arXiv subjects

Johannes E. Fröch

Publications and source records attributed to Johannes E. Fröch.

At least 37 records · Page 2Linked to original sources

arXiv2023

Large Field-of-View Thermal Imaging via All-Silicon Meta-Optics

A broad range of imaging and sensing technologies in the infrared require large Field-of-View (FoV) operation. To achieve this, traditional refractive systems often employ multiple elements to compensate for aberrations, which leads to excess size, weight, and cost. For many applications, including night vision eye-wear, air-borne surveillance, and autonomous navigation for unmanned aerial vehicles, size and weight are highly constrained. Sub-wavelength diffractive optics, also known as meta-optics, can dramatically reduce the size, weight, and cost of these imaging systems, as meta-optics are significantly thinner and lighter than traditional refractive lenses. Here, we demonstrate 80$^\circ$ FoV thermal imaging in the long-wavelength infrared regime (8-12 $μ$m) using an all-silicon meta-optic with an entrance aperture and lens focal length of 1 cm.

physics.optics↗

arXiv2022

Large FOV short-wave infrared meta-lens for scanning fiber endoscopy

The scanning fiber endoscope (SFE), an ultra-small optical imaging device with a large field-of-view (FOV) for having a clear forward view into the interior of blood vessels, has great potential in the cardio-vascular disease diagnosis and surgery assistance, which is one of the key applications for short-wave infrared (SWIR) biomedical imaging. The state-of-the-art SFE system uses a miniaturized refractive spherical lens doublet for beam projection. A meta-lens is a promising alternative which can be made much thinner and has fewer off-axis aberrations than its refractive counterpart. We report an SFE system with meta-lens working at 1310nm to achieve a resolution ($\sim 140μm$ at the center of field and the imaging distance of $15mm$), FOV ($\sim 70 \circ$), and depth-of-focus (DOF $\sim 15mm$), which are comparable to a state-of-the-art refractive lens SFE. The use of the meta-lens reduces the length of the optical track from $1.2mm$ to $0.86mm$. The resolution of our meta-lens based SFE drops by less than a factor of $2$ at the edge of the FOV, while the refractive lens counterpart has a $\sim 3$ times resolution degradation. These results show the promise of integrating a meta-lens into an endoscope for device minimization and optical performance improvement.

physics.optics↗

arXiv2022

Foveated Thermal Computational Imaging in the Wild Using All-Silicon Meta-Optics

Foveated imaging provides a better tradeoff between situational awareness (field of view) and resolution and is critical in long-wavelength infrared regimes because of the size, weight, power, and cost of thermal sensors. We demonstrate computational foveated imaging by exploiting the ability of a meta-optical frontend to discriminate between different polarization states and a computational backend to reconstruct the captured image/video. The frontend is a three-element optic: the first element which we call the "foveal" element is a metalens that focuses s-polarized light at a distance of $f_1$ without affecting the p-polarized light; the second element which we call the "perifoveal" element is another metalens that focuses p-polarized light at a distance of $f_2$ without affecting the s-polarized light. The third element is a freely rotating polarizer that dynamically changes the mixing ratios between the two polarization states. Both the foveal element (focal length = 150mm; diameter = 75mm), and the perifoveal element (focal length = 25mm; diameter = 25mm) were fabricated as polarization-sensitive, all-silicon, meta surfaces resulting in a large-aperture, 1:6 foveal expansion, thermal imaging capability. A computational backend then utilizes a deep image prior to separate the resultant multiplexed image or video into a foveated image consisting of a high-resolution center and a lower-resolution large field of view context. We build a first-of-its-kind prototype system and demonstrate 12 frames per second real-time, thermal, foveated image, and video capture in the wild.

physics.optics↗

arXiv2022

Coherent Manipulation with Resonant Excitation and Single Emitter Creation of Nitrogen Vacancy Centers in 4H Silicon Carbide

