Dispersion Contours Research Articles

Controlling behaviour of transparency and absorption in three-coupled multiple quantum wells via spontaneously generated coherence

This article presents a coherent phenomenon called spontaneously generated coherence (SGC) under the regime of electromagnetically induced transparency (EIT) in a three-coupled multiple quantum wells. We demonstrate that the presence of SGC in these quantum wells lead to intriguing modifications in the transparency window within the absorption spectrum. At the same time, modification of the dispersive nature is also demonstrated which enables the feasibility of the system in diverse applications based on light propagation. The absorption and dispersion responses are found to be varied by the individual strength of the first and second control fields in presence as well as in absence of SGC in the EIT regime. The positional shifting of the transparency window and simultaneous modifications in the dispersive profiles by tuning the control field detunings of both the first and second control fields are also revealed. Some absorption and dispersion contours are illustrated for getting better insights into the modifications of the optical responses via SGC. Finally, by manipulating the strength of the SGC parameter, we observe the changes in the respective position of the transparency window and dispersion curve. It is expected that the current investigations will pave novel ways for innovative applications in quantum communications, and fabrication of advanced photonic devices.

Ultrafast anisotropic dynamics of hyperbolic nanolight pulse propagation.

Polariton pulses-transient light-matter hybrid excitations-traveling through anisotropic media can lead to unusual optical phenomena in space and time. However, studying these pulses presents challenges with their anisotropic, ultrafast, and nanoscale field variations. Here, we demonstrate the creation, observation, and control of polariton pulses, with in-plane hyperbolic dispersion, on anisotropic crystal surfaces by using a time-resolved nanoimaging technique and our developed high-dimensional data processing. We capture and analyze movies of distinctive pulse spatiotemporal dynamics, including curved ultraslow energy flow trajectories, anisotropic dissipation, and dynamical misalignment between phase and group velocities. Our approach enables analysis of polariton pulses in the wave vector time domain, demonstrating a time-domain polaritonic topological transition from lenticular to hyperbolic dispersion contours and the ability to study the polariton-induced time-varying optical forces. Our findings promise to facilitate the study of diverse space-time phenomena at extreme scales and drive advances in ultrafast nanoimaging.

Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons.

Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)-which exist in much broader spectral ranges than phonon polaritons-has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag2Te platelets. The hybridization of the PPs with their mirror image-by placing the platelets above a Au layer-increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

Magnetical Manipulation of Hyperbolic Phonon Polaritons in Twisted Double-Layers of Molybdenum Trioxide.

Controlling the twist angle between double stacked van der Waals (vdW) crystals holds great promise for nanoscale light compression and manipulation in the mid-infrared (MIR) range. A lithography-free geometry has been proposed to mediate the coupling of phonon polaritons (PhPs) in double-layers of vdW α-MoO3. The anisotropic hyperbolic phonon polaritons (AHPhPs) are further hybridized by the anisotropic substrate environment of magneto-optic indium arsenide (InAs). The AHPhPs can be tuned by twisting the angle between the optical axes of the two separated layers and realize a topological transition from open to closed dispersion contours. Moreover, in the presence of external magnetic field, an alteration of the hybridization of PhPs will be met, which enable an efficient way for the control of light-matter interaction at nanoscale in the MIR region.

Doping-driven topological polaritons in graphene/α-MoO3 heterostructures

Control over charge carrier density provides an efficient way to trigger phase transitions and modulate the optoelectronic properties of materials. This approach can also be used to induce topological transitions in the optical response of photonic systems. Here we report a topological transition in the isofrequency dispersion contours of hybrid polaritons supported by a two-dimensional heterostructure consisting of graphene and α-phase molybdenum trioxide. By chemically changing the doping level of graphene, we observed that the topology of polariton isofrequency surfaces transforms from open to closed shapes as a result of doping-dependent polariton hybridization. Moreover, when the substrate was changed, the dispersion contour became dominated by flat profiles at the topological transition, thus supporting tunable diffractionless polariton propagation and providing local control over the optical contour topology. We achieved subwavelength focusing of polaritons down to 4.8% of the free-space light wavelength by using a 1.5-μm-wide silica substrate as an in-plane lens. Our findings could lead to on-chip applications in nanoimaging, optical sensing and manipulation of energy transfer at the nanoscale.

