Beaming light from a quantum emitter with a planar optical antenna

Efficient room-temperature molecular single-photon sources for quantum key distribution

Quantum key distribution (QKD) allows the distribution of cryptographic keys between multiple users in an information-theoretic secure way, exploiting quantum physics. While current QKD systems are mainly based on attenuated laser pulses, deterministic single-photon sources could give concrete advantages in terms of secret key rate (SKR) and security owing to the negligible probability of multi-photon events. Here, we introduce and demonstrate a proof-of-concept QKD system exploiting a molecule-based single-photon source operating at room temperature and emitting at 785 nm. With an estimated maximum SKR of 0.5 Mbps, our solution paves the way for room-temperature single-photon sources for quantum communication protocols.

Biosensing with a scanning planar Yagi-Uda antenna

We investigate a model bioassay in a liquid environment using a z-scanning planar Yagi-Uda antenna, focusing on the fluorescence collection enhancement of ATTO-647N dye conjugated to DNA (deoxyribonucleic acid) molecules. The antenna changes the excitation and the decay rates and, more importantly, the emission pattern of ATTO-647N, resulting in a narrow emission angle (41°) and improved collection efficiency. We efficiently detect immobilized fluorescently-labeled DNA molecules, originating from solutions with DNA concentrations down to 1 nM. In practice, this corresponds to an ensemble of fewer than 10 ATTO-647N labeled DNA molecules in the focal area. Even though we use only one type of biomolecule and one immobilization technique to establish the procedure, our method is versatile and applicable to any immobilized, dye-labeled biomolecule in a transparent solid, air, or liquid environment.

Photostable fluorescent molecules on layered hexagonal boron nitride: Ideal single-photon sources at room temperature

Photoblinking and photobleaching are commonly encountered problems for single-photon sources. Numerous methods have been devised to suppress these two impediments; however, either the preparation procedures or the operating conditions are relatively harsh, making them difficult to apply to practical applications. Here, we reported giant suppression of both photoblinking and photobleaching of a single fluorescent molecule, terrylene, via the utilization of hexagonal boron nitride (h-BN) flakes as substrates. Experimentally, a much-prolonged survival time of terrylene has been determined, which can have a photostable emission over 2 h at room temperature under ambient atmospheres. Compared with single molecules on a SiO₂/Si substrate or glass coverslip, a more than 100-fold increase in the total number of photons collected from each terrylene on h-BN flakes has been demonstrated. We also proved that the photostability of terrylene molecules can be well maintained for more than 6 months even under ambient conditions without any further protection. Our results demonstrate that the utilization of h-BN flakes to suppress photoblinking and photobleaching of fluorescent molecules has promising applications in the production of high-quality single-photon sources at room temperature.

Single organic molecules for photonic quantum technologies

Isolating single molecules in the solid state has allowed fundamental experiments in basic and applied sciences. When cooled down to liquid helium temperature, certain molecules show transition lines that are tens of megahertz wide, limited by only the excited-state lifetime. The extreme flexibility in the synthesis of organic materials provides, at low costs, a wide palette of emission wavelengths and supporting matrices for such single chromophores. In the past few decades, their controlled coupling to photonic structures has led to an optimized interaction efficiency with light. Molecules can hence be operated as single-photon sources and as nonlinear elements with competitive performance in terms of coherence, scalability and compatibility with diverse integrated platforms. Moreover, they can be used as transducers for the optical read-out of fields and material properties, with the promise of single-quanta resolution in the sensing of charges and motion. We show that quantum emitters based on single molecules hold promise to play a key role in the development of quantum science and technologies.

