Publications

2021
Gadalla, Mena N., Andrew S. Greenspon, Rodrick Kuate Defo, Xingyu Zhang, and Evelyn L. Hu. 2021. “Enhanced cavity coupling to silicon vacancies in 4H silicon carbide using laser irradiation and thermal annealing.” Proceedings of the National Academy of Sciences 118 (12). Publisher's Version Abstract
The negatively charged silicon monovacancy VSi in 4H silicon carbide (SiC) is a spin-active point defect that has the potential to act as a qubit in solid-state quantum information applications. Photonic crystal cavities (PCCs) can augment the optical emission of the VSi, yet fine-tuning the defect–cavity interaction remains challenging. We report on two postfabrication processes that result in enhancement of the V1' optical emission from our PCCs, an indication of improved coupling between the cavity and ensemble of silicon vacancies. Below-bandgap irradiation at 785-nm and 532-nm wavelengths carried out at times ranging from a few minutes to several hours results in stable enhancement of emission, believed to result from changing the relative ratio of VSi0 (“dark state”) to VSi (“bright state”). The much faster change effected by 532-nm irradiation may result from cooperative charge-state conversion due to proximal defects. Thermal annealing at 100 °C, carried out over 20 min, also results in emission enhancements and may be explained by the relatively low-activation energy diffusion of carbon interstitials CiCi, subsequently recombining with other defects to create additional VSis. These PCC-enabled experiments reveal insights into defect modifications and interactions within a controlled, designated volume and indicate pathways to improved defect–cavity interactions.
Just as “classical” information technology rests on a foundation built of interconnected information-processing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the “interconnect,” a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a “quantum internet” poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on “Quantum Interconnects” that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry, and national laboratories is required.
2020
The family of III-nitride materials has provided a platform for tremendous advances in efficient solid-state lighting sources such as light-emitting diodes and laser diodes. In particular, quantum dot (QD) lasers using the InGaN/GaN material system promise numerous benefits to enhance photonic performance in the blue wavelength regime. Nevertheless, issues of strained growth and difficulties in producing InGaN QDs with uniform composition and size pose daunting challenges in achieving an efficient blue laser. Through a review of two previous studies on InGaN/GaN QD microdisk lasers, we seek to provide a different perspective and approach in better understanding the potential of QD emitters. The lasers studied in this paper contain gain material where QDs are sparsely distributed, comprise a wide distribution of sizes, and are intermixed with “fragmented” quantum well (fQW) material. Despite these circumstances, the use of microdisk cavities, where a few distinct, high-quality modes overlap the gain region, not only produces ultralow lasing thresholds (∼6.2 μJ/cm2) but also allows us to analyze the dynamic competition between QDs and fQWs in determining the final lasing wavelength. These insights can facilitate “modal” optimization of QD lasing and ultimately help to broaden the use of III-nitride QDs in devices.
Craik, DPL Aude, P. Kehayias, Andrew S. Greenspon, Xinghu Zhang, J. M. Schloss, E. Bauch, CA Hart, EL Hu, and R. L. Walsworth. 2020. ““Microwave-assisted spectroscopy technique for studying charge state in nitrogen-vacancy ensembles in diamond” .” Physical Review Applied 14 (1): 1-17. Publisher's Version Abstract
We introduce a microwave-assisted spectroscopy technique to determine the relative ratio of fluorescence emitted by nitrogen-vacancy (N-V) centers in diamond that are negatively charged (N−V−) and neutrally charged (N−V0) and present its application to studying spin-dependent ionization in N-V ensembles and enhancing N-V-magnetometer sensitivity. Our technique is based on selectively modulating the N−V− fluorescence with a spin-state-resonant microwave drive to isolate, in situ, the spectral shape of the N−V− and N−V0 contributions to an N-V-ensemble sample’s fluorescence. As well as serving as a reliable means to characterize the charge state, the method can be used as a tool to study spin-dependent ionization in N-V ensembles. As an example, we apply the microwave technique to a high-N-V-density diamond sample and find evidence for an additional spin-dependent ionization pathway, which we present here alongside a rate-equation model of the data. We further show that our method can be used to enhance the contrast of optically detected magnetic resonance (ODMR) on N-V ensembles and may lead to significant sensitivity gains in N-V magnetometers dominated by technical noise sources, especially where the N−V0 population is large. With the high-N-V-density diamond sample investigated here, we demonstrate an up to 4.8-fold enhancement in the ODMR contrast. We also propose a second postprocessing method of increasing the ODMR contrast in shot-noise-limited applications. The techniques presented here may also be applied to other solid-state defects, as long as their fluorescence can be selectively modulated by means of a microwave drive. We demonstrate this utility by applying our method to isolate room-temperature spectral signatures of the V2-type silicon vacancy from an ensemble of V1 and V2 silicon vacancies in 4H silicon carbide.
