CMOS-compatible, plasmonic-metamaterial-enhanced single-photon source
The techniques for separating single diamond nano-crystals containing a single NV-center have been developed. It was experimentally established that such systems produce fluxes of uncorrelated (single) photons. It was also shown that positioning such a single-photon source close to the surface of a hyperbolic metamaterial (HMM) considerably increases its efficiency. The employed HMM contains titanium nitride, TiN, as its plasmonic component. TiN is a CMOS-compatible material, which will allow to manufacture single-photon emitters using standard nanofabrication techniques and will make it easier to integrate them with nanoelectronic components. These results constitute a significant step in development of commercial, CMOS-compatible, efficient single-photon source.
Plasmonics is a rapidly growing field of nano-science that employs surface plasmons (SP) to control the behavior of light with nanometer precision. SP are localized or propagating oscillations of free-electron gas in a material such as a metal or a semiconductor. While isolated nanoparticles can support localized surface plasmons (Figure 1), propagating SP exist at the interfaces between the materials with appropriately different optical properties (Figure 2), for example - a dielectric and a metal.
|Figure 1. Surface plasmons localized at an isolated nanoparticle. The black arrow indicates the direction of polarization of the incident light (red arrows). The electric field of the light causes separation of positive and negative charges, and the attraction of the charges of opposite signs creates a restoring force, leading to resonant interaction of electron gas with light.||Figure 2. Surface plasmons propagating along e.g. a metal-dielectric interface. The SP wavelength is much shorter than that of the incident light (straight red arrows). This opens up a possibility of controlling electromagnetic energy at optical frequencies (~1015 Hz) with much higher precision than that attainable with the traditional optical elements such as optical fibers, lenses, apertures, modulators, etc.|
Traditionally, gold and silver have been the preferred materials in the design and fabrication of nano-structures intended for supporting and guiding surface plasmons. The reason for this is that plasmon resonance frequencies of these metals lie close to the visible part of the electro-magnetic spectrum, allowing for efficient energy exchange between the surface plasmons and light propagating in free space or a dielectric medium.
An important technological limitation of noble metals, however, is their incompatibility with standard silicon manufacturing processes. This precludes plasmonic devices from being manufactured using well-established nanofabrication technologies and makes their integration with nanoelectronic components difficult. To overcome this limitation, researchers studied numerous alternative materials with different degrees of metallicity. Among such materials are
• metals diluted with nonmetals to produce silicides, oxides or nitrides
• semiconductors doped with metals such as aluminum or gallium.
These materials are already used in silicone CMOS technology, which resolves the above problem, and they have been shown to perform well as plasmonic materials . One of such alternative plasmonic materials is titanium nitride, TiN, which was used in an ingenious way to increase the efficiency of a diamond-based single-photon source, as described below.
Figure 3. An example of a hyperbolic metamaterial. A key factor is that the thickness of individual layers is much smaller than the wavelength of light. As a result, the light experiences average optical parameters of such a medium.
An efficient and stable source of isolated photons is necessary for a number of quantum information transmission and processing applications . A recently established R&D Company - Photonic Nano-Meta Technologies (headquartered in Moscow, Russia; Parent Company - Nano-Meta Technologies, Inc., West Lafayette, IN, USA) - is developing a commercial single-photon source employing Nitrogen-Vacancy (NV) color centers in diamond .
At this stage of development, the techniques for separating a single diamond nano-crystal containing a single NV-center have been developed, while previously the focus of attention was the HMM-based enhancement of spontaneous emission originating from an ensemble of nano-diamonds, each containing, generally, several NV-centers. After the fact of enhanced spontaneous emission by NV-centers through positioning them in the nano-vicinity of an HMM had been established, the main focus had become the demonstration of similar spontaneous emission enhancement of a true single-photon source. In the course of this work, the emission of a single NV-center was studied in detail, including the lifetime of the excited state, the number of photons emitted per unit time and the photons' auto-correlation function. The shape of the latter (Figure 4) proves uncorrelated, single-photon nature of the emitted radiation.
Figure 5 shows the time-dependence of fluorescence intensity for an NV-center excited by a laser pulse for the cases when the nano-diamond containing the NV-center is located on a glass surface (blue curve) and on the surface of an HMM sample (red curve). The data show a four-fold reduction of the excited NV-center's lifetime caused by the HMM. More importantly, the presence of HMM results in a two-fold increase of the rate of emission of photons by the NV-center i.e. in the increase of efficiency of the single-photon source by a factor of two.
Figure 4. Second-order autocorrelation function, g(2)(t), of the photons emitted by a single NV-center when containing it nano-diamond is located in the vicinity of HMM. A significant dip at zero time-delay indicates single-photon character of the emission . The blue curve shows experimental data; the red curve - theoretical fitting results.
Figure 5. Fluorescence decay for a single NV-center contained in a diamond nano-crystal located on a glass surface (blue curve - experimental data) and on TiN-based HMM (red curve - experiment); the black cures present corresponding theoretical fitting results.
These results indicate that single-photon source based on NV-centers in diamond nano-crystals is a viable concept and that the efficiency of such a single-photon source can be enhanced by employing a hyperbolic TiN-based metamaterial. This is a significant step in development of commercial, CMOS-compatible, efficient single-photon source.
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