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Ultralow-power integrated nanophotonics for future chips
Abstract
Recently, photonic crystals have enabled a variety of ultrasmall photonic devices with extremely small energy consumption of ~fJ/bit level, suggesting that we can integrate a vast number of nanophotonic devices in a single chip. This technology may give us a way to introduce high-speed integrated photonics in an information processing chip, which will be crucial in future ICT.
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Optical random-access memory (o-RAM) has been regarded as one of the most difficult challenges in terms of replacing its various functionalities in electronic circuitry with their photonic counterparts. Nevertheless, it constitutes a key device in optical routing and processing. Here, we demonstrate that photonic crystal nanocavities with an ultrasmall buried heterostructure design can solve most of the problems encountered in previous o-RAMs. By taking advantage of the strong confinement of photons and carriers and allowing heat to escape efficiently, we have realized all-optical RAMs with a power consumption of only 30 nW, which is more than 300 times lower than the previous record, and have achieved continuous operation. We have also demonstrated their feasibility in multibit integration. This paves the way for constructing a low-power large-scale o-RAM system that can handle high-bit-rate optical signals.
We investigate the spectral linewidth of a monolithic photonic crystal nanocavity laser. The nanocavity laser is based on a buried heterostructure cavity in which an ultra-small InGaAsP active region is embedded in an InP photonic crystal. Although it was difficult to achieve narrow linewidth operation in previously reported photonic crystal nanocavity lasers, we have successfully demonstrated a linewidth of 143.5 MHz, which is far narrower than the cold cavity linewidth and the narrowest value yet reported for nanolasers and photonic crystal lasers. The narrow linewidth is accompanied by a low power consumption and an ultrasmall footprint, thus making this particular laser especially suitable for use as an integrated multi-purpose sensor.
We have developed a wavelength-scale embedded active-region photonic-crystal laser using lateral p-i-n structure. Zn diffusion and Si ion implantation are used for p- and n-type doping. Room-temperature continuous-wave lasing behavior is clearly observed from the injection current dependence of the output power, 3dB-bandwidth of the peak, and lasing wavelength. The threshold current is 390 μA and the estimated effective threshold current is 9.4 μA. The output power in output waveguide is 1.82 μW for a 2.0-mA current injection. These results indicate that the embedded active-region structure effectively reduce the thermal resistance. Ultrasmall electrically driven lasers are an important step towards on-chip photonic network applications.
We have demonstrated an all-optical memory by using InGaAsP/InP buried heterostructure photonic crystal (BH-PhC) lasers. We achieved distinct optical injection locking bistability in an ultra-compact active region (4 × 0.3 × 0.16 μm3) with only 25 μW pump power in the PhC waveguide, which is two orders less than previously reported optical memories based on other bistable semiconductor lasers. Dynamic memory operations were achieved with pump power of 100 μW and switching power of 22 μW and 71 μW in the PhC waveguide. Fast switching times of about 60 ps were achieved. To the authors' best knowledge, this is the first demonstration of PhC laser-based all-optical memory.
We demonstrate channel selective 0.1-Gb/s photo-receiver operation at telecom wavelength using a silicon high-Q photonic crystal nanocavity with a laterally integrated p-i-n diode. Due to the good crystal property of silicon the measured dark current is only 15 pA. The linear and nonlinear characteristics are investigated in detail, in which we found that the photo-current is enhanced of more than 100,000 due to the ultrahigh-Q (>100,000). With the help of two-photon absorption, which is visible at a surprisingly low input power of 10 nW, the quantum efficiency of this device reaches about 10%. Comment: 3 pages, 4 figures, submitted to Appl. Phys. Lett
Photonic crystals were proposed over two de- cades ago to realize strong light confinement via their perfect photonic bandgaps, but the expected ultrahigh-Q wavelength- sized cavities were achieved just recently in a slightly different system that has only a partial bandgap, more specifically, a modulated mode-gap cavity in a 2-D photonic crystal. It is now possible to store photons for over a nanosecond in a wavelength-sized volume for this type of cavity, which has not been realized in other systems. The same confinement mechanism has provided various interesting cavities including air-core cavities, index-modulation-induced cavities, and dy- namic cavities. We discuss the impact and possible applications of the achieved strong light confinement. First, it has a strong impact on photonic integrated circuits and photonic network- on-chip (NoC) applications because these cavities enable us to realize tiny, low-power-consumption, and integratable pho- tonic devices, which are hard to realize conventionally. It is next shown that an ultrahigh-Q cavity system constitutes an extreme slow-light medium. It is also shown that ultrahigh-Q cavities enable the adiabatic tuning of light, which makes it possible to manipulate optical signals very differently from conventional optics. Finally, other possible systems promising for strong light confinement are investigated, such as 1-D pho- tonic crystals and photonic amorphous structures without any periodicity.
