Zhuoxian Wang

Broadband High-Efficiency Half-Wave Plate: A Supercell-Based Plasmonic Metasurface Approach

We design, fabricate, and experimentally demonstrate an ultrathin, broadband half-wave plate in the near-infrared range using a plasmonic metasurface. The simulated results show that the linear polarization conversion efficiency is over 97% with over 90% reflectance across an 800 nm bandwidth. Moreover, simulated and experimental results indicate that such broadband and high-efficiency performance is also sustained over a wide range of incident angles. To further obtain a background-free half-wave plate, we arrange such a plate as a periodic array of integrated supercells made of several plasmonic antennas with high linear polarization conversion efficiency, consequently achieving a reflection-phase gradient for the cross-polarized beam. In this design, the anomalous (cross-polarized) and the normal (copolarized) reflected beams become spatially separated, hence enabling highly efficient and robust, background-free polarization conversion along with broadband operation. Our results provide strategies for creating compact, integrated, and high-performance plasmonic circuits and devices.

Controlling Random Lasing with Three-Dimensional Plasmonic Nanorod Metamaterials

Plasmonics has brought revolutionary advances to laser science by enabling deeply subwavelength nanolasers through surface plasmon amplification. However, the impact of plasmonics on other promising laser systems has so far remained elusive. Here, we present a class of random lasers enabled by three-dimensional plasmonic nanorod metamaterials. While dense metallic nanostructures are usually detrimental to laser performance due to absorption losses, here the lasing threshold keeps decreasing as the volume fraction of metal is increased up to ∼0.07. This is ∼460 times higher than the optimal volume fraction reported thus far. The laser supports spatially confined lasing modes and allows for efficient modulation of spectral profiles by simply tuning the polarization of the pump light. Full-field speckle-free imaging at micron-scales has been achieved by using plasmonic random lasers as the illumination sources. Our findings show that plasmonic metamaterials hold potential to enable intriguing coherent optical sources.

MXenes for nanophotonic and metamaterial devices (Conference Presentation)

MXenes are a recently discovered family of two-dimensional nanomaterials formed of transition metal carbides and carbon nitrides with the general chemical form Mn+1XnTx, where ‘M’ is a transitional metal, ‘X’ is either C or N, and ‘T’ represents a surface functional group (O, -OH or -F). MXenes are derived from layered ternary carbides and nitrides known as MAX (Mn+1AXn) phases by selective chemical etching of the ‘A’ layers and addition of functional groups ‘T’. In our work, we focus on one of the most well studied MXene, titanium carbide (Ti3C2Tx). Single to few layer flakes of Ti3C2Tx (in a solution dispersed form) are used to create a continuous film on a desired substrate by using spin coating technique. Losses inherent to the bulk MXene and existence of strong localized SP resonances in Ti3C2Tx disks/pillar-like nanostructures at near-IR frequencies are utilized to design an efficient broadband absorber. For Ti3C2Tx MXene disk array sitting on a bilayer stack of Au/Al2O3, high efficiency (>90%) absorption across visible to near-IR frequencies (bandwidth ~1.55 μm), is observed experimentally. We also experimentally study random lasing behavior in a metamaterial constructed by randomly dispersing single layer nanosheets of Ti3C2Tx into a gain medium (rhodamine 101, R101). Sharp lasing peaks are observed when the pump energy reaches the threshold value of ~ 0.70 μJ/pulse. This active metamaterial holds a great potential to achieve tunable random lasing by changing the optical properties of Ti3C2Tx flakes.

Hyperbolic Metamaterials for Single-Photon Sources and Nanolasers

Hyperbolic metamaterials are anisotropic media that behave as metals or as dielectrics depending on light polarization. These plasmonic materials constitute a versatile platform for promoting both spontaneous and stimulated emission for a broad range of emitter wavelengths. We analyze experimental realizations of a single–photon source and of a plasmonic laser based on two different architectures of hyperbolic metamaterials. At the heart of this material capability lies the high broadband photonic density of states originating from a rich structure of confined plasmonic modes.

