The invention relates to the field of optical field concentrator, and in particular to a highly efficient optical field concentrator that is capable of confining optical field in nanometer-thin low-index media with very high optical confinement factor.
Silicon based photonic integration is a promising technology that can potentially integrate optical and electronic devices on one single silicon wafer through CMOS technology. One of the key components is the silicon-based electrically-pumped laser that can emit at telecommunication wavelength. Among the different silicon based gain materials developed so far, silicon nanostructures and rare earth doping of silicon in SiO2 have dominated scientific efforts towards the goal of achieving practical silicon lasers. However, directly using these oxide materials as laser core layers faces both electronic and optical difficulties: on one hand, making electrical carrier injection through the non-conductive thick oxide is very difficult; on the other hand, large optical confinement in low-index thin gain materials is also hard to achieve with conventional approaches. For the first issue, researchers have recently demonstrated that for the oxide thickness in the range of 5-10 nm, the efficient carrier injection can be realized by the field effect tunneling injection approach. In order to achieve high external quantum efficiency of the laser, confining and guiding light in such low-index nanometer-sized structures with very high confinement factor, hence, becomes a key technology for CMOS compatible light emitting devices.
Strong optical field concentration with very high optical confinement and power density in very small volume is also of great importance for the other guided wave optoelectronic devices, such as optical microcavities, switches, modulators, optical sensors and other applications requiring high field confinement in low-index medium. It increases the degree of control of light-matter interaction, allowing both enhancement and inhibition. With the development of photonic technology, the device size is approaching the nanometer range. It needs a new confining mechanism that is able to break the diffraction limit of the light and still able to guide and confine light.
Conventional optical field confining and concentrating are based on optical waveguides made of a high-index core surrounded by low-index claddings. For the guided modes with steady spatial pattern to exist in the waveguide, it requires total internal reflections (TIR) at the boundaries to ensure that the optical field mainly concentrates and propagates in the core region. Therefore, low-index guiding and confining are prohibited in conventional waveguides. In some other circumstances, such as in the photonic bandgap (PBG) structures and antiresonant reflecting optical waveguides (ARROWs), by utilizing the external reflection induced by the multiple-dielectric-layer interferences, light can be confined and guided in the low-index core.
However, because the external reflection is realized by interferences, the performances of PBG or ARROW-type waveguides are very sensitive to the physical parameters, such as layer thicknesses (or periods) and indices, as well as the operating wavelength. Furthermore, to confine light inside the core region, the resonant condition requires the core layer thickness to be in the order of half wavelength. It appears that the nanometer size low-index guiding is not possible by ARROW-type waveguides.
A slot waveguide has demonstrated the capability of guiding and confining light in low-index media in the nanometer size range. The structure consists of a thin low-index (nl) slot embedded between two rectangular high-index (nH) regions. Due to the large index contrast at interfaces, the normal electric field undergoes a large discontinuity, which results in a field enhancement in the low-index region with a ratio of nH2/nl2. A confinement factor about 30% has been demonstrated with a 50 nm wide Si—SiO2 slot configuration. However, in terms of the field concentration, measured by the normalized power density, this structure only provides about 20 μm−2, which is about two times larger than the value of an optimal conventional silicon-on-insulator (SOI) waveguide.
This small enhancement is mainly due to the relatively large width of the slot. Further increasing the slot width will saturate the confinement factor and result in a further decrease of the field concentration. On the other hand, decreasing the slot width can increase the field concentration. However, it also decreases the confinement factor drastically. About half of the confinement will be lost due to the slot thickness decreasing from 50 nm to 5 nm. This confinement reduction will eventually compromise the power density inside the slot region.