Evidence of asymmetric beaming in a piecewise-linear propagation channel.

Asymmetric beaming in a piecewise-linear propagation channel is demonstrated for a single photonic-crystal prism at Gaussian-beam illumination. The used hybrid refraction-diffraction mechanism exploits oblique incidence, the first-negative-order deflection at the longer interface, and asymmetry in coupling at the exit interfaces and does not need blocking of transmission by dispersion in the backward illumination case. The Floquet-Bloch mode with left-handed behavior and nearly circular equifrequency dispersion contours is utilized. The outgoing waves may have significantly different spatial distributions for the forward and backward illumination cases, yielding asymmetry in the beaming regime.

Ultrafast Topological Engineering in Metamaterials.

Transient optical heating provides an efficient way to trigger phase transitions in naturally occurring media through ultrashort laser pulse irradiation. A similar approach could be used to induce topological transitions in the photonic response of suitably engineered artificial structures known as metamaterials. Here, we predict a topological transition in the isofrequency dispersion contours of a layered graphene metamaterial under optical pumping. We show that the contour topology transforms from elliptic to hyperbolic within a subpicosecond timescale by exploiting the extraordinary photothermal properties of graphene. This new phenomenon allows us to theoretically demonstrate applications in engineering the decay rate of proximal optical emitters, ultrafast beam steering, and dynamical far-field subwavelength imaging. Our study opens a disruptive approach toward ultrafast control of light emission, beam steering, and optical image processing.

Topological polaritons and photonic magic angles in twisted α-MoO3 bilayers.

Twisted two-dimensional bilayer materials exhibit many exotic electronic phenomena. Manipulating the 'twist angle' between the two layers enables fine control of the electronic band structure, resulting in magic-angle flat-band superconductivity1,2, the formation of moiré excitons3-8 and interlayer magnetism9. However, there are limited demonstrations of such concepts for photons. Here we show how analogous principles, combined with extreme anisotropy, enable control and manipulation of the photonic dispersion of phonon polaritons in van der Waals bilayers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in bilayers of α-phase molybdenum trioxide (α-MoO3), arising when the rotation between the layers is at a photonic magic twist angle. These transitions are induced by polariton hybridization and are controlled by a topological quantity. At the transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with a resolution of less than λ0/40, where λ0 is the free-space wavelength. Our findings extend twistronics10 and moiré physics to nanophotonics and polaritonics, with potential applications in nanoimaging, nanoscale light propagation, energy transfer and quantum physics.

Determine Geothermal Fluid spreading pattern under subsurface using 2d Resistivity Imaging and geomagnetic method at Silaou Kahean

The Research has been carried out at geothermal Regional using 2d resistivity imaging and Gemagnetic method in Dusun Silou Kahean Subdistrict, Kabupaten Simalungun. The purpose of this study is to determine the distribution of earth magnetic anomalies, rock structures, and fluid distribution patterns. The measurement of geomagnetic method uses PPM (Proton Precession Magnetometer) type Elsec 770. Data retrieval have been done randomly with the number of points obtained by 40 measuring points, the data is processing by using surfer 10 to determine the contours and Mag2DC to obtain a magnetic anomaly cross section. The 2D resistivity imaging is used to determine the rock resistivity value. The results showed that the anomaly value with the geomagnetic method had the lowest value of 20.51 nT and the highest value of 67.18 nT. The susceptibility value of 0.00051 to 0,00098 cgs is indicated by sandstone and susceptibility value of 0.0013 to 0.0016 cgs is indicated by limestone. The 2D Resistivity imaging method has a resistivity value of 12.5 to 37.3 Ωm is indicated by Silts layer with a depth of 1.25-3.75 meters, and a resistivity value of 1.26 to 8.72 Ωm is indicated by c lay layer with a depth of 6.38-12.4 meters . from the results it can be concluded that the fluid dispersion contour using geomagnetic and geoelectric methods is spread across from the southwest to the northeast.