Lasing boosted with plasmonic nanostructures

Plasmonics, a technique to tightly concentrate light down to the nanoscaleby coupling photons to surface plasmons,has been employed to downscale lasers to sub-wavelength dimensions. The resultant device is called a spaser, short for surface plasmon amplification by stimulated emission of radiation, or more generally plasmonic nanolaser, which is being explored for applications in areas such as sensing and biomedical imaging. In this talk we will first make an overview of the major advances achieved thus far in this emerging field. Then, we will present our work addressing two major challenges regarding wavelength tunablility and lasing directionality of plasmonic nanolasers. To this date, single plasmonic nanoparticles, two-dimensional arrays of nanoapertures in plasmonic metasurfaces, and bulk plasmonic metamaterials have been designed and utilized as plasmonic nanocavities. Lasing wavelength tunability and directionality have been achieved with these nanocavities. Interesting effects have been also observed in other geometries. For instance, the 3-D random nanostructures bring in a possibility to control the lasing resonance by simply tuning the polarization of the pump laser, which is out of reach of conventional dielectric-cavity based lasers. By engineering the absorption and scattering properties, the laser can operate without degradation of lasing performance in the presence of densely packed metal nanostructures; consequently, spatial confinement of lasing modes at micron-scales has been obtained. We also present our two approaches to numerically model realistic spasers and nanolaser arrays. In the frequency domain, the spasers that utilize plasmonic modes of metallic nano-particles are modeled within the classical electrodynamics scattering framework using an intensity-dependent Lorentzian dielectric function, which rigorously accounts for quantum-mechanical saturation effects. In the time domain, we have developed a systematic approach to study lasing in plasmonic nanostructures using a finite difference model coupled to the rate equations of a multi-level gain system (4- and 6-level system). The modeling results show good agreement with experimental data.

Plasmonic random lasing in strongly scattering regime with slanted silver nanorod array

We present a plasmonic approach employing a slanted silver nanorod array for achieving controllable random lasing in a strongly scattering regime. Such random lasers can serve as a bright optical source for speckle-free imaging.

Time-domain model of 4-level gain system fitted to nanohole array lasing experiment

We developed an accurate three dimensional time domain model of a 4-level gain system fitted to lasing experiment with a silver nanohole array. The simulated emission intensity showed clear lasing effects confirmed by optical experiments.

Time-resolved lasing dynamics for plasmonic system with gain (Presentation Recording)

To study the light-matter interaction between plasmonic systems and gain media, numerous theoretical and numerical methods have been proposed. Among them, because of its accurate treatment of the quantum property of gain media, the time domain (TD) multi-physics approach is viewed as the most powerful method, especially for analysis of transient dynamics. Even though the finite difference, finite-volume and finite element TD methods can be readily coupled to a multi-level atomic system through auxiliary differential equations, for each of them however there is limited information on accurate TD kinetic parameters fitted with experimental measurements. In this work, we develop a multi-physics time domain model to inspect our most recent lasing experiment with a silver nanohole array. We use a classical finite difference time-domain (FDTD) model coupled to the rate equations of a 4-level gain system. To retrieve kinetic energy parameters for accurate modeling, we first fit 1-D simulations with pump-probe experiments studying Rhodamine-101 (R-101) dye embedded in epoxy on an indium tin oxide silica substrate. The retrieved parameters are then fed into a 3-D model to study the lasing behavior of the R-101-coated nanohole array. The simulated emission intensity shows a clear lasing effect, which is in good agreement with the experimental measurements. By tracing the population inversion and polarization dynamics, the amplification and lasing regimes inside the nanohole cavity can be clearly distinguished. With the help of our systematic approach, we can further improve understanding of the time-resolved physics of active plasmonic nanostructures with gain.

Unidirectional Surface Plasmon Polariton Coupler in the Visible Using Metasurfaces

We have theoretically investigated a metasurface as a unidirectional surface plasmon polariton (SPP) coupler. The structure can work over a broad bandwidth in the visible region.

Incident field image reconstruction using speckle intensity correlations over position

Speckle images are used to form speckle intensity spatial correlations over incident light position. These correlations allow the reconstruction of the field incident on and transmitted through a heavily scattering medium.