Applications In Quantum Photonics Research Articles

Interfacial Modulated Lattice-Polarity-Controlled Epitaxy of III-Nitride Heterostructures on Si(111)

Monolithic integration of wurtzite III-nitrides with nonpolar silicon (Si), the two most-produced semiconductor materials, is essential and critical for a broad range of applications in electronics, optoelectronics, quantum photonics, and renewable energy. To date, however, it has remained challenging to achieve III-nitride heterostructures on Si with controlled lattice-polarity. Herein, we show that such critical challenges of III-nitrides on Si can be fundamentally addressed through a unique interfacial modulated lattice-polarity-controlled epitaxy (IMLPCE). It is discovered that the lattice-polarity of aluminum nitride (AlN) grown on Si(111) is primarily determined by the AlSiN interlayer: N-polar and Al-polar AlN can be achieved by suppressing and promoting the AlSiN interlayer formation, respectively. Furthermore, we develop a unique active-nitrogen-free in situ annealing process to mitigate the AlSiN layer formation at the GaN/AlN interface, which can eliminate the inverted domain formation commonly seen in N-polar GaN on AlN/Si. This study provides an alternative approach for controlling the lattice-polarity of III-nitrides on Si substrates and will enable their seamless integration with the mature Si-based device technology.

Design and demonstration of an efficient pump rejection filter for silicon photonic applications.

Photon pair generation via spontaneous four-wave mixing in silicon waveguides/microring resonators integrated with a high extinction pump rejection filter is very much in demand for futuristic large-scale integrated quantum photonics circuits. Ideally, a distributed Bragg reflector (DBR) can be designed to offer desired pump rejection. However, fabricated DBRs suffer degradation in pump extinction due to roughness-induced unwanted scattering waves in the forward direction around the Bragg wavelength. It is therefore inferred that the roughness-induced forward scattering can be reduced significantly by integrating a DBR structure in one of the sidewalls (instead of two sidewalls) of a multimode rib waveguide (instead of a single mode strip waveguide). Therefore, we studied a single-stage DBR filter with this design which exhibits a significantly higher stop band extinction (∼63 dB), in comparison with that of earlier reported results (<50 dB). To validate the pump rejection efficiency of such fabricated devices in quantum photonic applications, we have carried out on-chip stimulated four-wave mixing experiments and shown that the pump laser within the rejection band could be attenuated to the level of idler power.

Quantum emitters in 2D materials: Emitter engineering, photophysics, and integration in photonic nanostructures

Quantum emitters have become a vital tool for both fundamental science and emerging technologies. In recent years, the focus in the field has shifted to exploration and identification of new quantum systems enabled by the emerging library of atomically thin, two dimensional materials. In this review, we highlight the current state of the art in engineering of quantum emitters in 2D systems, with an emphasis on transition metal di-chalcogenides (TMDCs) and hexagonal boron nitride. We start by reviewing progress in TMDCs, with focus on emitter engineering, ability to tune their spectral properties, and observation of interlayer excitons. We then discuss emitters in hBN and focus on emitters' origin, engineering, and emerging phenomena—spanning super-resolution imaging and optical spin readout. We summarize by discussing practical advances of integration of emitters in 2D hosts with plasmonic and dielectric photonic cavities, underpinned by quantum light–matter interactions. We conclude by outlining pathways for practical on-chip quantum photonics applications and highlight challenges and opportunities within this field of research.

Towards Integrated Single Photon Avalanche Detectors for Visible Light (Invited)

Integrated avalanche photodetectors (APDs) are essential and ubiquitous devices in quantum photonics applications. While free-space APDs are a mature technology, the development of integrated APDs for visible light is still in its infancy. In this invited talk, we review our work on integrated photodetectors – the Germanium photodetector for O band, and the first integrated silicon (Si) APD for visible light. A unique feature of the integrated Si APD system for visible light is the end-fire coupling between the silicon nitride (SiN) waveguide and the Si APD in the same layer. This allows for broadband and high-efficiency coupling of light from the SiN waveguide to the Si APD for light detection, without the drawbacks of conventional interlayer coupling. Our work on integrated Si APDs based on Geiger-mode and our progress towards achieving single photon detection for visible light is discussed in the talk.

Efficient and tunable blue light generation using lithium niobate nonlinear photonics

Thin-film lithium niobate (LN) has recently emerged as a playground for chip-scale nonlinear optics and leads to efficient frequency conversions from near-infrared to near-visible bands. For many nonlinear and quantum photonics applications, it is desirable to operate deep into the visible band within LN's transparency window. However, the strong material dispersion at short wavelengths makes phase-matching difficult, necessitating sub-micrometer scale control of domain structures for efficient phase-matching. Here, we report the operation of thin film LN in the blue wavelength and high fidelity poling of the thin-film LN waveguide to this regime. As a result, quasi-phase matching is realized between IR (871 nm) and blue (435.5 nm) wavelengths in a straight waveguide and prompts strong blue light generation with a conversion efficiency (1040% ± 140%/W). This blue second harmonic generator exhibits stable temperature tunability, which is important for applications that require precise frequency alignment, such as atomic clocks.

