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Nanoscale and Quantum Materials: From Synthesis and Laser Processing to Applications

Laser written nitrogen vacancy centers in diamond integrated with transfer print GaN solid immersion lenses

Xingrui Cheng, Nils Kolja Wessling, Saptarsi Ghosh, Andrew R. Kirkpatrick, Menno J. Kappers, Yashna Lekhai, Gavin W. Morley, Rachel A. Oliver, Martin D. Dawson, Jason M. Smith, Patrick S. Salter, Michael John Strain

Published Mar 12, 2024• 0 citations
Publisher

Abstract

Laser-written Nitrogen Vacancy (NV−) centers are combined with transfer-printed GaN micro-lenses to increase fluorescent light collection by reducing total internal reflection at the planar diamond interface. We find a 2x improvement of fluorescent light collection using a 0.95 NA air objective at room temperature, in agreement with FDTD simulations. The nature of the transfer print micro-lenses leads to better performance with lower Numerical Aperture (NA) collection, as confirmed by results with a 0.5NA air objective which show improvement greater than 5x. The approach is attractive for scalable integrated quantum technologies.

DOI
10.1117/12.3000474

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Abstract Solid‐state quantum emitters, such as nitrogen‐vacancy (NV) centers in diamond, offer a promising route toward scalable quantum technologies. However, their random spatial distribution and inherently broad dipole emission severely hinder high‐purity single‐photon extraction—particularly in low‐numerical‐aperture (NA) optical systems, which are essential for compact and scalable quantum photonic systems. Here, a defect‐selective metalens integration approach is presented that allows precise and efficient single photon collection from individual NV centers, even under ultra‐low‐NA optical systems. Using an in situ transfer‐printing process, high‐purity silicon dioxide metalenses are stamped precisely and deterministically onto selected NV centers located 25 µm below the diamond surface. This integration reshapes the broad dipolar emission of the targeted NV center into a tightly collimated, low‐divergence beam, achieving a 40‐fold enhancement in photon collection efficiency with an objective lens of NA = 0.07. Notably, emission is detected exclusively from metalens‐coupled NV centers, effectively filtering out background noise from neighboring defects. Second‐order correlation measurements yield g⁽ 2 ⁾(0) = 0.04, unambiguously confirming the generation of high‐purity single photons. This scalable, site‐specific approach addresses key limitations in low‐NA operation, opening the door to compact and fiber‐integrated solid‐state quantum photonics.

