Harish N. S. Krishnamoorthy

Topological transitions in metamaterials

Manipulating Optical Topology Phase transitions in solid-state systems are often associated with a drastic change in the properties of that system. For example, metal-to-insulator transition or magnetic-to-nonmagnetic states find wide application in memory storage technology. An exotic electronic phase transition is the Lifshitz transition, whereby the Fermi surface undergoes a change in topology and a drastic change in the electronic density of states. Krishnamoorthy et al. (p. 205) now show that the notion of such a phase transition can be carried over to the optical regime by the suitable design of a metamaterial structure. This effect could be used to control the interaction between light and matter. Metamaterials can undergo an optical analog of electronic phase transitions that impacts local light-matter interactions. Light-matter interactions can be controlled by manipulating the photonic environment. We uncovered an optical topological transition in strongly anisotropic metamaterials that results in a dramatic increase in the photon density of states—an effect that can be used to engineer this interaction. We describe a transition in the topology of the iso-frequency surface from a closed ellipsoid to an open hyperboloid by use of artificially nanostructured metamaterials. We show that this topological transition manifests itself in increased rates of spontaneous emission of emitters positioned near the metamaterial. Altering the topology of the iso-frequency surface by using metamaterials provides a fundamentally new route to manipulating light-matter interactions.

Active hyperbolic metamaterials: enhanced spontaneous emission and light extraction

Hyperbolic metamaterials (HMMs) have recently garnered much attention because they possess the potential for broadband manipulation of the photonic density of states and subwavelength light confinement. These exceptional properties arise due to the excitation of electromagnetic states with high momentum (high-k modes). However, a major hindrance to practical applications of HMMs is the difficulty in coupling light out of these modes because they become evanescent at the surface of the metamaterial. Here we report the first demonstration, to our knowledge, of simultaneous spontaneous emission enhancement and outcoupling of high-k modes in an active HMM using a high-index-contrast bullseye grating. Quantum dots embedded inside the metamaterial are used for local excitation of high-k modes. This demonstration could pave the way for the development of photonic devices such as single-photon sources, ultrafast LEDs, and true nanoscale lasers.

Tunable hyperbolic metamaterials utilizing phase change heterostructures

We present a metal-free tunable anisotropic metamaterial where the iso-frequency surface is tuned from elliptical to hyperbolic dispersion by exploiting the metal-insulator phase transition in the correlated material vanadium dioxide (VO2). Using VO2-TiO2 heterostructures, we demonstrate the transition in the effective dielectric constant parallel to the layers to undergo a sign change from positive to negative as the VO2 undergoes the phase transition. The possibility to tune the iso-frequency surface in real time using external perturbations such as temperature, voltage, or optical pulses creates new avenues for controlling light-matter interaction.

Organometallic perovskite metasurfaces

Organometallic perovskites, solution-processable materials with outstanding optoelectronic properties and high index of refraction, provide a platform for all-dielectric metamaterials operating at visible frequencies. Perovskite metasurfaces with structural coloring tunable across visible frequencies are realized through subwavelength structuring. Moreover, a threefold increase of the luminescence yield and comparable reduction of luminescence decay time are observed.

Metamaterial based broadband engineering of quantum dot spontaneous emission

We report the broadband (~ 25 nm) enhancement of radiative decay rate of colloidal quantum dots by exploiting the hyperbolic dispersion of a one-dimensional nonmagnetic metamaterial structure.

Nonreciprocity and one-way topological transitions in hyperbolic metamaterials

Control of the electromagnetic waves in nano-scale structured materials is crucial to the development of next generation photonic circuits and devices. In this context, hyperbolic metamaterials, where elliptical isofrequency surfaces are morphed into surfaces with exotic hyperbolic topologies when the structure parameters are tuned, have shown unprecedented control over light propagation and interaction. Here we show that such topological transitions can be even more unusual when the hyperbolic metamaterial is endowed with nonreciprocity. Judicious design of metamaterials with reduced spatial symmetries, together with the breaking of time-reversal symmetry through magnetization, is shown to result in nonreciprocal dispersion and one-way topological phase transitions in hyperbolic metamaterials.

Bidirectional reconfiguration and thermal tuning of microcantilever metamaterial device operating from 77 K to 400 K

We experimentally report the bidirectional reconfiguration of an out-of-plane deformable microcantilever based metamaterial for advanced and dynamic manipulation of terahertz waves. The microcantilever is made of a bimaterial stack with a large difference in the coefficient of thermal expansion of the constituent materials. This allows for the continuous deformation of microcantilevers in upward or downward direction in response to positive or negative temperature gradient, respectively. The fundamental resonance frequency of the fabricated microcantilever metamaterial is measured at 0.4 THz at room temperature of 293 K. With decreasing temperature, the resonance frequency continuously blue shifts by 30 GHz at 77 K. On the other hand, with increasing temperature, the resonance frequency gradually red shifts by 80 GHz and saturates at 0.32 THz for 400 K. Furthermore, as the temperature is increased above room temperature, which results in the downward actuation of the microcantilever, a significant resonan...

A Superconducting Dual‐Channel Photonic Switch

The mechanism of Cooper pair formation and its underlying physics has long occupied the investigation into high temperature (high-Tc ) cuprate superconductors. One of the ways to unravel this is to observe the ultrafast response present in the charge carrier dynamics of a photoexcited specimen. This results in an interesting approach to exploit the dissipation-less dynamic features of superconductors to be utilized for designing high-performance active subwavelength photonic devices with extremely low-loss operation. Here, dual-channel, ultrafast, all-optical switching and modulation between the resistive and the superconducting quantum mechanical phase is experimentally demonstrated. The ultrafast phase switching is demonstrated via modulation of sharp Fano resonance of a high-Tc yttrium barium copper oxide (YBCO) superconducting metamaterial device. Upon photoexcitation by femtosecond light pulses, the ultrasensitive cuprate superconductor undergoes dual dissociation-relaxation dynamics, with restoration of superconductivity within a cycle, and thereby establishes the existence of dual switching windows within a timescale of 80 ps. Pathways are explored to engineer the secondary dissociation channel which provides unprecedented control over the switching speed. Most importantly, the results envision new ways to accomplish low-loss, ultrafast, and ultrasensitive dual-channel switching applications that are inaccessible through conventional metallic and dielectric based metamaterials.

Dataset for Ultra-confined surface phonon polaritons in molecular layers of van der Waals dielectrics

Dataset supporting: Dubrovkin, Alexander et al. (2018) Ultra-confined surface phonon polaritons in molecular layers of van der Waals dielectrics. Nature Communications

Dataset for Superconducting dual-channel photonic switch

Data for all the plots in the publication Srivastava, Y. K. et al (2018). Superconducting dual-channel photonic switch. Advanced Materials, 1-8. [1801257].