Can a chip guide single plasmons from diamond light sources?
Single germanium-vacancy emitters placed inside plasmonic waveguides on crystalline silver sent over half their light into the guide, which carried it tens of micrometres.
Source
On-chip excitation of single germanium vacancies in nanodiamonds embedded in plasmonic waveguides
Study at a glance
- Design
- Other — Single GeV nanodiamonds located on silver, then embedded in HSQ plasmonic waveguides on silver film or single-crystal silver flakes; lifetime, spectra, autocorrelation and out-coupled intensity measured, with finite-element simulation.
- N
- A handful of individually selected single-emitter nanodiamond devices; no overall sample count given.
- Population
- Germanium-vacancy colour centres in HPHT-grown nanodiamonds on silver
- Outcome
- Emitter lifetime (Purcell factor), coupling efficiency (beta factor), plasmon propagation length, and single-photon statistics
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What they did
The authors grew nanodiamonds containing germanium-vacancy (GeV) colour centres, picked ones that emitted single photons, and fabricated dielectric-loaded surface plasmon waveguides around them on silver. They compared a polycrystalline silver film with smooth single-crystal silver flakes, measured lifetimes, spectra and photon correlations before and after coupling, and in a second experiment sent green laser light down the waveguide to excite the emitter remotely. Finite-element simulations predicted the coupling.
What they found
Coupling to the waveguide shortened the emitter lifetime from about 12.3 to 3.8 ns, giving roughly a six-fold Purcell enhancement including the silver surface. On crystalline silver the measured coupling efficiency was 56%, close to the simulated 62%, and the emission propagated 33 ± 3 μm, versus 9 ± 3 μm on the silver film. The emitters stayed single-photon sources after coupling, and the combined figure of merit of 180 exceeded earlier emitter-plasmonic waveguide systems.
The limits
What it doesn't show
Results come from a few hand-selected devices, and the lifetimes of GeV centres varied widely between nanodiamonds, so typical performance is uncertain. The beta factor is an apparent value estimated from out-coupled intensities, not a direct measurement. All measurements were at room temperature, so the cooperativity is an upper estimate and photon indistinguishability was not tested; single-plasmon detection and interference on the chip were not shown.
Key terms
- Surface plasmon polariton
- A light wave bound to electron oscillations at a metal surface, confined below the diffraction limit but lossy.
- Purcell enhancement
- The speed-up of an emitter's spontaneous emission caused by its electromagnetic environment, seen as a shorter lifetime.
- Beta factor
- The fraction of an emitter's decay that goes into the desired guided mode.
- Germanium-vacancy centre
- A diamond defect where a germanium atom sits between two empty lattice sites, emitting narrow light near 602 nm.
- Antibunching
- A dip in photon coincidences at zero delay; g2(0) below 0.5 indicates a single emitter.
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Quiz yourself
What limits placement precision of the nanodiamonds in this method?
Common questions
Why use single-crystal silver flakes?
They have much lower plasmon damping than deposited films, so both the green pump and the emitted light travel several times farther.
What does remote excitation mean here?
The green laser is coupled into the waveguide at one end and travels on-chip as a plasmon to excite the embedded emitter, rather than being focused directly on it.
How do they know the emitter is still a single-photon source?
The second-order correlation dips below 0.5 at zero delay both before and after coupling.
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