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Advanced Quantum Technologies

Tunable Fano Resonance in Diamond Cavity Coupled Optical Nanowire

Satyajit Murmu, Triranjita Srivastava, Subrat Sahu, Rajan Jha

Published Aug 29, 2025• 5 citations
Publisher

Abstract

Abstract A fiber inline compatible hybrid diamond cavity system manifesting Fano resonance in transmission, as well as emission spectra, is presented. This system comprises a 1D Bragg cavity structured elliptically faceted diamond nanowire coupled to an optical nanowire. The device exhibits ≈47% transmission variation within a wavelength range ≈522 pm with a moderate Q ‐factor ≈1100. The dual supermode‐based photon coupling enables ≈80% photon coupling efficiency. The tunability of the Fano lineshape and the Purcell factor as high as 223 is attained by altering the dipole position of the quantum emitter. Such significantly high Purcell factor results in ≈150 times reduction in the decay time of quantum emitter emission, thereby allowing faster data transmission. It is believed that the proposed hybrid cavity Fano system may provide a new pathway for quantum optical switches, sensors, and applications at a single photon level.

DOI
10.1002/qute.202500408

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Abstract A fiber inline compatible hybrid diamond cavity system manifesting Fano resonance in transmission, as well as emission spectra, is presented. This system comprises a 1D Bragg cavity structured elliptically faceted diamond nanowire coupled to an optical nanowire. The device exhibits ≈47% transmission variation within a wavelength range ≈522 pm with a moderate Q ‐factor ≈1100. The dual supermode‐based photon coupling enables ≈80% photon coupling efficiency. The tunability of the Fano lineshape and the Purcell factor as high as 223 is attained by altering the dipole position of the quantum emitter. Such significantly high Purcell factor results in ≈150 times reduction in the decay time of quantum emitter emission, thereby allowing faster data transmission. It is believed that the proposed hybrid cavity Fano system may provide a new pathway for quantum optical switches, sensors, and applications at a single photon level.
Abstract The directional coupling of single photon emission from nitrogen vacancy (NV – ) emitters to optical systems for various applications requires a suitable interface between NV – emitters and optical waveguides. Unidirectional coupling of photons may have significant importance in next generation quantum technology such as quantum and complex optical networks, quantum communication, and multi‐scale quantum photonics devices including quantum non‐linearity and quantum metrology. Here, a hybrid asymmetric structure of elliptically faceted (ELFA) diamond nanowire with Bragg grating (BG) containing negatively charged NV – center for efficient and unidirectional optical coupling to optical nanowire (waveguide) is proposed. The calculations indicate that the structure can provide coupling efficiency of ≈90% toward the elliptically facet direction and ≈1% in the opposite direction. Further, Purcell factor is enhanced due to the augmented electric field intensity in the ELFA structure. This structure is significantly robust against imperfect placement and rotational alignment of dipole from its intended position.
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 elliptically faceted (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 structure -- an 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.
Tunable cavities have proven to be highly attractive systems in cavity quantum electrodynamics thanks to their performance and flexibility. The possibility to form a cavity around any emitter while simultaneously spectrally matching the chosen transitions makes these cavities an important tool in photonic quantum technology. In this paper, we report on the experimental realization and theoretical description of a fiber-based resonator with two spatially and spectrally distinct cavity modes. The careful design of the structures was performed via finite-element simulations. Thanks to the intrinsic tunability of the system, one mode can be brought in resonance with the second one, forming a supermode resulting in a hybridized two-mode pattern in the emission spectrum. For its realization, we combine a monolithic bottom cavity, formed by two distributed Bragg reflectors, with a top movable fiber mirror forming an externally tunable cavity mode. When tuning the top cavity in resonance with the monolithic bottom one, the cavity modes exhibit a pronounced anticrossing behavior typical for mode hybridization. Differently from a standard open cavity, when embedding single emitters---in this case, InGaAs quantum dots---in the structure, the Purcell factor is not uniform for each wavelength. Furthermore, we find a strong influence of the simulated Purcell factor, as well as of the measured light extraction, depending on the placement of the emitter in the top or in the bottom cavity. The discussed two-mode cavity could be employed for simultaneously Purcell-enhancing multiple transitions of an emitter while preserving the single-photon nature of the emitters inside the device.
We propose a geometry-driven strategy for achieving tunable single-photon coupling in an optical nanofiber cavity (ONFC), where the resonance wavelength is controlled by tailoring the optical nanofiber (ONF) diameter and the grating pitch. By adjusting these structural parameters, the cavity response can be systematically tuned without modifying the material platform or by using any external control schemes. There is a wide spectral resonance wavelength variation of 60, 106, and 70 nm that can be achieved by varying the ONF diameter in the range of 150 nm, the grating pitch of 60 nm, and the elongation of 400 µm, respectively. The integration of a quantum emitter (QE) within the cavity region retains the geometry-dependent tunability of the system. This enables controlled spectral alignment between the QE emission wavelength and the cavity resonance. For a typical QE placed on the ONFC operating around 640 nm, it facilitates a strong spontaneous emission enhancement with a Purcell factor of ∼20 and a coupling efficiency of ∼94% into the ONF-guided mode. The scattering-limited cavity performance yields a quality factor, finesse, and one-pass power loss of 2930, 104, and 2.9%. Owing to its geometry-controlled design, the proposed device can be readily adapted to different QE wavelengths through appropriate structural optimization, offering a compact and fiber-integrated platform for quantum information and communication applications.
Nitrogen-Vacancy (NV) centers in diamond are promising solid-state quantum emitters that can be utilized for photonic quantum applications. Various diamond nanophotonic devices have been fabricated for efficient extraction of single photons emitted from NV centers to a single guided mode. However, for constructing scalable quantum networks, further efficient coupling of single photons to a guided mode of a single-mode fiber (SMF) is indispensable and a difficult challenge. Here, we propose a novel efficient hybrid system between an optical nanofiber and a cylindrical-structured diamond nanowire. The maximum coupling efficiency as high as 75% for the sum of both fiber ends is obtained by numerical simulations. The proposed hybrid system will provide a simple and efficient interface between solid-state quantum emitters and a SMF suitable for constructing scalable quantum networks.
We employ a fiber-based optical microcavity with high finesse to study the enhancement of phonon sideband fluorescence of nitrogen-vacancy centers in nanodiamonds. Harnessing the full tunability and open access of the resonator, we explicitly demonstrate the scaling laws of the Purcell enhancement by varying both the mode volume and the quality factor over a large range. While changes in the emission lifetime remain small in the regime of a broadband emitter, we observe an increase of the emission spectral density by up to a factor of 300. This gives a direct measure of the Purcell factor that could be achieved with this resonator and an emitter whose linewidth is narrower than the cavity linewidth. Our results show a method for the realization of wavelength-tunable narrow-band single-photon sources and demonstrate a system that has the potential to reach the strong-coupling regime.