Tunable Microcavity Research Articles

Deterministic Enhancement of Coherent Photon Generation from a Nitrogen-Vacancy Center in Ultrapure Diamond

The nitrogen-vacancy (NV) center in diamond has an optically addressable, highly coherent spin. However, an NV center even in high quality single-crystalline material is a very poor source of single photons: extraction out of the high-index diamond is inefficient, the emission of coherent photons represents just a few per cent of the total emission, and the decay time is large. In principle, all three problems can be addressed with a resonant microcavity. In practice, it has proved difficult to implement this concept: photonic engineering hinges on nano-fabrication yet it is notoriously difficult to process diamond without degrading the NV centers. We present here a microcavity scheme which uses minimally processed diamond, thereby preserving the high quality of the starting material, and a tunable microcavity platform. We demonstrate a clear change in the lifetime for multiple individual NV centers on tuning both the cavity frequency and anti-node position, a Purcell effect. The overall Purcell factor $F_{\rm P}=2.0$ translates to a Purcell factor for the zero phonon line (ZPL) of $F_{\rm P}^{\rm ZPL}\sim30$ and an increase in the ZPL emission probability from $\sim 3 \%$ to $\sim 46 \%$. By making a step-change in the NV's optical properties in a deterministic way, these results pave the way for much enhanced spin-photon and spin-spin entanglement rates.

Electrically Tunable Dye Emission via Microcavity Integrated PDMS Gel Actuator.

Electrically tunable microcavities are essential elements for tunable laser sources indispensable for modern telecommunication and spectroscopy. However, most device concepts suffer from extensive lithography or etching for membrane processing. Here, we present an electrically and continuously tunable, multi-half-wavelength microcavity with a quality factor > 1000 as an easy-to-fabricate platform with potential use for vertical-cavity surface-emitting lasers. The microcavity has a Fabry-Pérot structure consisting of ultrasoft PDMS gel with a thickness of 14-15 μm and capped by a distributed Bragg reflector on the bottom end and a silver layer serving as top mirror and electrode. Additionally, we have embedded a pyrromethene dye into the PDMS matrix to prove efficient gain medium integration. By means of an integrated dielectric elastomer actuator, the microcavity thickness is varied 1.3 μm (9%) with a driving voltage of 70 V. The subsequent silver mirror deflection achieves a reversible 40 nm tuning of the cavity resonance wavelength. The tuning range is limited by the lateral bending of the electrodes for increasing voltages. This characteristic bending is confirmed by simulations with finite elements method. The dynamic behavior of the microcavity is characterized by capacitance measurements and modeled by viscoelastic theory. Our research provides in-depth examinations of electrically tunable, PDMS gel-based microcavities with the future goal of building simple, miniaturized, and cost-efficient laser sources with high tuning range.

Directional release of the stored ultrashort light pulses from a tunable Bragg-grating microcavity.

We demonstrate numerically the ability for directionally releasing the stored ultrashort light pulse from a microcavity by means of two-pulse nonlinear interaction in a cascading Bragg grating structure. The setting is built by a chirped grating segment which is linked through a uniform segment, including a tunable microcavity located at the junction between the two components. Our simulations show that stable trapping of an ultrashort light pulse can be achieved in the setting. The stored light pulse in a microcavity can be possibly released, by nonlinearly interacting with the lateral incident control pulse. Importantly, by breaking the symmetry of potential cavity, the stably trapped light pulse can be successfully released from the microcavity to the expected direction. Owing to the induced optical nonlinearity, the released ultrashort light pulses could preserve their shapes, propagating in a form of Bragg grating solitons through the uniform component, which is in contrast to the extensively studied light pulse trappings in photonic crystal cavities which operate at the linear regime.

Exploiting One-Dimensional Exciton-Phonon Coupling for Tunable and Efficient Single-Photon Generation with a Carbon Nanotube.

