Squeezing light in a wavelength scale volume is a challenging goal. However, strong light confinement is associated to high electromagnetic field density and therefore strong light-matter interaction which is rewarded for many phenomena like light generation, detection or interaction with chemical & bio species. The planar geometry of the Fabry-Perot cavity, know as a microcavity, has aroused a great interest and lead to the invention of the VCSEL. In the quest of ultra-high Q/V semiconductor cavities, the SiNaPS laboratory worked out the integration of FP nanocavities in sub-micron sized waveguides. By the means of new geometries reducing modal mismatch losses at cavity-mirrors interfaces, this allowed to reach record Q factor for SOI based nanocavities. The tight localization of the electromagnetic field in those cavities was also probed at nanometer scale using scanning near field microscopy. Furthermore, the possibility of cavity resonance tuning by near field interaction between the SNOM nanometric tip and the nanocavity was demonstrated.
Fig. 1a : SEM picture of the nanocavity (hole diameter 200nm and period 370nm). Fig. 1b : Measured cavity transmittance. Fig 1c : optical field recorded above the nanocavity by SNOM.
The concept of waveguide integrated microcavity was introduced by a MIT team in the late 90's, but till now the quality factor Q, characteristic of the time scale during which photons are bouncing within the cavity, remained low. If it was well understood that the Q factor was limited by light transmission through the cavity mirrors, we took also into account modal mismatch between the propagating cavity mode and the mirror Bloch mode to manage photon confinement within the cavity. A new cavity design has then been proposed based on tapered mirrors. It allowed to reach Q as high as 60,000 with a modal volume of about 0.6 (l/n)3 - fig. 1a and 1c.Near field microscopy was then chosen to explore at sub wavelength scale field localization within the nanocavity and evidence the tight field confinement Fig. 1c.
Fig 2a : schematic view of the experiment, 2b : cavity transmission recorded for the two tip locations (300nm above cavity - green and 4nm above cavity - red). The high-frequency oscillations are due to the bouncing of light between the sample cleaved facets.
If the nanometric tip allows us to finely map the resonant field distribution, its interactions with strongly localized fields was also considered. As matter of fact the cavity-tip interactions resulted as a means to tune the cavity resonant wavelength. As shown in fig. 2b, a red shift of the resonance wavelength occurs in the presence of the tip close to the cavity. It is noticeable that this shift occurs without alteration of the Q factor. We believe that such effect may open the way to a new class of nano-mechanical optical switchs.
Fig. 3a : mode profile in monomode silicon waveguide. 3b : mode profile in slot waveguide stressing the confinement in air gap. 3c : SEM view of the Silicon-On-Insulator photonic crystal coupled nanocavities and measured profile of optical near field in TE polarization.
In order to even decrease light confinement volume, we considered so-called slot waveguides which support guided light modes in sub l (100nm width) air slot - fig.3a & b and realized air slot coupled nanocavities - fig. 3c
We believe that this geometry will not only allow to reach ultra low volume light confinement but also strong light material interaction since the air slot provide an efficient way to integrate low optical index materials or biological species in the high density electromagnetic field region .
Maj : 27/09/2016 (721)