Transverse-mode coupling effects in scanning cavity microscopy

Tunable open-access Fabry-P\'erot microcavities enable the combination of cavity enhancement with high resolution imaging. To assess the limits of this technique originating from background variations, we perform high-finesse scanning cavity microscopy of pristine planar mirrors. We observe spatially localized features of strong cavity transmission reduction for certain cavity mode orders, and periodic background patterns with high spatial frequency. We show in detailed measurements that the localized structures originate from resonant transverse-mode coupling and arise from the topography of the planar mirror surface, in particular its local curvature and gradient. We further examine the background patterns and find that they derive from non-resonant mode coupling, and we attribute it to the micro roughness of the mirror. Our measurements and analysis elucidate the impact of imperfect mirrors and reveal the influence of their microscopic topography. This is crucial for the interpretation of scanning cavity images, and could provide relevant insight for precision applications such as gravitational wave detectors, laser gyroscopes, and reference cavities

Quantitative Determination of the Complex Polarizability of Individual Nanoparticles by Scanning Cavity Microscopy

The complex polarizability describes the complete optical properties of a nanoobject in the Rayleigh limit, including its absorption, scattering, and dispersion. A large range of applications would benefit from the capability to infer the polarizability on a single-particle level; however, it requires two complementary measurements to fully determine this quantity, and the smallness of the signals makes this highly challenging. Here we use signal enhancement in a tunable high finesse fiber cavity and apply noise-rejecting differential measurement techniques to simultaneously obtain the extinction cross section and the dispersion of individual gold nanospheres, which allows us to quantitatively obtain the real and imaginary part of the polarizability with high precision. We achieve a detection limit for extinction cross sections of 1.8 nm2 and for the polarizability of α/ϵ0 = (28 000 + 200i) nm3. Our method opens the way to a full characterization of the optical properties of individual nanosystems, with applications ranging from nanomaterial science to biology

A highly stable and fully tunable open microcavity platform at cryogenic temperatures

Open-access microcavities are a powerful tool to enhance light-matter interactions for solid-state quantum and nano systems and are key to advance applications in quantum technologies. For this purpose, the cavities should simultaneously meet two conflicting requirements - full tunability to cope with spatial and spectral inhomogeneities of a material, and highest stability under operation in a cryogenic environment to maintain resonance conditions. To tackle this challenge, we have developed a fully-tunable, open-access, fiber-based Fabry-P\'erot microcavity platform which can be operated also under increased noise levels in a closed-cycle cryostat. It comprises custom-designed monolithic micro- and nanopositioning elements with up to mm-scale travel range that achieve a passive cavity length stability at low temperature of only 15 pm rms in a closed-cycle cryostat, and 5 pm in a more quiet flow cryostat. This can be further improved by active stabilization, and even higher stability is obtained under direct mechanical contact between the cavity mirrors, yielding 0:8 pm rms during the quiet phase of the closed-cycle cryo cooler. The platform provides operation of cryogenic cavities with high finesse and small mode volume for strong enhancement of light-matter interactions, opening up novel possibilities for experiments with a great variety of quantum and nano materials

Laser written mirror profiles for open-access fiber Fabry-P\'erot microcavities

We demonstrate laser-written concave hemispherical structures produced on the endfacets of optical fibers that serve as mirror substrates for tunable open-access microcavities. We achieve finesse values of up to 250, and a mostly constant performance across the entire stability range. This enables cavity operation also close to the stability limit, where a peak quality factor of $1.5\times 10^4$ is reached. Together with a small mode waist of $2.3\; \mathrm{\mu m}$, the cavity achieves a Purcell factor of $C \sim 2.5$, which is useful for experiments that require good lateral optical access or otherwise large separation of the mirrors. Laser-written mirror profiles can be produced with a tremendous flexibility in shape and on various surfaces, opening new possibilities for microcavities

A Diamond-Photonics Platform Based on Silicon-Vacancy Centers in a Single Crystal Diamond Membrane and a Fiber-Cavity

We realize a potential platform for an efficient spin-photon interface, namely negatively-charged silicon-vacancy centers in a diamond membrane coupled to the mode of a fully-tunable, fiber-based, optical resonator. We demonstrate that introducing the thin ($\sim 200 \, \text{nm}$), single crystal diamond membrane into the mode of the resonator does not change the cavity properties, which is one of the crucial points for an efficient spin-photon interface. In particular, we observe constantly high Finesse values of up to $3000$ and a linear dispersion in the presence of the membrane. We observe cavity-coupled fluorescence froman ensemble of SiV$^{-}$ centers with an enhancement factor of $\sim 1.9$. Furthermore from our investigations we extract the ensemble absorption and extrapolate an absorption cross section of $(2.9 \, \pm \, 2) \, \cdot \, 10^{-12} \, \text{cm}^{2}$ for a single SiV$^{-}$ center, much higher than previously reported

Cavity-enhanced spectroscopy of a few-ion ensemble in Eu3+:Y2O3

We report on the coupling of the emission from a single europium-doped nanocrystal to a fiber-based microcavity under cryogenic conditions. As a first step, we study the sample properties and observe a strong correlation between emission lifetime and brightness, as well as a lifetime reduction for nanocrystals embedded in a polymer film. This is explained by differences in the local density of states. We furthermore quantify the scattering loss of a nanocrystal inside the cavity and use this to deduce the crystal size. Finally, by resonantly coupling the cavity to a selected transition, we perform cavity-enhanced spectroscopy to measure the inhomogeneous linewidth, and detect the fluorescence from an ensemble of few ions in the regime of power broadening. We observe an increased fluorescence rate consistent with Purcell enhancement. The results represent an important step towards the efficient readout of single rare-earth ions with excellent optical and spin coherence properties, which is promising for applications in quantum communication and distributed quantum computation