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Physical review. B./Physical review. B

Realization of a tunable fiber-based double cavity system

T. Herzog, Sascha Böhrkircher, Steffen Both, Matthias Fischer, Robert Sittig, Michael Jetter, Simone Luca Portalupi, T. Weiss, Peter Michler

Published Dec 30, 2020• 5 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.102.235306

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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.
Abstract Cavity quantum electrodynamics is widely used in many solid-state systems for improving quantum emitter performances or accessing specific physical regimes. For these purposes it is fundamental that the non-classical emitter, like a quantum dot or an NV center, matches the cavity mode, both spatially and spectrally. In the present work, we couple single photons stemming from In(Ga)As quantum dots into an open fiber-based Fabry–Pérot cavity. Such a system allows for reaching an optimal spatial and spectral matching for every present emitter and every optical transition, by precisely tuning the cavity geometry. In addition to that, the capability of deterministically and repeatedly locating a single quantum dot enables to compare the behavior of the quantum emitter inside the cavity with respect to before it is placed inside. The presented open-cavity system shows full flexibility by precisely tuning in resonance different QD transitions, namely excitons, biexcitons and trions. A measured Purcell enhancement of 4.4 ± 0.5 is obtained with a cavity finesse of about 140, while still demonstrating a single-photon source with vanishing multi-photon emission probability.
We demonstrate non-perturbative coupling between a single selfassembled InGaAs quantum dot and an external fiber-mirror-based microcavity.Our results extend the previous realizations of tunable microcavities while ensuring spatial and spectral overlap between the cavity mode and the emitter by simultaneously allowing for deterministic charge control of the quantum dots.Using resonant spectroscopy, we show that the coupled quantum dot cavity system is at the onset of strong coupling, with a cooperativity parameter of C ≈ 2.0 ± 1.3.Our results constitute a milestone in the progress toward the realization of a high-efficiency solid-state spin-photon interface.
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.
Open microcavities offer great potential for the exploration and utilization of efficient spin-photon interfaces with Purcell-enhanced quantum emitters thanks to their large spectral and spatial tunability combined with high versatility of sample integration. However, a major challenge for this platform is the sensitivity to cavity length fluctuations in the cryogenic environment, which leads to cavity resonance frequency variations and thereby a lowered averaged Purcell enhancement. This work presents a closed-cycle cryogenic fiber-based microcavity setup, which is in particular designed for a low passive vibration level, while still providing large tunability and flexibility in fiber and sample integration, and high photon collection efficiency from the cavity mode. At temperatures below 10 K, a stability level of around 25 pm is reproducibly achieved in different setup configurations, including the extension with microwave control for manipulating the spin of cavity-coupled quantum emitters, enabling a bright photonic interface with optically active qubits.
Fiber Fabry-Perot cavities, formed by micro-machined mirrors on the end-facets of optical fibers, are used in an increasing number of technical and scientific applications. Some of the most promising areas of application of these optical micro-resonators with high finesse and small mode volume are in the field of quantum communication and information. The resonator-enhanced light-matter interaction, for instance, provide basis for the realization of efficient optical interfaces between stationary matter-based quantum nodes and flying single-photon qubits. To date fiber Fabry-Perot cavities have been successfully applied in experiments interfacing single photons with a wide range of quantum systems, including cold atoms, ions and solid state emitters as well as quantum optomechanical experiments. Here we address some important practical questions that arise during the experimental implementation of high finesse fiber Fabry-Perot cavities: How can optimal fiber cavity alignment be achieved and how can the efficiency of coupling light from the optical fibers to the cavity mode and vice versa be characterized? How should optical fiber cavities be constructed and stabilized to fulfill their potential for miniaturization and integration into robust scientific and technological devices that can operate outside of dedicated laboratory environments in the future? The first two questions we answer with an analytic mode matching calculation that relates the alignment dependent fiber-to-cavity mode-matching efficiency to the easily measurable dip in the reflected light power at the cavity resonance. Our general analysis provides a simple recipe for the optimal alignment of fiber Fabry-Perot cavities and moreover for the first time explains the asymmetry in their reflective line shapes. The latter question we explore by investigating a novel, intrinsically rigid fiber cavity design that makes use of the high passive stability of a monolithic cavity spacer and employs thermal self-locking and external temperature tuning. The performance of this novel approach is compared to a more traditional, piezomechanically-actuated resonator that is feedback stabilized to a reference laser based on the Pound-Drever-Hall locking technique. Finally, with the application of single photon interfacing in mind we discuss the issue of fiber-generated background photons in Fiber Fabry-Perot cavities. We believe that, due to their simplicity, compactness and robustness, rigid fiber cavities could be a promising solution for miniaturized, integratable and scalable cavity quantum electrodynamic devices and further applications of FFPC with similar requirements.
A cavity quantum electrodynamics (cQED) system consisting of a confined single photon and a single quantum emitter serves as a fundamental block for quantum optics and photonic quantum technologies. The canonical optical mode employed in the conventional cavity quantum electrodynamics features a uniform polarization distribution, leading to the scalar light-matter interaction in most existing experiments. Despite the rapid progress in the generation of structured light with spatially varied polarizations, the structured light-matter interaction, especially at the single quanta level, is highly intriguing yet largely unexplored. Here, we present the structured light-matter interaction at the single-photon level in a semiconductor cavity quantum electrodynamics system. Four distinct structured cavity modes that are spectrally close to each other are constructed in a micropillar cavity. By spatially locating a single epitaxial quantum dot (QD) at the periphery of a semiconductor micropillar cavity and spectrally tuning the QD emission wavelength into the resonances of the structured cavity modes, cavity-enhanced single-photon emissions with spin-locked chiral orbital angular momentum (OAM) and engineerable spin-orbit entanglements are achieved within a single wavelength-scale device. Our work opens an unexplored paradigm of structured quantum light-matter interactions and may further advance chiral quantum optics and high-dimensional photonic quantum technology.