A composite microcavity of diamond nanopillar and ... - OSA Publishing

A composite microcavity of diamond nanopillar and deformed silica microsphere with enhanced evanescent decay length Russell J. Barbour,* Khodadad N. Dinyari, and Hailin Wang Department of Physics and Oregon Centre for Optics,University of Oregon, Eugene, Oregon 97403, USA *[email protected]

Abstract: We report the experimental realization of a composite microcavity system, in which negatively-charged nitrogen vacancy (NV) centers in diamond nanopillars couple evanescently to whispering-gallery modes (WGMs) in a deformed, non-axisymmetric silica microsphere. We show that the deformed microsphere can feature an evanescent decay length four times larger than that of a regular silica microsphere. With the enhanced evanescent coupling, WGMs can in principle couple to NV centers that are 100 to 200 nm beneath the diamond pillar surface, providing a promising avenue for exploring evanescently-coupled cavity QED systems of NV centers in ultrahigh purity diamond. © 2010 Optical Society of America OCIS codes: (140.3948) Microcavity devices; (270.5580) Quantum electrodynamics.

References and links 1.

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Received 25 Jun 2010; revised 27 Jul 2010; accepted 30 Jul 2010; published 20 Aug 2010

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Recent experimental progress on the full quantum control of a coupled electron-nuclear spin system associated with negatively-charged nitrogen vacancy (NV) centers in ultrahigh purity diamond [1–7] have opened up the exciting possibility of combining quantum control of individual photons, electron spins, and nuclear spins in a solid-state cavity QED system. Such a NV-based cavity QED system can provide us with a platform to pursue quantum information processing, especially quantum networks, by generating entanglement between distant electron spins with optical processes and by using nuclear spins for quantum memories. A variety of cavity QED systems have been actively pursued for NV centers. While strongcoupling cavity QED with NV centers in diamond nanocrystals has been realized, further progress has been hindered by the poor quality of the nanocrystals [8–11]. The use of NV centers in bulk diamond for cavity QED studies has been challenging due to the difficulty of fabricating high quality photonic structures from diamond crystals [12]. Recent experimental efforts have focused on composite microcavity systems where NV centers in bulk diamond couple evanescently to an optical resonator, such as silica microspheres [13], GaP microdisks [14], or SiN photonic crystals [15]. For these resonators, the evanescent decay length at the wavelength (λ = 637 nm) of the NV zero-phonon transition is approximately 100 nm. A major obstacle for composite microcavity systems based on evanescent coupling is that for an evanescent decay length of 100 nm, the relevant NV centers need to be within a few tens of nanometers from the diamond crystal surface. Nitrogen implantation has been successfully used in order to generate NV centers near the surface of ultrahigh purity diamond [16]. However, the zero-phonon emission from the implanted NV centers exhibits spectral fluctuations that are much greater than the intrinsic zero-phonon linewidth. This makes these NV centers unsuitable for the intended application [17]. An alternative remedy is to develop and exploit optical resonators that can feature greater evanescent decay length while still retaining high Q-factor for cavity QED studies. In this letter, we report experimental studies of a composite microcavity system, in which NV centers in a diamond nanopillar are coupled to whispering gallery modes (WGMs) in a deformed silica microsphere. We show that the evanescent decay length in the deformed optical resonator exhibits strong position dependence. An evanescent decay length four times larger than that of a regular spherical resonator has been achieved in the deformed resonator. With the enhanced evanescent decay length, it becomes feasible to develop an evanescent-coupled

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cavity QED system with NV centers in ultrapure diamond without the complication of nitrogen implantation. In a deformed whispering gallery resonator, the angle of incidence of a light ray is no longer conserved, leading to corresponding variations in the evanescent decay length. The dependence of the evanescent decay length on the angle of incidence, θ, is illustrated by the well-known result for total internal reflection from a planar interface [18],

1 2π = λ le

sin 2 (θ ) −1 sin 2 (θ c )

