Tunable phonon-plasmon hybridization in α-MoO3-graphene based van der Waals heterostructures.

The plasmon-phonon hybridization behavior between anisotropic phonon polaritons (APhP) of orthorhombic phase Molybdenum Trioxide (α - MoO3) and the plasmon-polaritons of Graphene layer - forming a van der Waals (vdW) heterostructure is investigated theoretically in this paper. It is found that in-plane APhP shows strong interaction with graphene plasmons lying in their close vicinity, leading to large Rabi splitting. Anisotropic behavior of biaxial MoO3 shows the polarization-dependent response with strong anti-crossing behavior at 0.55 eV and 0.3 eV of graphene's Fermi potential for [100] and [001] crystalline directions, respectively. Numerical results reveal unusual electric field confinement for the two arms of enhanced hybrid modes: the first being confined in the graphene layer representing plasmonic-like behavior. The second shows volume confined zigzag pattern in hyperbolic MoO3. It is also found that the various plasmon-phonon hybridized modes could be wavelength tuned, simply by varying the Fermi potential of the graphene layer. The coupling response of the hybrid structure is studied analytically using the coupled oscillator model. Furthermore, we also infer upon the coupling strength and frequency splitting between the two layers with respect to their structural parameters and interlayer spacing. Our work will provide an insight into the active tunable property of hybrid van der Waals (vdW) structure for their potential application in sensors, detectors, directional spontaneous emission, as well as for the tunable control of the propagating polaritons in fields of flat dispersion where strong localization of photons can be achieved, popularly known as the flatband optics.

Modulation of coherence-polarization property of speckles using a birefringent scatterer.

The change of the spatial coherence-polarization (CP) property of a speckle pattern due to the modulation of birefringence in a scattering medium is investigated experimentally. The birefringence is introduced to the scattering medium by attaching layers of overhead projector (OHP) sheets to it. It is shown that the spatial polarization distribution can be tuned from a uniformly polarized case to a randomly polarized case by increasing the number of OHP sheets. The change of the spatial CP property with the number of OHP sheets is investigated from the study of the spatial degree of coherence and the degree of polarization, and these variations are further confirmed from the probability density function of the intensity of the speckle pattern and from the measurement of the generalized Stokes parameters. The effect of the change of the number of OHP sheets on the visibility of the intensity correlation function and the quality of the object retrieved through the scattering medium are also investigated.

Unified analysis of coherence property of a Stokes wave generated via a stimulated Raman process in optical fiber.

We investigate the evolution of coherence property of a noise-seeded Stokes wave in short (<1ps) and long pulse (>1ps) regimes numerically through a set of coupled nonlinear equations. The simulations include quantum noise by incorporating noise seed in the pump field. The spectral phase fluctuations of the Stokes wave for both regimes are characterized, and the degrees of first-order mutual spectral coherence are calculated for different conditions. Statistical analysis demonstrates the effect of spectral coherence of the Stokes wave in optical fiber on pump power, fiber length, and pump pulse width for short and long pulse regimes. It is observed that the noise-seeded stimulated Raman process causes degradation of spectral coherence with the increase in pump power, fiber length, and pulse width of the pump wave. The degradation of the spectral coherence is manifested by the transition of the Stokes wave from a quasi-coherent to incoherent spectrum.

Design of CMOS compatible and compact, thermally-compensated electro-optic modulator based on off-axis microring resonator for dense wavelength division multiplexing applications.

In this work, we propose and demonstrate the performance of silicon-on-insulator (SOI) off-axis microring resonator (MRR) as electro-optic modulator (EOM). Adding an extra off-axis inner-ring in conventional microring structure provides control to compensate thermal effects on EOM. It is shown that dynamically controlled bias-voltage applied to the outer ring has the potency to quell the thermal effects over a wide range of temperature. Thus, besides the appositely biased conventional microring, off-axis inner microring with pre-emphasized electrical input message signal enables our proposed structure suitable for high data-rate dense wavelength division multiplexing scheme of optical communication within a very compact device size.

Triple-core collinear and noncollinear plasmonic photonic crystal fiber couplers.

