Dries Van Thourhout

We report the first demonstration of a lead zirconate titanate (PZT) thin-film electro-optic modulator operating at 635 nm and 950 nm, enabled by micro-transfer printing of the PZT layer onto a silicon nitride photonic platform. The devices exhibit a V _π L between 0.94 V cm and 1.5 V cm, depending on the operating wavelength, thereby providing proof-of-concept electro-optic modulation in the near-infrared and visible wavelength range. We characterize the performance of the devices and discuss future improvements. This work highlights the feasibility of micro-transfer-printed ferroelectric electro-optic modulators and opens a pathway toward the heterogeneous integration of low-loss, wide-bandgap electro-optic materials into photonic integrated circuits.

Colloidal semiconductor nanocrystals (NCs) are widely explored as optical gain materials, and throughout over 20 years of research their performance is evaluated through transient absorption spectroscopy (TAS) using parameters such as gain magnitude, lifetime, and threshold. In this work, we discuss some of the literature values reported and couple that to different gain mechanisms. Next, we take a closer look at how these actually relate to the physical parameters that govern lasing, as described by the semiconductor laser rate equations, and found some discrepancy with what is usually reported and what is necessary. We introduce a new analytical framework that connects TAS data to two new fundamental quantities: the stimulated emission rate, representing the intrinsic light–matter interaction speed, and the carrier recombination time, which determines the competing nonradiative loss channel. Together with the already typically reported transparency density, these parameters enable direct prediction of key laser characteristics, like threshold behavior for both femto- and quasi-CW excitation, without requiring full device fabrication. Applying this methodology to a range of colloidal NC systems, we demonstrate that their intrinsic gain performance approaches that of epitaxially grown quantum wells. Analysis of colloidal NC laser literature data further confirms the consistency and predictive power of this approach. This work establishes a unified basis for interpreting ultrafast spectroscopy of NCs and provides a more quantitative route to benchmarking emerging materials for integrated laser technologies.

Bulk nanocrystals (BNCs) exhibit exceptional optical gain because of strong Auger suppression and band gap renormalization effects, but this advantage comes at the cost of losing the size-dependent spectral tunability that underpins conventional colloidal nanocrystals. As a result, CdS BNCs remain largely limited to the cyan-green spectral region (480–520 nm), restricting access to technologically important wavelengths such as true green (520–550 nm). Here, we show that compositional alloying restores spectral tunability to the BNC regime while preserving its favorable gain characteristics. By incorporating selenium into CdS, we obtain CdS0.87Se0.13 alloyed BNCs that emit in the true-green region while retaining bulk-like suppression of Auger recombination. Transient-absorption spectroscopy reveals strong optical gain with multinanosecond lifetimes, including a red-shifted alloy band-edge gain feature together with a higher-energy CdS-derived contribution due to a CdS shell. Thin films of the alloyed BNCs exhibit amplified spontaneous emission with a threshold of 215 μJ/cm2. When integrated into photonic crystal surface-emitting lasers, the same material supports single-mode, highly collimated lasing across 520–540 nm under both femtosecond and quasi-continuous-wave excitation, with line widths of ∼ 0.7 nm and thresholds as low as 10 kW/cm2. These results establish alloyed BNCs as solution-processable gain media that combine bulk-like gain physics with compositional wavelength tunability, supporting the development of compact visible lasers in spectral regions inaccessible to single-composition BNC materials.

Quantum photonic integrated circuits (QPICs) are a promising platform for scalable quantum technologies. A major outstanding challenge is the efficient interconnection of diverse quantum photonic components, where optical losses must remain below the ∼10% threshold required for advanced quantum applications. In this paper, the optical coupling between GaAs single‐photon sources and a low‐loss SiN interposer is investigated, where reliable high‐efficiency transitions have not yet been achieved. To address this, a fully numerical framework is introduced for the optimization of waveguide mode couplers that directly incorporates experimentally characterized fabrication variations critically impacting performance, including waveguide misalignment, width deviation, and sidewall roughness. Using this approach, low‐loss GaAs/SiN transitions are designed and experimentally demonstrated with reproducible coupling efficiencies of –0.43 dB ± 0.14 dB. These results establish a viable path toward low‐loss heterogeneous QPICs tailored to advanced quantum applications.

