Metallic nanodevices for chip-scale optical interconnects

The use of optics to make connections within and between electronic chips has been the subject of research for over 20 years because it could solve many of the problems experienced in electrical systems [1]. A critical challenge for the convergence of optics and electronics is that the micrometer scale of optics is significantly larger than the nanometer scale of modern electronic devices. In the conversion between photons and electrons, this size incompatibility often leads to substantial penalties in power consumption, area, latency and noise. Among all the requirements, low power is crucial for chip-scale interconnects. On-chip global interconnects typically consume 1 pJ per bit (1mW/(Gb/s)). The system energy of optical interconnects needs to be 10 times lower for sufficient advantages over their electrical counterpart. Given the other energies required for the total system, the optical output device (a laser or modulator) should target energies ~ 10 % of this amount (10 fJ). In such a system we might expect ~ 10% (1 fJ) of transmitted energy arrives at the receiving detector, given a reasonable optical loss in the system. To achieve such aggressive energy targets requires miniaturized optoelectronic devices [2]. Metallic nanostructures can confine light to dimensions considerably smaller than the wavelength of the light. They have the potential to play a unique and important role in solving this size-mismatch problem. The strong near fields associated with metallic nanostructures have been studied extensively in recent years. However, until recently, very little research had been done in the interaction of these strong fields with semiconductors, i.e., active metallic nanodevices [3]. Attenuation in metals has limited the propagation length of metallic nanowaveguides to tens of microns, but it has not become a dominating factor for active devices such as light emitters and detectors. For emitters, the radiative emission rate can be amplified by adopting a small mode volume. Hill et al. reported the first metallic nanolaser emitting at telecom wavelengths with dimensions of 200 to 300 nm [4]. This electrically pumped nanolaser exhibits a low threshold current smaller than 6 μA. The structure consists of an InP/InGaAs/InP pillar surrounded by a thin insulating SiN layer encapsulated by gold. This nanolaser has a mode volume of 0.38(λ/2n), possibly smaller than that of a typical photonic crystal cavity. At low temperatures (< 77K) where loss in metals is reduced, unambiguous lasing operation was achieved. The Q factor of the metal cavity is very small at room temperature, and therefore lasing was not observed. More recently, a silver cavity with Q factor over 1000 was demonstrated, paving the way for such a laser to operate at room temperature [5]. The receiving end of an optical link typically consists of a detector and an electronic receiver circuit. The detector converts photons into electric current and then the receiver circuit amplifies the current into a large signal suitable for digital logic. If the detector is sufficiently small and thus with a small capacitance, an optical signal can generate large enough voltage swings to drive logic inputs directly from the terminals of the detectors. In this scheme we do not require any receiving amplifier circuit at all, and therefore are Journal of Nanophotonics, Vol. 3, 030302 (12 March 2009)