Linear and Passive Silicon Diodes, Isolators, and Logic gates

Zhi-Yuan Li, Chen Wang, and Xiao-Lan Zhong

Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences P. O. Box 603, Beijing 100190, China Email address: lizy@aphy.iphy.ac.cn

Abstract—Silicon photonic integrated devices and circuits have offered a promising means to revolutionalize information processing and computing technologies. In this talk we present our recent efforts of design, fabrication, and characterization of ultracompact, linear, passive on-chip optical diodes, isolators and logic gates based on silicon two-dimensional photonic crystal slabs. Both simulation and experiment results show high performance of these novel designed devices. The success of these devices would help to construct nanophotonic on-chip processor architectures for future optical computers.

Keywords-silicon photonic crystal; optical integration; optical diodes; optical isolator; optical logic gates; linear and passive

Silicon photonic integrated devices and circuits have offered a promising means to revolutionalize information processing and computing technologies. One important reason is that these devices are compatible with conventional CMOS processing technology that overwhelms current microelectronics industry. Yet, the dream to build optical computers has yet to come without the breakthrough of several key elements including optical diodes, isolators, and logic gates with low power, high signal contrast, and large bandwidth. Photonic crystal has a great power to mold the flow of light in micrometer/nanometer scale and is a promising platform for optical integration because of the unique properties of photonic band gaps, defect states, and band dispersions.

Silicon has a large refraction index and low loss in infrared wavelengths, which makes it an important optical material that has been widely used for integrated photonics applications in the near infrared regime around 1550 nm [1-7]. In this talk we present our recent efforts of design, fabrication, and characterization of ultracompact, linear, passive on-chip optical diodes, isolators and logic gates based on silicon two-dimensional photonic crystal slabs. Both simulation and experiment results show high performance of these novel designed devices. These linear and passive silicon devices have the unique properties of small fingerprint, low power request, large bandwidth, fast response speed, easy for fabrication, and being compatible with COMS technology. Further improving their performance would open up a road towards photonic logics and optical computing and help to

construct nanophotonic on-chip processor architectures for future optical computers.

Optical isolation is a long pursued object with fundamental difficulty in integrated photonics. Although widely used in lasers and optical communications, such devices are still lacking in semiconductor integrated photonic systems because of challenges in both materials integration and device design. Conventionally, an efficient routine to create optical isolation is via time-reversal symmetry breaking. However, the traditional schemes in this category, including magneto-optical isolators, nonlinear optical structures, and time-dependent optical structures, have disadvantages such as large loss of isolation signal, a relatively large size, and slow response. In addition, these schemes are incompatible with conventional CMOS processing. The need to overcome these difficulties is becoming increasingly urgent with the emergence of silicon nano-photonics.

We recently reported the demonstration of on-chip linear and passive silicon optical diode and isolator [8,9] in the patform of silicon photonic crystal slabs. The diode is made from a photonic crystal heterojunction with directional bandgap mismatch and spatial inversion symmetry breaking. It

has a ultrasmall size scale of only about 26 6μm , which is

the world-record smallest all-optical diode so far. The experimental transmission spectra show an average of 21.3% of the forward peak transmissivity and 0.885 of the signal contrast at 1,550 nm, which has reached the level of conventional electronic diodes (interestingly, they are made from semiconductor pn junctions). The performance could be improved by high nanofabrication precision. Further experimental studies show that the silicon photonic crystal heterojunction diode also exhibits promising performance of optical isolation [9], with a round-trip transmissivity two orders of magnitude smaller than the forward transmissivity for in-plane infrared light across the structure, as can be found in Fig. 1. The scattering matrix analysis indicates that the unidirectional transport of in-plane signal light can be attributed to the information dissipation and selective modal conversion in the multiple-channel spatial-inversion symmetry breaking structure and has no conflict with the reciprocal principle for a time-reversal symmetric structure. The achievement of the linear, passive, and ultracompact on-chip silicon optical diode and isolator would open up a road

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towards photonic logics and optical computing in silicon integrated optical devices and circuits.