(1)

where le is the evanescent decay length, θc is the critical angle, λ is the optical wavelength in the medium of lesser refractive index. As shown in Eq. (1), the evanescent decay length approaches infinity when the angle of incidence approaches θc. Earlier studies have shown that for WGMs near the equatorial plane in a slightly deformed, non-axisymmetric silica microsphere, the angle of incidence most closely approaches θc in regions 45° away from either the major or minor axis [19]. The enhanced evanescent escape in these regions leads to highly directional far-field emission patterns from the WGMs [19]. However, no experimental studies have been carried out on enhanced evanescent decay in these deformed resonators. We have fabricated silica microspheres by heating an optical fibre tip using a Synrad CO2 laser. Deformation is induced by heating a microsphere from two opposing sides with a 20 ms CO2 laser pulse [20]. The degree of deformation is controlled by adjusting the intensity of the laser pulse. For the silica microspheres used in this study, the sphere diameter is approximately, d = 40 µm. The deformation (determined from optical images) is kept to approximately 2% in order to retain the ultrahigh optical Q-factor of the resonator. Note that the minor or major axis in Fig. 1a corresponds to the symmetry axis in the equatorial plane. The deformed sphere has no 3D symmetry axis due to deformation induced by the CO2 laser pulse and that by the presence of a fibre stem.

Fig. 1. (a) Schematic of the experimental setup for free-space excitation. WGMs in a deformed silica microsphere (deformation is exaggerated for clarity) are excited with a tunable laser. A fiber tip is also used to probe the evanescent field of the WGMs. (b) A WGM resonance near λ = 800 nm and with l ≈ m and a deformation < 2%. The data is fitted with a Lorentzian lineshape (red line).

We have used a free-space evanescent excitation technique [8,21], to excite WGMs near the sphere equator (with l ≈ m , where l and m are the angular momentum and azimuthal mode numbers, respectively). Figure 1a shows a schematic of the experimental setup. For these experiments, a laser beam was focused to within 1 micron of the edge of the sphere in a region 45° away from a symmetry axis. A frequency-stabilized Ti:Sapphire ring laser (Coherent 89921) with a linewidth close to 250 kHz and with λ near 800 nm was used. Figure 1b shows, as an example, a WGM resonance with a cavity linewidth of 12 MHz (corresponding to Q = #130415 - $15.00 USD

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3x107), where the emission from the excited WGM is measured as a function of the excitation laser frequency. To probe the evanescent decay length, we placed a fiber tip (d≈1 µm) in the evanescent field of the relevant WGMs [22]. The fiber tip, mounted on a 3D piezoelectric stage, is in the equatorial plane and is normal to the sphere surface. The stem attached to the sphere was mounted on a rotation stage, enabling us to control the equatorial position of the tip relative to the relevant symmetry axis. Scattering of the evanescent wave by the fiber tip leads to a degradation of the optical Q-factor. Figure 2 shows the Q-factors obtained as a function of the tipsphere distance. The tip was positioned at the antinode of a WGM with by monitoring the emission intensity and the linewidth of the WGM. Due to the relatively large tip diameter (~1 µm), precise positioning along the azimuthal direction was not required. As the tip enters the evanescent field, a clear reduction in the emission intensity was observed. With the tip approaching the sphere surface, we set the contact point or the zero tip-sphere distance as the point at which Q-spoiling started to saturate. Once in contact with the sphere, the tip was pulled back in steps of 50 nm using a calibrated piezo-electric stage. The linewidth of the WGM was recorded at each position and the corresponding Q-factor was calculated. No mode splitting due to the presence of the tip was observed, indicating that the back-scatteringinduced mode splitting is small compared with the WGM linewidth. The solid circles in Fig. 2 show the experimental results obtained with the fiber tip positioned in an equatorial region along a symmetry axis of the sphere. The open circles in Fig. 2 show the results obtained when the tip was positioned in an equatorial region 45° away from the symmetry axis; where one would expect maximum evanescent decay length. For the solid circles, the onset of the tip-induced Q-spoiling begins around 100 nm from the surface, which is in general agreement with the theoretically expected evanescent decay length. In contrast, for the open circles, the Q-spoiling begins when the tip is approximately 400 nm from the spheres surface, indicating a factor of 4 enhancement in the evanescent decay length.

Fig. 2. Q-spoiling of the WGM as a function of the tip-sphere distance. Solid circles: The tip approaches a surface area near a symmetry axis. Open circles: The tip approaches a surface area that is 45° away from a symmetry axis, with the inset showing the Q-factor over a longer distance scale.

