Diamond nanotechnology

Diamond has many material qualities that make it attractive for the diverse palette of nanoscale science and engineering. Applications in electronics, for example, benefit from its high thermal conductivity, low thermal-expansion coefficient, large breakdown field, and high carrier mobility.¹ It can be both n- and p-type doped, thus allowing realization of optoelectronic devices.² Diamond is also an ultrahard material with a large Young's modulus,³ which is advantageous for high-frequency nanoscale electro- and optomechanical systems. Photonics devices benefit from its large refractive index (n=/2.4), large band gap (5.5eV), and wide transparency window (from UV to IR). Moreover, active optical devices can be created using its vast inventory of luminescent defects (color centers).⁴ As a member of the carbon family, diamond-device platforms are naturally extended to biological and chemical contexts.

One exciting application at the forefront of diamond research is in quantum science. Nonclassical (single-photon) light sources based on individual color centers in diamond, most notably the nitrogen-vacancy (NV) center,⁵ have been used for secure communication based on quantum-cryptography protocols.^5,6 Coupling between the NV center's electronic spin and nearby nuclear spins can be used to form a large qubit register,^7,8 an essential ingredient for a quantum computer. Recently, techniques designed to manipulate the NV center have been applied to nanoscale magnetic-field sensing based on single spins.^9,10 But practical implementations of these technologies require efficient excitation and extraction of single photons from NV centers using a simple optical system. This is a challenge because of the high refractive index of the diamond host, so that the majority of photons emitted from an embedded color center are not accessible even to sophisticated setups.

To overcome this obstacle, we have been developing a diamond-nanophotonics platform that can provide an efficient interface between the atomic-scale NV center and the macroscopic optical system. We recently demonstrated a single-photon source based on a diamond nanowire^11,12 fabricated directly into a single-crystal diamond substrate. (High-quality synthetic diamond samples are now available from companies such as E6 Technologies, Apollo Diamond, and Sumitomo Electric. Unfortunately, optically thin single-crystal diamond films on low-index or sacrificial substrates, needed for realization of high-quality-factor optical resonators, are not yet available.) The diamond nanowire with an embedded NV center acts as an antenna that enables efficient incoupling of the pump power used to drive the NV center's optical transition, as well as efficient outcoupling of emitted photons to an objective lens: see Figure 1(a). The diamond nanowires, ~2μm long and ~200nm in diameter, are fabricated from type Ib diamond (which contains randomly distributed NV centers) using electron-beam lithography and reactive ion etching: see Figure 1(b).

Figure 1. (a) Scanning-electron-microscope (SEM) image of a typical diamond nanowire device. Finite-difference time-domain simulation of the E_r component of dipole emission at 637nm wavelength shows directional emission towards a microscope objective located above the nanowire when the nitrogen-vacancy (NV) center is polarized horizontally and located in the center of the device. Inset: Energy-level diagram of an NV center. (b) SEM image of a large array of nanowire devices.

Applications of our devices in quantum science require that the nanowires contain a single NV center. This can be verified by measuring the intensity (I) autocorrelation function g⁽²⁾(τ) = 〈I(t)I(t+τ)〉/〈I(t)〉² of the nanowire fluorescence as a function of time, t (τ is the delay time), in a Hanbury Brown and Twiss experiment: see Figure 2(a). An individual NV can emit only one photon at a time and would be characterized by g⁽²⁾(0)=/0 in an ideal case, or g⁽²⁾(0)</0.5 if background fluorescence is nonnegligible. We observe strong photon antibunching—g⁽²⁾(0)~0.14— for our diamond nanowires, indicating single-photon emission from an individual color center embedded within a single nanowire, as shown in Figure 2(b).

Figure 2. (a) Hanbury Brown and Twiss experiment used to measure the intensity autocorrelation function g⁽²⁾(τ)as a function of delay time τ. NV-center emission (not shown) from 650–800nm is collected. (b) Representative g⁽²⁾(τ)data shows strong photon antibunching in the fluorescence of a single NV center in a nanowire. APD: Avalanche photodiode detector.

A critical figure of merit for the single-photon source is the overall photon flux accessible to an external optical system. To investigate the advantages offered by the nanowire-antenna geometry, we compared single-photon sources based on NV centers in both bulk diamond crystals and nanowires. For each device, we fitted the number of collected photons I as a function of pump power P to a curve of the form I(P)=/P×CPS_SAT/(P+P_SAT): see Figure 3(a). We found—see Figure 3(b)—that the nanowire geometry allows for an order of magnitude increase in the number of single photons that can be collected at saturation (CPS_SAT), using an order of magnitude less pump power (P_SAT). These results are in excellent agreement with our theoretical predictions.

Figure 3. (a) Number of single-photon counts per second measured from an individual NV center in a bulk diamond crystal (blue) and diamond nanowire (red) as a function of pump-laser power. (b) Characterization of single-photon device parameters unambiguously shows an order-of-magnitude improvement in light incoupling (Xaxis) and outcoupling (Y axis) from a single NV center. Sat.: Saturation.

The diamond-nanowire antenna provides a natural and efficient interface for an individual color center and significantly increases the collection efficiency of emitted photons. However, we are working on further improvements. For example, coupling emission directly to an optical fiber could realize more compact systems with larger overall photon-extraction efficiencies. Photon flux can further be improved by increasing the photon-generation rate (Purcell effect) using plasmonic nanostructures or optical cavities fabricated directly in diamond. We believe that these developments will enable realization of large-scale quantum-information-processing systems as well as sensitive magnetometers based on nanostructured diamonds.