Silicon carbide (SiC) has become a key player in realization of scalable quantum technologies due to its ability to host optically addressable spin qubits and wafer-size samples. Here, we have demonstrated optically detected magnetic resonance (ODMR) with resonant excitation, and clearly identified the ground state energy levels of the NV centers in 4H-SiC. Coherent manipulation of NV centers in SiC has been achieved with Rabi and Ramsey oscillations. Finally, we show the successful generation and characterization of single nitrogen vacancy (NV) center in SiC employing ion implantation. Our results are highlighting the key role of NV centers in SiC as a potential candidate for quantum information processing.

cond-mat.mes-hall↗

arXiv2022

Inverse-Designed Meta-Optics with Spectral-Spatial Engineered Response to Mimic Color Perception

Meta-optics have rapidly become a major research field within the optics and photonics community, strongly driven by the seemingly limitless opportunities made possible by controlling optical wavefronts through interaction with arrays of sub-wavelength scatterers. As more and more modalities are explored, the design strategies to achieve desired functionalities become increasingly demanding, necessitating more advanced design techniques. Herein, the inverse-design approach is utilized to create a set of single-layer meta-optics that simultaneously focus light and shape the spectra of focused light without using any filters. Thus, both spatial and spectral properties of the meta-optics are optimized, resulting in spectra that mimic the color matching functions of the CIE 1931 XYZ color space, which links the distributions of wavelengths in light and the color perception of a human eye. Experimental demonstrations of these meta-optics show qualitative agreement with the theoretical predictions and help elucidate the focusing mechanism of these devices.

physics.optics↗

arXiv2022

Monolithic Silicon Carbide Metalenses

Silicon carbide has emerged as a promising material platform for quantum photonics and nonlinear optics. These properties make the development of integrated photonic components in high-quality silicon carbide a critical aspect for the advancement of scalable on-chip networks. In this work, we numerically design, fabricate and demonstrate the performance of monolithic metalenses from SiC suitable for on-chip optical operations. We engineer two distinct lenses, with parabolic and cubic phase profiles, operating in the near infrared spectral range, which is of interest for quantum and photonic applications. We support the lens fabrication by optical transmission measurement and characterize the focal points of the lenses. Our results will accelerate the development of SiC nanophotonic devices and aid in an on chip integration of quantum emitters with meta-optical components.

physics.optics↗

arXiv2022

Broadband nonvolatile electrically programmable silicon photonic switches

Programmable photonic integrated circuits (PICs) have recently gained significant interest due to their potential in creating next-generation technologies ranging from artificial neural networks and microwave photonics to quantum information processing. The fundamental building block of such programmable PICs is a tunable 2 x 2 switch, traditionally controlled by the thermo-optic or free-carrier dispersion. Yet, these implementations are power-hungry, volatile, and have a large footprint (typically > 100 um). Therefore, a truly 'set-and-forget' type 2 x 2 switch with zero static power consumption is highly desirable for large-scale PICs. Here, we report a broadband nonvolatile electrically programmable 2 x 2 silicon photonic switch based on the phase-change material Ge2Sb2Te5. The directional coupler switch exhibits a compact coupling length (64 um), small insertion loss (<2 dB), and minimal crosstalk (<-8 dB) across the entire telecommunication C-band while maintaining a record-high endurance of over 2,800 switching cycles. This demonstrated switch constitutes a critical component for realizing future generic programmable silicon photonic systems.

physics.app-ph↗

arXiv2021

Purcell enhancement of a cavity-coupled emitter in hexagonal boron nitride

Integration of solid state quantum emitters into nanophotonic circuits is a critical step towards fully on-chip quantum photonic based technologies. Among potential materials platforms, quantum emitters in hexagonal boron nitride have emerged over the last years as viable candidate. While the fundamental physical properties have been intensively studied over the last years, only few works have focused on the emitter integration into photonic resonators. Yet, for a potential quantum photonic material platform, the integration with nanophotonic cavities is an important cornerstone, as it enables the deliberate tuning of the spontaneous emission and the improved readout of distinct transitions for that quantum emitter. In this work, we demonstrate the resonant tuning of an integrated monolithic hBN quantum emitter in a photonic crystal cavity through gas condensation at cryogenic temperature. We resonantly coupled the zero phonon line of the emitter to a cavity mode and demonstrate emission enhancement and lifetime reduction, with an estimation for the Purcell factor of ~ 15.

