All-silicon Photonic Crystal Receiver for 1.55-µm Light Detection Utilizing Two Photon Absorption


Takasumi Tanabe, Hisashi Sumikura, Hideaki Taniyama, Akihiko Shinya, and Masaya Notomi
Optical Science Laboratory

 Silicon (Si) is widely used to constitute electrical circuits, but it is also a good material for constructing optical integrated circuits. Indeed, optical waveguides and nanocavities have been fabricated on Si chips. However, Si cannot be used to detect 1.5-µm light because it is transparent in this wavelength region. Therefore, germanium (Ge) on Si detectors have been fabricated, but these detectors exhibit a relatively large dark current because of the 4 % lattice mismatch between Ge and Si [1]. In addition, ion-implanted Si detectors have been studied, but they also suffer from a large dark current because of the presence of defects [2]. If we can fabricate an all-Si detector, we should benefit from a low dark current because of the good crystal quality of Si. For this purpose, we need to employ two-photon absorption (TPA) whose coefficient is usually very small. In this study, we used a very high-Q photonic crystal (PhC) nanocavity to compensate for the low TPA coefficient and to enable us to detect very weak optical light in an all-Si device [3].
 Figure 1 is an illustration of our pin integrated PhC nanocavity. The Q of the fabricated device was 4.3×105 and the transmittance was 24 % [Fig. 2(a) inset]. The dark current was just -15 pA when we applied a -3 V bias. Figure 2(a) shows photocurrent versus input power. Due to the strong light confinement of the nanocavity, the TPA current is visible at an extremely low input power of 10-8 W. The QE at an input power of 1.17 µW was very high at ~10 % (we define QE as 100 % when one photon generates one electron). This value corresponds to 44 % of the cavity-coupled light being sufficiently absorbed. Such a high detection efficiency with 1.55 µm light is obtained because of the high Q of the PhC nanocavity.
 We also demonstrated 0.1-Gb/s photo receiver operation using the same device [Fig. 2(b)]. Only light that can resonate with the cavity was detected electrically. This demonstration shows that our Si-chip integrated device can be used for telecom light detection.

[1] S. Assefa, F. Xia, and Y. Vlasov, Nature 262 (2010) 80.
[2] M. Geis et al., Opt. Express 17 (2009) 5193.
[3] T. Tanabe et al., Appl. Phys. Lett. 96 (2010) 101103.

Fig. 1. Schematic illustration of pin integrated photonic
crystal nanocavity.
Fig. 2. (a) Input power versus photocurrent. Inset
is the transmittance spectrum. (b) 0.1-Gb/s
photo receiver demonstration.

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