Realisation of the complete photonic integrated circuit

5.2.3.1 Optical absorption of the SNSPD

Figure 5.6: (a)Schematic of the waveguide cross section with its dimensions and the materials used. (b) 3D FDTD simulation of near field intensity distribution (normalized) of the fundamental quasi-TE mode along 5µm of NbTiN super-conducting nanowire. The light is coupled to the waveguide from the left. (c) Simulated cross section of the electric field (normalized) in the Si³N⁴ waveguide before reaching the nanowire detector. (d) Simulated cross section of the electric field (normalized) in the Si³N⁴ waveguide with NbTiN nanowires after 2.5µm of propagation.

A key feature for waveguide integrated detectors is the high detection

efficiency due to the strong evanescent coupling to the detector. To ver-ify this, we perform 3D FDTD simulations of the optical absorption for our 25 µm long NbTiN superconductor. Figure 5.6(a) shows a waveguide cross-section with the dimensions and the material used for the evanescent coupling configuration. We injected a TE(TM) polarized mode into a silicon nitride waveguide, as shown in Figure 5.6(b) and recorded the transmitted power after introducing the NbTiN nanowire below the waveguide as well as the total scattered power.

The optical mode propagating in the waveguide is displayed in Figure 5.6(c) and the dimensions chosen for the waveguide (800 ∗ 200 nm²) provide a good confinement of the fundamental TE mode. Additionally, the selected air cladding delocalizes the optical mode slightly into the substrate where the SNSPD is located, resulting in a strong absorption of the incoming mode in the first micrometer as depicted in Figure 5.6(b). Figure 5.6(d) shows the eigen mode at the SNSPD region where the electric field intensity is max-imum near the NbTiN wires. Figure 5.7 shows the simulated absorption

Figure 5.7: Simulated absorption of TE and TM mode for NbTiN nanowires located below silicon nitride waveguides as function of the length of the nanowire for 890 nm. The dashed lines show an exponential fit to the simulated absorption data.

for TE and TM polarization, overlaid with an exponential fit of the data points. For the TE mode, the absorption of the superconducting detector reaches a plateau at a value of 97% for only 10 µm of nanowire length.

The simulation gives a scattered power of 3.5% for this mode. As shown in Figure 5.7, the absorption attains 40% for the TM mode for 25µm long nanowire. This is due to significant scattering and reflection of the optical mode with the superconducting film (∼12%).

5.2.3.2 Layout of the photonic integrated circuit

The sketch of the fabricated full-quantum transceiver on-chip is shown in Figure 5.8. It consists in a waveguide-integrated SNSPD ((I)), a ring

res-Figure 5.8: Sketch of the photonic circuit with an optical picture of the selected quantum dots nanowires and a section of the ring resonator (II) and in the right a SEM image of the fabricated waveguide on top of the SNSPD (I). The emitted photons from the QD nanowire are coupled to the silicon nitride waveguide, then filtered by a ring resonator and finally detected by the single photon detector.

onator filter and selected nanowire QDs(II). The waveguide circuit was pat-terned using e-beam lithography and the pattern was then transferred to Si³N⁴ by dry etching in a CHF³/Ar chemistry [Figure 5.1[7]]. The optical losses of the photonic waveguides are 4.0±0.3 dB/cm and 2.5±0.4 dB/cm for TE and TM respectively [16, 28]. The drop port of the resonator filter is terminated by a waveguide coupled superconducting detector while the input port is coupled to a nanowire quantum dot. We designed the ring res-onator with a radius of 140µm and a gap of 180 nm, so that the resonances have critical coupling around the emission wavelength of the quantum dot transition at 886.5 nm (Figure 5.9(a)). The loaded quality factor for TE mode is around 20000 [28]. The total suppression of the laser is estimated to be 100 dB.

The deterministic transfer of the quantum source from the growth chip to the photonic circuit chip is the following. The chip containing the nanowire QDs has been grown at the National Research Council of Canada in Ottawa.

The emission spectrum of several nanowire QDs are reviewed individually with an experimental set up similar to the one which is described in sec-tion 5.3.4. The selecsec-tion of the QD is based on its brightness, the width and the number of spectral line that composed its spectrum. Then, the QD was deterministically transferred using a nano-manipulation technique [16, 17] from the growth chip to the photonic circuit chip. The instrument used is a nano-manipulator consisting in a tungsten tip mounted on a x-y-z movable stage and imaged by a high resolution optical microscope. The nanowire is detached at its base from the growth chip, then due to the Van Der Waals forces, it adheres to the tungsten chip. Finally, the quantum source was transferred to our chip at the chosen position, as shown on the left of Figure 5.8.

5.2.3.3 On-chip measurements

Figure 5.9: (a) Spectrum of a selected quantum dot nanowire. (b) Lifetime mea-surement of the quantum dot nanowire performed on chip.

The nanowire quantum dot was excited from the top using a femtosecond 515 nm pulsed laser with a repetition rate of 20 MHz. The experimental set up used for the measurements is described in the next section (section 5.3.4). The emission spectrum of the waveguide-coupled nanowire quantum dot is shown in Figure 5.9(a). After excitation of the QD, the emitted photons are coupled to the silicon nitride waveguide, then, they are filtered byt the ring resonator, and finally detected by the superconducting detector.

Figure 5.9(b) shows a time-resolved start-stop correlation measurement with the laser signal and the SNSPD provides a high time-resolution of 23 ps.

We extracted the QD signal decay time of 0.62 ± 0.02 ns, in agreement with previous measurements performed on similar quantum dot nanowires

configuration [16, 29].