Search for: All records

Cavities in large-scale photonic integrated circuits (PICs) 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, which we named mode index engineering (MIE). 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 post-fabrication step, providing robust passive tuning in large-scale PICs. We show that the proposed method achieves a tuning resolution below 1/Q and a range of up to 10³/Q for visible to near-infrared wavelengths. Furthermore, this method can be applied and explored in various integrated photonic cavities and material configurations. 

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 film 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. This roughness, as perturbations couple light from guided to far-field radiation modes, leads to scattering losses that can be estimated from theoretical models. Typically, with UV-based lithography, sidewall roughness is significantly larger than wafer-top surface roughness. Atomic force microscopy (AFM) imaging measurement gives a 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 (SEM) imaging. We characterized the loss on two homemade nitride and oxide films on 3-inch silicon wafers with 12 waveguide devices on each and correlated the scattering loss estimated from a 2D image-based sidewall profile and theoretical Payne model. The lowest loss of guided fundamental transverse electric (TE₀) mode is found at 0.075 dB/cm at 633 nm across 24 devices, a record at visible wavelength. Our work shows 100% success (edge continuity span exceeding 95% of image width/height) in edge detection in image processing of all images to estimate autocorrelation function and optical mode loss. These demonstrations offer valuable insights into waveguide sidewall roughness and a comparison of experimental and 2D SEM image processing based loss estimations with applications in loss characterization at wafer-scale PICs. 

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 requires a buildup of coherence in an ensemble of incoherently excited QDs via their coupling to a common radiation mode. In perovskite QDs, superradiance has been rarely observed, unlike superfluorescence, due to the challenge in QD electronic coupling. Here, we report superradiance with a very narrow linewidth (<5 meV) and a large redshift (∼200 meV) from the strongly coupled CsPbBr3 QD superlattice achieved through the combination of quantum confinement and ligand engineering. The superradiance is polarized in contrast to the uncoupled exciton emission from the same superlattice, indicating anisotropic electronic coupling in superlattices. This finding suggests the potential of a perovskite QD superlattice with structurally controllable interdot coupling as the polarized cooperative photon emitters