Front Cover: Scalable Creation of Deep Silicon‐Vacancy Color Centers in Diamond by Ion Implantation through a 1‐µm Pinhole (Adv. Quantum Technol. 12/2021)

Diamond color centers represent a promising resource for quantum photonics applications. Various approaches based on ion implantation have been developed for their controlled creation, but only at shallow depths. In article number 2100079, Lukas Hunold, Mario Agio, and co-workers show the ability to produce diamond color centers with a lateral precision of 1 µm at depths down to 1 µm and they discuss methods to improve the resolution further. These findings facilitate the post-fabrication of photonic structures and post-processing of diamond-based devices.

Bright Silicon Carbide Single-Photon Emitting Diodes at Low Temperatures: Toward Quantum Photonics Applications.

Color centers in silicon carbide have recently emerged as one of the most promising emitters for bright single-photon emitting diodes (SPEDs). It has been shown that, at room temperature, they can emit more than 109 photons per second under electrical excitation. However, the spectral emission properties of color centers in SiC at room temperature are far from ideal. The spectral properties could be significantly improved by decreasing the operating temperature. However, the densities of free charge carriers in SiC rapidly decrease as temperature decreases, which reduces the efficiency of electrical excitation of color centers by many orders of magnitude. Here, we study for the first time the temperature characteristics of SPEDs based on color centers in 4H-SiC. Using a rigorous numerical approach, we demonstrate that although the single-photon electroluminescence rate does rapidly decrease as temperature decreases, it is possible to increase the SPED brightness to 107 photons/s at 100 K using the recently predicted effect of hole superinjection in homojunction p-i-n diodes. This gives the possibility to achieve high brightness and good spectral properties at the same time, which paves the way toward novel quantum photonics applications of electrically driven color centers in silicon carbide.

Nested nonconcentric microring resonators with high-Q and large fabrication tolerance

Microring resonators are one of the most sought-after optical components for realizing several on-chip functionalities that include sensing, data routing, new frequency generation, and quantum photonic applications. Many of these applications demand a high quality factor and large notch depth (high extinction ratio), which can be achieved by critical coupling. However, the critical coupling is very sensitive to fabrication accuracies and thermal drift. Geometrical parameters of the resonators are generally swept to attain critical coupling, where a few designs can pass the critical coupling condition criteria. In this work, we propose a methodology to circumvent this vital issue. The proposed technique is based on coupled-resonator systems where two different microrings are embedded into a larger microring, referred to as a nonconcentric nested microring resonator (NN-MRR). The NN-MRR configuration relaxes the requirement of the critical coupling condition by 20% when the strip optical waveguide has either smooth or rough sidewalls. Numerical simulations reveal that, unlike standard MRR, a high Q -factor ( &gt; --> 1 0 5 ) and a large transmission notch depth &gt; --> 10 d B can be maintained irrespective of the rings’ coupling conditions for the nested MRRs. Besides the extra degree of freedom of design provided by the inner rings, the other significant advantage of the proposed NN-MRR is its compactness. We believe that the nested MRR arrangement could be highly efficient for biosensing, nonlinear, and quantum applications within a broad ambient temperature range. We have fabricated the NN-MRRs in a silicon-on-insulator platform through electron beam lithography and experimentally demonstrated the theoretical and numerical findings.

Advances in Chip‐Scale Quantum Photonic Technologies

AbstractQuantum photonic system has made remarkable achievements in computing and communication. Serving as quantum information carriers, photons are flying qubits and robust against decoherence, emerging as a desirable platform to make quantum processor a reality. However, with system's complexity and functionality scaling up, the requirements for stability, programmability, and manufacturability will be in high demand. Integrated photonics, compatible with complementary metal‐oxide‐semiconductor fabrication, has overwhelming dominance in terms of density and performance, making it an unrivaled contender for large‐scale quantum information processing (QIP). To improve the performance of individual blocks and integrate them on a common substrate is one of the core tasks for a practical quantum processor. Here, the recent advances in components that constitute quantum photonic systems on silicon photonics platform, including sources, modulators, and detectors are reviewed. Burgeoning quantum photonics applications, such as multi‐dimensional, multi‐photon QIP and integrated quantum key distribution are highlighted.

Arbitrary optical wave evolution with Fourier transforms and phase masks.