The experimental results shown in Fig. 2 were obtained with a single sphere. These experiments were carried out in open air. Q-factors were first measured with a fiber tip positioned along a symmetry axis. It was verified (by rechecking the Q-factor without the fiber tip at the end of a set of measurements) that the intrinsic Q-factors did not deteriorate during this set of measurements. After the completion of this set of measurements, Q-factors were then measured with the fiber tip positioned in an equatorial region 45° degrees away from the symmetry axis. It was verified that the Q-factors did not deteriorate during this second set of measurements. In between these two sets of measurements, however, Q-spoiling of the sphere oc-

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curred, due to moisture or dust contamination on the surface of the sphere. In order to recover the Q-factor, the sphere was gently reheated with a CO2 laser beam before the start of the second set of measurements. This procedure helps remove moisture and contaminants but does not significantly alter the spheres deformation. The evanescent decay length along a symmetry axis, which serves as a reference, is not sensitive to deformation. We have repeated these measurements many times, nearly the same evanescent decay length was observed for deformed spheres along a symmetry axis and for undeformed spheres, which agrees with the theoretical expectation. Note that because of the long time it takes to complete a set of measurements, Q-spoiling often occurs during a set of measurements or between the two sets. Nevertheless, the same behavior of evanescent decay length has been observed for all these measurements. To take advantage of this enhanced evanescent decay length for cavity QED studies of NV centers, one can simply replace the fiber tip with a diamond nanopillar with the pillar positioned at 45° away from a symmetry axis of the deformed microsphere. Figure 3a shows a schematic of the composite microsphere-nanopillar system. The diamond nanopillars were fabricated from a chemical vapor deposition (CVD) grown bulk diamond crystal. The fabrication process consists of patterning the diamond crystal with an aluminum mask and etching the exposed diamond with a reactive ion etcher. The nanopillars protrude ~1 µm from the substrates surface and have diameters ranging from 100 nm to more than 1 µm. Details of the nanopillar fabrication have been presented in an earlier study [13]. Similar to the experiments with the fiber tip discussed earlier, we measured the Q-factor of the composite microspherenanopillar system as a function of the nanopillar-sphere gap. For these measurements, a nanopillar with a diameter of 200 nm is used. As shown in Fig. 3b, the onset of Q-spoiling begins when the pillar is about 400 nm from the spheres surface, in agreement with the evanescent decay length measurement obtained with a fiber tip. Similar results have been obtained with pillars with varying sizes (diameter < 500 nm) and shapes. This experiment demonstrates the feasibility of exploiting enhanced evanescent decay length of a deformed microsphere to couple WGMs to NV centers that are within 200 nm of the diamond surface. The enhanced evanescent decay length also increases the effective interaction volume of a nanopillar. NV centers within this volume can couple efficiently to relevant WGMs. This is highly beneficial since for nanopillars fabricated from ultrahigh purity diamond, the likelihood of finding a high quality NV center within the effective interaction volume is relatively small. We also note that similar Q-spoiling was observed in both Fig. 2 and Fig. 3b, in spite of the large difference between the size of the fiber tip used for Fig. 2 and that of the pillar for Fig. 3b. This is in part due to the much higher refractive index of diamond (n = 2.38) than that of silica (n = 1.46). In addition, since the size of the fiber tip exceeds λ, Q-spoiling obtained with the fiber tip results from an average effect of scattering from both the node and antinode regions of a WGM. Detailed discussions on Q-spoiling induced by diamond nanopillars have been presented in [13]. While the experimental results discussed above have shown enhanced evanescent decay length, quantitative measurements on the coupling between NV centers and relevant WGMs require detailed studies on enhanced spontaneous emission rates or on behaviors of strong coupling cavity QED. These experiments will need to be carried out at low temperature. For conventional WGM resonators, input and output coupling of WGMs are realized with a tapered fiber (or with frustrated total internal reflection in a high index optical prism). It is feasible, but highly challenging especially in a low temperature environment, to control simultaneously and precisely the relative position between a resonator and a pillar and that between a resonator and a tapered fiber. The use of a deformed microsphere not only enhances the evanescent decay length, but also enables efficient input and output coupling of WGMs without the use of a tapered optical fiber. The ability to carry out experimental studies without the use of a tapered fiber will great-