Short‐wave infrared (SWIR) technologies are vital for optical communication, all‐weather imaging, and advanced spectroscopy. Using a single diode for light detection and emission can reduce device footprints, lower cost and enable fast, bidirectional communication. However, dual detection/emission operation often involves incompatible choices of material and stack design, which result in devices with inferior performance. Here, we introduce a bifunctional PbS QD diode stack that achieves, for the first time, nanosecond‐fast photodetection and emission in a single SWIR device. Using thin active layers with high charge‐carrier mobility and compact geometries, we demonstrate a record 2 ns response in photodiode (QDPD) mode and 11.7 ns rise time/6.5 ns fall time in light‐emitting diode (QDLED) mode. This high‐speed operation is combined with a 49% external quantum efficiency (EQE) and 2.6 × 10 ^{1 2} cm·Hz ^{0 . 5} ·W ^{− 1} detectivity for QDPD operation, while the QDLED reaches 8.1% EQE. These high‐speed, compact devices unify imaging and detection with a single, CMOS‐compatible platform, and set the stage for integrated SWIR photonics and bidirectional optical interconnects. This work demonstrates the potential of QDs to unify imaging, detection, and communication in a single materials platform.

Although graphene's THz absorption rate increases with number of graphene layers, the absorbance is limited and unsatisfactory as a practicable absorber. Patterned graphene or graphene‐based surface plasmon resonance has been proved to enhance THz absorption, but the performance is prone to be affected by graphene defects and local multilayer stacks. In this paper, we proposed to use a double circular metal ring (DCMR) array to enhance both the graphene's THz absorption with an ultrawide bandwidth and the tolerance of graphene's physical impurities. A DCMR array is patterned on the top surface of graphene layer with dielectric substrates, which excites a broadband spoof surface plasmons polaritons (SPPs) in the THz frequency range. The broadband spoof SPPs locally confine and strengthen THz wave interaction with graphene, and further enhance graphene's THz absorption, which is confirmed by numerically simulated dispersion relations. A four‐layer graphene composite structure is fabricated to validate the scenario, which consists of graphene, a SiO ₂ ‐capped doped‐silicon substrate covered by graphene, and the DCMR array on the top surface of graphene. Time domain spectroscopy test shows that the THz absorption enhancement of the graphene composite with DCMR array covers the frequency range from 0.3 to 1.1 THz with 110% bandwidth. The THz absorption of the proposed graphene composite increases to more than 70% over a wide bandwidth of 77% at 0.65 THz compared with the graphene composite without DCMR array.

Colloidal quantum dot photodiodes (QDPDs) are becoming an increasingly mature technology for infrared sensing and imaging that can offer high detectivity, short response times, and micrometer-scale pixelation. Even so, QDPDs suffer from a limited linear dynamic range (LDR), with photocurrents saturating at modest light intensities. Here, we analyze this remaining QDPD bottleneck in PbS-based QDPDs through operando transient absorption spectroscopy. As compared to isolated QDs, we find that recombination of photogenerated charge carriers accelerates in device-ready QD films and full QDPD stacks. Supported by kinetic Monte Carlo simulations, we assign this loss pathway to trion recombination mediated by hopping and doping-induced background charges, and we show that the photocurrent saturates when the rate of trion recombination and charge separation match. On the basis of this result, we argue that the widely varying literature data on the linear dynamic range of QDPDs mainly reflect different carrier extraction rates and conclude that faster carrier extraction is essential to extend the linear dynamic range.

This paper highlights micro-transfer printing (MTP) as a promising scalable approach to heterogeneous integration for silicon photonics. MTP uniquely achieves high integration density, high throughput, and high material efficiency through a low-temperature, back-end integration process. Current demonstrations, including integrated III-V lasers and thin-film electro-optic modulators, confirm MTP's potential. Industrial adoption requires resolving challenges related to final integration yield and throughput, device reliability, and supply chain maturity.