Optical interconnects are widely used because of the low loss and high bandwidth of optical fiber. As Internet traffic continues to increase, shortening the distance of optical interconnects is expected to greatly increase transmission speed and reduce power consumption. For this purpose, it is important to reduce the power consumption and cost of semiconductor lasers as optoelectronic conversion devices.

The direct modulation of semiconductor lasers is the best way to reduce costs. On the other hand, volume reduction of the active region has long been studied to achieve low power consumption. As shown in Figure 1(a), membrane lasers have a thin film with a thickness of 200 to 350 nm, about 1/10 that of conventional lasers, and the top and bottom layers are sandwiched by low refractive index materials (1,2). In particular, the recipients proposed a -scale embedded active-region photonic crystal laser (LEAP laser) with an active layer of a few microns embedded in the InP photonic crystal slab shown in Figure 1(b), and realized the world's first photonic crystal laser oscillating with room-temperature continuous-wave operation (3). Furthermore, they demonstrated that the laser can operate with a modulation energy per bit of 4.8 fJ, which is 1/10 of the energy of a vertical-cavity surface-emitting laser (VCSEL), and showed the possibility of future applications for chip-to-chip and intra-chip optical interconnects (4).

High-density integration with silicon (Si) photonics devices is also important for cost reduction. The integration of compound semiconductor devices on Si substrates has the problem of the degradation of crystal quality caused by differences in lattice constants and thermal expansion coefficients, a problem that has not been solved for more than 30 years since the concept was proposed. To overcome this problem, the recipients have proposed (Figure 2(a)) and demonstrated (Figure 2(b)) a novel membrane structure and fabrication method (1,5,6) in which re-growth is performed on a thin InP layer directly bonded to Si. They have established an epitaxial growth technique for InP film on an Si substrate by reducing the thickness of the InP film to a thickness that does not cause degradation of the laser active layer due to the difference in the thermal expansion coefficient. They have also realized a lateral current injection structure in the thin film. These technologies have made it possible to fabricate a buried heterostructure on Si substrates with a high design flexibility, which is essential for a high-performance semiconductor lasers.

In addition, it has been shown that laser active layers with different bandgap can be grown simultaneously over a wavelength width of 150 nm using selective epitaxial growth (Figure 2(c)) (7). This is an indispensable technology for the integration of lasers and modulators and for the realization of photonic integrated circuits (PICs) using wavelength division multiplexing (WDM) technology. In addition, high-speed modulation using the membrane laser on high-thermal-conductivity SiC substrates has been achieved (8), and the technology can be applied to a wide range of applications, including next-generation 800 Gbit and 1.6 Tbit Ethernet transmission devices.

Furthermore, this technology is expected to be one of the key devices to support NTT's IOWN concept, and will be used in data centers in the future, greatly contributing to the realization of a carbon-neutral society. This technology can also be applied to devices for sensing and computing. These achievements of the recipients are significant both for the progress of the technology and for the industrial implementation of silicon integrated photonic circuits, enough to make them worthy of the Society’s Achievement Award.

References

1. S. Matsuo, T. Fujii, K. Hasebe, T. Takeda, and T. Kakitsuka, “Directly modulated buried heterostructure DFB laser on SiO2/Si substrate fabricated by regrowth of InP using bonded active layer,” Optics Express, vol. 22, no. 10, pp. 12139-12147, 2014.
2. S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi and M. Notomi, “High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted,” Nature Photonics, vol. 4, pp. 648-654, 2010.
3. S. Matsuo, K. Takeda, T. Sato, M. Notomi, A. Shinya, K. Nozaki, H. Taniyama, K. Hasebe, and T. Kakitsuka, “Room-temperature continuous-wave operation of lateral current injection wavelength-scale embedded active-region photonic-crystal laser,” Optics Express, vol. 20, no. 4, pp. 3773-3780, 2012.
4. K. Takeda, T. Sato, A. Shinya, K. Nozaki, W. Kobayashi, H. Taniyama, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “A few-fJ/bit data transmissions using directly modulated lambda-scale embedded active region photonic-crystal lasers,” Nature Photonics, vol. 7, no. 7, pp. 569 – 575, 2013.
5. T. Fujii, T. Sato, K. Takeda, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Epitaxial growth of InP to bury directly bonded thin active layer on SiO2/Si substrate for fabricating distributed feedback lasers on silicon,” IET Optoelectronics 9, 151–157 (2015).
6. K. Takeda, T. Tsurugaya, T. Fujii, A. Shinya, Y. Maeda, T. Tsuchizawa, H. Nishi, M. Notomi, T. Kakitsuka, and S. Matsuo, “Optical links on silicon photonic chips using ultralow-power consumption photonic-crystal lasers,” Opt. Express 29, 26082 (2021).
7. T. Fujii, K. Takeda, H. Nishi, N.-P. Diamantopoulos, T. Sato, T. Kakitsuka, T. Tsuchizawa, and S. Matsuo, “Multiwavelength membrane laser array using selective area growth on directly bonded InP on SiO2/Si,” Optica 7, 838 (2020).
8. S. Yamaoka, N.-P. Diamantopoulos, H. Nishi, R. Nakao, T. Fujii, K. Takeda, T. Hiraki, T. Tsurugaya, S. Kanazawa, H. Tanobe, T. Kakitsuka, T. Tsuchizawa, F. Koyama, and S. Matsuo, “Directly modulated membrane lasers with 108 GHz bandwidth on a high-thermal-conductivity silicon carbide substrate,” Nature Photonics 15, 28 (2021).