Turner, M., N. Langelier, R. Bainbridge, D. Walters, S. Meesala, T. Babinec, P. Kehauas, et al. 2020. “Magnetic Field Fingerprinting of Integrated-Circuit Activity with a Quantum Diamond Microscope.” Physical Review Applied 14 (014097). Publisher's Version
2020_435_turner_supplementary.pdf
Crook, Alexander L., Christopher P. Anderson, Kevin C. Miao, Alexandre Bourassa, Hope Lee, Sam L. Bayliss, David O. Bracher, et al. 2020. “Purcell enhancement of a single silicon carbide color center with coherent spin control.” NanoLetters 20 (5): 3427-3434. Publisher's Version Abstract
Silicon carbide has recently been developed as a platform for optically addressable spin defects. In particular, the neutral divacancy in the 4H polytype displays an optically addressable spin-1 ground state and near-infrared optical emission. Here, we present the Purcell enhancement of a single neutral divacancy coupled to a photonic crystal cavity. We utilize a combination of nanolithographic techniques and a dopant-selective photoelectrochemical etch to produce suspended cavities with quality factors exceeding 5000. Subsequent coupling to a single divacancy leads to a Purcell factor of ∼50, which manifests as increased photoluminescence into the zero-phonon line and a shortened excited-state lifetime. Additionally, we measure coherent control of the divacancy ground-state spin inside the cavity nanostructure and demonstrate extended coherence through dynamical decoupling. This spin-cavity system represents an advance toward scalable long-distance entanglement protocols using silicon carbide that require the interference of indistinguishable photons from spatially separated single qubits.
2020_433_crook_nanoletters.pdf 2020_433_crook_supplementary.pdf
Gadalla, Mena, Chaudhary Kundan, Federico Capasso, and Evelyn Hu. 2020. “Imaging of surface plasmon polaritons and spoof plasmons in low-loss highly metallic titanium nitride thin films in visible and infrared regimes.” Optics Express 28 (10): 14536-14546. Publisher's Version Abstract
Titanium nitride (TiN) has been identified as a promising refractory material for high temperature plasmonic applications such as surface plasmon polaritons (SPPs) waveguides, lasers and light sources, and near field optics. Such SPPs are sensitive not only to the highly metallic nature of the TiN, but also to its low loss. We have formed highly metallic, low-loss TiN thin films on MgO substrates to create SPPs with resonances between 775-825 nm. Scanning near-field optical microscopy (SNOM) allowed imaging of the SPP fringes, the accurate determination of the effective wavelength of the SPP modes, and propagation lengths greater than 10 microns. Further, we show the engineering of the band structure of the plasmonic modes in TiN in the mid-IR regime and experimentally demonstrate, for the first time, the ability of TiN to support Spoof Surface Plasmon Polaritons in the mid-IR (6 microns wavelength).