A large number of applications in classical and quantum photonics require the capability of implementing arbitrary linear unitary transformations on a set of optical modes. In a seminal work by Reck et al. [Phys. Rev. Lett.73, 58 (1994)10.1103/PhysRevLett.73.58], it was shown how to build such multiport universal interferometers with a mesh of beam splitters and phase shifters, and this design became the basis for most experimental implementations in the last decades. However, the design of Reck et al. is difficult to scale up to a large number of modes, which would be required for many applications. Here we present a deterministic algorithm that can find an exact and efficient implementation of any unitary transformation, using only Fourier transforms and phase masks. Since Fourier transforms and phase masks are routinely implemented in several optical setups and they do not suffer from some of the scalability issues associated with building extensive meshes of beam splitters, we believe that our design can be useful for many applications in photonics.

Brightening of Dark States in CsPbBr3 Quantum Dots Caused by Light-Induced Magnetism.

Lead halide perovskite quantum dots (QDs) have shown great potential for optoelectronic and quantum photonic applications. Although controversy remains about the electronic fine structures of bulk perovskites due to the strong spin-orbit coupling affecting the conduction bands, compelling evidence indicates that the ground states of perovskite QDs remain dark, limiting their applications in optoelectronic devices. Here, it is demonstrated that photoexcitation can induce large intrinsic magnetic fields in Mn-doped CsPbBr3 perovskite QDs. Equivalent to applying an external magnetic field, the light-induced field causes giant Zeeman splitting to the bright triplet states and brightens the dark singlet ground state, thus effectively rendering a partially bright ground state in the doped QDs. These findings here may contribute to the understanding of the electronic fine structures in perovskite QDs and demonstrate a potential approach for creating semiconductor nanostructures that can serve as bright light sources.

Simultaneous type-I and type-II phase matching for second-order nonlinearity in integrated lithium niobate waveguide.

Second-order optical nonlinearity is widely used for both classical and quantum photonic applications. Due to material dispersion and phase matching requirements, the polarization of optical fields is pre-defined during the fabrication. Only one type of phase matching condition is normally satisfied, and this limits the device flexibility. Here, we demonstrate that phase matching for both type-I and type-II second-order optical nonlinearity can be realized simultaneously in the same waveguide fabricated from thin-film lithium niobate. This is achieved by engineering the geometry dispersion to compensate for the material dispersion and birefringence. The simultaneous realization of both phase matching conditions is verified by the polarization dependence of second-harmonic generation. Correlated photons are also generated through parametric down conversion from the same device. This work provides a novel approach to realize versatile photonic functions with flexible devices.

Coupling Spin Defects in Hexagonal Boron Nitride to Monolithic Bullseye Cavities.

Color centers in hexagonal boron nitride (hBN) are becoming an increasingly important building block for quantum photonic applications. Herein, we demonstrate the efficient coupling of recently discovered spin defects in hBN to purposely designed bullseye cavities. We show that boron vacancy spin defects couple to the monolithic hBN cavity system and exhibit a 6.5-fold enhancement. In addition, by comparative finite-difference time-domain modeling, we shed light on the emission dipole orientation, which has not been experimentally demonstrated at this point. Beyond that, the coupled spin system exhibits an enhanced contrast in optically detected magnetic resonance readout and improved signal-to-noise ratio. Thus, our experimental results, supported by simulations, constitute a first step toward integration of hBN spin defects with photonic resonators for a scalable spin-photon interface.

Fabrication of Photonic Resonators in Bulk 4H‐SiC

AbstractThe design and engineering of photonic architectures, suitable to enhance, collect and guide light on chip is needed for applications in quantum photonics and quantum optomechanics. In this work, a Faraday cage‐based oblique angle etch method is applied to fabricate various functional photonic devices from 4H Silicon Carbide (SiC)—a material that has attracted attention in recent years, due to its potential in optomechanics, nonlinear optics, and quantum information. The processing conditions are detailed and the geometrical and optical characteristics of the fabricated devices are thoroughly addressed. Employing photoluminescence measurements high‐quality factors are demonstrated for suspended microring resonators of up to 3500 in the visible range. Such devices will be applicable in the future to augment the properties of SiC in integrated on chip quantum photonics.

Tunable dual-channel ultra-narrowband Bragg grating filter on thin-film lithium niobate.

We demonstrate dual-channel phase-shifted Bragg grating filters in the telecom band on thin-film lithium niobate. These integrated tunable ultra-narrow linewidth filters are crucial components for optical communication and sensing systems, as well as future quantum-photonic applications. Thin-film lithium niobate is an emerging platform suitable for these applications and has been exploited in this Letter. The demonstrated device has an extinction ratio of 27 dB and two channels with close linewidths of about 19 pm (quality factor of ${8} \times {{10}^4}$), separated by 19 GHz. The central wavelength could be efficiently tuned using the high electro-optic effect in lithium niobate with a tuning factor of 3.83 pm/V. This demonstration can be extended to tunable filters with multiple channels, along with desired frequency separations and optimized tunability, which would be useful for a variety of complex photonic integrated circuits.

