Ultrafast Spontaneous Emission from Copper-Doped Silicon Nanocavity


Hisashi Sumikura1,2, Eiichi Kuramochi1,2, Hideaki Taniyama1,2, and Masaya Notomi1,2
1Nanophotonics Center, 2Optical Science Laboratory

   Impurities binding an exciton in Si show intense emission without phonon emission and attract a lot of attention as quantum bits (qubits) manipulated by light because of their long-lived spins. However, the light-impurity interaction is inefficient due to nonradiative processes including the Auger process. In this study, we focused on the combination of copper isoelectronic centers (Cu-IECs) and photonic crystal (PhC) nanocavities. The IEC originates from a bound exciton of deep impurities. Since the IEC is free from the Auger process, its emission quantum efficiency (QE) is high [1]. In addition, the high-Q nanocavity accelerates spontaneous emission from the Cu-IECs, which is known as the Purcell effect, and enhances the light-impurity interaction.
   We have developed a novel method for doping the Cu-IECs to a SOI wafer. The Si PhC nanocavities were fabricated on the Cu-doped SOI wafer. Figure 1 shows photoluminescence (PL) spectra for the cavity with Q = 7200 and PL decays of the Cu-IECs. At 1227.5 nm, the emission peak of the Cu-IECs is found. The blue-side peak originates from the cavity resonance. When the cavity resonance approaches to the Cu-IEC line, the PL intensity and the decay rate of the Cu-IECs increase and take maximum values at zero detuning. Figure 2 shows PL decays and decay rates of the Cu-IECs in resonant cavities as a function of the Q/V value of the cavity. It is found that the decay rate is almost proportional to Q/V. This result proves the Purcell effect on the Cu-IECs because the theoretical estimation agrees well to the experimental data. In the highest-Q cavity (Q = 16000), the PL lifetime of the Cu-IECs is 1.1 ns, which is 30 times shorter than that in an unpatterned SOI sample. Since this PL lifetime is sufficiently smaller than the nonradiative lifetime around 40 ns, the QE may close to unity.
   These results enable to develop novel fast and energy-efficient Si light emitting devices and quantum optical devices based on impurity qubits in Si.

[1] S. P. Watkins et al., Solid State Commun. 43 (1982) 687.

Fig. 1. (a) PL spectra for Cu-doped nanocavity. Inset shows the intensity of the Cu-IEC line.
(b) PL decays and decay rates of the Cu-IECs.
Fig. 2. (a) PL decays of the Cu-IECs in resonant cavities. (b) PL decay rates as a function of the Q/V values of the cavities. V is a mode volume.