Fiber-taper collected emission from NV centers in high-Q/V diamond microdisks.

Fiber-coupled microdisks are a promising platform for enhancing the spontaneous emission from color centers in diamond. The measured cavity-enhanced emission from the microdisk is governed by the effective volume (V) of each cavity mode, the cavity quality factor (Q), and the coupling between the microdisk and the fiber. Here we observe room temperature photoluminescence from an ensemble of nitrogen-vacancy centers into high Q/V microdisk modes, which when combined with coherent spectroscopy of the microdisk modes, allows us to elucidate the relative contributions of these factors. The broad emission spectrum acts as an internal light source facilitating mode identification over several cavity free spectral ranges. Analysis of the fiber taper collected microdisk emission reveals spectral filtering both by the cavity and the fiber taper, the latter of which we find preferentially couples to higher-order microdisk modes. Coherent mode spectroscopy is used to measure Q ∼ 1 × 105 - the highest reported values for diamond microcavities operating at visible wavelengths. With realistic optimization of the microdisk dimensions, we predict that Purcell factors of ∼50 are within reach.

Semiconductor-on-diamond cavities for spin optomechanics.

Optomechanical cavities are powerful tools for classical and quantum information processing that can be realized using nanophotonic structures that co-localize optical and mechanical resonances. Typically, phononic localization requires suspended devices that forbid vertical leakage of mechanical energy. Achieving this in some promising quantum photonic materials such as diamond requires non-standard nanofabrication techniques, while hindering integration with other components and exacerbating heating related challenges. As an alternative, we have developed a semiconductor-on-diamond platform that co-localizes phononic and photonic modes without requiring undercutting. We have designed an optomechanical crystal cavity that combines high optomechanical coupling with low dissipation, and we show that this platform will enable optomechanical coupling to spin qubits in the diamond substrate. These properties demonstrate the promise of this platform for realizing quantum information processing devices based on spin, phonon, and photon interactions.

Processing light with an optically tunable mechanical memory.

Mechanical systems are one of the promising platforms for classical and quantum information processing and are already widely-used in electronics and photonics. Cavity optomechanics offers many new possibilities for information processing using mechanical degrees of freedom; one of them is storing optical signals in long-lived mechanical vibrations by means of optomechanically induced transparency. However, the memory storage time is limited by intrinsic mechanical dissipation. More over, in-situ control and manipulation of the stored signals processing has not been demonstrated. Here, we address both of these limitations using a multi-mode cavity optomechanical memory. An additional optical field coupled to the memory modifies its dynamics through time-varying parametric feedback. We demonstrate that this can extend the memory decay time by an order of magnitude, decrease its effective mechanical dissipation rate by two orders of magnitude, and deterministically shift the phase of a stored field by over 2π. This further expands the information processing toolkit provided by cavity optomechanics.

Two-colour interferometry and switching through optomechanical dark mode excitation.

Efficient switching and routing of photons of different wavelengths is a requirement for realizing a quantum internet. Multimode optomechanical systems can solve this technological challenge and enable studies of fundamental science involving widely separated wavelengths that are inaccessible to single-mode optomechanical systems. To this end, we demonstrate interference between two optomechanically induced transparency processes in a diamond on-chip cavity. This system allows us to directly observe the dynamics of an optomechanical dark mode that interferes photons at different wavelengths via their mutual coupling to a common mechanical resonance. This dark mode does not transfer energy to the dissipative mechanical reservoir and is predicted to enable quantum information processing applications that are insensitive to mechanical decoherence. Control of the dark mode is also utilized to demonstrate all-optical, two-colour switching and interference with light separated by over 5 THz in frequency.

Hexagonal Boron Nitride Cavity Optomechanics.

Hexagonal boron nitride (hBN) is an emerging layered material that plays a key role in a variety of two-dimensional devices, and has potential applications in nanophotonics and nanomechanics. Here, we demonstrate the first cavity optomechanical system incorporating hBN. Nanomechanical resonators consisting of hBN beams with average dimensions of 12 µm × 1.2 µm × 28 nm and minimum predicted thickness of 8 nm were fabricated using electron beam induced etching and positioned in the optical near-field of silicon microdisk cavities. Of the multiple devices studied here a maximum 0.16 pm/[Formula: see text] sensitivity to the hBN nanobeam motion is demonstrated, allowing observation of thermally driven mechanical resonances with frequencies between 1 and 23 MHz, and largest mechanical quality factor of 1100 for a 23 MHz mode, at room temperature in high vacuum. In addition, the role of air damping is studied via pressure dependent measurements. Our results constitute an important step toward realizing integrated optomechanical circuits employing hBN.

Nanocavity optomechanical torque magnetometry and radiofrequency susceptometry.

Nanophotonic optomechanical devices allow the observation of nanoscale vibrations with a sensitivity that has dramatically advanced the metrology of nanomechanical structures and has the potential to impact studies of nanoscale physical systems in a similar manner. Here we demonstrate this potential with a nanophotonic optomechanical torque magnetometer and radiofrequency (RF) magnetic susceptometer. Exquisite readout sensitivity provided by a nanocavity integrated within a torsional nanomechanical resonator enables observations of the unique net magnetization and RF-driven responses of single mesoscopic magnetic structures in ambient conditions. The magnetic moment resolution is sufficient for the observation of Barkhausen steps in the magnetic hysteresis of a lithographically patterned permalloy island. In addition, significantly enhanced RF susceptibility is found over narrow field ranges and attributed to thermally assisted driven hopping of a magnetic vortex core between neighbouring pinning sites. The on-chip magnetosusceptometer scheme offers a promising path to powerful integrated cavity optomechanical devices for the quantitative characterization of magnetic micro- and nanosystems in science and technology.