Metasurface-Enabled Generation of Circularly Polarized Single Photons

Single photons carrying spin angular momentum (SAM), i.e., circularly polarized single photons generated typically by subjecting a quantum emitter (QE) to a strong magnetic field at low temperatures, are at the core of chiral quantum optics enabling nonreciprocal single-photon configurations and deterministic spin-photon interfaces. Here, a conceptually new approach to the room-temperature generation of SAM-coded single photons (SSPs) is described, which entails QE nonradiative coupling to surface plasmons being transformed, by interacting with an optical metasurface, into a collimated stream of SSPs with the designed handedness. Design, fabrication, and characterization of SSP sources, consisting of dielectric circular nanoridges with azimuthally varying widths deterministically fabricated on a dielectric-protected silver film around a nanodiamond containing a nitrogen-vacancy center, are reported. With properly engineered phases of QE-originated fields scattered by nanoridges, the outcoupled photons are characterized by a well-defined SAM (with the chirality >0.8) and high directionality (collection efficiency up to 92%).

Electrically-driven Yagi-Uda antennas for light

Yagi-Uda antennas are a key technology for efficiently transmitting information from point to point using radio waves. Since higher frequencies allow higher bandwidths and smaller footprints, a strong incentive exists to shrink Yagi-Uda antennas down to the optical regime. Here we demonstrate electrically-driven Yagi-Uda antennas for light with wavelength-scale footprints that exhibit large directionalities with forward-to-backward ratios of up to 9.1 dB. Light generation is achieved via antenna-enhanced inelastic tunneling of electrons over the antenna feed gap. We obtain reproducible tunnel gaps by means of feedback-controlled dielectrophoresis, which precisely places single surface-passivated gold nanoparticles in the antenna gap. The resulting antennas perform equivalent to radio-frequency antennas and combined with waveguiding layers even outperform RF designs. This work paves the way for optical on-chip data communication that is not restricted by Joule heating but also for advanced light management in nanoscale sensing and metrology as well as light emitting devices.

Planar antenna designs for efficient coupling between a single emitter and an optical fiber

Fluorescence detection is a well-established readout method for sensing, especially for in-vitro diagnostics (IVD). A practical way to guide the emitted signal to a detector is by means of an optical fiber. However, coupling fluorescence into a fiber is challenging and commonly lacks single-molecule sensitivity. In this work, we investigate specific fiber geometries, materials and coatings that in combination with a planar Yagi-Uda antenna reach efficient excitation and collection. The simulation of a practical setting determines more than 70% coupling efficiency for a horizontally oriented dipole, with respect to the planar antenna, emitting at 700 nm and embedded in polyvinyl alcohol (PVA). Moreover, the coupling efficiency would only scale by a factor of 2/3 for emitters with random orientation, as a result of the antenna geometry. These findings are relevant for single-molecule detection with fiber optics and have implications for other applications involving the coupling of light with nano-scale sources and detectors. Scanning the surface of a sample with such fibers could also be advantageous for imaging techniques to provide a low background noise and a high resolution.

On the emission pattern of nanoscopic emitters in planar anisotropic matrix and nanoantenna structures

Single nanoscopic emitters embedded in the crystalline matrix have become a valuable resource for emerging nanophotonics and quantum technologies. The generally anisotropic nature of the matrix strongly affects the emission properties of the quantum emitters, in particular, when the matrix is assembled in nanophotonic structures. We report on rigorous analysis and engineering of spontaneous emission from single emitters coupled to nanoantenna and planar anisotropic antenna structures. By developing a convenient theoretical method with efficient numerical implementation, we show that accurate modeling of the anisotropy is essential in predicting the emission pattern for many important systems, such as single molecules in the solid-state matrix, isolated defects in 2D materials and so on. In particular, we illustrate the amplified effects of material anisotropy and geometrical anisotropy for emitters coupled to planar antenna and nanoantenna structures. We show that with an appropriate design of the anisotropy, a strong enhancement of the emission rate and a nearly collimated beam from single emitters can be simultaneously achieved.

