Tal Galfsky

Strong light–matter coupling in two-dimensional atomic crystals

Microcavity polaritons—the bosonic quasiparticles that result from strong light–matter coupling—are observed for the first time in a dielectric cavity containing a monolayer of molybdenum disulphide at room temperature.

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.

Broadband Enhancement of Spontaneous Emission in Two-Dimensional Semiconductors Using Photonic Hypercrystals

The low quantum yield observed in two-dimensional semiconductors of transition metal dichalcogenides (TMDs) has motivated the quest for approaches that can enhance the light emission from these systems. Here, we demonstrate broadband enhancement of spontaneous emission and increase in Raman signature from archetype two-dimensional semiconductors: molybdenum disulfide (MoS2) and tungsten disulfide (WS2) by placing the monolayers in the near field of a photonic hypercrystal having hyperbolic dispersion. Hypercrystals are characterized by a large broadband photonic density of states due to hyperbolic dispersion while having enhanced light in/out coupling by a subwavelength photonic crystal lattice. This dual advantage is exploited here to enhance the light emission from the 2D TMDs and can be utilized for developing light emitters and solar cells using two-dimensional semiconductors.

Photonic hypercrystals for control of light–matter interactions

Significance Light–matter interaction lies at the heart of several fundamental phenomena and technological applications ranging from photosynthesis to lasers. Current approaches to control this interaction such as optical cavities, photonic crystals, and metamaterials either rely on frequency resonance mechanisms which limit the bandwidth or suffer from poor light-coupling issues. Here we report a class of artificial media: photonic hypercrystals to control light–matter interactions. Both bandwidth and outcoupling limitations are overcome using hypercrystals. This characteristic is demonstrated through simultaneous enhancement of spontaneous emission rate (20×) and outcoupling (100×) from quantum dots embedded in the hypercrystal. This platform for broadband control of light–matter interaction will push the boundaries of applications such as ultrafast light-emitting diodes, photovoltaics, and quantum informatics. Photonic crystals (PCs) have emerged as one of the most widely used platforms for controlling light–matter interaction in solid-state systems. They rely on Bragg scattering from wavelength-sized periodic modulation in the dielectric environment for manipulating the electromagnetic field. A complementary approach to manipulate light–matter interaction is offered by artificial media known as metamaterials that rely on the average response of deep-subwavelength unit cells. Here we demonstrate a class of artificial photonic media termed “photonic hypercrystals” (PHCs) that combine the large broadband photonic density of states provided by hyperbolic metamaterials with the light-scattering efficiency of PCs. Enhanced radiative rate (20×) and light outcoupling (100×) from PHCs embedded with quantum dots is observed. Such designer photonic media with complete control over the optical properties provide a platform for broadband control of light–matter interaction.

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.

Preferential emission into epsilon-near-zero metamaterial [Invited]

We report the use of epsilon near zero (ENZ) metamaterial to control spontaneous emission from Zinc-Oxide (ZnO) excitons. The ENZ material consists of alternating layers of silver and alumina with subwavelength thicknesses, resulting in an effective medium where one of the components of the dielectric constant approach zero between 370nm-440nm wavelength range. Bulk ZnO with photoluminescence maximum in the ENZ regime was deposited via atomic layer deposition to obtain a smooth film with near field coupling to the ENZ metamaterial. Preferential emission from the ZnO layer into the metamaterial with suppression of forward emission by 90% in comparison to ZnO on silicon is observed. We attribute this observation to the presence of dispersionless plasmonic modes in the ENZ regime as shown by the results of theoretical modeling presented here. Integration of ENZ metamaterials with light emitters is an attractive platform for realizing a low threshold subwavelength laser.

Enhanced spontaneous emission in photonic hypercrystals

We demonstrate a new class of nanostructure termed photonic hypercrystal which combines properties of photonic crystals and metamaterials. Enhanced radiative decay by factor of 20 and light extraction by 100 is observed from embedded quantum dots.

Control of Light-matter Interaction Using Photonic Hypercrystals

We demonstrate broadband enhancement of light emission from quantum dots, single photon emitters and 2D semiconductors using photonic hypercrystals (PHC). Both out-coupling and spontaneous emission rate are increased from the different emitters.

Preferential emission into epsilon-near-zero metamaterial

We report preferential emission from ZnO nanoparticles into an epsilon near zero metamaterial. The structure is designed to have the parallel component of its dielectric constant approach zero at the maximum emission wavelength of ZnO.

Simultaneous enhancement of decay rate and light extraction from active hyperbolic metamaterial

We demonstrate simultaneous enhancement in spontaneous emission rate and light extraction from hyperbolic metamaterials embedded with quantum dots using a high contrast grating. Enhancements of twenty-fold in extraction and thirteen-fold in emission rate are observed.