Condensed-matter emitters offer enriched cavity quantum electrodynamical effects due to the coupling to external degrees of freedom. In the case of carbon nanotubes, a very peculiar coupling between localized excitons and the one-dimensional acoustic phonon modes can be achieved, which gives rise to pronounced phonon wings in the luminescence spectrum. By coupling an individual nanotube to a tunable optical microcavity, we show that this peculiar exciton-phonon coupling is a valuable resource to enlarge the tuning range of the single-photon source while keeping an excellent exciton-photon coupling efficiency and spectral purity. Using the unique flexibility of our scanning fiber cavity, we are able to measure the efficiency spectrum of the very same nanotube in the Purcell regime for several mode volumes. Whereas this efficiency spectrum looks very much like the free-space luminescence spectrum when the Purcell factor is small (large mode volume), we show that the deformation of this spectrum at lower mode volumes can be traced back to the strength of the exciton-photon coupling. It shows an enhanced efficiency on the red wing that arises from the asymmetry of the incoherent energy exchange processes between the exciton and the cavity. This allows us to obtain a tuning range up to several hundred times the spectral width of the source.

Design and low-temperature characterization of a tunable microcavity for diamond-based quantum networks

We report on the fabrication and characterization of a Fabry-Perot microcavity enclosing a thin diamond membrane at cryogenic temperatures. The cavity is designed to enhance resonant emission of single nitrogen-vacancy centers by allowing spectral and spatial tuning while preserving the optical properties observed in bulk diamond. We demonstrate cavity finesse at cryogenic temperatures within the range of F=4000–12 000 and find a sub-nanometer cavity stability. Modeling shows that coupling nitrogen-vacancy centers to these cavities could lead to an increase in remote entanglement success rates by three orders of magnitude.

Cavity-Enhanced Single-Photon Source Based on the Silicon-Vacancy Center in Diamond

Single photon sources are an integral part of various quantum technologies, and solid state quantum emitters at room temperature appear as a promising implementation. We couple the fluorescence of individual silicon vacancy centers in nanodiamonds to a tunable optical microcavity to demonstrate a single photon source with high efficiency, increased emission rate, and improved spectral purity compared to the intrinsic emitter properties. We use a fiber-based microcavity with a mode volume as small as $3.4~\lambda^3$ and a quality factor of $1.9\times 10^4$ and observe an effective Purcell factor of up to 9.2. We furthermore study modifications of the internal rate dynamics and propose a rate model that closely agrees with the measurements. We observe lifetime changes of up to 31%, limited by the finite quantum efficiency of the emitters studied here. With improved materials, our achieved parameters predict single photon rates beyond 1 GHz.

Fluorescent Proteins: Strong Coupling in Fully Tunable Microcavities Filled with Biologically Produced Fluorescent Proteins (Advanced Optical Materials 1/2017)

Fluorescent Proteins: Strong Coupling in Fully Tunable Microcavities Filled with Biologically Produced Fluorescent Proteins (Advanced Optical Materials 1/2017)

Purcell-Enhanced Single-Photon Emission from Nitrogen-Vacancy Centers Coupled to a Tunable Microcavity

Optical microcavities are a powerful tool to enhance spontaneous emission of individual quantum emitters. However, the broad emission spectra encountered in the solid state at room temperature limit the influence of a cavity, and call for ultra-small mode volume. We demonstrate Purcell-enhanced single photon emission from nitrogen-vacancy (NV) centers in nanodiamonds coupled to a tunable fiber-based microcavity with a mode volume down to $1.0\,\lambda^{3}$. We record cavity-enhanced fluorescence images and study several single emitters with one cavity. The Purcell effect is evidenced by enhanced fluorescence collection, as well as tunable fluorescence lifetime modification, and we infer an effective Purcell factor of up to 2.0. With numerical simulations, we furthermore show that a novel regime for light confinement can be achieved, where a Fabry-Perot mode is combined with additional mode confinement by the nanocrystal itself. In this regime, effective Purcell factors of up to 11 for NV centers and 63 for silicon vacancy centers are feasible, holding promise for bright single photon sources and efficient spin readout under ambient conditions.

Strong Coupling in Fully Tunable Microcavities Filled with Biologically Produced Fluorescent Proteins

We thank C. Schneider for fruitful discussions and A. Clemens and K. Ostermann (TU Dresden, Germany) for technical support with protein preparation. We acknowledge financial support from the European Research Council (ERC StG ABLASE, 640012), the Scottish Funding Council (via SUPA), the European Union Marie Curie Career Integration Grant (PCIG12-GA-2012-334407) and the EPSRC Hybrid Polaritonics program grant (EP/M025330/1). M.S. acknowledges funding from the German Science Foundation (DFG) through a Research Fellowship (SCHU 3003/1-1) and from the European Commission for a Marie Sklodowska-Curie Individual Fellowship (659213). S.H. gratefully acknowledges support by the Royal Society and the Wolfson Foundation.