On the dynamics of vortex–droplet interactions, dispersion and breakup in a coaxial swirling flow

This paper discusses the fundamental mechanisms of vortex–droplet interactions leading to flow distortion, droplet dispersion and breakup in a complex swirling gas flow field. In particular, the way in which the location of droplet injection determines the degree of inhomogeneous dispersion and breakup modes has been elucidated in detail using high-fidelity laser diagnostics. The droplets are injected as monodispersed streams at various spatial locations such as the vortex breakdown bubble and the shear layers (inner and outer) exhibited by the swirling flow. Simultaneous time-resolved particle image velocimetry ($3500~\text{frames}~\text{s}^{-1}$) and high-speed shadowgraphy measurements are employed to delineate the two-phase interaction dynamics. These measurements have been used to evaluate the fluctuations in instantaneous circulation strength $\unicode[STIX]{x1D6E4}^{\prime }$ caused by the flow field eddies and the resultant angular dispersion in the droplet trajectories $\unicode[STIX]{x1D703}^{\prime }$. The droplet–flow interactions show two-way coupling at low momentum ratios ($MR$) and strong one-way coupling at high momentum ratios. The gas phase flow field is globally altered at low airflow rates (low $MR$) due to impact of droplets with the vortex core. The flow perturbation is found to be minimal and mainly local at high airflow rates (high $MR$). Spectral coherence analysis is carried out to understand the correlation between eddy circulation strength $\unicode[STIX]{x1D6E4}^{\prime }$ and droplet dispersion $\unicode[STIX]{x1D703}^{\prime }$. The droplet dispersion shows strong coherence with the flow in certain frequency bands. Subsequently, proper orthogonal decomposition (POD) is implemented to elucidate the governing instability mechanism and frequency signatures associated with the turbulent coherent structures. The POD results suggest dominance of the Kelvin–Helmholtz (KH) instability mode (axial and azimuthal shear). The frequency range pertaining to high coherence between dispersion and circulation shows good agreement with KH instability quantified from POD analysis. The droplets injected at the inner shear layer (ISL) and outer shear layer (OSL) show different interaction dynamics. For instance, droplet dispersion at the OSL exhibits secondary frequency (shedding mode) coupling in addition to the KH mode, whereas ISL injection couples only in a single narrow frequency band (i.e. KH mode). Further, high-speed shadow imaging ($7500~\text{frames}~\text{s}^{-1}$) is employed to visualize the breakup dynamics of the droplets. The effect of coherent structures on the droplet breakup modes is shown as a function of the Weber number ($We$) defined based on the circulation strength. The wide fluctuations caused in the instantaneous circulation strength lead to different breakup modes (bag, multimodal, shear thinning, catastrophic) even for fixed airflow rates. These fluctuations also lead to inhomogeneous spatial dispersion of the droplets in the swirling gas flow field. We are able to present the dispersion contours in terms of the Stokes number and a spatial homogeneity parameter. In essence, the dispersion inhomogeneity is found to be a strong function of the injection location, the phase relationship with the eddies and the momentum ratio ($MR$).

VOC level control by ventilation improvement of Flexography printing room using CFD modeling

Using Computational Fluid Dynamics (CFD) technique, the dispersion contours and the exposure rate of Flexographic printing workers to VOCs in a printing department is evaluated. Firstly, VOCs distribution is determined in the printing room due to the existing ventilation system. Through next steps, 4 scenarios for lowering VOCs concentration and its exposure rate to workers are analyzed. Concentration distributions of ethylene glycol (MEG) as a representative of VOCs are determined for 4 scenarios. The results show that, regarding the existing ventilation, the concentration of MEG at the breathing height is 1×10-5 mg/m3 and it is higher than the standard permissible level. Finally, the findings of this study lead to lowered VOCs concentrations to 13.87×10-9 mg/m3 via changing the ventilation system for the Flexography Printing Room.

