Metal-Clad InP Cavities for Nanolasers on Si
Photonic integrated circuits offer high-speed data transmission and reduced energy per bit consumption. To meet the energy requirements for on-chip communication, a small form factor of the integrated III-V semiconductor emitters and detectors is crucial. Ding et al. suggest a laser volume of less than ~2(λ/n)  to lower the energy to 10 fJ/bit . Scaling to these dimensions, however, is not possible for purely photonic devices, hence hybrid plasmonic-photonic cavities are being explored . Using template-assisted selective epitaxy (TASE), we can integrate high quality III-Vs epitaxially on Si, despite their lattice and thermal mismatch . We have demonstrated InP microdisk lasers grown via TASE  and are currently integrating InGaAs quantum wells  for emission above the Si absorption edge. Moreover, we have used intensity measurements of scattered photoluminescence (PL) in Au waveguides on InP to confirm that PL couples to surface plasmons and is guided in this materials system. In this work, we explore Au cladded III-V whispering gallery mode (WGM) cavities to further scale down the footprint of lasers. We design and fabricate InP cavities with resonances in the near-infrared using direct wafer bonding. InP is bonded on Si and (WGM) cavities with diameters from 100nm to 1000nm are dry etched subsequently. In a second step selected cavities are cladded with Au and the performance of purely photonic and metal-clad devices is compared. μ-PL spectroscopy is performed using a ps-pulsed supercontinuum laser at 750nm. A 100× objective is used to focus light on the sample from the top and collect the PL response. Measurements show evidence of lasing with resonant emission around 900 nm and a far field radiation pattern with fringes for cavities above 500 nm in diameter in both cases, with and without Au. For the Au cladded case, the threshold is higher. Indication of lasing is not observed for 400 nm wide purely photonic structures. However, for the cavities cladded with Au, a resonant peak emerges, indicating lasing at around 920 nm. Finite difference time domain simulations fit with our experimental observations and confirm a decrease in quality factor with decreasing diameter for the photonic cavities. Around 600 nm diameter, Q-factors become comparable to Au cladded cavities. When further reducing the cavity size, purely photonic cavities support no resonances anymore while cladded cavities do. In summary, we observe evidence of lasing at room temperature for WGM cavities upon pulsed optical excitation. Preliminary results show that for smaller cavities one benefits from metal cladded designs and is able to scale down the emitter beyond the limit of purely photonic cavities. In a next step we will apply the same approach to InP cavities directly grown in silicon using TASE. This might further improve performance as the TASE growth results in atomically smooth sidewalls with reduced defect density . This work was supported by H2020 ERC project PLASMIC #678567 and by the Korea MSIT #2017R1A2B4007219. 1. Ding, K. et al., Las&Phot Rev, 9, No. 5, 488 - 497 (2015). 2. Oulton, R. et al., Nat. Phot. 2, 496–500 (2008). 3. Schmid, H. et al., Appl. Phys. Lett. 106, 233101 (2015). 4. Mauthe, S. et al., IEEE JSTQE. 25, 1–7 (2019). 5. Baumgartner, Y. et al., IEEE 14th International Conference on Group IV Photonics, Berlin (2017). 6. Tiwari, P. et al., 10th international conference on Metamaterials, Photonic Crystals and Plasmonics, Lisbon (2019).