On-chip infrared photonic waveguides excitation by quantum dots

On-chip light generation is highly desirable for photonic waveguide-based biosensing applications which aim at improving efficiency and reducing costs and size of the instrument into which the chip will embark. The integration of fluorescent semiconducting nanocrystals (colloidal quantum-dots) as on-chip light source for waveguide excitation with sufficient output power would reduce the need for bulky optical excitation setups and would greatly reduce the complexity of the alignment configuration in which in/out fibers need to be aligned to the waveguides on the chip via grating couplers or by butt-coupling. This idea is pursued in our European project GRACED [https://graced.tech]. As compared to on-chip technologies like VCSELs or other on-chip lasers, quantum dot sources are less technology demanding and simply require an optical pumping beam much larger than a fiber, which can be easily realized with an LED or laser diode. In the literature a demonstration of lasing under optical pumping has been demonstrated with QDs integrated with SiN micro-disks [1]. We have also reported on QD with micro-ring resonators but coupled into waveguide configurations in the visible range [2]. The QDs we use in this work have a PbS core designed to emit light peaking near 1550 nm and a CdS shell that provides more stability and durability. The novelty we report is about working at 1550 nm combined with coupling of the emitted light into on-chip photonic waveguides by exploiting original cumulative designs for improved photons harvesting.

We fabricate single mode photonic waveguides in SiN (n~2.0) at 1550 nm. The deposited QD are excited by a UV led pump beam delivered though a fiber optic placed close above the sample. Images are taken from top with an IR camera. Many challenges need to be addressed, like the reproducibility of the QD deposition process, their stability versus the different fabrication steps but the main challenge is to find an efficient design to get enough power out of the excited waveguide. There is an intrinsic lack of efficiency in collecting the fluorescent light emitted spherically in all directions and coupling it into a unidirectional photonic waveguide mode. To mitigate this, QDs should preferably be symmetrically encapsulated into the waveguide to get a maximum overlap of the emitters with the guided mode. We have experimentally compared encapsulation with the configuration where QDs are on top of the waveguides, easier to fabricate but expected to be less performant. We have explored cumulative designs like simple or Archimedean spiral waveguides, as well as waveguide cascaded junctions. We also studied simple 2D disc structures (planar waveguide) to get a basic estimation of the photon collection efficiency without the constraint of the unidirectional waveguide.

Si chips with 2 µm of SiO₂ are first covered with RF-sputtered SiN thin film. The 600 nm thick 1200 nm large SiN single mode waveguides and disc waveguides are patterned by e-beam lithography (EBL) and RIE. QDs patterns are realized by EBL followed by spin coating of the QD solution and lift-off [2,3]. In the encapsulated configuration, the consecutive steps are the following: after deposition of 300 nm of SiN, gold alignment markers are patterned by EBL and lift-off, QDs are deposited by another EBL step and lift-off, another 300 nm of SiN is deposited, EBL and RIE is finally run to etch the full-height waveguides.

A representative result obtained with a spiral waveguide is presented in Fig. 1. The light emitted by QDs in the pumped area is coupled into the waveguide and propagates with no loss, until reaching the grating coupler on the far right, which couples out the light in the perpendicular direction towards the camera. Fig 2 shows the different disc structures fabricated in both the encapsulated and top configurations. In the IR images, the central illuminated disc represents the emission escaping perpendicularly from the pumping area whereas the illuminated disc rim is the result of planar propagation and scattering from the edge of the SiN disc, thus representing the output. The figure of merit chosen is the out/pump integrated intensity ratio and results are plotted in the diagram in fig 2. The results show that the encapsulation gives an important improvement factor of 2 to 8 with respect to the top configuration. This is a very good result in spite of the observation that the encapsulated QDs display a much larger emission inhomogeneity. These results will allow us to further refine the configuration and design to make this concept even more performant towards applications.

We acknowledge T. Pons and C. Roux-Byl at ESPCI Paris for the synthesis of the QDs.