Numerical analysis of single and multiple gold nanowires embedded in triple cores arranged in collinear and noncollinear configurations in photonic crystal fibers (PCFs) is reported. A full-vectorial finite element method is used to achieve coupling characteristics of plasmonic PCF couplers for both x and y polarizations. It is demonstrated numerically that the PCF plasmonic couplers exhibit polarization-independent tunable broadband filter characteristics that can be tuned according to the diameter of the embedded gold rod(s).

Realistic nonlocal refrigeration engine based on Coulomb-coupled systems.

We investigate, in detail, a triple quantum dot system that exploits Coulomb coupling to achieve nonlocal refrigeration. The system under investigation is a derivative of the nonlocal thermodynamic engine, originally proposed by Sánchez and Büttiker [Phys. Rev. B 83, 085428 (2011)PRBMDO1098-012110.1103/PhysRevB.83.085428], that employs quadruple quantum dots to attain efficient nonlocal heat harvesting. Investigating the cooling performance and operating regime using the quantum master equation approach, we point out some crucial aspects of the refrigeration engine. In particular, we demonstrate that the maximum cooling power for the setup is limited to about 70% of the optimal design. Proceeding further, we point out that to achieve a target reservoir temperature lower than the average temperature of the current path, the applied voltage must be greater than a given threshold voltage V_{TH} that increases with the decrease in the target reservoir temperature. In addition, we demonstrate that the maximum cooling power, as well as the coefficient of performance, deteriorates as one approaches a lower target reservoir temperature. The triple quantum dot system, investigated in this paper, combines fabrication simplicity along with descent cooling power and may pave the way towards the practical realization of efficient nonlocal cryogenic refrigeration systems.

Variational approach to study soliton dynamics in a passive fiber loop resonator with coherently driven phase-modulated external field.

We report a detailed semianalytical treatment to investigate the dynamics of a single cavity soliton (CS) and two copropagating CSs separately in a Kerr mediated passive optical fiber resonator which is driven by a phase-modulated pump. The perturbation is dealt with by introducing Rayleigh's dissipation function in the framework of a variational principle that results in a set of coupled ordinary differential equations describing the evolution of individual soliton parameters. We further derive closed-form expressions for quick estimation of the temporal trajectory, drift velocity, and the phase shift accumulated by the CS due to the externally modulated pump. We also extend the variational approach to solve a two-soliton interaction problem in the absence as well as in the presence of the externally modulated field. In the absence of a phase-modulated field, the two copropagating solitons can attract, repulse, or propagate independently depending on their initial delay. The final state of interaction can be predicted through a second-order differential equation which is derived by the variational method. While in the presence of the phase-modulated field, the two-soliton interaction can result in annihilation, merging, breathing, or a two-soliton state depending on the detuning frequency and the pump power. Variational treatment analytically predicts these states and portrays the related dynamics that agrees with the full numerical simulation carried out by solving the normalized Lugiato-Lefever equation. The results obtained through this variational approach will enrich the understanding of complex pulse dynamics under a phase-modulated driving field in passive dissipative systems.

Dispersion, birefringence, and amplification characteristics of newly designed dispersion compensating hole-assisted fibers.

We propose a new design of hole-assisted fiber (HAF) that can compensate for the accumulated dispersion in single-mode fiber link along with dispersion slope, thus providing broadband dispersion compensation over C-band as well as can amplify the signal channels by utilizing the stimulated Raman scattering phenomena. The proposed dispersion-compensating HAF (DCHAF) exhibits the lowest dispersion coefficient of -550 ps/nm/km at 1550 nm with an effective mode area of 15.6 microm(2). A 2.52 km long module of DCHAF amplifies incoming signals by rendering a gain of 4.2 dB with +/-0.8 dB gain flatness over whole C-band. To obtain accurate modal properties of DCHAF, a full-vector finite element method (FEM) solver is employed. The macro-bend loss characteristics of the proposed DCHAF are evaluated using FEM solver in cylindrical coordinate systems of a curved DCHAF, and low bending losses (<10(-2) dB/m for 1 cm bending radius) are obtained for improved DCHAF design while keeping intact its dispersion compensation and Raman amplification properties. We have further investigated the birefringence characteristics that can give significant information on the polarization mode dispersion of DCHAF by assuming a certain deformation (eccentricity e = 7%) either in air-holes or in the doped core or in both at a same time. It is noticed that the distortion in air-holes induces a birefringence of 10(-5), which is larger by a factor of 10 than the birefringence caused due to the core ellipticity. A PMD of 11.3 ps/ radicalkm is obtained at 1550 nm for distorted air-holes DCHAF structure.

