Boron Nitride Membranes Unlock New Photonics Potential

The quest to integrate atomically thin materials into optical devices has taken a significant step forward with research focused on hexagonal boron nitride (hBN). Patrick Maier, Alexander Kubanek, and colleagues at Ulm University have developed a method to isolate and transfer membrane-like flakes of hBN, which naturally host single-defect centers emitting single photons. This achievement addresses a key challenge in the field, as commercially available hBN flakes, while promising, suffer from surface irregularities that cause light scattering and hinder integration into sensitive optical resonators. By carefully extracting these membranes and coupling a single photon emitter to an open optical cavity, the team observes a dramatic enhancement of the emitted light, up to 100 times stronger at room temperature, opening new possibilities for advanced optomechanical systems and quantum photonics.

HBN and Diamond Defect-Based Single Photon Sources

Research focuses on harnessing defect centers within hexagonal boron nitride (hBN) and diamond to create single-photon emitters, crucial components for quantum technologies. Investigations center on identifying and controlling these defects to produce stable and efficient light sources, with diamond, specifically containing Nitrogen-Vacancy (NV) and Silicon-Vacancy (SiV) centers, serving as a comparative material. Researchers enhance the performance of these emitters by integrating them with optical cavities, utilizing techniques like cavity quantum electrodynamics and exploring diverse cavity designs, including fiber, photonic crystal, and microcavities, to maximize light emission and control its direction. Scanning cavity microscopy maps the interaction between emitters and cavities, while the Purcell effect increases the rate of light emission.

A key goal is to create indistinguishable photons, possessing identical properties, for applications in quantum information processing. Researchers also investigate coupling these emitters to nanomechanical resonators, such as membranes and beams, to control their energy levels through mechanical strain, opening possibilities for quantum sensing and information processing. Understanding and controlling the vibrational modes of the material is crucial for optimizing coupling and minimizing unwanted interactions that degrade performance. Techniques like exfoliation and transfer obtain high-quality hBN and diamond membranes, while scanning probe microscopy characterizes materials and maps emitter locations.

Ultimately, this research aims to develop bright, efficient, and indistinguishable single-photon sources for quantum cryptography, computing, and sensing. Utilizing emitters coupled to nanomechanical resonators promises new ways to detect weak forces and magnetic fields. The ability to control decoherence, the loss of quantum information, is paramount for building robust quantum technologies. Recognizing that commercially available hBN flakes often contain imperfections that scatter light, the team prioritized techniques for extracting pristine, membrane-like structures. This process begins with identifying promising hBN particles containing emitters using a confocal microscope, followed by delicate manipulation with a tungsten tip that physically lifts the desired hBN structure from its original substrate and transfers it onto a macroscopic mirror, effectively creating a free-standing membrane. Refined through observation with both optical and electron microscopes, this process overcomes the challenges of handling such fragile materials and preserves the embedded light-emitting defects.

A key innovation lies in the design of an open Fabry-Perot fiber cavity (FPFC) used to enhance the emitted light, consisting of a microscopic curved mirror at the end of an optical fiber facing a macroscopic planar mirror onto which the transferred hBN membranes are placed. The fiber mirror is meticulously crafted using focused ion beam milling and laser smoothing to achieve optimal curvature and minimize imperfections, further enhanced by a dielectric coating to maximize light confinement and amplification. The entire system is housed within a vacuum chamber and vibration isolation stage, minimizing external disturbances that could degrade the signal. To precisely align the hBN emitter with the cavity mode, researchers implemented a scanning cavity microscope utilizing piezo-driven actuators to move the planar mirror with nanometer precision.

By illuminating the system and observing the resulting signal, they identify regions with minimal scattering, indicating a high-quality membrane. Once a promising particle is identified, active stabilization of the resonator using a Pound-Drever-Hall locking scheme ensures a stable and consistent signal. The team then characterized the coupled system by carefully overlapping the emitter’s emission wavelength with a resonant mode of the cavity, demonstrating a significant enhancement of the emitted light, up to 100-fold at room temperature, and confirming the successful integration of the hBN emitter into the photonic device.

Room Temperature Brightness Enhancement in Boron Nitride

Researchers have successfully integrated a nanoscale light source, a defect within a hexagonal boron nitride (hBN) membrane, with an optical cavity to dramatically enhance its light emission. This achievement overcomes significant challenges in manipulating and positioning these delicate membranes, which naturally host these light-emitting defects. The team developed techniques to extract membrane-like structures of hBN containing single photon emitters and carefully couple them to a specifically designed fiber optic cavity, demonstrating a remarkable 100-fold increase in the brightness of the light emitted by the defect, achieved by funneling the light into the cavity mode. This enhancement is particularly noteworthy because it occurs at room temperature, simplifying potential applications.

Furthermore, the cavity narrowed the emission spectrum of the light source by a factor of 160, indicating a significant improvement in the purity of the emitted light. Detailed analysis confirms the single-photon nature of the emitted light, crucial for secure communication and quantum technologies. The lifetime of the emitted photons remained comparable to that of the free-standing emitter, indicating the cavity integration did not significantly degrade the light source’s performance. Measurements of the cavity’s finesse reveal that the hBN membrane introduces some losses, but these are now understood and could be further minimized. This work paves the way for creating efficient and compact optomechanical systems and for developing advanced quantum light sources with improved brightness and spectral purity.

HBN Integration Boosts Quantum Emitter Performance

Researchers have developed nanomanipulation techniques to isolate and transfer thin hexagonal boron nitride (hBN) membranes containing single photon emitters into optical devices. These techniques successfully reduce scattering losses typically associated with hBN flakes, enabling strong coupling of a single photon emitter within the membrane to an open Fabry-Perot fiber cavity. This integration results in significant spectral enhancement, with observed narrowing of the emission spectrum by a factor of 160 and an increase in spectral density up to 100-fold compared to measurements taken without the cavity. These findings represent a step towards integrating pre-selected quantum emitters within hBN into practical quantum photonic devices and lay the groundwork for future optomechanical experiments.