Harshawardhan Wanare

Coherently controlling metamaterials.

Two independent significant developments have challenged our understanding of light-matter interaction, one, involves the artificially structured materials known as metamaterials, and the other, relates to the coherent control of quantum systems via the quantum interference route. We theoretically demonstrate that one can engineer the electromagnetic response of composite metamaterials using coherent quantum interference effects. In particular, we predict that these composite materials can show a variety of effects ranging from dramatic reduction of losses to switchable ultraslow-to-superluminal pulse propagation. We propose parametric control of the metamaterials by active tuning of the capacitance of the structures, which is most efficiently engineered by embedding the metamaterial structures within a coherent atomic/molecular medium. This leads to dramatic frequency dependent features, such as significantly reduced dissipation accompanied by enhanced filling fraction. For a Split-ring resonator medium with magnetic properties, the associated splitting of the negative permeability band can be exploited for narrow band switching applications at near infrared frequencies involving just a single layer of such composite metamaterials.

Negative and positive hysteresis in double-cavity optical bistability in a three-level atom

We present dual hysteretic behavior of a three-level ladder system exhibiting optical bistability in a double-cavity configuration in the mean-field limit. The two fields coupling the atomic system experience competing cooperative effects along the two transitions. We observe a hump-like feature in the bistable curve arising due to cavity-induced inversion, which transforms into a negative-hysteresis loop. Apart from negative- and positive-hysteresis regions, the system offers a variety of controllable nonlinear dynamical features, ranging from switching, periodic self-pulsing to chaos.

Switching a plasmalike metamaterial via embedded resonant atoms exhibiting electromagnetically induced transparency

We theoretically demonstrate control of the plasmalike effective response of a metamaterial composed of aligned metallic nanorods when the electric field of the incident radiation is parallel to the nanorods. By embedding this metamaterial in a coherent atomic/molecular medium, for example, silver nanorod arrays submerged in sodium vapor, we can make the metamaterial transmittive in the forbidden frequency region below its plasma frequency. This phenomenon is enabled by having Lorentz absorbers or other coherent processes in the background medium, which provide a large positive dielectric permittivity in the vicinity of the resonance, thereby rendering the effective permittivity positive. In particular, processes such as electromagnetically induced transparency are shown to provide additional control to switch and tune the new transmission bands.

Commentary: Controlling electromagnetic metamaterials

The last two decades have been witness to two exciting and independent developments that have forever changed our conventional view of how light interacts with matter. One relates to coherent control via quantum interference, wherein the possibility of making an otherwise opaque medium transparent [1], now known as electromagnetically induced transparency (EIT), set off intense research activity. EIT essentially requires careful creation of atomic coherence, that results in diverse effects varying from almost freezing light in its tracks (slow light) to freezing atoms to nanoKelvin temperatures via velocity-selective coherent population trapping. The second development relates to metamaterials whose origins are very classical in nature. In electromagnetics, these designer materials were originally proposed for realizing a super-lens wherein the evanescent field becomes the work horse that accords sub-wavelength resolution in imaging [2]. Since then a variety of metamaterials have been proposed, where even the propagating fields can be dramatically controlled, as in electromagnetic cloaks wherein the fields are maneuvered around an obstacle so as to make it invisible. The biggest technological contraints in realizing large-scale device applications of metamaterials have been two. The first is the large dissipation associated with an inherently resonant phenomenon. The second arises due to the very design of metamaterial; once the metamaterial structures (inclusions) are fabricated, they offer little maneuverability in terms of the operating frequency. However, both EIT and metamaterials have truly lifted the tedium associated with the usual classical linear phenomena involving light. It is commonly believed this is just the beginning of a long journey, where our inherent drive to control gainfully these and many other wondrous effects will be the prime mover of future developments. The marriage of these two diverse developments is highlighted here, with a word of caution: one can only naively guess the surprises this relationship will bring forth. In order to achieve a composite material combining the attributes of EIT and metamaterials, one simple design involves immersing the metamaterial in a dilute atomic gas whose frequency-selective absorption can be exploited to manipulate the metamaterial response [3]. Furthermore, a combination of light fields accords extra control over the metamaterial via the absorption and dispersion of the atomic gas through the atomic coherence (quantum) route, based on effects like EIT. The price of working at near-resonant conditions is the large dispersion with frequency accompanied by large loss. EIT-based control exploits the large frequency-dispersion and yet provides substantially decreased loss which is even lower than the metallic losses in a narrow-bandwidth regime. The large variation of the refractive index of the EIT medium results in the freezing of currents in the metallic inclusions of the metamaterial, thereby lowering loss. The most critical issues that govern this alliance arise from the very nature of the two partners. EIT is a quantum phenomenon, whereas metamaterials are described classically. Quantum phenomena are extremely susceptible to the surroundings, and using a solid or liquid medium severely restricts the performance of the quantum partner. For example, rareearth ions implanted in crystals have been shown to exhibit EIT only at extremely low temperatures where the phonon noise is sufficiently suppressed such that the ground-state