Asymmetric transmission in prisms using structures and materials with isotropic-type dispersion.

It is demonstrated that strong asymmetry in transmission can be obtained at the Gaussian beam illumination for a single prism based on a photonic crystal (PhC) with isotropic-type dispersion, as well as for its analog made of a homogeneous material. Asymmetric transmission can be realized with the aid of refraction at a proper orientation of the interfaces and wedges of the prism, whereas neither contribution of higher diffraction orders nor anisotropic-type dispersion is required. Furthermore, incidence toward a prism wedge can be used for one of two opposite directions in order to obtain asymmetry. Thus, asymmetric transmission is a general property of the prism configurations, which can be obtained by using simple geometries and quite conventional materials. The obtained results show that strong asymmetry can be achieved in PhC prisms with (nearly) circular shape of equifrequency dispersion contours, in both cases associated with the index of refraction 0<n<1 and n>1. For the comparison purposes, results are also presented for solid uniform non-magnetic prisms made of a material with the same value of n. It is shown in zero-loss approximation that the PhC prism and the ultralow-index material prism (0<n<1) can replace each other in some cases without affecting the scenario of asymmetric transmission. Moreover, the PhC prism and the solid dielectric prism can show the same scenario at n>1. Possible contributions of scattering on the individual rods and diffraction on the wedge to the resulting mechanism are discussed. Analogs of unidirectional splitting and unidirectional deflection regimes, which are known from the studies of PhC gratings, are obtained in PhC prisms and solid uniform prisms, i.e. without higher diffraction orders.

NOX EMISSION MODELLING FROM INDUSTRIAL STEAM BOILERS

Previous researches often emphasize on the sources and effects of air pollutants in the environment and human population. A part of those studies were done in order to explore the spread or distribution pattern of those pollutants, especially regarding the emission from industrial steam boilers. It is very important to evaluate the transfer trend of air pollutants at both local and global scales. In this study, Industrial Source Complex Short Term Version 3 (ISCST3) model has been used to predict the distribution of NOx emitted from industrial steam boilers in the District of Hulu Langat, Selangor. The result of analyses indicates that the emission rates for steam boilers were ranging from 0.0083 kg NOx/hour to as high as 0.2771 kg NOx/hour while the total emission load was 1.9969 kg NOx­/hour. The evaluation on dispersion contour shows that the concentration of NOx was higher in 1-hour reading than 24-hour value. The highest concentration of NOx was predicted to be within 104.65 km2 from the sources of NOx (steam boilers).

All-Angle Collimation for Spin Waves

We studied the effect of collimation for monochromatic beams of spin waves, resulting from the refraction at the interface separating two magnetic half-planes. The collimation was observed in broad range of the angles of ncidence for homogenous Co and Py half-planes, due to significant intrinsic anisotropy of spin wave propagation in these materials. The effect exists for the sample saturated by in plane magnetic field tangential to the interface. The collimation for all possible angles of incidence was found in the system where the incident spin wave is refracted on the interface between homogeneous and periodically patterned layers of YIG. The refraction was investigated by the analysis of isofrequency dispersion contours of both pairs materials, i.e., uniform YIG/patterned YIG and Co/Py, which are calculated with the aid of the plane wave method. Besides, the refraction in Co/Py system was studied using micromagnetic simulations.

Defect-mode-like transmission and localization of light in photonic crystals without defects

It is shown that far- and near-field effects, which are similar to those arising in transmission through finite-thickness photonic crystals (PCs) with structural defects, can be obtained in the conventional defect-free dielectric square-lattice PCs. Multiple narrow peaks of total transmission can appear within a frequency range, where the noncircular isofrequency dispersion contours of one type are quickly transformed for a higher-order Floquet-Bloch wave to the noncircular contours of the other type, leading to a frequency-domain passband being very narrow within a certain range of variation of the angles of incidence. In this regime, the mirror reflectance of the equivalent Fabry-P\'erot resonator takes rather large values, which correspond to large values of $Q$ factor and group index of refraction, strong field localization, and good isolation of the transmission peaks from each other. In some examples presented, $Q$ factor exceeds ${10}^{4}$.

