Sarvagya Dwivedi

In this work, we report the growth of 300 nm c-axis oriented barium titanate (BTO) films on (001)-strontium titanate (STO) substrates via off-axis radio frequency (RF) sputtering. The as-grown films exhibit exceptional crystallinity, with a rocking curve full width at half maximum of ∼0.03°, and atomically smooth surface morphology with a root mean square roughness of 0.23 nm, ensuring minimal optical losses. Transmission ellipsometric characterization of the film reveals a record-high electro-optic (EO) coefficient, r ₅₁ ∼ 550 pm V⁻¹ that exceeds by a factor of 3 compared to the previously reported values for RF-sputtered BTO. This high-quality, industry-compatible material paves the way for the development of efficient and compact EO modulators based on hybrid BTO-on-silicon waveguides.

We demonstrate efficient on-chip green light generation via frequency upconversion in silicon nitride–thin-film lithium niobate (SiN-TFLN) hybrid waveguides, obtained by transfer printing LN coupons on selected areas of photonic integrated circuits (PICs). By utilizing modal phase matching (MPM), our devices achieve a high normalized conversion efficiency of 42.5% W⁻¹cm⁻² in a single-pass, 2.4-mm-long waveguide configuration. The SiN–LN transition in the waveguide inherently facilitates mode conversion, transforming a higher-order second-harmonic mode into a fundamental TE mode, ensuring coherent, narrow-linewidth, green light emission. Our waveguide platform gives rise to a wavelength shift of ∼1 nm for every 10 nm of waveguide width variation and temperature-induced wavelength tuning of ∼0.02 nm/°C.

A laser source that is both narrow-linewidth and frequency-agile is essential for FMCW applications, such as LIDAR or distributed optical fiber sensing (DOFS). In this work, we present the design and characterization of a laser architecture based on the heterogeneous integration of III-V gain material on a SiN photonic circuit that meets these specifications. By incorporating a dual-ring resonator mirror in the cavity, we achieved a linewidth of 2.4 kHz and a large frequency excursion. We obtained chirps with an amplitude of 20 GHz, with residual nonlinearities of less than 1%. A preliminary DOFS measurement was carried out, demonstrating the excellent performance of the laser.

We present the design and experimental evaluation of low-loss advanced bends in silicon nitride (SiN) waveguides for the C-band. The advanced bends, with a radius of 25 μm, exhibit a bending loss of approximately 0.025 dB per 90°, comparable to the loss of a circular bend with a radius of 50 μm. Consequently, the 25 μm radius advanced bend is proposed for routing in SiN photonic integrated circuits to reduce the overall footprint. Furthermore, the use of these advanced bends in micro ring resonators results in quality factors of 5.8×10³ and 5.5×10⁴, with relatively large free spectral ranges and extinction ratios for radii of 15 μm and 25 μm, respectively.

Leveraging its superior waveguide properties, silicon-nitride (Si₃N₄) photonics is emerging to expand the applications of photonic integrated circuits to optical systems where bulk optics and fibers today still dominate. In order to fully leverage its advantages, heterogeneous integration of III-V gain elements on Si₃N₄ is one of the most critical steps. In this paper, we demonstrate a III-V-on-Si₃N₄ widely tunable narrow-linewidth laser based on micro-transfer printing. Detailed design considerations of the tolerant III-V-to-Si₃N₄ vertical coupler, Si₃N₄-based micro-ring resonators (MRRs), and micro-heaters are discussed. By introducing the dispersion of Si₃N₄ waveguide in the design, the proposed Vernier MRRs enable an extended tuning range over multiple Vernier periods. The laser shows a wavelength tuning range of 54 nm in C and L bands with intrinsic linewidth less than 25 kHz. Within the tuning range, the side mode suppression ratio is larger than 40 dB and the output power in the Si₃N₄ waveguide reaches 6.3 mW. The integration process allows for the fabrication and quality control of both the Si₃N₄ circuits and III-V devices in its own foundry, which greatly enhances the integration yield and paves the way for large-scale integration.

