Gain-enhanced nanoplasmonic metamaterials: ultrafast sub-wavelength emitters

Nanoplasmonics and optical metamaterials have in the last 10–15 years emerged as a new paradigm in condensed matter optics and nanoscience, offering a fresh perspective on the world of optics. They enable efficient coupling of electromagnetic fields to the nanoscale: the world of biological and nonbiological molecules.¹ This tight localization of light to truly nanoscopic dimensions—well below the diffraction limit for visible light—enhances its interaction with matter, paving the way for a multitude of classical and quantum nano-optics applications. However, metal optics suffer from inherent dissipative losses, which have persistently hampered many of the envisaged uses. Advances in the theoretical understanding and experimental fabrication of gain-enhanced nanoplasmonic metamaterials now promise to overcome these hindrances, potentially leading to novel nanophotonic components and devices.

Dissipative losses in nanoplasmonics arise from the interaction of incident photons with quasi-free conduction electrons in metals, and therefore constitute an inherent feature of the responses of metal-based nanodevices. For truly sub-wavelength plasmonic structures, these losses follow universal laws, i.e., they do not depend on a particular geometric configuration, but rather only on the type of (usually noble) metal used. Typical damping rates (Γ) are of the order of Γ∼100ps⁻¹, requiring gain coefficients Γ/c∼10³–10⁴s⁻¹ to compensate for the losses. As an alternative strategy, optical metamaterials pave the way toward nanoscale control of light at the fabrication level. However, such control needs to be modulated dynamically, on-demand and in real time. Both of the above- mentioned challenges can be met by metamaterials with gain incorporated directly into their fabric.

Pioneering theoretical^2,3 and experimental⁴ work has recently shown that it is realistically possible to overcome dissipative losses of nanoplasmonic metamaterials, even in the exotic negative-index regime. However, whether steady-state net amplification can be accomplished is yet to be answered. Another question is what kind of active nanocomponents can we ultimately expect by enhancing metamaterials and nanoplasmonics with gain?

We recently showed that strong coupling of excited (bright) modes of optical metamaterials to the electromagnetic continuum opens up a broad window of wavelengths within which we can achieve full loss compensation and amplification, even in the steady state.³ Higher gain densities result in compensation of both dissipative and radiative losses, leading into the full lasing regime, where bright and dark lasing states dynamically compete with each other, giving rise to ultrafast nonlinear responses.

Figure 1 depicts an example of an active (gain-enhanced) nanoplasmonic metamaterial comprising two thin silver nanofilms, periodically perforated with holes, and laser dyes inserted between the films. The structure has been designed so that there is an optimum coupling of the plasmonic excitations to the gain molecules, ensuring efficient harnessing of the gain. An intense pump pulse of 2ps duration (λ_pump=680nm) inverts the gain medium and dynamically creates a 3D occupation inversion profile closely matching the spatial distribution of the electric field at the probe wavelength (λ_probe=710nm). After 7ps, a weak broadband pulse of 12fs duration probes the active structure, and its far-field spectrum is recorded at the two sides of the planar metamaterial. Detailed calculations have shown that in the regime of full loss compensation, the real part of the effective refractive index of the nanostructure becomes more negative compared to the passive case. By prolonging the duration of the probe pulse so that the energy inside the nano-fishnet becomes constant over time, we have also ascertained that there can be net outflux of optical energy through a volume encompassing the metamaterial, i.e., there is more energy radiated away from the volume than energy incident on the volume. Hence, our quantum plasmonic amplifier can operate not only transiently, but also in a steady-state mode.

Figure 1. Schematic of a gain-enhanced plasmonic nano-fishnet metamaterial together with example profiles of the inversion (lower left) and electric-field amplitude. h_m, h_c, and h_ddenote the height of the metal, cladding, and dielectric layers, respectively, a_xand a_y are the width of the rectangular holes in the x and y direction, and p is the periodicity of the material.

Such active nanostructures can function as powerful light sources, either coherent (nanolasers) or incoherent (diodes), delivering intense optical power within ultrasmall volumes or on ultrathin flat surfaces. We have also been able to embed the active medium in a quasi-2D active metamaterial and produce a gain sufficient to overcome dissipative and radiative losses. This design allows the structure to function as a coherent emitter of surface plasmons over the whole ultrathin 2D area, well below the diffraction limit for visible light.^5,6 Both bright and dark plasmonic lasing states can be generated, coupling either strongly or weakly to the continuum. The dominant lasing state can be controlled by the design of the metamaterial. Figure 2(a) and (b) shows the competition between these lasing states. One can see the initial buildup of the bright-mode energy as a red trace that occurs both externally (a) and internally (b), followed by damped-amplitude, ps-period relaxation oscillations. The subsequent steady-state emission is interrupted after around 50ps by the instability of the dark mode, shown in a yellow trace, occurring internally in (b). This lasts until steady-state emission is reached again.

Figure 2. Dynamics of bright and dark lasing states in gain-enhanced plasmonic nano-fishnet. (a) External (emission): far-field intensity (black), pump light (light green, dashed), and bright mode (red). (b) Internal: energy density (black), average inversion (dark green), pump intensity (light green), bright mode (red), and dark mode (yellow, dash-dotted).

Gain-enhanced plasmonics and optical metamaterials constitute an exciting new frontier in nanophotonics and nanoscience, and are precursors to active, integrated quantum nano-optics.¹ Achieving gain on the nanoscale is anticipated to improve the performance of a host of active nanocomponents, such as electro-optic modulators and light sources, but also passive ones, such as plasmonic waveguides and sensors featuring intensified plasmonic hotspots for single-emitter spectroscopy. We next aim to advance semiconductor metamaterials using nanoscale gain architectures, such as quantum dots and quantum wires. This will open up possibilities including ultrafast and ultrathin meta-lasers for on-chip pulse generation, quantum-squeezed state emitters, and cavity-free lasing in the stopped-light regime.

We gratefully acknowledge financial support provided by the Leverhulme Trust, the Engineering and Physical Sciences Research Council, and the Royal Academy of Engineering.