Session B18: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- Metamaterials with Gain and Active

Metamaterials with Gain

Nanoplasmonic metamaterials are the key to an extreme control of light and allow us to conceive materials with negative or vanishing refractive index. Indeed, metamaterials enable a multitude of exciting and useful applications, such as subwavelength focusing, invisibility cloaking, and ``trapped rainbow'' stopping of light. The realization of these materials has recently advanced from the microwave to the optical regime. However, at optical wavelengths, metamaterials may suffer from high dissipative losses owing to the metallic nature of their constituent nanoplasmonic meta-molecules. It is therefore not surprising that overcoming loss restrictions by gain is currently one of the most important topics in metamaterials' research. At the same time, providing gain on the nanoplasmonic (metamolecular) level opens up exciting new possibilities such as a whole new type of metamaterial nano-laser with a cavity length of about a tenth of the wavelength. The talk gives an overview of the state of the art of gain-enhanced metamaterials. Particular focus will be placed on nano-plasmonic metamaterials (such as double-fishnet metamaterials) with integrated laser dyes as gain medium. The successful compensation of loss by gain is demonstrated on the meta-molecular level. On the basis of a comprehensive, microscopic Maxwell-Bloch Langevin approach of spatio-temporal light amplification and lasing in gain-enhanced nanoplasmonic (negative-index) metamaterials a methodology based on the discrete Poynting's theorem is introduced that allows dynamic tracing of the flow of electromagnetic energy into and out of ``microscopic'' channels (light field, plasmons, gain medium). It is shown that steady-state amplification can be achieved in nanoplasmonic metamaterials. Finally, a complex spatio-temporal interplay of light-field and coherent absorption dynamics is revealed in the lasing dynamics of a nanoplasmonic gain-enhanced double-fishnet metamaterial.   

Metamaterials with gain and interpretation of transmission in pump-probe experiments

We establish a new approach for pump-probe simulations of metallic metamaterials coupled to the gain materials. It is of vital importance to understand the mechanism of the coupling of metamaterials with the gain medium. Using a four-level gain system, we have studied light amplification of arrays of metallic split-ring resonators (SRRs) with a gain layer underneath. We find that that the differential transmittance $\Delta T/T$ can be negative for SRRs on the top of the gain substrate, which is not expected, and $\Delta T/T$ is positive for the gain substrate alone. These simulations agree with pump-probe experiments and can help to design new experiments to compensate the losses of metamaterials.We numerically investigate loss compensation and transmission in pump-probe experiments in resonant metamaterials with gain using an FDTD algorithm coupled to semiclassical rate equations. We explain experimentally observed negative differential transmission by gain-dependent impedance.   

Spontaneous emission enhancement in a plasmonic nanocavity

Recently, metallic optical cavities containing coupled emitters have been fabricated that operate at visible frequencies. These cavities are capable of tightly confining light, greatly modifying the electromagnetic density of states in the location of the optical emitters. Here, we present measurements from a metal-based optical cavity that greatly modifies both the spectral and temporal characteristics of the coupled emitters. Our design is based on plasmonic coupling between a silver nanowire and a planar silver substrate, with a layer of optical emitters within the gap between the two silver components. The field confinement of the structure results in a 1000-fold enhancement of the spontaneous emission rate of the coupled emitters. These results suggest that metal-based optical cavities can allow quantum cavity electrodynamics of emitters such as colloidal quantum dots and organic dyes.   

Directionally emitting plasmon laser and circuits

Scaling down of the laser promises unprecedented ultra-dense and ultra-fast integrated photonics. The research of nanoscale lasers is rapidly advancing and a variety of approaches have been explored including whispering gallery lasers, photonic crystal lasers, metallic lasers. However, a major obstacle of integrating nanolasers with other components is the strong divergence of emission from a sub-wavelength laser cavity due to the diffraction of light. Here, we demonstrate a deep sub-wavelength plasmon laser that directs more than 70{\%} of its radiation into an embedded semiconductor waveguide. The laser naturally integrates photonic and electronic functionality allowing both efficient electrical modulation and wavelength multiplexing. A maximum modulation depth of 11 dB for a small 1 V of bias sweep is achieved. We demonstrate an ultra-compact plasmonic circuit integrating five independently modulated multi-colored laser sources multiplexed onto a single semiconductor waveguide, illustrating the potential of plasmon lasers for large scale, ultra-dense photonic integration.   

