PerspectiveApplied Physics

Metamaterials with Quantum Gain

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Science  08 Feb 2013:
Vol. 339, Issue 6120, pp. 654-655
DOI: 10.1126/science.1231254

Optical metamaterials and nanoplasmonics offer extreme control and localization of light within volumes that can be smaller than a cubic light wavelength by more than three orders of magnitude, but they suffer from appreciable dissipative losses. This weakness is thought to constitute the prime impediment before many of the envisaged applications can succeed in practice. However, recent breakthroughs in the theoretical understanding and experimental fabrication of gain-enhanced metamaterials and nanoplasmonic heterostructures promise to overcome these hindrances, while allowing for new ways to control spontaneous and stimulated emission of light on the nanoscale (1, 2).

Resistive losses in nanoplasmonic metamaterials arise from the interaction of the incident photons with the quasi-free conduction electrons of the metals, thereby constituting an inherent feature of the response of metal-based nanodevices. For truly subwavelength plasmonic structures, these losses follow universal laws; that is, they do not depend on the particular geometric configuration but only on the metal used (usually noble metals) (3). Meanwhile, there has been an increased emphasis on two-dimensional (2D) metasurfaces, which are much more convenient to fabricate than their full-3D metamaterial counterparts but can steer light in equally dramatic ways, well below the fundamental diffraction limit and over broad, flat areas (4).

Playing for gain.

(A) An illustration of the gain-enhanced optical metamaterial, with a magnified unit cell and an example of plasmonic field enhancement at two vertical planes inside the cell. (B) Energy and average inversion (green solid line, right axis) inside the lasing nanofishnet over time. The signals, time-averaged over 0.4 ps (black), are decomposed into the pump mode (green dashed line), the bright mode (red solid line), and the dark mode (yellow dash-dotted line).

In such a 2D nanostructure with laser dyes (gain medium) incorporated into its fabric (see the figure, panel A), the objective is to obtain optimum coupling of the plasmonic excitations to the gain molecules, so that maximum harnessing of the gain medium can be achieved—a requirement due to the high losses. When the pattern of the nanoholes periodically perforating the two silver nanofilms is engineered such that the desired plasmonic resonance coincides with the emission wavelength of the dyes, full loss compensation and amplification can be achieved, even in the exotic negative refractive index regime (57). By extending the duration of the incident probe pulse such that the energy inside the nanostructure becomes constant with time, the quantum plasmonic amplifier can operate transiently and also in a steady-state mode (8).

If the gain supplied by the active medium is sufficient to overcome both dissipative and radiative losses, then the structure can function as a coherent emitter of surface plasmons over the ultrathin 2D area, deep below the diffraction limit for visible light (1, 9). Here, controlling the spontaneous emission rate is crucial (10) as both bright and dark plasmonic lasing states exist, giving rise to a strong, nonlinear competition. Which one eventually dominates can be controlled by the design and excitation of the metamaterial (see the figure, panel B). In this example, where bright plasmonic emission dominates, the bright-mode energy (red solid line) builds up initially, followed by picosecond-period relaxation oscillations and steady-state emission, interrupted (at ∼50 ps) by an instability of the dark mode (yellow dash-dotted line) until again steady-state emission is reached.

These 2D active nanostructures can function as powerful on-chip light sources, either coherent (nanolasers) or incoherent (light-emitting diodes, LEDs), delivering intense optical power. Although the typical plasmonic cavity Q factors are rather small (∼50), the attained Purcell factors (a measure of the spontaneous emission rate enhancement that varies as Q/Vm) can be large because the mode volume Vm can be extremely small. A major goal in the field is to enhance the Purcell factor so as to accelerate spontaeous emission to the degree that it becomes faster than stimulated emission, so that ultrafast, low-energy LEDs can be attained and integrated within nanoscale circuits (11).

Large Purcell factors can also be obtained in plasmonic slow-light heterostructures for special cases where the group velocity of the plasmons is close to zero (1). Computations reveal that at these zero–group velocity points, full lasing operation can be reached in completely uniform (minimalistic) structures that do not require a cavity to confine light (by reflections or Bragg scattering) because successive light pulses can be stopped and strongly localized (12). Without any cavity walls restricting the lasing mode size to be above the diffraction limit, the mode can be pushed into a deep-subwavelength regime, with sizes (in 2D) of λ2/1000 being realistically attainable. Because spontaneous emission can be efficiently channeled into the stopped-light mode, the result is low-threshold operation.

The marriage of nanoplasmonic metamaterials with quantum gain media represents an exciting frontier in nanophotonics and nanoscience, and is a precursor of active, intergrated quantum nano-optics. Bringing gain in the nanoscale will open a platform for practical, loss-free nanodevices—not only electro-optic modulators and intense light sources, but also plasmonic waveguides and nanosensors exploiting intensified plasmonic hot spots for single-emitter spectroscopy or nanoscale lithography.

References and Notes

  1. Acknowledgments: Supported by the Leverhulme Trust, the Royal Academy of Engineering, and the UK Engineering and Physical Sciences Research Council.
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