Wavelength-scale All-optical Photonic Crystal Nanocavity Memory Operating at Nanowatt-level

Eiichi Kuramochi1,2, Kengo Nozaki1,2, Akihiko Shinya1,2, Hideaki Taniyama1,2, Koji Takeda1,3, Tomonari Sato1, Shinji Matsuo1,3, and Masaya Notomi1,2
1NTT Nanophotonics Center, 2Optical Science Laboratory,
3NTT Device Technology Laboratories

 We are developing ultralow-power-consumption large-scale photonic integrated circuits on a chip using nanophotonic technologies. All-optical memory is one of the key devices. Since we need a considerably large memory, reducing power consumption is a crucial issue. We have realized an optical RAM with 30-nW operation using a wavelength-scale photonic crystal (PhC) nanocavity with a small buried heterostructure (BH) [1]. Here we describe a further one order of magnitude reduction of power consumption by employing a novel multi-hole-tuned three-missing-hole (L3) nanocavity with a BH placed at the cavity center [Fig. 1(a)] [2]. We used the same design as in [3], which enables large quality factor (Q) enhancement and multibit optical RAM operation [3]. In this study, a two-port filter design [Fig. 1(b)] was employed and Q was enhanced to 45,000 and 210,000 with and without the InGaAsP BH, where the latter is the record high value among all InP-based nanocavities. Two multi-hole-tuned BH-L3 cavities (L3LM, L3M3) were operated as an all-optical bistable memory as shown in Fig. 1(c) and compared to the operation of mode-gap-confined BH nanocavity (MG1) reported before [1]. As shown in Fig. 1(d), L3M3 that had Q comparable to MG1 decreased onset bias power to 10 nW thanks to the smaller mode volume (V) of the L3 nanocavity. Furthermore, L3M1 having much higher Q (42,000) exhibited onset bias power of 2.3 nW, which is nearly 1/13 of MG1’s, and the corresponding average number of photons in the cavity was only 0.1. Even at the 2.3-nW bias power, L3M1 operated with a very good "1"/"0" switching contrast [Fig. 1(e)]. This study paves the way for ultralow power consumption integrated photonic crystal devices and encourages the employment of multi-hole-tuned L-type nanocavities for fundamental and device studies.

Fig. 1. (a) Design of systematically-tuned BH-L3. (Lattice constant (a): 426 nm. Shifts: 0.09a, 0.35a, 0.175a, and 0.045a for A, B, C and D. BH is InGaAsP.) (b) SEM image of L3 optical memory. (c) Schematics of all-optical bistable memory operation. (d) Relation between detuning (δ) and bias power for memory operation. (e) Input/output waveforms in memory operation at 2.3 nW bias power.