Electromagnetically induced transparency and absorption in metamaterials: self-consistent theory and experiments

There has recently been a lot of interest in slow-light metamaterials that provide transparency windows combining low absorption with high group delay. This phenomenon was explained by a two-resonator model involving a radiative resonator that couples directly to the incident field and a dark resonator that can only be excited through coupling with the radiative resonator. However, in our most recent experiments on wire/SRR metamaterials, we have observed a much richer behavior---we measure not only transparency windows with incisions in the absorption spectrum (electromagnetically induced transparency), but also narrow spectral features with absorption larger than the background absorption of the radiative element (electromagnetically induced absorption). We have developed a model in which the coupling of the electromagnetic waves to the radiative resonator is treated explicitly. An important attribute of this model, which is in excellent agreement with our experiments and full-wave simulations, is the self-consistent treatment of the spectral broadening of the bright resonator originating from the dipole radiation as opposed to the bare linewidth due to dissipation. We discuss the conditions under which electromagnetically induced transparency/absorption can be observed.   

Lighting up dark plasmon modes with electron beam

Dark plasmon modes couple weakly with free-space photons. It is, therefore, generally believed that far-field detection of dark plasmon modes is difficult. In this talk, we discuss how an electron beam can light up dark plasmon modes and show theoretically and experimentally that bright and dark modes in a single metal bowtie nanoantenna can be studied at the far field using cathodoluminescence spectroscopy.   

Direct modulation of lanthanide emission at sub-lifetime scales using electric and magnetic dipole transition

Lanthanide ions, such as trivalent Europium (Eu3+) and Erbium (Er3+) are technologically important, high quantum yield light emitters that exhibit both magnetic dipole (MD) and electric dipole (ED) transitions. It is well know that the transition rate of an emitter in an inhomogenous optical environment, e.g. an emitter near a mirror, is modified due to self-interference effects, leading to either enhancement or inhibition of spontaneous emission. However, due to the opposite symmetry of their emitted fields, ED and MD transitions exhibit differing self-interference. Here, we leverage this difference to show large spectral tuning and sub-lifetime dynamic modulation of Eu3+ emission. Specifically, we use a moving gold mirror to selectively enhance the ED and MD transitions in Eu3+ doped Y2O3. Controlling the emitter-mirror distance allows us to tune the emission spectra from 580 nm to 715 nm. Modulating the mirror position with a piezoelectric crystal allows us to dynamically tune the Eu3+ emission at speeds faster than the excited state lifetime.   

Strong Coupling of Molecular Absorption and Mid-Infrared Metamaterial Resonances

Here we present numerical, analytical, and experimental evidence for strong optical coupling between a mid-infrared perfect absorber thin-film metamaterial and a molecular absorption resonance within a dielectric medium. Clear anti-crossing behavior is exhibited numerically and experimentally when the metamaterial resonance is scanned through the dielectric molecular resonance; a coupled oscillator model is used to present a further analytical description. The anti-crossing is used to demonstrate and quantify the strength of the optical coupling within the thin film. Such a device can potentially be developed for mid-infrared sensing applications and actively tunable metamaterial optical components.   

