Asymmetric Diamond Emitters for Unidirectional Photon Coupling

Abstract

The demand of single photon coupling to optical systems requires a suitable interface between negatively charged Nitrogen Vacancy (NV-) emitters in diamond to waveguides or optical circuits for applications in quantum network systems and in unidirectional emission with controlled photon states. Here, we propose a hybrid asymmetric structure of ellipticallyfaceted (ELFA) diamond nanowire with Bragg Grating containing negatively charged NV center for efficient and unidirectional optical coupling to optical nanowire. Our calculations indicate that the structure can provide coupling efficiency of ~90% towards the elliptically facet direction and ~1% towards the opposite direction. Further, Purcell factor is enhanced due to the augmented electric field intensity in the Bragg Grating assisted ELFA structure. Further, we observe higher chirality constant for ELFA+BG structurean indicative of efficient unidirectional photon coupling. By integrating two ELFA diamond nanowire having opposite aligned elliptical facet can be used to design and develop complex optical circuits. Further, this structure can also be used as an inline polarizer filter where it can reflect only one particular mode (e.g. TE mode or TM mode) with higher extinct ratio. The hybrid structure can potentially be used for various applications in quantum photonics, quantum-nonlinear systems. Nitrogen Vacancy (NV-) defect center in diamond is an example of quantum emitters possessing extraordinary optical and magnetic properties [1], such as coherent optical interface between electronic and nuclear spin [2, 3], narrow optical transition [4], and high temporal coherence of paramagnetic electron spin resonance at room temperature [5]. Further, it emits single photons with high photostability through both continuous and pulse excitation. The intrinsic single-photon emission properties of the emitters over a wide temperature range enable them for their wide-field applications in quantum memories [6], magnetometry [7], quantum computing [8], opto-spin based chemical sensing [9], or in general a singlephoton source [10, 11]. Some optical systems such as photonic crystal cavities [12], micro cavities [13], solid immersion lenses [14] are used as interface for photon coupling. Further, processed single mode fiber (SMF) i.e. optical nanowire has also been used to collect single photon from NVcenters in diamond [15,16]. Interfacing of the quantum emitters of chiral molecules or quantum dots for unidirectional light coupling is of significant interest as it provides non-classical mapping of matter state into the optical state [17]. Generally, in those systems, spin-momentum locking between spin of exciton states and angular momentum of light emission enables the spin states to control the propagation direction of light. Similarly, in plasmonic systems also an ideal unidirectional coupler has been designed with a circularly polarized dipole [18]. Further, the free space light has been unidirectionally guided into waveguides using sub-grating technique by implementation of the spin-momentum coupling concept [19]. The unidirectionality concept is needed to encode the information in photon states as qubits and also applied to communication, and complex optical circuits. Here, we theoretically report a unidirectional light coupling of a linearly polarized dipole (NVcenter) emitter embedded in a diamond nanowire coupled to an optical nanofiber. We propose an elliptically faceted (ELFA) diamond nanowire integrated with an optical nanowire in order to achieve enhanced unidirectional light coupling. The ELFA structure exhibits the unidirectional coupling and by incorporating a Bragg grating (BG) in the ELFA (ELFA+BG) structure further enhances the unidirectional coupling efficiency as the spontaneous emission rate of NVcenter increases due to the presence of BG. The ELFA+BG structure shows a maximum unidirectional coupling efficiency of 90 % with significant Purcell factor. Integrating two ELFA diamond nanowire having opposite aligned elliptical facet can be used to design and develop complex optical circuits. Further, this structure can also be used as an inline polarizer filter where it can reflect only one particular mode (e.g. TE mode or TM mode) with higher extinct ratio. The hybrid structure can potentially be used for various applications in quantum photonics, quantum-nonlinear systems [20].