Near‐Zero Index Photonic Crystals with Directive Bound States in the Continuum

AbstractNear‐zero‐index platforms arise as a new opportunity for light manipulation with boosting of optical nonlinearities, transmission properties in waveguides, and constant phase distribution. In addition, they represent a solution to impedance mismatch faced in photonic circuitry offering several applications in quantum photonics, communication, and sensing. However, their realization is limited to the availability of materials that could exhibit such low index. For materials used in the visible and near‐infrared wavelengths, the intrinsic losses annihilate most of near‐zero index properties. The design of all‐dielectric photonic crystals with specific electromagnetic modes overcomes the issue of intrinsic losses while showing effective mode index near zero. Nonetheless, these modes strongly radiate to the surrounding environment, greatly limiting the devices applications. Here, a novel all‐dielectric photonic crystal structure is explored that is able to sustain effective near‐zero‐index modes coupled to directive bound‐states in the continuum in order to decrease radiative losses, opening extraordinary opportunities for radiation manipulation in nanophotonic circuits. Moreover, its relatively simple design and phase stability facilitate integration and reproducibility with other photonic components.

Racetrack microresonator based electro-optic phase shifters on a 3C silicon-carbide-on-insulator platform.

We report, to the best of our knowledge, the first demonstration of integrated electro-optic (EO) phase shifters based on racetrack microresonators on a 3C silicon-carbide-on-insulator (SiCOI) platform working at near-infrared wavelengths. By applying DC voltage in the crystalline axis perpendicular to the waveguide plane, we have observed optical phase shifts from the racetrack microresonators whose loaded quality ($ Q $) factors are $\sim\! {30,\!000}$. We show voltage-length product (${{V}_{\pi}} \cdot {{L}_{ \pi}}$) of ${118}\;{{\rm V}\cdot{\rm cm}}$, which corresponds to an EO coefficient ${{r}_{41}}$ of 2.6 pm/V. The SiCOI platform can be used to realize tunable silicon carbide integrated photonic devices that are desirable for applications in nonlinear and quantum photonics over a wide bandwidth that covers visible and infrared wavelengths.

Metasurfaces for quantum photonics

Rapid progress in the development of metasurfaces allowed to replace bulky optical assemblies with thin nanostructured films, often called metasurfaces, opening a broad range of novel and superior applications to the generation, manipulation, and detection of light in classical optics. Recently, these developments started making a headway in quantum photonics, where novel opportunities arose for the control of nonclassical nature of light, including photon statistics, quantum state superposition, quantum entanglement, and single-photon detection. In this Perspective, we review recent progress in the field of quantum-photonics applications of metasurfaces, focusing on innovative and promising approaches to create, manipulate, and detect nonclassical light.

Scalable and Deterministic Fabrication of Quantum Emitter Arrays from Hexagonal Boron Nitride.

We demonstrate the fabrication of large-scale arrays of single-photon emitters (SPEs) in hexagonal boron nitride (hBN). Bottom-up growth of hBN onto nanoscale arrays of dielectric pillars yields corresponding arrays of hBN emitters at the pillar sites. Statistical analysis shows that the pillar diameter is critical for isolating single defects, and diameters of ∼250 nm produce a near-unity yield of a single emitter at each pillar site. Our results constitute a promising route toward spatially controlled generation of hBN SPEs and provide an effective and efficient method to create large-scale SPE arrays. The results pave the way to scalability and high throughput fabrication of SPEs for advanced quantum photonic applications.

Nano-structure made of donor-acceptor quantum dots embedded in three dimensional photonic crystals for all optical tunable quantum memory/switch applications

A robust, energy efficient, tunable as well as self-directed quantum structure made of two different sized QDs is proposed for switching and memory applications in quantum photonics. Three dimensional photonic crystal (PC) made of nonlinear polystyrene spheres is considered as the substrate to carry the closely located dots. Theoretical model based on density matrix approach is introduced to evaluate the physical features of the proposed system by taking in to consideration of the near field optical energy transfers whose spatial nature is characterized by a Yukawa-type potential. Furthermore, all couplings of excitons to thermal bath besides of the possible intra-dot relaxations are considered in modellings. Probabilities of the quantum states and switchings in between the states are studied systematically over time by exploring the populations of 8 possible configurations. In order to better understand the potential of the suggested structure to be utilized as a tunable quantum element, dependence of the possible absorption/dispersions and the population of each quantum state, to the structure temperature and features of the illuminating lights are discussed in detail. Results clearly reflect that energy transfer in subwavelength range is possible using the suggested system via combined effects of near field interactions and relaxations of energy levels. Then this study provides insight into design and fabrication of structures based on network of QDs embedded in PC that are both self-tunable as well as being controllable using external factors and thus meet the goals of integrated photonic circuits.