Unidirectional transmission in non-symmetric gratings containing metallic layers

The mechanism of achieving unidirectional transmission in the gratings, which only contain isotropic dielectric and metallic layers, is suggested and numerically validated. It is shown that significant transmission in one direction and nearly zero transmission in the opposite direction can be obtained in the same intrinsically isotropic gratings as those studied recently in A. E. Serebryannikov and E. Ozbay, Opt. Express 17, 278 (2009), but at a non-zero angle of incidence. The tilting, non-symmetric features of the grating and the presence of a metallic layer with a small positive real part of the index of refraction are the conditions that are necessary for obtaining the unidirectionality. Single- and multibeam operational regimes are demonstrated. The frequency and angle ranges of the unidirectional transmission can be estimated by using the conventional framework based on isofrequency dispersion contours and construction lines that properly take into account the periodic features of the interfaces, but should then be corrected because of the tunneling arising within the adjacent ranges. After proper optimization, this mechanism is expected to become an alternative to that based on the use of anisotropic materials.

Superprism properties in 2D polymer photonic crystals at high normal frequencies

The superprism effects of higher bands, i.e., for normal frequencies of higher than one, in two-dimensional (2D) polymer photonic crystals (PCs) are investigated. It is shown that in a polymer PC of triangular symmetry with filling factor of about 31%, the gradual transition of the hexagonal into triangular equi-frequency dispersion contours leads to a strong superprism in the 6th band at a normal frequency of 1.2. This dispersion is more prominent than those observed in the lower bands in 2D PCs. Also, this requires a lattice constant longer than the concerned wavelength. Furthermore, in a 2D polymer PC with a filling factor of about 83% a strong discontinuous superprism effect occurs at normal frequencies higher than one, which is due to an abrupt transition between two modes with the refraction angles of opposite signs. The effect can be exploited for switching applications as demonstrated in the paper.

Collisional interference of lines in the spectra of symmetric top molecules: I. A theory of relaxation parameters of the shape of the spectrum in the impact approximation

The peculiarities of broadening of spectral lines of symmetric top molecules that are related to the double degeneracy of JK levels at K ≠ 0 are considered. It is shown that, irrespective of the potential barrier height for the inversion vibration, transitions between such levels should be considered as doublets whose components are coupled with each other by the collision-induced spectral exchange. The shape of the doublet is described by the well-known Ben-Reuven formula, and, in this case of molecules with a high potential barrier for the inversion vibration, is a dispersion contour with a half-width equal to the difference between the halfwidth of the component of the doublet and the cross-relaxation parameter.

The superprism effect in lithium niobate photonic crystals for ultra-fast, ultra-compact electro-optical switching

We numerically analyze ultra-refraction and slow-light in lithium niobate photonic crystals in order to investigate and then optimize the efficiency of a tunable photonic crystal superprism. In contrast to a passive superprism 1-to-N demultiplexer, we describe a tunable bandpass filter with only three output ports. The electro-optic effect in lithium niobate is used to achieve tunability, with the filter bandwidth shifting in wavelength as the refractive index of the superprism is modified by an externally applied electric field. Such a device could be used to realize a compact and fast wavelength multiplexer/demultiplexer for telecommunications or optical interconnect applications. We calculate constant frequency dispersion contours (plane-wave expansion) to identify initial configurations that show significant ultra-refraction, and verify the expected behavior of light propagation inside the structure using 2D FDTD (finite difference time domain) simulations. We show that the voltage requirements of such an electro-optically tunable superprism could potentially be relaxed by exploiting the enhancement of the electro-optic effect recently discovered by our group [M. Roussey, M.-P. Bernal, N. Courjal, D. Van Labeke, F.I. Baida, Electro-optic effect exaltation on lithium niobate photonic crystals due to slow photons. Appl. Phys. Lett. 89 (24) (2006) 241110], which we believe to be due to the presence of slow-light in the nanostructure. We present a methodology that readily identifies superprism design points showing both strong ultra-refraction as well as low group velocity. However, we find that this improved voltage efficiency comes at the cost of reduced operating bandwidth and increased insertion losses due to proximity to the band-edge.