Magnetic Dipole Emission Lines in Trivalent Lanthanide Ions

Magnetic dipole (MD) transitions offer a unique opportunity to study light-matter interactions beyond the electric dipole approximation. Modified spontaneous emission from MD transitions can serve as an ideal way to study optical magnetic fields in metamaterials and related nanostructures. While many MD absorption lines have been identified, only a few MD emission lines in solid state systems are widely known. Here we seek to expand the known MD emission lines by calculating all such transitions in the trivalent lanthanide ions. We use intermediate coupling to construct a detailed free-ion Hamiltonian including electrostatic, spin-orbit, two- and three-body, spin-spin, spin-other-orbit, and electrostatically correlated spin-orbit interactions. Determining the free-ion energy levels allows the calculation of ground-state MD absorption lines, and their respective oscillator strengths, as well as spontaneous emission rates for all MD mediated transitions. Many strong MD emission lines are predicted throughout the visible and near infrared spectrum. Corresponding electric quadrupole transitions were also calculated and found to have negligible contribution. If time permits, we will also present direct experimental measurements characterizing these new MD emission lines.   

Self-assembled quantum dots as active emitters for silicon photonic crystal nanocavities

We present a study of the properties of active (Si)Ge self-assembled quantum dot (QD) emitters coupled to Si photonic crystal (PC) nanocavities up to room temperature and evaluate the potential of such nanostructures for the realization of efficient silicon based light sources in the near-infrared regime. The Si-Ge system allows the formation of type I and type II band alignment QD, depending on the epitaxy conditions. We first discuss the emission properties of different types of QD. In particular we present type I SiGe islands designed to produce a strong emission due to a large wave function overlap of confined electrons and holes. We then investigate the coupling properties of (Si)Ge QD emitters to 2D Si PC cavities. We interpret the experimentally observed correlation between the photoluminescence intensity of the QDs and the quality factor of the PC cavity using simulations based on a dissipative master-equation approach [1]. As a further step towards efficient light sources, we currently study the coupling of electrically pumped (Si)Ge QDs to 2D PC in contacted diode structures. [1] N. Hauke, et al. Phys. Rev. B 84, 085320 (2011)   

Broadband Transparency of Graded Anisotropic Metamaterial

We have investigated the scattering of electromagnetic waves from a graded anisotropic sphere whose dielectric permittivity is radially anisotropic, with different radial and tangential components described by the graded Drude model. The Rayleigh scattering cross section (RSCS) has been calculated analytically and numerically by the Rayleigh scattering theory. The electric polarization and the electromagnetic field distribution are also examined in a quasi-static condition. Due to the scattering cancellation mechanism, the results reveal that the RSCS can be rather smaller over a wider frequency range, indicating the so called broadband transparency. Furthermore, compared with the non-graded and graded isotropic structure, the anisotropic structure leads a better broadband transparency.   

Reconfigurable Gradient Index using VO$_2$ Memory Metamaterials

We have demonstrated tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial was achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. Through design of the device and contact geometry, we achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device was observed on spatial scales on the order of one wavelength at 1 THz. Thus we have demonstrated the viability of elements for use in future devices with potential applications in beamforming and communications.\footnote{M. D. Goldflam, \textit{et. al.}, Appl. Phys. Lett. 99, 044103 (2011).} Various contact geometries are currently being investigated with the goal of implementing finer control over gradients and expanding on the possible applications of such devices.   

Waves and rays in meta-microcavities: from positive ``n'' to negative ``n'' and from order to chaos

The emergence of metamaterials and, in particular, negative index metamaterials (NIMs) triggers reconsideration of many fundamental physical phenomena. Importantly, the majority of unique properties of NIMs stand out when NIMs are combined with conventional positive index materials (PIMs). In this talk, we consider the wave and ray properties of electromagnetic wave interaction in two-dimensional microcavities that contain a combination of PIM and NIM with negative dielectric permittivity and magnetic permeability. We consider closed and open cavities and total and partial reflection and refraction at the boundaries between the media with different indices of refraction. By using a combination of analytic and numerical methods we are able to classify the different types of possible solutions in this type of mixed PIM-NIM regions. In particular, we derive the special properties of whispering gallery modes as well as the constructive and destructive interference due to wave refraction/reflection across different refractive media boundaries. Finally, we discuss possible practical applications of this type of mixed refractive indices microcavities.