2019
Phonons at gigahertz frequencies interact with electrons, photons, and atomic systems in solids, and therefore, have extensive applications in signal processing, sensing, and quantum technologies. Surface acoustic wave (SAW) resonators that confine surface phonons can play a crucial role in such integrated phononic systems due to small mode size, low dissipation, and efficient electrical transduction. To date, it has been challenging to achieve a high quality (Q) factor and small phonon mode size for SAW resonators at gigahertz frequencies. We present a methodology to design compact high-Q SAW resonators on lithium niobate operating at gigahertz frequencies. We experimentally verify designs and demonstrate Q factors in excess of 2×104 at room temperature (6×104 at 4 Kelvin) and mode size as low as 1.87 λ2. This is achieved by phononic band structure engineering, which provides high confinement with low mechanical loss. The frequency Q products (fQ) of our SAW resonators are greater than 1013. These high-fQ and small mode size SAW resonators could enable applications in quantum phononics and integrated hybrid systems with phonons, photons, and solid-state qubits.
Gadalla, Mena N., Andrew S. Greenspon, Michele Tamagnone, Federico Capasso, and Evelyn L. Hu. 2019. “Excitation of Strong Localized Surface Plasmon Resonances in Highly Metallic Titanium Nitride Nano-Antennas for Stable Performance at Elevated Temperatures.” ACS Applied Nanomaterials. Publisher's Version Abstract
New opportunities for plasmonic applications at high temperatures have stimulated interest in refractory plasmonic materials that show greater stability at elevated temperatures than the more commonly used silver and gold (Au). Titanium nitride (TiN) has been identified as a promising refractory material, with deposition of TiN thin films through techniques ranging from plasma-enhanced atomic laser deposition to sputter deposition to pulsed laser deposition, on a variety of substrates, including MgO, polymer, SiO2, and sapphire. A variety of plasmonic devices have been evaluated, including gratings, nanorods, and nanodisks. An implicit metric for TiN behavior has been the comparison of its plasmonic performance to that of Au, in particular at various elevated temperatures. This paper carries out a one-to-one comparison of bowtie nanoantennas formed of TiN and Au (on both Si and MgO substrates), examining the far-field characteristics, related to the measured near-field resonances. In both cases, the optical constants of the TiN films were used to simulate the expected plasmonic responses and enjoyed excellent agreement with the experimental measurements. Furthermore, we examined the consistency of the plasmonic response and the morphological changes in the TiN and Au nanoantennas at different temperatures up to 800 °C in the atmosphere. This comparison of the measured plasmonic response from nanoscale resonances to the far-field response allows for anomalies or imperfections that may be introduced by the nanofabrication processes and provides a more accurate comparison of TiN plasmonic behavior relative to the Au standard.
Supporting.pdf Manuscript.pdf video2_snom_3d_au_mgo.mp4 video1_snom_3d_tin_mgo.mp4
2018
Defo, Rodrick Kuate, Xingyu Zhang, David Bracher, Gunn Kim, Evelyn Hu, and Efthimios Kaxiras. 2018. “Energetics and kinetics of vacancy defects in 4H-SiC.” Physical Review B 98 (10): 104103. Abstract
Defect engineering in wide-gap semiconductors is important in controlling the performance of single-photon emitter devices. The effective incorporation of defects depends strongly on the ability to control their formation and location, as well as to mitigate attendant damage to the material. In this study, we combine density functional theory, molecular dyamics (MD), and kinetic Monte Carlo (KMC) simulations to study the energetics and kinetics of the silicon monovacancy VSi and related defects in 4H-SiC. We obtain the defect formation energy for VSi in various charge states and use MD simulations to model the ion implantation process for creating defects. We also study the effects of high-temperature annealing on defect position and stability using KMC and analytical models. Using a larger (480-atom) supercell than previous studies, we obtain the temperature-dependent diffusivity of VSi in various charge states and find significantly lower barriers to diffusion than previous estimates. In addition, we examine the recombination with interstitial Si and conversion of VSi into CSiVC during annealing and propose methods for using strain to reduce changes in defect concentrations. Our results provide guidance for experimental efforts to control the position and density of VSi defects within devices, helping to realize their potential as solid-state qubits.