Unidirectional Enhanced Dipolar Emission with an Individual Dielectric Nanoantenna

Light manipulation at the nanoscale is the vanguard of plasmonics. Controlling light radiation into a desired direction in parallel with high optical signal enhancement is still a challenge for designing ultracompact nanoantennas far below subwavelength dimensions. Here, we theoretically demonstrate the unidirectional emissions from a local nanoemitter coupled to a hybrid nanoantenna consisting of a plasmonic dipole antenna and an individual silicon nanorod. The emitter near-field was coupled to the dipolar antenna plasmon resonance to achieve a strong radiative decay rate modification, and the emitting plasmon pumped the multipoles within the silicon nanorod for efficient emission redirection. The hybrid antenna sustained a high forward directivity (i.e., a front-to-back ratio of 30 dB) with broadband operating wavelengths in the visible range (i.e., a spectral bandwidth of 240 nm). This facilitated a large library of plasmonic nanostructures to be incorporated, from single element dipole antennas to gap antennas. The proposed hybrid optical nanorouter with ultracompact structural dimensions of 0.08 λ² was capable of spectrally sorting the emission from the local point source into distinct far-field directions, as well as possessing large emission gains introduced by the nanogap. The distinct features of antenna designs hold potential in the areas of novel nanoscale light sources, biosensing, and optical routing.

Shaping the Nonlinear Emission Pattern of a Dielectric Nanoantenna by Integrated Holographic Gratings

We demonstrate the shaping of the second-harmonic (SH) radiation pattern from a single AlGaAs nanodisk antenna using coplanar holographic gratings. The SH radiation emitted from the antenna toward the-otherwise forbidden-normal direction can be effectively redirected by suitably shifting the phase of the grating pattern in the azimuthal direction. The use of such gratings allows increasing the SH power collection efficiency by 2 orders of magnitude with respect to an isolated antenna and demonstrates the possibility of intensity-tailoring for an arbitrary collection angle. Such reconstruction of the nonlinear emission from nanoscale antennas represents the first step toward the application of all-dielectric nanostructures for nonlinear holography.

Highly Directive Hybrid Metal-Dielectric Yagi-Uda Nanoantennas

A hybrid metal-dielectric nanoantenna promises to harness the large Purcell factor of metallic nanostructures while taking advantage of the high scattering directivity and low dissipative losses of dielectric nanostructures. Here, we investigate a compact hybrid metal-dielectric nanoantenna that is inspired by the Yagi-Uda design. It comprises a metallic gold bowtie nanoantenna feed element and three silicon nanorod directors, exhibiting high unidirectional in-plane directivity and potential beam redirection capability in the visible spectral range. The entire device has a footprint of only 0.38 λ², and its forward directivity is robust against fabrication imperfections. We use the photoluminescence from the gold bowtie nanoantenna itself as an elegant emitter to characterize the directivity of the device and experimentally demonstrate a directivity of ∼49.2. In addition, we demonstrate beam redirection with our device, achieving a 5° rotation of the main emission lobe with a feed element displacement of only 16 nm. These results are promising for various applications, including on-chip wireless communications, quantum computing, display technologies, and nanoscale alignment.

Self-Assembled Nanocrystals of Polycyclic Aromatic Hydrocarbons Show Photostable Single-Photon Emission

Quantum technologies could largely benefit from the control of quantum emitters in sub-micrometric size crystals. These are naturally prone to integration in hybrid devices, including heterostructures and complex photonic devices. Currently available quantum emitters in nanocrystals suffer from spectral instability, preventing their use as single-photon sources for most quantum optics operations. In this work we report on the performances of single-photon emission from organic nanocrystals (average size of hundreds of nm), made of anthracene (Ac) and doped with dibenzoterrylene (DBT) molecules. The source has hours-long photostability with respect to frequency and intensity, both at room and at cryogenic temperature. When cooled to 3 K, the 00-zero phonon line shows linewidth values (50 MHz) close to the lifetime limit. Such optical properties in a nanocrystalline environment recommend the proposed organic nanocrystals as single-photon sources for integrated photonic quantum technologies.