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ly simplify or facilitate cavity QED measurements at low temperature. Figures 1 and 2 show that WGMs in deformed microspheres can be excited via direct free-space excitation. As discussed in detail below, we can take advantage of directional emission patterns of WGMs in a deformed microsphere to probe WGM emissions from a composite pillar-microsphere system [19]. For a proof-of-principle demonstration, a diamond nanopillar with a diameter of 400 nm fabricated from a type Ib diamond bulk crystal was used. The pillar with a relatively large diameter was used for the ease of positioning in a cryogenic environment. In order to minimize background emissions from the bulk crystal, the diamond substrate was also covered with a 200 nm layer of gold, leaving the pillars uncoated. The composite nanopillar-microsphere system was mounted on the cold finger of a helium flow cryostat, with the nanopillar positioned in an equatorial antinode region 45° away from a symmetry axis. The relative position between the pillar and the microsphere was controlled with a 3D nanopositioning stage. The nanopillar was excited directly with a laser at λ = 532 nm and with a focused spot size of ~1 µm. The excitation beam was focused through the sphere onto the top of the diamond nanopillar.

Fig. 3. (a) Schematic plan view of the setup for coupling NV centers to the evanescent field of a deformed microsphere (deformation is exaggerated for clarity). The emission from the sphere (red arrow) is collected and sent to the spectrometer via freespace optics. (b) The induced Qspoiling by a 200 nm diamond nanopillar positioned in the evanescent field maximum. (c) PL spectra from the diamond nanopillar coupled to a deformed microsphere obtained at T = 150 K.

Figure 3c shows the PL spectrum obtained at 150 K from the composite microspherenanopillar system, where the PL was detected along a direction 45° away from a symmetry axis of the deformed sphere (see the schematic in Fig. 3a). This detection scheme takes advantage of the strong directional evanescent escape of the WGMs in these regions. The PL spectrum features a periodic WGM structure and shows the characteristic NV zero-phonon resonance along with a spectrally broad phonon sideband. The measured free spectral range (FSR) of 2.9 nm (2100 GHz) is in good agreement with the FSR of a silica microsphere with a di-

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ameter of 50 µm. Each peak in Fig. 3c is expected to consist of a set of spectrally narrower WGM resonances with mode separation much smaller than the spectrometer resolution (0.2 nm). In comparison, for a composite microsphere-nanopillar system where a regular silica microsphere is used, WGM structures in the PL spectrum are observable only when emission from the WGMs is coupled out though a tapered optical fiber [13]. Note that additional experiments carried out at room temperature indicate that the background emission (emissions that show no WGM structures) can be further reduced with pillars with smaller diameters. In many aspects, the composite system of nanopillar and deformed microsphere is very similar to the nanocrystal-microsphere system developed in earlier studies [8], with the nanocrystals replaced by nanopillars (Q-spoiling induced by the nanopillar can be avoided or reduced by using a nanopillar with sufficiently small diameters [13]). There are, however, essential differences. Nanopillars can be positioned with picometer precision on the sphere surface. Perhaps more importantly, NV centers formed naturally in nanopillars of ultrahigh purity diamond can feature much better optical properties than those of NV centers in nanocrystals. In conclusion, we have demonstrated that the evanescent decay length in a deformed silica microsphere is enhanced by a factor of four in the equatorial regions at 45° degrees from a symmetry axis. The enhanced evanescent decay length offers a practical solution for developing an evanescently-coupled cavity QED system utilizing NV centers in diamond nanopillars, in which the cavity field can couple to NV centers that are 100 to 200 nm, instead of tens of nm, beneath the crystal surface. In addition, the use of a deformed microsphere in the composite system should also enable cavity QED measurements without incorporating a tapered optical fiber, greatly reducing the experimental complexity in a cryogenic environment. Finally, we note that the enhanced evanescent decay length demonstrated here can also be of interest to other evanescent field based QED studies, such as atomic cavity QED studies, in which cold atoms are dropped through or trapped in the evanescent field of a toroidal resonator [23]. The enhanced evanescent decay length can greatly expand the effective spatial region where the atoms can couple strongly to the relevant WGM. Acknowledgments

This work is supported by DARPA-MTO. KND acknowledges financial support from the National Science Foundation GK-12 program under Grant No. DGE-0742540.

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