On the other hand, all-optical integrated circuits for computing and information processing have been pursued for decades as a potential strategy to overcome the speed limitations intrinsic to electronics. However feasible on-chip integrated logic units and devices still have been limited by its size, quality, scalability, and reliability. Very recently we demonstrated all-passive on-chip optical AND and NAND logic gates made from a directional emitting cavity connecting two ultrasmall photonic crystal heterojunction diodes [10]. The measured transmission spectra show more than 10dB contrast of the logic transport with a high phase tolerance, agreeing well with numerical simulations. The building of linear, passive, and ultracompact silicon optical logic gates might pave the way to construct novel nanophotonic on-chip processor architectures for future optical computing technologies.

Our works presented in this review show that new concepts are very important to bring closer the fundamental goal of ultrasmall optical integration for more powerful information processing and optical computing technologies in the framework of silicon photonics. As photonic crystals have a strong power of controlling propagation of light at micrometer/nanometer scale and possess a great potential of applications in integrated photonic circuits, it can also be a platform for constructing optical processors and computers. With the demonstration of several key elements as linear and passive optical diodes, isolators, and logic gates, and with the hope to further improving their performance, one might see a big step towards building optical computers. Design is always the top key issue of reaching the ultimate dream of optical computers.

This work was supported by the National Basic Research Foundation of China under Grant Nos. 2011CB922002.

REFERENCES

[1] L. Gan, C. Z. Zhou, C. Wang, R. J. Liu, D. Z. Zhang, and

Z. Y. Li, “Two-dimensional air-bridged silicon photonic crystal slab devices”, Physica Status Solidi A 207, 2715-2725 (2010).

[2] Y. Z. Liu, R. J. Liu, C. Z. Zhou, D. Z. Zhang, and Z. Y. Li, “ M waveguides in two-dimensional triangular-lattice photonic crystal slabs”, Optics Express 16, 21483-21491 (2008).

[3] Y. Z. Liu, R. J. Liu, S. Feng, C. Ren, H. F. Yang, D. Z. Zhang, and Z. Y. Li, “Multi-Channel filters via K and

M Waveguide coupling in two-dimensionaltTriangular-lattice photonic crystal slabs”, Appl. Phys. Lett. 93, 241107 (2008).

[4] L. Gan, Y. Z. Liu, J. Y. Li, Z. B. Zhang, D. Z. Zhang, and Z. Y. Li, “Ray trace visualization of negative refraction of light in two-dimensional air-bridged silicon photonic crystal slabs at 1.55 um”, Optics Express 17, 9962-9970 (2009).

[5] L. Gan, F. Qin, and Z. Y. Li, “Broadband large-angle self-collimation in two-dimensional silicon photonic crystal”, Opt. Lett. 37, 2412-2414 (2012).

[6] Y. Liu, F. Qin, Z. M. Meng, F. Zhou, Q. H. Mao, and Z.Y. Li, “All-optical logical gates based on two-dimensional nonlinear photonic crystal slabs”, Optics Express 19, 1945 (2011).

[7] F. Qin, Z. M. Meng, X. L.Zhong, Y. Liu, and Z.Y. Li, “Fabrication of semiconductor-polymer compound nonlinear photonic crystal slab with highly uniform infiltration based on nano-imprint lithography technique”, Optics Express 20, 13091-13099(2012).

[8] C. Wang, C. Z. Zhou, and Z. Y. Li, “On-chip optical diode based on silicon photonic crystal heterojunctions”, Optics Express 19, 26948-25955 (2011).

[9] C. Wang， X. L. Zhong, and Z. Y. Li, “Linear and passive silicon optical isolator”, Scientific Reports 2, 674(1-6) (2012).

[10] C. Wang and Z. Y. Li, “Linear ultracompact on-chip silicon optical logic gates with phase insensitivity”, submitted (2013).

Fig. 1. Demonstration of a linear, passive, and ultracompact optical isolator made from silicon photonic crystal heterojunction diode. Both theory and experiment show that the round-trip transmissivity is far smaller than the forward transmissivity, which confirms the excellent isolation performance.

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