Small•Nov 2, 2025•6 citations
Laser-written Nitrogen Vacancy (NV−) centers are combined with transfer-printed GaN micro-lenses to increase fluorescent light collection by reducing total internal reflection at the planar diamond interface. We find a 2x improvement of fluorescent light collection using a 0.95 NA air objective at room temperature, in agreement with FDTD simulations. The nature of the transfer print micro-lenses leads to better performance with lower Numerical Aperture (NA) collection, as confirmed by results with a 0.5NA air objective which show improvement greater than 5x. The approach is attractive for scalable integrated quantum technologies.
Nitrogen vacancy centres in diamond have shown strong potential for use as qubits in quantum computing. However, nitrogen vacancy centres have a low percentage of overall emission emitted through the Zero Phonon Line (ZPL). In addition, the fabrication of NV centres with controlled positioning is limited. This thesis presents the ZPL linewidth characterisation of NV centres fabricated using laser writing and laser diffusion. Only one out of the six NV centres characterised showed a <100 MHz average single scan linewidth, which suggests that the laser diffusion process needs to improved. The low ZPL emission can be enhanced by cavity coupling to Fabry-Pérot microcavities. Fabry-Pérot microcavities are susceptible to vibrations, which will decouple the cavity from the NV centre and reduce the photon enhancement. A cavity stabilisation system based on cavity fringe stabilisation is implemented, and the results of the stability are presented. The Fabry-Pérot microcavity can be stabilised with 0.13 nm using fringe stabilisation. High collection efficiency of the emission is also desired for maximising the entanglement rate and is reduced by optical aberrations. For NV centres in bulk diamond, using Solid Immersion Lenses (SILs) removes the spherical aberration that is introduced when collecting NV emission through a planar interface. This thesis presents the results of modelling the aberrations for NV centres cavity coupled to a Fabry-Pérot microcavity and for hemispherical SILs on diamond. The wavefronts of the aberrations introduced by the planar mirror of the microcavity and the aberrations introduced by the positioning error of the SIL are modelled numerically. A deformable mirror is implemented into a confocal microscope to monitor the aberrations of the photonic devices and the results are compared with the numerical model. For Fabry-Pérot microcavities, the intensity loss caused by the aberrations introduced by the planar mirror was predicted to be less than 10 \% due to the thin mirror substrate and low numerical aperture of the cavity mode. This prediction was consistent with the experimental results where an effective 3 \% increase in light collection was observed with aberration correction. For the SILs, the model predicts that the collection efficiency is more sensitive to the lateral positioning of the SIL relative to the NV centre than the axial positioning error.
The efficiency of photon collection from optically active defect centers in bulk diamond is greatly reduced by refraction at the diamond-air interface. We report on the fabrication and measurement of a geometrical solution to the problem; integrated solid immersion lenses (SILs) etched directly into the surface of diamond. An increase of a factor of 10 was observed in the saturated count-rate from a single negatively charged nitrogen-vacancy (NV−) within a 5 μm diameter SIL compared with NV−’s under a planar surface in the same crystal. Such a system is potentially scalable and easily adaptable to other defect centers in bulk diamond.
The negatively charged nitrogen-vacancy (NV−\mathrm{NV}^-NV−) defect in diamond can be used for quantum information processing via the readout of the radiative decay of the excited state into the ground state. The large refractive index mismatch of diamond and air causes low photon extraction efficiency due to total internal reflection losses. Here, we investigate the photon extraction numerically and report additive solid immersion lenses (SILs) consisting of the high-k dielectric yttria-stabilized-zirconia (YSZ) for surface modification and enhanced light extraction of shallow implanted nitrogen-vacancy centers. We demonstrate an average increase of 63% in saturation intensity using a scalable bottum up room temperature pulsed laser deposition and photolithography structurization process. The average necessary laser intensity for saturation is reduced by 33% by the SILs.
We report experimental studies of microsphere-enhanced fluorescence collection of nitrogen vacancy (NV) centers using silica microspheres with diameters ranging between 15 and 50 μm and employing 20× and 40× objectives with numerical aperture of 0.42 and 0.64, respectively. Photoluminescence-excitation saturation counts as high as 95 kHz have been observed. These studies show that due to the effective collimation of fluorescence by the microsphere, objectives with relatively low numerical aperture (NA) can be used without sacrificing collection enhancement, in agreement with a theoretical model based on Mie scattering. The large enhancement of fluorescence collection with relatively low NA objectives, which feature extralong working distance and are relatively inexpensive, can potentially enable wider use of NV-based quantum sensing in real world applications.
Single Nitrogen-Vacancy centers (NV-centers) in diamond can be operated at ambient conditions as qubits in quantum memories and nanoscale quantum sensors. At TH Nürnberg, we plan to deterministically generate single NV-centers via femtosecond-laser pulses and integrate them into photonic platforms via 3D-printed microoptics and fs-written waveguides. This presentation gives a summary about our activites: The design and analysis of micro-lens systems to be manufactured via Two-Photon Polymerization on top of individual, self-written NV-centers for increased photon-collection efficiency, the programming of a spatial light modulator for wavefront optimization, the setup of a confocal scanning fluorescence microscope that addresses individual NV-Centers and laser material processing that allows to shape the diamond surface and to fabricate waveguides in the diamond for efficient optical coupling.
Abstract Solid‐state quantum emitters, such as nitrogen‐vacancy (NV) centers in diamond, offer a promising route toward scalable quantum technologies. However, their random spatial distribution and inherently broad dipole emission severely hinder high‐purity single‐photon extraction—particularly in low‐numerical‐aperture (NA) optical systems, which are essential for compact and scalable quantum photonic systems. Here, a defect‐selective metalens integration approach is presented that allows precise and efficient single photon collection from individual NV centers, even under ultra‐low‐NA optical systems. Using an in situ transfer‐printing process, high‐purity silicon dioxide metalenses are stamped precisely and deterministically onto selected NV centers located 25 µm below the diamond surface. This integration reshapes the broad dipolar emission of the targeted NV center into a tightly collimated, low‐divergence beam, achieving a 40‐fold enhancement in photon collection efficiency with an objective lens of NA = 0.07. Notably, emission is detected exclusively from metalens‐coupled NV centers, effectively filtering out background noise from neighboring defects. Second‐order correlation measurements yield g⁽ 2 ⁾(0) = 0.04, unambiguously confirming the generation of high‐purity single photons. This scalable, site‐specific approach addresses key limitations in low‐NA operation, opening the door to compact and fiber‐integrated solid‐state quantum photonics.