Elastomer based electrically tunable, optical microcavities

Tunable optical elements are mostly realized by microelectromechanical systems, which require expensive and complex lithography during processing. We demonstrate an alternative device based on an electrically tunable microcavity employing a dielectric soft elastomer actuator. The cavity resonance is varied by changing the physical cavity thickness due to electrostriction of the soft elastomer. We realize a tunable metal-elastomer-distributed Bragg reflector multi-half wavelength microcavity with a cavity layer thickness around 12 μm and quality factors up to 700. Applying a voltage up to 60 V between bottom ITO and top metal electrode tunes the wavelength of the cavity modes up to Δλ=14 nm, which relates to a cavity thickness change of about 200 nm. This concept allows the implementation of tunable optical elements like tunable filters or resonators with low cost and simple processing.

Photothermal effects in ultra-precisely stabilized tunable microcavities.

We study the mechanical stability of a tunable high-finesse microcavity under ambient conditions and investigate light-induced effects that can both suppress and excite mechanical fluctuations. As an enabling step, we demonstrate the ultra-precise electronic stabilization of a microcavity. We then show that photothermal mirror expansion can provide high-bandwidth feedback and improve cavity stability by almost two orders of magnitude. At high intracavity power, we observe self-oscillations of mechanical resonances of the cavity. We explain the observations by a dynamic photothermal instability, leading to parametric driving of mechanical motion. For an optimized combination of electronic and photothermal stabilization, we achieve a feedback bandwidth of 500 kHz and a noise level of 1.1 × 10-13 m rms.

Zero-Dimensional Organic Exciton–Polaritons in Tunable Coupled Gaussian Defect Microcavities at Room Temperature

We demonstrate strong light-matter interaction at ambient conditions between a ladder-type conjugated polymer and the individual modes of a vertical microcavity with tunable resonance frequencies. Zero-dimensional wavelength-scale confinement for the polaritons is achieved through a sub-micron sized Gaussian defect, resulting in a vacuum Rabi splitting of the polariton branches of 2g = 166 meV. By placing a second Gaussian defect nearby, we create a polaritonic molecule with tunnel coupling strength of up to 2J ~ 50 meV. This platform enables the creation of tailor-made potential landscapes with wavelength-scale dimensions and tunable coupling strengths beyond the thermal energy, opening a route towards room-temperature polariton-based quantum simulators.

Zero-Dimensional Organic Exciton?Polaritons in Tunable Coupled Gaussian Defect Microcavities at Room Temperature

Zero-Dimensional Organic Exciton?Polaritons in Tunable Coupled Gaussian Defect Microcavities at Room Temperature

Cavity-enhanced Raman microscopy of individual carbon nanotubes.

Raman spectroscopy reveals chemically specific information and provides label-free insight into the molecular world. However, the signals are intrinsically weak and call for enhancement techniques. Here, we demonstrate Purcell enhancement of Raman scattering in a tunable high-finesse microcavity, and utilize it for molecular diagnostics by combined Raman and absorption imaging. Studying individual single-wall carbon nanotubes, we identify crucial structural parameters such as nanotube radius, electronic structure and extinction cross-section. We observe a 320-times enhanced Raman scattering spectral density and an effective Purcell factor of 6.2, together with a collection efficiency of 60%. Potential for significantly higher enhancement, quantitative signals, inherent spectral filtering and absence of intrinsic background in cavity-vacuum stimulated Raman scattering render the technique a promising tool for molecular imaging. Furthermore, cavity-enhanced Raman transitions involving localized excitons could potentially be used for gaining quantum control over nanomechanical motion and open a route for molecular cavity optomechanics.

Perovskite—Cavity Complex: the Modulated Optical Characteristics of Organometallic Halide Perovskite CH3NH3PbIxCl3–x Doped in Length Tunable Optical Microcavities

When organometallic halide perovskite semiconductors with continuous transition band were placed into an optical microcavity, interaction/modulation effect between confined photonic mode supported by cavity and free charge carriers mode triggered in perovskite occurred. In this metal–insulator–metal (MIM) microcavity, given the broad absorption region of perovskite CH3NH3PbIxCl3–x, the cavity resonant energy can be selectively designed to modulate the photophysical characteristics of perovskite charge carriers at different levels. Another advantage of using perovskite is that its band filling effect (charge carrier’s accumulation process) and relaxation process could last for few and hundreds picoseconds respectively, allowing us to accurately examine the time-resolved couple modulating effect by microcavities. Steady-state and transient absorption (TA) spectra were carried out to experimentally confirm the modulation effect of microcavity on optical characteristics of perovskite free charge carriers. It ...

