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Photonic integrated circuits (PICs) are vital for developing affordable, high-performance optoelectronic devices that can be manufactured at an industrial scale, driving innovation and efficiency in various applications. Optical loss of modes in thin film waveguides and devices is a critical measure of their performance. Thin films growth, lithography, masking, and etching processes are imperfect processes that introduce significant sidewall and top-surface roughness and cause dominating optical losses in waveguides and photonic structures. These roughness as perturbations couple light from guided to far-field radiation modes, leading to scattering losses that can be estimated from theoretical models. Typically, with UV-based lithography sidewall roughness is found to be significantly larger than wafer-top surface roughness. Atomic force microscopy (AFM) imaging measurement gives 3D and high-resolution roughness profile but the measurement is inconvenient, costly, and unscalable for large-scale PICs and at wafer-scale. Here, we evaluate the sidewall roughness profile based on 2D high-resolution scanning electron microscope imaging. We characterized the loss on two homemade nitride and oxide films on 3-inch silicon wafers with 12 waveguide devices on each and co-related the scattering loss estimated from a 2D image-based sidewall profile and theoretical Payne model. The lowest loss of guided fundamental transverse electric (TE$_{0}$) is found at 0.075 dB/cm at 633 nm across 24 devices, which is a record at visible wavelength. Our work shows a 100% success in edge detection in image processing to estimate autocorrelation function and optical mode loss. These demonstrations offer valuable insights into waveguide sidewall roughness and comparison of experimental and 2D SEM image-processing-based loss estimations., Comment: 10 pages, 6 figures

Cooperative emission of photons from an ensemble of quantum dots (QDs) as superradiance can arise from the electronically coupled QDs with a coherent emitting excited state. This contrasts with superfluorescence (Dicke superradiance), where the cooperative photon emission occurs via a spontaneous buildup of coherence in an ensemble of incoherently excited QDs via their coupling to a common radiation mode. While superfluorescence has been observed in perovskite QD systems, reports of superradiance from the electronically coupled ensemble of perovskite QDs are rare. Here, we demonstrate the generation of polarized superradiance with a very narrow linewidth (<5 meV) and a large redshift (~200 meV) from the electronically coupled CsPbBr3 QD superlattice achieved through a combination of strong quantum confinement and ligand engineering. In addition to photon bunching at low excitation densities, the superradiance is polarized in contrast to the uncoupled exciton emission from the same superlattice. This finding suggests the potential for obtaining polarized cooperative photon emission via anisotropic electronic coupling in QD superlattices even when the intrinsic anisotropy of exciton transition in individual QDs is weak.

Photonic integrated circuits require a cladding material on top to prevent any outside interaction with the photonic circuit elements and electromagnetic modes that could cause damage. Mohit et al. proposed selective tuning of the resonance of a cavity by using the mode-index engineering method. However, their work did not consider adding a cladding (upper cladding) material on top of the tuned cavity. In this study, I aim to build upon their work by investigating the impact of depositing cladding material on the frequency distribution of tuned cavities through analytical studies and numerical experiments. I identify crucial calculation parameters and discuss the optimum conditions for high-resolution tuning and large tuning range., Comment: 5 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:2409.04422

We present the characterization of the photonic waveguide resonator using confocal laser scanning microscopy imaging method. Free space TEM$_{00}$ laser mode is coupled into quasi-TE$_{0}$ waveguide mode using confocal microscopy via a diffractive grating coupler and vice versa. Our work includes the design, fabrication, and experimental characterization of a silicon nitride racetrack-shaped resonator of length ~ 165 um. We illustrate clear evidence of resonance excitation from the confocal microscope image and demonstrate loaded Q-factor and finesse ~ 8.2 \pm 0.17 * 10^4 and ~ 180 \pm 3.5, respectively. We further demonstrate its one application in air density sensing by measuring the resonance wavelength shifts with variation in environment air pressure. Our work impacts spectroscopy, imaging, and sensing applications of single or ensemble atoms or molecules coupled to photonic devices. Additionally, our study highlights the potential of confocal microscopy for analyzing photonic components on large-scale integrated circuits, providing high-resolution imaging and spectral characterization., Comment: 12 pages, 19 figures