Wang, Danqing, Tongtong Zhu, Rachel A. Oliver, and Evelyn L. Hu. 2018. “Ultra-low-threshold InGaN/GaN quantum dot micro-ring lasers.” Optics Letters 43 (4): 799-802. Publisher's Version
Greenspon, Andrew S., Brandt L Marceaux, and Evelyn L. Hu. 2018. “Robust lanthanide emitters in polyelectrolyte thin films for photonic applications.” Nanotechnology 29 (7): 075302. Publisher's Version Abstract
Trivalent lanthanides provide stable emission sources at wavelengths spanning the ultraviolet through the near infrared with uses in telecommunications, lighting, and biological sensing and imaging. We describe a method for incorporating an organometallic lanthanide complex within polyelectrolyte multilayers, producing uniform, optically active thin films on a variety of substrates. These films demonstrate excellent emission with narrow linewidths, stable over a period of months, even when bound to metal substrates. Utilizing different lanthanides such as europium and terbium, we are able to easily tune the resulting wavelength of emission of the thin film. These results demonstrate the suitability of this platform as a thin film emitter source for a variety of photonic applications such as waveguides, optical cavities, and sensors.
2017
Shi, B., S. Zhu, Q. Li, Y. Wan, E. Hu, and K. Lau. 2017. “1.55 mm room-temperature lasing from subwavelength quantum-dot microdisks directly grown on (001) Si.” Applied Physics Letters 110 (12). Publisher's Version
Shi, B., S. Zhu, Q. Li, Y. Wan, E. Hu, and K. Lau. 2017. “1.55 mm band low-threshold continuous-wave lasing from InAs/InAlGaAs quantum dot microdisks.” Optics Letters 42 (4): 679-682. Publisher's Version
Shi, B., S. Zhu, Q. Li, Y. Wan, E. Hu, and K. Lau. 2017. “Continuous-Wave optically pumped 1.55 mm InAs/InAlGaAs quantum dot microdisk lasers epitaxially grown on silicon.” ACS Photonics 4 (2): 204-210. Publisher's Version
Bracher, David O, Xingyu Zhang, and Evelyn L Hu. 2017. “Selective Purcell enhancement of two closely linked zero-phonon transitions of a silicon carbide color center.” Proceedings of the National Academy of Sciences 114 (16): 4060-4065. Publisher's Version Abstract
Point defects in silicon carbide are rapidly becoming a platform of great interest for single-photon generation, quantum sensing, and quantum information science. Photonic crystal cavities (PCCs) can serve as an efficient light–matter interface both to augment the defect emission and to aid in studying the defects’ properties. In this work, we fabricate 1D nanobeam PCCs in 4H-silicon carbide with embedded silicon vacancy centers. These cavities are used to achieve Purcell enhancement of two closely spaced defect zero-phonon lines (ZPL). Enhancements of >80-fold are measured using multiple techniques. Additionally, the nature of the cavity coupling to the different ZPLs is examined.
2016
Li, Q., Y. Wan, A. Liu, A. Gossard, J. Bowers, E. Hu, and K. Lau. 2016. “1.3 mm InAs quantum-dot micro-disk lasers on V-groove patterned and unpatterned (001) silicon.” Optics Express 24 (18). Publisher's Version
Gui, L., S. Bagheri, N. Strohfeldt, M. Hentschel, C. Zgrabik, B. Metzger, H. Linnenbank, E. Hu, and H. Giessen. 2016. “Nonlinear refractory plasmonics with Titanium Nitride Nanoantennas.” NanoLetters 16 (9): 5708-13. Publisher's Version
Wan, Y., Q. Li, A. Gossard, J. Bowers, E. Hu, and K. Lau. 2016. “Temperature characteristics of epitaxially grown InAs quantum dot micro-disk lasers on silicon for on-chip light sources.” Applied Physics Letters 109 (1). Publisher's Version

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