Optimization of pump spectra for gain-flattened photonic crystal fiber Raman amplifiers operating in C-band.

This paper focuses on the optimization of pump spectra to achieve low Raman gain ripples over C-band in ultra-low loss photonic crystal fiber (PCF) and dispersion compensating PCFs (DCPCFs). Genetic algorithm (GA), a multivariate stochastic optimization algorithm, is applied to optimize the pump powers and the wavelengths for the aforesaid fiber designs. In addition, the GA integrated with full-vectorial finite element method with curvilinear edge/nodal elements is used to optimize the structural parameters of DCPCF. The optimized DCPCF provides broadband dispersion compensation over C-band with low negative dispersion coefficient of -530 ps/nm/km at 1550 nm, which is five times larger than the conventional dispersion compensating fibers with nearly equal effective mode area (21.7 mum(2)). A peak gain of 8.4 dB with +/-0.21 dB gain ripple is achieved for a 2.73 km long DCPCF module when three optimized pumps are used in the backward direction. The lowest gain ripple of +/-0.36 dB is attained for a 10 km long ultra-low loss PCF with three backward pumps. Sensitivity analysis has been performed and it is found that within the experimental fabrication tolerances of +/-2%, the absolute magnitude of dispersion may vary by +/-16%, while the Raman gain may change by +/-7%. Through tolerance study, it is examined that the ring core's hole-size is more sensitive to the structural deformations.

Design and analysis of a broadband dispersion compensating photonic crystal fiber Raman amplifier operating in S-band.

This paper presents an optimized design of a dispersion compensating photonic crystal fiber (PCF) to achieve gain-flattened Raman performances over S-band using a single pump. Genetic algorithm interfaced with an efficient full-vectorial finite element modal solver based on curvilinear edge/nodal elements is used as an optimization tool for an accurate determination of PCF design parameters. The designed PCF shows high negative dispersion coefficient (-264 ps/nm/km to -1410 ps/nm/km) and negative dispersion slope, providing coarse dispersion compensation over the entire S-band. The module comprised of 1.45-km long optimized PCF exhibits +/-0.46 dB gain ripples over 50 nm wide bandwidth and shows a very low double Rayleigh backscattering value (-59.8 dB). The proposed module can compensate for the dispersion accumulated in one span (80-km) of standard single mode fiber with a residual dispersion of +/-700 ps/nm, ensuring its applicability for 10 Gb/s WDM networks. Additionally, the designed PCF remains single mode over the range of operating wavelengths.

The impact of elliptical deformations for optimizing the performance of dual-core fluorine-doped photonic crystal fiber couplers.

In this paper we study the impact of elliptically-deformed features such as cladding air-holes and elliptically-modulated cores, as ingredients for optimizing the coupling characteristics of dual-core fluorine-doped photonic crystal fiber (PCF) couplers. We provide a detailed numerical investigation by using a trial and error approach for optimizing the propagation characteristics of fluorine-doped PCF couplers. Typical characteristics of the newly proposed PCF coupler structure are: wavelength-flattened coupling characteristics between 0.7 mum and 1.6 mum wavelength range, coupling efficiency of 50+/-1 % from 0.9 mum to 1.6 mum, and a reasonably small coupling length of 1.3 cm. In addition we have elaborately derived the design parameters so that our proposed dual-core PCF coupler exhibits polarization-insensitive characteristics verified by using a full-vectorial beam propagation method. The proposed dual-core PCF can be effectively used as a 3-dB coupler, over a wide wavelength range.

Raman amplification characteristics of As2Se3 photonic crystal fibers.

We present the dispersion and Raman amplification characteristics of As2Se3 photonic crystal fibers (PCFs). We compare the gain characteristics with conventional As2Se3 fibers and find that the Raman gain efficiency in PCFs can be improved by a factor of more than 4. This allows us to either use a small length of the fiber or to use the low pump power to attain similar gain characteristics. Numerical simulations reveal that a peak gain of 10 dB can be achieved in a 1.1 m long PCF when it is pumped at 1.5 microm in wavelength with an input power of 500 mW.