Coupled microresonators are expected to play a key role in slow-light engineering and various types of light-matter interaction enhancement, especially if they are based on small and high-Q cavities. Although rapid progress has been made on microresonator performance, large-scale arrays of coupled resonators based on high-Q wavelength-sized cavities have not yet been realized. Here, we show large-scale (N > 100) ultrahigh-Q coupled nanocavity arrays based on photonic crystals. This is the first demonstration of large-scale coupled resonator arrays based on wavelength-sized cavities, in which tight-binding sinusoidal dispersion is seen. We confirm that an ultrahigh value of Q (similar to 1 x 10(6)) is maintained, even when N is large, and the resonators exhibit very low loss characteristics with regard to light propagation. The ultrahigh value of Q and small size has enabled us to achieve ultraslow light pulse propagation with a group velocity well below 0.01c and a long group delay.
Recently, strongly modulated photonic crystals, fabricated by the state-of-the-art semiconductor nanofabrication process, have realized various novel optical properties. This paper describes the way in which they differ from other optical media, and clarifies what they can do. In particular, three important issues are considered: light confinement, frequency dispersion and spatial dispersion. First, I describe the latest status and impact of ultra-strong light confinement in a wavelength-cubic volume achieved in photonic crystals. Second, the extreme reduction in the speed of light is reported, which was achieved as a result of frequency dispersion management. Third, strange negative refraction in photonic crystals is introduced, which results from their unique spatial dispersion, and it is clarified how this leads to perfect imaging. The last two sections are devoted to applications of these novel properties. First, I report the fact that strong light confinement and huge light–matter interaction enhancement make strongly modulated photonic crystals promising for on-chip all-optical processing, and present several examples including all-optical switches/memories and optical logics. As a second application, it is shown that the strong light confinement and slow light in strongly modulated photonic crystals enable the adiabatic tuning of light, which leads to various novel ways of controlling light, such as adiabatic frequency conversion, efficient optomechanics systems, photon memories and photons pinning.
The authors review their recent studies on various nanophotonic devices including all-optical switches, optical memories, electro-optic modulators, photo-detectors and lasers, all of which are based on photonic crystal (PhC) nanocavities. The strong light confinement achieved in PhC nanocavities has enabled these devices with ultrasmall footprint and ultralow power/energy consumption. These characteristics are ideally suited for constructing dense photonic network on chip, which will overcome the limitation of future CMOS chips in terms of high-speed operation with less energy consumption and heat generation.
The ability to directly modulate a nanocavity laser with ultralow power consumption is essential for the realization of a CMOS-integrated, on-chip photonic network, as several thousand lasers must be integrated onto a single chip. Here, we show high-speed direct modulation (3-dB modulation bandwidth of 5.5 GHz) of an ultracompact InP/InGaAsP buried heterostructure photonic-crystal laser at room temperature by optical pumping. The required energy for transmitting one bit is estimated to be 13 fJ. We also achieve a threshold input power of 1.5 µW, which is the lowest observed value for room-temperature continuous-wave operation of any type of laser. The maximum single-mode fibre output power of 0.44 µW is the highest output power, to our knowledge, for photonic-crystal nanocavity lasers under room-temperature continuous-wave operation. Implementing a buried heterostructure leads to excellent device performance, reducing the active region temperature and effectively confining the carriers inside the cavity.
Although high-speed all-optical switches are expected to replace their electrical counterparts in information processing, their relatively large size and power consumption have remained obstacles. We use a combination of an ultrasmall photonic-crystal nanocavity and strong carrier-induced nonlinearity in InGaAsP to successfully demonstrate low-energy switching within a few tens of picoseconds. Switching energies with a contrast of 3 and 10 dB of 0.42 and 0.66 fJ, respectively, have been obtained, which are over two orders of magnitude lower than those of previously reported all-optical switches. The ultrasmall cavity substantially enhances the nonlinearity as well as the recovery speed, and the switching efficiency is maximized by a combination of two-photon absorption and linear absorption in the InGaAsP nanocavities. These switches, with their chip-scale integratability, may lead to the possibility of low-power, high-density, all-optical processing in a chip.
We have fabricated high-Q photonic crystal nanocavities with a lateral p-i-n structure to demonstrate low-power and high-speed electro-optic modulation in a silicon chip. GHz operation is demonstrated at a very low (microW level) operating power, which is about 4.6 times lower than that reported for other cavities in silicon. This low-power operation is due to the small size and high-Q of the photonic crystal nanocavity.