Strong coupling of surface plasmon resonances to molecules on a gold grating

The intense electromagnetic fields generated by a surface plasmon resonance can strongly couple to molecules in the vicinity of the surface, causing significant line shifts. By measuring the angle dependent transmission spectrum through gold gratings with Rhodamine-6G molecules deposited on them, the coupling of the surface plasmon resonance to the molecular levels at different frequencies was determined. The strong coupling within the absorption and fluorescence bands of the molecules leads to anti-crossing of the states of the coupled system evidenced by the transmission minima. In particular, simultaneous existence of two distinct resonances at different wavevectors for a given wavelength over the absorption and fluorescence bands is observed. The multiplicity of the molecular levels and the coupling fields involved in the process is captured in a three-level Λ-system model coherently driven by the enhanced surface plasmon fields. The enhanced surface plasmon fields and the resonant absorption/fluorescent fields form the two arms of the Λ-system with approriate detuning.

Nonlinear dynamics of double-cavity optical bistability of a three-level ladder system

We present nonlinear dynamical features of two-photon double-cavity optical bistability exhibited by a three-level ladder system in the mean-field limit at low input light levels. The system exhibits a hump-like feature in the lower branch of the bistable response, wherein a region of instability develops. The system displays a range of dynamical features varying from normal stable switching to periodic self-pulsing and chaos. The inclusion of two competing cooperative atom-field couplings leads to such rich, nonlinear dynamical behavior. We provide a domain map that clearly delineates the various regions of stability, as well as bifurcation diagrams with associated supporting evidence that identifies the period-doubling route to chaos.

Magic frequency enabled by quantum interference for a dual atomic device

We present a magic frequency associated with two-photon processes in room temperature atomic vapour at the coherent population trapping (CPT) condition for an elliptically polarized light. It is realized by combining the symmetries of the lineshape functions arising from the atomic coherence. In practice, the magic frequency is synthesized from a polarimetric measurement involving the reflected and transmitted CPT signal at the polarizing beam splitter, which provides an atomic frequency standard for near zero magnetic fields. We further develop a magnetometer, which concurrently measures the magnetic field and its direction from the enhanced reflected signal with significantly improved performance. Additionally, the magic frequency and its dependence on the polarization content provide a handle to precisely locate the single photon resonance.

Harnessing quantum superposition and interference in atomic systems

We propose resilient quantum superposition states in closed-loop multilevel system which result in myriad quantum interference phenomena. An interplay of these superposition states results in a whole gamut of atomic phenomena including coherent population trapping (CPT), electromagnetically induced transparency (EIT), electromagnetically induced absorption (EIA), amplification without inversion (AWI) and enhancement of refractive index accompanied with negligible absorption. The polarization and the phases of the fields transform the underlying superposition of the excited states leading to all these effects, where, given the macroscopic nature of these phenomena the quantum superposition states as well as the synergy between them can be ascertained. Numerical simulations for D1 transition in room temperature Rb87 atomic vapour system bear out these findings.

Bio-organism detection in one-dimensional photonic crystals using electromagnetically induced transparency

We propose an optical sensor that allows site-selective detection of a refractive index change occurring due to any infiltration such as a bio-organism in a porous one-dimensional photonic crystal (PC). We use the electromagnetically induced transparency (EIT) to detect and locate the infiltration. With a localized change in the refractive index, the maximum of the peak EIT transmission shifts, which is determined by tuning the control field frequency. The strong dispersion and the narrowing of the absorption free response associated with EIT within the PC form the basis of such enhanced sensitivity.

Gain without population inversion in V-type systems driven by a frequency-modulated field

We obtain the gain of the probe field at multiple frequencies in a closed three-level V-type system using a frequency-modulated pump field. There is no associated population inversion among the atomic states of the probe transition. We describe both the steady-state and transient dynamics of this system. Under suitable conditions, the system exhibits a large gain simultaneously at a series of frequencies far removed from resonance. Moreover, the system can be tailored to exhibit multiple frequency regimes where the probe experiences anomalous dispersion accompanied by negligible gain or absorption over a large bandwidth, a desirable feature for obtaining superluminal propagation of pulses with negligible distortion.