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arXiv:2404.02563 · Physical Review A

Probing microcavity resonance spectra with intracavity emitters

C. Koks, Martin P. van Exter

Published Apr 23, 2024• 1 citation
arXivHTMLPDF

Abstract

We measure the fluorescence spectrum of broadband emitters in an open optical microcavity with radius of curvature $R=17.7(3)\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}$ and finesse $F\ensuremath{\approx}1000$. This geometry enables a combined measurement of emission spectra versus cavity length, which has several benefits over measurements at fixed wavelength or fixed cavity length alone. We demonstrate the role of the optical penetration depths on the cavity modes and provide practical working equations for its analysis. Furthermore, we show the ability to measure the coupling of cavity modes within a small scan range of the cavity length. By measuring these cavity emission spectra as a function of cavity length, we thus obtain a rich and complete picture of the optical microcavity.

DOI
10.1103/physreva.109.043523

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•Jan 1, 2017•0 citations
We measure the fluorescence spectrum of broadband emitters in an open optical microcavity with radius of curvature R=17.7(3)\textmumR=17.7(3)\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}R=17.7(3)\textmum and finesse F\ensuremath≈1000F\ensuremath{\approx}1000F\ensuremath≈1000. This geometry enables a combined measurement of emission spectra versus cavity length, which has several benefits over measurements at fixed wavelength or fixed cavity length alone. We demonstrate the role of the optical penetration depths on the cavity modes and provide practical working equations for its analysis. Furthermore, we show the ability to measure the coupling of cavity modes within a small scan range of the cavity length. By measuring these cavity emission spectra as a function of cavity length, we thus obtain a rich and complete picture of the optical microcavity.
Optical microcavities are structures which confine light to a small region in the range of one wavelength. The radiation of a quantum emitter is coupled to cavity resonances which leads to an optical confinement of the broadband fluorescence. A practical design for this single-mode microcavity is formed by two silver mirrors enclosing a transparent dielectric medium with single quantum emitters. Steiner et al. have shown that the fluorescence spectra and decay lifetimes of single molecules in this Fabry-Perot type microcavity are strongly dependent on the resonator length.
Open Fabry-Perot microcavities represent a promising route for achieving a quantum electrodynamics (cavity-QED) platform with diamond-based emitters. In particular, they offer the opportunity to introduce high-purity, minimally fabricated material into a tunable, high quality factor optical resonator. Here, we demonstrate a fiber-based microcavity incorporating a thick ($>10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\text{m}$) diamond membrane with a finesse of 17 000, corresponding to a quality factor $Q\ensuremath{\sim}{10}^{6}$. Such minimally fabricated thick samples can contain optically stable emitters similar to those found in bulk diamond. We observe modified microcavity spectra in the presence of the membrane, and we develop analytic and numerical models to describe the effect of the membrane on cavity modes, including loss and coupling to higher-order transverse modes. We estimate that a Purcell enhancement of approximately 20 should be possible for emitters within the diamond in this device, and we provide evidence that better diamond surface treatments and mirror coatings could increase this value to 200 in a realistic system.
We investigate experimentally and theoretically the fluorescence emitted by molecular ensembles as well as spatially isolated, single molecules of an organic dye immobilized in a quasi-planar optical microresonator at room temperature. The optically excited dipole emitters couple simultaneously to on- and off-axis cavity resonances of the microresonator. The multi-spectral radiative contributions are strongly modified with respect to free (non-confined) space due to enhancement and inhibition of the molecular spontaneous emission (SpE) rate. By varying the mirror spacing of the microresonator on the nanometer-scale, the SpE rate of the cavity-confined molecules and, consequently, the spectral line width of the microresonator-controlled broadband fluorescence can be tuned by up to one order of magnitude. Stepwise reducing the optical confinement, we observe that the microresonator-controlled molecular fluorescence line shape converges towards the measured fluorescence line shape in free space. Our results are important for research on and application of broadband emitters in nano-optics and -photonics as well as microcavity-enhanced (single molecule) spectroscopy.
The standard theory of resonance fluorescence is a weak-coupling theory.1 If n ¯ is the mean photon number for the driving field, g is the atom field coupling constant, and γ is the Einstein A coefficient, formally the standard theory of resonance fluorescence assumes the limit n ¯ → ∞ , g/ γ → 0, with the ratio of Rabi frequency to atomic linewidth 2 n ¯ n g / γ finite. The limit g/γ → 0 allows the variation of the Rabi frequency 2 n g across the photon number distribution of the driving field to be neglected. By placing the atom inside an optical cavity the ratio g/γ can be changed. In this paper we calculate spectra for intracavity resonance fluorescence in the strong-coupling limit g/γ ≫ 1, g/κ ≫ 1, where κ is the cavity linewidth. We consider a single two-level atom interacting on resonance with a single cavity mode driven by a resonant coherent field. The cavity subtends a small solid angle at the atom, so that the spontaneous emission into free-space modes is not negligible. We calculate the spectrum of the light transmitted by the cavity (the transmitted spectrum) and the spectrum of the light radiated by the atom into free space (the fluorescent spectrum). The transmitted and fluorescent spectra show fundamental differences from the (weak-coupling) spectrum of free-space resonance fluorescence.
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.
This thesis investigates the coupling of the fluorescence of nitrogen-vacancy (NV) centers in diamond to tunable optical microresonators at ambient conditions, in particular in the regime of Purcell enhancement. We use fiber-based, open-access Fabry-Pérot cavities optimized for high finesse and ultra-small mode volume. Different regimes of cavity enhancement are studied that are complementary to each other: A first experiment relies on a high-finesse cavity with dielectric mirrors. The scaling laws of Purcell enhancement are explicitly demonstrated by a large-range variation of both the cavity mode volume (V = 16 − 600 µm^3 ) and the quality factor (Q = 6 · 10^3 − 2 · 10^6). We detect an enhancement of the emission spectral density by up to a factor of 300. The full potential of this resonator can be exploited with emitters having a linewidth which is narrower than the resonance linewidth of the cavity. This concept holds promise for the implementation of wavelength-tunable, narrow-band single-photon sources as well as the generation of indistinguishable single-photons at ambient conditions. However, for broad-band emitters like the NV center at room temperature, the emission lifetime is not affected noticeably in this configuration. In order to directly observe lifetime changes and Purcell-enhanced single-photon emission, we manufacture fiber-based cavities with silver-coated mirrors having ultra-small mode volumes, as small as V = 1.0 λ^3 = 0.34 µm^3. We demonstrate cavity-enhanced fluorescence imaging, which allows to locate and analyze several single NV centers with one cavity. The Purcell effect is evidenced by an enhanced fluorescence collection of up to 1.6 · 10^6 photons per second from single-NV centers and a tunable variation of the emission lifetime corresponding to an effective Purcell factor of up to 2. We furthermore investigate a benefcial regime of optical confinement where the Fabry-Pérot cavity mode is combined with additional mode confinement by the diamond nanocrystal itself, enabling sub-λ^3 mode volumes. We perform simulations that predict effective Purcell factors of up to 11 for NV centers and of up to 63 for silicon-vacancy centers, revealing a great potential for bright single-photon sources and effcient spin readout at ambient conditions.