physics.optics↗

arXiv2021

Coupling Spin Defects in a Layered Material to Nanoscale Plasmonic Cavities

Spin defects in hexagonal boron nitride, and specifically the negatively charged boron vacancy (VB) centres, are emerging candidates for quantum sensing. However, the VB defects suffer from low quantum efficiency and as a result exhibit weak photoluminescence. In this work, we demonstrate a scalable approach to dramatically enhance the VB- emission by coupling to a plasmonic gap cavity. The plasmonic cavity is composed of a flat gold surface and a silver cube, with few-layer hBN flakes positioned in between. Employing these plasmonic cavities, we extracted two orders of magnitude in photoluminescence enhancement associated with a corresponding 2 fold enhancement in optically detected magnetic resonance contrast. The work will be pivotal to progress in quantum sensing employing 2D materials, and realisation of nanophotonic devices with spin defects in hexagonal boron nitride.

physics.optics↗

arXiv2021

Integration of hBN quantum emitters in monolithically fabricated waveguides

Hexagonal boron nitride (hBN) is gaining interest for potential applications in integrated quantum nanophotonics. Yet, to establish hBN as an integrated photonic platform several cornerstones must be established, including the integration and coupling of quantum emitters to photonic waveguides. Supported by simulations, we study the approach of monolithic integration, which is expected to have coupling efficiencies that are 4 times higher than those of a conventional hybrid stacking strategy. We then demonstrate the fabrication of such devices from hBN and showcase the successful integration of hBN single photon emitters with a monolithic waveguide. We demonstrate coupling of single photons from the quantum emitters to the waveguide modes and on-chip detection. Our results build a general framework for monolithically integrated hBN single photon emitter and will facilitate future works towards on-chip integrated quantum photonics with hBN.

physics.app-ph↗

arXiv2021

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 the all monolithic hBN cavity system exhibits an order of magnitude enhancement in the emission of the coupled boron vacancy spin defects. In addition, by comparative finite difference time domain modelling, 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 towards integration of hBN spin defects with photonic resonators for a scalable spin photon interface.

physics.optics↗

arXiv2021

Recoil Implantation Using Gas-Phase Precursor Molecules

Ion implantation underpins a vast range of devices and technologies that require precise control over the physical, chemical, electronic, magnetic and optical properties of materials. A variant termed recoil implantation - in which a precursor is deposited onto a substrate as a thin film and implanted via momentum transfer from incident energetic ions - has a number of compelling advantages, particularly when performed using an inert ion nano-beam [Fröch et al., Nat Commun 11, 5039 (2020)]. However, a major drawback of this approach is that the implant species are limited to the constituents of solid thin films. Here we overcome this limitation by demonstrating recoil implantation using gas-phase precursors. Specifically, we fabricate nitrogen-vacancy (NV) color centers in diamond using an Ar ion beam and the nitrogen-containing precursor gases N2, NH3 and NF3. Our work expands the applicability of recoil implantation to most of the periodic table, and to applications in which thin film deposition or removal is impractical.

physics.app-ph↗

arXiv2021

Fabrication of photonic resonators in bulk 4H-SiC

The 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 we apply a Faraday cage based oblique angle etch method to fabricate various functional photonic devices from 4H Silicon Carbide - a material that has attracted attention in recent years, due to its potential in optomechanics, nonlinear optics and quantum information. We detail the processing conditions and thoroughly address the geometrical and optical characteristics of the fabricated devices. Employing photoluminescence measurements we demonstrate high quality factors 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.

physics.optics↗

arXiv2021

Nanofabrication of high Q, transferable, diamond resonators

Advancement of diamond based photonic circuitry requires robust fabrication protocols of key components, including diamond resonators and cavities. Here, we present 1D (nanobeam) photonic crystal cavities generated from single crystal diamond membranes utilising a metallic tungsten layer as a restraining, conductive and removable hard mask. The use of tungsten instead of a more conventional silicon oxide layer enables good repeatability and reliability of the fabrication procedures. The process yields high quality diamond cavities with quality factors (Q factors) approaching 10$^$4. Finally, we show that the cavities can be picked up and transferred onto a trenched substrate to realise fully suspended diamond cavities. Our fabrication process demonstrates the capability of diamond membranes as modular components for broader diamond based quantum photonic circuitry.

