The burgeoning field of nanophotonics has grown to be a major research area, primarily because of the ability to control and manipulate single quantum systems (emitters) and single photons on demand. For many years studying nanophotonic phenomena was limited to traditional semiconductors (including silicon and GaAs) and experiments were carried out predominantly at cryogenic temperatures. In the last decade, however, diamond has emerged as a new contender to study photonic phenomena at the nanoscale. Offering plethora of quantum emitters that are optically active at room temperature and ambient conditions, diamond has been exploited to demonstrate super-resolution microscopy and realize entanglement, Purcell enhancement and other quantum and classical nanophotonic effects. Elucidati>ing the importance of diamond as a material, this review will highlight the recent achievements in the field of diamond nanophotonics, and convey a roadmap for future experiments and technological advancements.
Examples of hybrid approaches to diamond nanophotonics: (a) Silver Nanowires as plasmonic waveguides and beam splitter for single photons: an NV center is placed in close proximity to a silver nanowire (sample topography see upper panel). Light is guided by the nanowire as evident from the fluorescence image in the lower panel: fluorescence is not only recorded from the position of the NV center, but also from the ends of the wire (points B and C). Furthermore, the beam is split as evident by light emission from the second wire´s end (point D) (b)-(d) three-dimensional photonic structures produced via direct laser writing into photoresist containing nanodiamonds. (b) shows a scanning electron microscopy image of a disc resonator and an arc waveguide. Length of scale bars is 5 µm. (c) Fluorescence characterization of the device: the excitation laser spot is scanned across the microsdisc resonator, simultaneously; the fluorescence photons are collected at one end of the waveguide. The position of a single NV center is highlighted with a dashed circle. (d) a corresponding autocorrelation measurement is shown, the strong antibunching around zero delay confirms that a single NV center is addressed.
Optically active impurities in diamond, so called color centers, have attracted intense research interest in recent years. The success of these emitters builds upon their unique properties that include highly stable fluorescence (photostability) of single color centers even at room temperature and feasible control of single, highly coherent spins associated with color centers in diamond. Over 500 different color centers in diamond are known; their emission wavelengths span a spectral range from the ultraviolet to the near infrared. Furthermore, the diamond host material provides beneficial qualities like chemical inertness, biocompatibility, high transparency from the ultraviolet to infrared spectral range as well as a high mechanical strength (high Young`s modulus) and exceptionally high thermal conductivity. Furthermore, color centers in diamond have to be considered as potential building blocks of future quantum information processing (QIP) architectures and integrated nanophotonic devices as detailed in the following.
Single quanta of light, single photons, are the fundamental constituents of QIP. First, single photons can be utilized to encode and transmit information in a way that the laws of quantum mechanics ensure a secure exchange of information (quantum cryptography, for review see). Second, photons can serve as fast carriers of quantum information connecting distant nodes in so called quantum networks. In a quantum network, long lived quantum bits (qubits) store quantum information in so called quantum nodes and the information is exchanged via flying qubits, photons. Furthermore, in all optical quantum computing schemes, indistinguishable photons are proposed as building blocks of a quantum computer that could potentially outperform classical computers.
Conclusions and Outlook
Despite the challenging nanofabrication of diamond, the progress in diamond nanophotonics has been extremely rapid. In less than a decade, scientists and engineers managed to transform basic polycrystalline devices into high quality photonic crystal cavities suitable for enhancing and studying light matter interactions. New applications are constantly emerging a very recent one being a realization of a nonlinear photonics platform from diamond, enabled due to the sophisticated cavity fabrication]. The time is ripe, therefore, for more detailed and dedicated advanced quantum optical experiments with diamond. These should include proper integrated photonic networks, full exploration of weak coupling and demonstration of strong coupling between a single emitter and a cavity field and multi qubit entanglement. Finally, a deterministic placement of an emitter within the cavity field should be routinely established, either through curving a cavity around a predefined emitter, or a post processing using deterministic chemical or physical doping. The enormous progress toward entangling distant NV centers, strong coupling between the NV center and a superconducting flux qubit, improvements in quantum error corrections in conjunction with better optical resonators may (should) result in the demonstration of a solid state, quantum computer in the near future. In the meantime, applications such as quantum key distribution should not be forgotten.
Novel nanofabrication techniques for diamond, in addition to the fabrication methods in this review.