the optical system and various levels of deliberately induced transverse disorder in its refractive index periodicity has been studied. We show that the PBG indeed plays an important role and its simultaneous presence could catalyze realization of localized light even when strength of disorder is not sufficiently strong to independently cause localization of light. An important outcome of this study revealed that PBG could be gainfully exploited to tailor the spectral window for localization of light in potential applications like lasing in a disordered optical lattice.

physics.optics↗

arXiv2009

Chemical Equilibration and Transport Properties of Hadronic Matter near $T_c$

We discuss how the inclusion of Hagedorn states near $T_c$ leads to short chemical equilibration times of proton anti-proton pairs, $K\bar{K}$ pairs, and $Λ\barΛ$ pairs, which indicates that hadrons do not need to be "born" into chemical equilibrium in ultrarelativistic heavy ion collisions. We show that the hadron ratios computed within our model match the experimental results at RHIC very well. Furthermore, estimates for $η/s$ near $T_c$ computed within our resonance gas model are comparable to the string theory viscosity bound $η/s=1/4π$. Our model provides a good description of the recent lattice results for the trace anomaly close to $T_c=196$ MeV.

nucl-th↗

arXiv2009

Particle Ratios as a Probe of the QCD Critical Temperature

We show how the measured particle ratios can be used to provide non-trivial information about the critical temperature of the QCD phase transition. This is obtained by including the effects of highly massive Hagedorn resonances on statistical models, which are used to describe hadronic yields. The inclusion of Hagedorn states creates a dependence of the thermal fits on the Hagedorn temperature, $T_H$, which is assumed to be equal to $T_c$, and leads to an overall improvement of thermal fits. We find that for Au+Au collisions at RHIC at $\sqrt{s_{NN}}=200$ GeV the best square fit measure, $χ^2$, occurs at $T_c \sim 176$ MeV and produces a chemical freeze-out temperature of 172.6 MeV and a baryon chemical potential of 39.7 MeV.

nucl-th↗