Lasing Dynamics of A Novel Silicon Photonic Crystal Cavity

In this paper we proposed novel silicon laser design based on the dispersion engi- neered photonic crystals. With the unique self-collimation property, we design an optical cavity with a Q factor of 2000. The strong mode conflnement in the low index active material ofiers an opportunity to realize a lasing mechanism. To investigate the lasing dynamics we introduce the rate equations of atomic system into the electromagnetic polarization. With these auxil- iary difierential equations we solve the time evolutions of the electromagnetic waves and atomic populations by using the FDTD method. DOI: 10.2529/PIERS060907132027 Silicon, the leading material in microelectronics during the last four decades, also promises to be the key material in the future. Recent activities have focused on the achieving active functionalities, mostly light ampliflcation and generation. However, due to the fundamental limitation related to the indirect bandgap of the bulk Si, the development of silicon gain and laser becomes one of most challenging goal in the silicon photonics and optoelectronics. Many difierent approaches have been taken to achieve this goal. Recently stimulated Raman scattering efiect has been used to demonstrate the light ampliflcation and lasing in the silicon both in pulse and continuous-wave operation. (1,_ 2)In the presented work, we attempt to design a novel silicon laser. To this end, we will design a novel optical cavity based on the dispersion engineered photonic crystals (PhCs). To observe the lasing dynamics, we will incorporate the rate equations of a four level atomic system into the device to simulate the gain and absorption of the active material. By solving the Maxwell's equations with these auxiliary difierential equations with the Finite-difierence Time-domain (FDTD) method, we will track the time evolutions of the electromagnetic waves and atomic populations. 2. DISPERSION BASED PHOTONIC CRYSTAL CAVITY The light propagation in a PhC is most appropriately interpreted through a dispersion diagram, which characterizes the relationship between the frequency of the wave, !, and its associated wavevector, k. Dispersion surfaces provide the spatial variation of the spectral properties of a certain band within the photonic crystal structure. Electromagnetic wave vectors propagate at directions normal to the dispersion surface as shown in Fig. 1, which stems from the relation vg = rk!(k). The equil frequency contour (EFC) can lead to beam divergence or convergence as shown in the flgure. The ability to shape the EFCs, and thereby engineer the dispersion properties of the PhC, opens up a new paradigm for the design of optical devices. (3{5) For self-collimation, we desire a ∞at EFC, in which case the wave is only allowed to propagate along those directions normal to the sides of the straight curvatures. As such, it is possible to vary the incident wavevector over a wide range of angles and yet maintain a narrow range of propagating angles within the PhC. Consider silicon photonic crystals perforated by a square lattice with holes back-fllled by the gain medium, i.e., Er-doped glasses, as shown in the inset of Fig. 2(a). The hole has radius of 0.3a, where a is the lattice constant. The silicon and glass have refractive indices of 3.5 and 1.5, respectively. The dispersion surface of flrst band diagram is plotted in Fig. 1(a). By carefully selecting the frequency, one can obtain a ∞at curvature within certain angular range at specifled frequency, i.e., 0.18c/a, as depicted in Fig. 1(b), where the blue curvature is the dispersion contour in the free space with frequency of 0.18a. Such ∞atness of the curvature ofiers self-collimation along iM direction within a wide incident angular range. Self-collimation efiect has been widely used in the applications of optical routing, sensors, vari- able beam splitters, self collimated optical emitters to enhance the emission from a light emitting diode. In the presented paper we propose a novel design of silicon laser based on the self-collimation phenomena. Fig. 2 depicts the schematic chart of our design. A photonic crystal cavity consists of