physics.optics↗

arXiv2020

Versatile Direct Writing of Dopants in a Solid State Host Through Recoil Implantation

Modifying material properties at the nanoscale is crucially important for devices in nanoelectronics, nanophotonics and quantum information. Optically active defects in wide band gap materials, for instance, are vital constituents for the realisation of quantum technologies. Yet, the introduction of atomic defects through direct ion implantation remains a fundamental challenge. Herein, we establish a universal method for material doping by exploiting one of the most fundamental principles of physics - momentum transfer. As a proof of concept, we direct-write arrays of emitters into diamond via momentum transfer from a Xe+ focused ion beam (FIB) to thin films of the group IV dopants pre-deposited onto a diamond surface. We conclusively show that the technique, which we term knock-on doping, can yield ultra-shallow dopant profiles localized to the top 5 nm of the target surface, and use it to achieve sub-50 nm lateral resolution. The knock-on doping method is cost-effective, yet very versatile, powerful and universally suitable for applications such as electronic and magnetic doping of atomically thin materials and engineering of near-surface states of semiconductor devices.

physics.app-ph↗

arXiv2020

Valley Polarization Enhancement Induced by a Single Chiral Nanoparticle

Valley polarization is amongst the most critical attributes of atomically thin materials. However, achieving a high contrast from monolayer transition metal dichalcogenides (TMDs) has so far been challenging. In this work, a giant valley polarization contrast up to 45% from a monolayer WS2 has been achieved at room temperature by using a single chiral plasmonic nanoparticle. The increased contrast is attributed to the selective enhancement of both the excitation and the emission rate having one particular handedness of the circular polarization. The experimental results were corroborated by the optical simulation using finite-difference time-domain (FDTD) method. Additionally, the single chiral nanoparticle enabled the observation of valley-polarized luminescence with a linear excitation. Our results provide a promising pathway to enhance valley contrast from monolayer TMDs and utilize them for nanophotonic devices.

physics.app-ph↗

arXiv2020

Engineering spin defects in hexagonal boron nitride

Two-dimensional hexagonal boron nitride offers intriguing opportunities for advanced studies of light-matter interaction at the nanoscale, specifically for realizations in quantum nanophotonics. Here, we demonstrate the engineering of optically-addressable spin defects based on the negatively-charged boron vacancy center. We show that these centers can be created in exfoliated hexagonal boron nitride using a variety of focused ion beams (nitrogen, xenon and argon), with nanoscale precision. Using a combination of laser and resonant microwave excitation, we carry out optically detected magnetic resonance spectroscopy measurements, which reveal a zero-field ground state splitting for the defect of ~3.46 GHz. We also perform photoluminescence excitation spectroscopy and temperature dependent photoluminescence measurements to elucidate the photophysical properties of the center. Our results are important for advanced quantum and nanophotonics realizations involving manipulation and readout of spin defects in hexagonal boron nitride.

physics.app-ph↗

arXiv2020

Photonic nanobeam cavities with nano-pockets for efficient integration of fluorescent nanoparticles

Integrating fluorescent nanoparticles with high-Q, small mode volume cavities is indispensable for nanophotonics and quantum technologies. To date, nanoparticles have largely been coupled to evanescent fields of cavity modes, which limits the strength of the interaction. Here, we developed both a cavity design and a fabrication method that enable efficient coupling between a fluorescent nanoparticle and a cavity optical mode. The design consists of a fishbone-shaped, one-dimensional photonic crystal cavity with a nano-pocket located at the electric field maximum of the fundamental optical mode. Furthermore, the presence of a nanoparticle inside the pocket reduces the mode volume substantially and induces subwavelength light confinement. Our approach opens exciting pathways to achieve strong light confinement around fluorescent nanoparticles for applications in energy, sensing, lasing and quantum technologies.

physics.app-ph↗