Thermally tunable silicon photonic microdisk resonator with transparent graphene nanoheaters

Efficient tunable photonic integrated devices are important for the realization of reconfigurable photonic systems. Thermal tuning is a convenient and effective approach, and silicon’s large heat conductivity, thermo-optical coefficient, and CMOS fabrication compatibility make it a good candidate material for tunable optical microcavities, which are versatile elements in low-cost, large-scale photonic integrated circuits. Metal heaters are traditionally used for tuning, and a thick SiO2 upper-cladding layer is usually needed to prevent light absorption by the metal since that could reduce response speed and heating efficiency. In this paper, we propose and experimentally demonstrate thermally tunable silicon photonic microdisk resonators by introducing transparent graphene nanoheaters, which contact the silicon core directly without any isolator layer. The theoretical and experimental results show that the transparent graphene nanoheaters improve the heating efficiency, the temporal response, and the achievable temperature in comparison with a traditional metal heater. Furthermore, the graphene nanoheater is convenient for use in ultrasmall nanophotonic integrated devices due to its single-atom thickness and excellent flexibility. Both experiments and simulations show that the transparent graphene nanoheater is a promising option for other thermally tunable photonic integrated devices such as optical filters and switches.

Tunable open-access microcavities for on-chip cavity quantum electrodynamics

We report on the development of on-chip microcavities and show their potential as a platform for cavity quantum electrodynamics experiments. Microcavity arrays were formed by the controlled buckling of SiO2/Ta2O5 Bragg mirrors and exhibit a reflectance-limited finesse of 3500 and mode volumes as small as 35λ3. We show that the cavity resonance can be thermally tuned into alignment with the D2 transition of 87Rb and outline two methods for providing atom access to the cavity. Owing to their small mode volume and high finesse, these cavities exhibit single-atom cooperativities as high as C1=65. A unique feature of the buckled-dome architecture is that the strong-coupling parameter g0/κ is nearly independent of the cavity size. Furthermore, strong coupling should be achievable with only modest improvements in mirror reflectance, suggesting that these monolithic devices could provide a robust and scalable solution to the engineering of light-matter interfaces.

Spin Textures of Exciton-Polaritons in a Tunable Microcavity with Large TE-TM Splitting.

We report an extended family of spin textures of zero-dimensional exciton-polaritons spatially confined in tunable open microcavity structures. The transverse-electric-transverse-magnetic (TE-TM) splitting, which is enhanced in the open cavity structures, leads to polariton eigenstates carrying quantized spin vortices. Depending on the strength and anisotropy of the cavity confining potential and of the TE-TM induced splitting, which can be tuned via the excitonic or photonic fractions, the exciton-polariton emissions exhibit either spin-vortex-like patterns or linear polarization, in good agreement with theoretical modeling.

Controlled waveguide coupling for photon emission from colloidal PbS quantum dot using tunable microcavity made of optical polymer and silicon

A tunable microcavity device composed of optical polymer and Si with a colloidal quantum dot (QD) is proposed as a single-photon source for planar optical circuit. Cavity size is controlled by electrostatic micromachine behavior with the air bridge structure to tune timing of photon injection into optical waveguide from QD. Three-dimensional positioning of a QD in the cavity structure is available using a nanohole on Si processed by scanning probe microscope lithography. We fabricated the prototype microcavity with PbS-QD-mixed polymenthyl methacrylate on a SOI (semiconductor-on-insulator) substrate to show the tunability of cavity size as the shift of emission peak wavelength of QD ensemble.

Gain Spectroscopy of Solution‐Based Semiconductor Nanocrystals in Tunable Optical Microcavities

The lasing behavior of solution‐based colloidal quantum dots within an open microcavity is reported. The small size and wide tunability of the cavity provide single mode lasing over a wavelength range in excess of 25 nm. By extracting the lasing threshold and differential gain for the fundamental cavity mode over this spectral range, gain spectroscopy of the quantum dot solution is demonstrated. This new approach could help in the optimization of laser gain media and provides a way of constructing miniature laser arrays for on‐chip integration.