Cavities in large-scale photonic integrated circuits often suffer from a wider distribution of resonance frequencies due to fabrication errors. It is crucial to adjust the resonances of cavities using post-processing methods to minimize the frequency distribution. We have developed a concept of passive tuning by manipulating the mode index of a portion of a microring cavity. Through analytical studies and numerical experiments, we have found that depositing a thin film of dielectric material on top of the cavity or etching the material enables us to fine-tune the resonances and minimize the frequency distribution. This versatile method allows for the selective tuning of each cavity's resonance in a large set of cavities in a single fabrication step, providing robust passive tuning in large-scale photonic integrated circuits. We show that proposed method achieves tuning resolution below 1/Q and range upto 10^3/Q for visible to near-infrared wavelengths. Furthermore, this method can be applied and explored in various optical cavities and material configurations., Comment: 17 pages, 14 figures

Devices based on photonic integrated circuits play a crucial role in the development of low-cost, high-performance, industry-scale manufacturable sensors. We report the design, fabrication, and application of a silicon nitride waveguide-based integrated photonic sensor in Young's interferometer configuration combined with Complementary Metal-Oxide-Semiconductor (CMOS) imaging detection. We use a finite-difference time-domain method to analyze the performance of the sensor device and optimize the sensitivity of the fundamental transverse-electric (TE) mode. We develop a low-cost fabrication method for the photonic sensor chip, using photolithography-compatible dimensions, and produce the sensing region with wet-etching of silicon dioxide. We demonstrate the sensor's functioning by measuring the optical phase shift with glucose concentration in an aqueous solution. We obtain consistent interference patterns with fringe visibility exceeding 0.75 and measure the phase differences for glucose concentrations in the 10 ug/ml order, corresponding to the order of 10^7 molecules in the sensing volume. We envision extending this work to functionalized surface sensors based on molecular binding. Our work will impact biosensing applications and, more generally, the fabrication of interferometric-based photonic devices., Comment: 16 pages, 12 figures

Hot electrons play a crucial role in enhancing the efficiency of photon-to-current conversion or photocatalytic reactions. In semiconductor nanocrystals, energetic hot electrons capable of photoemission can be generated via the upconversion process involving the dopant-originated intermediate state, currently known only in Mn-doped cadmium chalcogenide quantum dots. Here, we report that Mn-doped CsPbBr3 nanocrystals are an excellent platform for generating hot electrons via upconversion that can benefit from various desirable exciton properties and the structural diversity of metal halide perovskites (MHP). 2-dimensional Mn-doped CsPbBr$_3$ nanoplatelets are particularly advantageous for hot electron upconversion due to the strong exciton-dopant interaction mediating the upconversion process. Furthermore, nanoplatelets reveal evidence for the hot electron upconversion via long-lived dark exciton in addition to bright exciton that may enhance the upconversion efficiency. This study not only establishes the feasibility of hot electron upconversion in MHP host but also demonstrates the potential merits of 2-dimensional MHP nanocrystals in hot electron upconversion.

We present the design, fabrication and characterization of high quality factor silicon nitride nanobeam PhC cavities at visible wavelengths for coupling to diamond color centers in a cavity QED system. We demonstrate devices with a quality factor of about 24;000 around the zero-phonon line of the germanium-vacancy center in diamond. We also present an efficient fiber-to-waveguide coupling platform for suspended nanophotonics. By gently changing the corresponding effective indices at the fiber-waveguide interface, we achieve an efficiency of about 96% at the cavity resonance.

Dark ground state exciton in semiconductor nanocrystals has been a subject of much interest due to its long lifetime attractive for applications requiring long-lived electronic or spin states. Significant effort has been made recently to explore and access the dark exciton level in metal halide perovskite nanocrystals, which are emerging as a superior source of photons and charges compared to other existing semiconductor nanocrystals. However, the direct observation of long-lived photoluminescence from dark exciton has remained elusive in metal halide perovskite nanocrystals. Here, we report the observation of the intense emission from dark ground state exciton with 1-10 us lifetime in strongly quantum confined CsPbBr3 nanocrystals, which contrasts the behavior of weakly confined system explored so far. The study in CsPbBr3 nanocrystals with varying degree of confinement has revealed the crucial role of quantum confinement in enhancing the bright-dark exciton level splitting which is important for accessing the dark exciton. Our work demonstrates the future potential of strongly quantum-confined perovskite nanocrystals as a new platform to utilize dark excitons.