Effects of fabrication methods on spin relaxation and crystallite quality in Tm-doped Y3AI5O12 powders studied using spectral hole burning.

High-quality rare-earth-ion (REI) doped materials are a prerequisite for many applications such as quantum memories, ultra-high-resolution optical spectrum analyzers and information processing. Compared to bulk materials, REI doped powders offer low-cost fabrication and a greater range of accessible material systems. Here we show that crystal properties, such as nuclear spin lifetime, are strongly affected by mechanical treatment, and that spectral hole burning can serve as a sensitive method to characterize the quality of REI doped powders. We focus on the specific case of thulium doped Y3AI5O12 (Tm:YAG). Different methods for obtaining the powders are compared and the influence of annealing on the spectroscopic quality of powders is investigated on a few examples. We conclude that annealing can reverse some detrimental effects of powder fabrication and, in certain cases, the properties of the bulk material can be reached. Our results may be applicable to other impurities and other crystals, including color centers in nano-structured diamond.

Diamond photonics platform enabled by femtosecond laser writing.

Diamond is a promising platform for sensing and quantum processing owing to the remarkable properties of the nitrogen-vacancy (NV) impurity. The electrons of the NV center, largely localized at the vacancy site, combine to form a spin triplet, which can be polarized with 532 nm laser light, even at room temperature. The NV's states are isolated from environmental perturbations making their spin coherence comparable to trapped ions. An important breakthrough would be in connecting, using waveguides, multiple diamond NVs together optically. However, still lacking is an efficient photonic fabrication method for diamond akin to the photolithographic methods that have revolutionized silicon photonics. Here, we report the first demonstration of three dimensional buried optical waveguides in diamond, inscribed by focused femtosecond high repetition rate laser pulses. Within the waveguides, high quality NV properties are observed, making them promising for integrated magnetometer or quantum information systems on a diamond chip.

High-Q/V Monolithic Diamond Microdisks Fabricated with Quasi-isotropic Etching.

Optical microcavities enhance light-matter interactions and are essential for many experiments in solid state quantum optics, optomechanics, and nonlinear optics. Single crystal diamond microcavities are particularly sought after for applications involving diamond quantum emitters, such as nitrogen vacancy centers, and for experiments that benefit from diamond's excellent optical and mechanical properties. Light-matter coupling rates in experiments involving microcavities typically scale with Q/V, where Q and V are the microcavity quality-factor and mode-volume, respectively. Here we demonstrate that microdisk whispering gallery mode cavities with high Q/V can be fabricated directly from bulk single crystal diamond. By using a quasi-isotropic oxygen plasma to etch along diamond crystal planes and undercut passivated diamond structures, we create monolithic diamond microdisks. Fiber taper based measurements show that these devices support TE- and TM-like optical modes with Q > 1.1 × 10(5) and V < 11(λ/n) (3) at a wavelength of 1.5 µm.

Optical design of split-beam photonic crystal nanocavities.

We design high-quality-factor photonic crystal nanobeam cavities formed by two mechanically isolated cantilevers. These "split-beam" cavities have a physical gap at the center, allowing mechanical excitations of one or both of the cavity halves. They are designed by analyzing the optical band structures and mode profiles of waveguides perforated by elliptical holes and rectangular gaps and are predicted to support optical resonances with quality factors exceeding 10(6) at wavelengths of ~1.6 µm.

Hybrid photonic crystal cavity and waveguide for coupling to diamond NV-centers.

A design for an ultra-high Q photonic crystal nanocavity engineered to interact with nitrogen-vacancy (NV) centers located near the surface of a single crystal diamond sample is presented. The structure is based upon a nanowire photonic crystal geometry, and consists of a patterned high refractive index thin film, such as gallium phosphide (GaP), supported by a diamond substrate. The nanocavity supports a mode with quality factor Q > 1.5 x 10(6) and mode volume V < 0.52(lambda/nGaP)(3), and promises to allow Purcell enhanced collection of spontaneous emission from an NV located more than 50 nm below the diamond surface. The nanowire photonic crystal waveguide can be used to efficiently couple light into and out of the cavity, or as an efficient broadband collector of NV phonon sideband emission. The proposed structures can be fabricated using existing materials and processing techniques.

Coherent interference effects in a nano-assembled diamond NV center cavity-QED system.

Diamond nanocrystals containing NV color centers are positioned with 100-nanometer-scale accuracy in the near-field of a high-Q SiO(2) microdisk cavity using a fiber taper. The cavity modified nanocrystal photoluminescence is studied, with Fano-like quantum interference features observed in the far-field emission spectrum. A quantum optical model of the system is proposed and fit to the measured spectra, from which the NV(-) zero phonon line coherent coupling rate to the microdisk is estimated to be 28 MHz for a nearly optimally placed nanocrystal.

Observation of the dynamic Jahn-Teller effect in the excited states of nitrogen-vacancy centers in diamond.

The optical transition linewidth and emission polarization of single nitrogen-vacancy (NV) centers are measured from 5 K to room temperature. Interexcited state population relaxation is shown to broaden the zero-phonon line and both the relaxation and linewidth are found to follow a T(5) dependence for T < 100 K. This dependence indicates that the dynamic Jahn-Teller effect is the dominant dephasing mechanism for the NV optical transitions at low temperatures.

Efficient input and output fiber coupling to a photonic crystal waveguide.

The efficiency of evanescent coupling between a silica optical fiber taper and a silicon photonic crystal waveguide is studied. A high-reflectivity mirror on the end of the photonic crystal waveguide is used to recollect, in the backward-propagating fiber mode, the optical power that is initially coupled into the photonic crystal waveguide. An outcoupled power in the backward-propagating fiber mode of 88% of the input power is measured, corresponding to a lower bound on the coupler efficiency of 94%.