We propose an all-optical neural network where signal processing requires no optical-to-electrical conversion. Weight multiplication, addition, and nonlinear activation of artificial neurons are performed in the photonic domain. Successful implementation will advance photonic neuromorphic computing, enabling practical solutions in artificial intelligence-driven tasks.

A fully-functional photonic integrated circuit (PIC) platform with supporting active and passive components in the extended short- and mid-wave infrared spectral regime is of significant research interest for next-generation optical systems. Here we design offset quantum well-based photonic integrated circuits which primarily consist of four section-based widely tunable single-mode lasers emitting at 2560 nm. The platform requires the selective removal of InGaAsSb multi-quantum wells located above a GaSb-based optical waveguide layer and then subsequent single blanket GaSb regrowth. Encouraging preliminary experimental results on regrowth are also reported to confirm the feasibility of the proposed PICs. The simulation result for the tunable laser design shows that a tuning range as wide as ~120 nm is possible. The quasi-theoretical work performed here is an initial step towards demonstrating complex non-telecommunication PICs which could offer a comprehensive range of photonic functionalities.

Optical phased arrays (OPAs) with phase-monitoring and phase-control capabilities are necessary for robust and accurate beamforming applications. This paper demonstrates an on-chip integrated phase calibration system where compact phase interrogator structures and readout photodiodes are implemented within the OPA architecture. This enables phase-error correction for high-fidelity beam-steering with linear complexity calibration. A 32-channel OPA with 2.5-µm pitch is fabricated in an Si–SiN photonic stack. The readout is done with silicon photon-assisted tunneling detectors (PATDs) for sub-bandgap light detection with no-process change. After the model-based calibration procedure, the beam emitted by the OPA exhibits a sidelobe suppression ratio (SLSR) of −11 dB and a beam divergence of 0.97° × 0.58° at 1.55-µm input wavelength. Wavelength-dependent calibration and tuning are also performed, allowing full 2D beam steering and arbitrary pattern generation with a low complexity algorithm.

Optical phase errors are inherent to practical waveguides and will degrade the beam quality of optical phased arrays. This paper demonstrates a scalable approach to on-chip phase calibration of optical phased arrays using a compact phase interrogator structure. This phase interrogator is designed for optical phased arrays with a 2.5μm pitch and demonstrated in a 64-channel optical phased array realized in a Si/SiN photonic stack. The optical phased array exhibits 38° of unambiguous beam steering range and a side lobe level of at most -9.8 dB after calibration. The calibration algorithm is model-based and does not rely on any iterative optimization algorithm. This enables calibration with linear scaling of both the computational complexity and the area of the calibration structures. By including wavelength dependent effects, the model allows for 2D beam steering without any need for re-calibration.

A fully-functional photonic integrated circuit (PIC) platform with supporting active and passive components in the green part of the visible spectral regime is of significant research interest for next-generation optical systems. Here we design highly-integrated ~3.5 mm long PICs at green wavelengths, which consist of on-chip GaAs-based near-infrared pump lasers, SiN-LiNbO3 hybrid waveguides and ring resonators for nonlinear frequency conversion. The waveguides in the PICs are designed to eliminate etching of LiNbO3, reducing fabrication challenges. Efficient up-conversion of 1062 nm pump to 531 nm coherent light is achieved by employing modal phase matching. Unlike a quasi-phase matching technique, modal phase matching enables poling-free operation and further eases device fabrication with comparable performance in terms of second harmonic generation efficiency. The effective nonlinear mode-overlap factor between 1062-nm-TE00 and 531-nm-TE01 modes in the hybrid waveguide is calculated to be 26%. For robust on-chip light coupling between pump laser-to-waveguide, a calculated maximum coupling efficiency of -2.3 dB is achieved. The theoretical work presented is an initial step towards demonstrating complex non-telecom PICs which could offer a comprehensive range of photonic functionalities.