High-Q silicon photonic crystal cavity for enhanced optical nonlinearities

We fabricate and experimentally characterize an H0 photonic crystal slab nanocavity with a design optimized for maximal quality factor, Q = 1.7 x 10(6). The cavity, fabricated from a silicon slab, has a resonant mode at lambda = 1.59 mu m and a measured Q-factor of 400 000. It displays nonlinear effects, including high-contrast optical bistability, at a threshold power among the lowest ever reported for a silicon device. With a theoretical modal volume as small as V = 0.34(lambda/n)(3), this cavity ranks among those with the highest Q/V ratios ever demonstrated, while having a small footprint suited for integration in photonic circuits. (C) 2014 AIP Publishing LLC.

Epitaxial Piezoelectric MEMS on Silicon

Don Isarakorn

Integration of new functional materials into silicon microsystems is a key factor to enable technology for a wide range of innovative MEMS devices. Piezoelectric materials are of primary interest for integrating sensing and actuation functions in MEMS due to their high forces and high energy densities. The use of PZT thin films in MEMS applications offers the possibility of increasing the sensitivity or actuation capabilities of the devices compared to alternatives such as AlN and ZnO. In general, PZT thin films exhibit smaller piezoelectric coefficients and polarizations than PZT bulk materials due to grain size, composition, crystallographic orientation, non-defined stoichiometry and mechanical boundary conditions. Moreover, PZT thin films are typically grown onto amorphous surfaces resulting in polycrystalline structures, which often lead to degraded performance due to fatigue and aging characteristics. Since epitaxial PZT films exhibit properties, including piezoelectric coefficients, polarizations, and dielectric constants, generally superior to polycrystalline films, it is of high interest to consider their application in MEMS devices, however, there are some real challenges to be solved. Once these issues are overcome, epitaxial PZT thin films could offer interesting potentials in the realization of high performance piezoelectric MEMS. In this thesis, several aspects related to the development of epitaxial piezoelectric MEMS on silicon are investigated, which cover the following topics: the deposition and integration of high quality epitaxial PZT thin films on silicon wafers; the establishment of microfabrication techniques with associated process flows; and the FEM supported design, and characterization of epitaxial piezoelectric MEMS. A short overview is first given on the current state-of-the-art of piezoelectric MEMS. The integration of epitaxial oxide films on silicon wafers and their properties is then briefly described. The epitaxial oxide thin film heterostructures are based on a piezoelectric Pb(Zr0.2Ti0.8)O3 layer grown on 2'' silicon wafers through two oxide layers: SrTiO3 used as buffer and metallic SrRuO3 used as bottom electrode. The optimized microfabrication process for these oxide layers with specific attention in maintaining the piezoelectric properties of the epitaxial PZT films is presented. The polarization was measured to optimize their processing with at the end no degradation of the piezoelectric properties throughout the process. The epitaxial PZT thin films exhibit a large piezoelectric coefficient d31 of 130-140 pm V-1, allowing the realization of MEMS devices with enhanced actuation/detection properties, such as large amplitude actuation with lower driving voltage, high sensitivity, and high efficiency in energy conversion. The superior properties of the epitaxial PZT thin film and the effectiveness of the optimized microfabrication techniques have been demonstrated by two examples of epitaxial PZT MEMS devices. First, different epitaxial PZT cantilevers with and without a Si proof mass have been developed for vibration energy harvesting applications. A high power density of up to 14 µW g-2 was obtained with high current generation and usable voltage, while maintaining lower optimal resistive load. The second application is based on an epitaxial PZT membrane to produce a resonating device. The study of basic characteristics of such device has shown excellent results as it shows a strong harmonic oscillation response with a high quality factor at atmospheric pressure. The finite element model of the epitaxial PZT membrane has then been developed for localized-mass sensing application to determine the resonant frequency, and the effect of the position of the mass and of the resonant mode on the mass sensitivity. The mass sensitivity is of the order 10-12 g Hz-1, which is in excellent agreement with the simulated value. The minimum detectable mass of ∼5 ng can be achieved. These results indicate that the integration of epitaxial thin films on silicon is a promising technology, which improves the performances of piezoelectric MEMS devices. Finally, the concept of charge integration technique for static measurement in piezoelectric sensors is proposed. This technique improves the detection sensitivity of piezoelectric sensors in low frequency measurements, which makes them suitable for chemical and biological detection in a liquid environment.

EPFL2011

GaN-based photonic crystal cavities on silicon for visible and near infrared applications

Noelia Vico Triviño

Photonic crystal (PhC) cavities combine ultra-high quality (Q) factors with small mode volumes, resulting in an enhancement of the light-matter interaction at the nanoscale, which, beyond fundamental studies is advantageous for countless applications in photonics. In addition, III-nitride (III-N) semiconductors offer unique optical properties including a direct wide bandgap and tunable emission spanning the ultraviolet to near infrared (NIR) spectral region. Despite these advantages the development of III-N based PhC cavities has been hindered owing to several processing issues inherent to such hard and chemically inert materials. This work aims at investigating GaN-based PhC cavities operating at both visible and NIR frequencies. In the first part of this work a fabrication process that fulfills the low-defect requirements of PhCs, while remaining compatible with silicon for future integration prospects, was developed. It is based on the growth of GaN layers on Si(111) substrates, which are then patterned by e-beam lithography and dry etching techniques, including the substrate undercut in order to create an airgap below the PhC area, which guarantees light confinement into the resulting planar waveguide. This approach allowed achieving airgaps > 3 µm, which are essential for structures operating in the NIR, and hardly achievable by sacrificial layer techniques. The ultimate assessment of the optical quality of such nanostructures is given by the Q-factor of the cavity. In this regard, spectroscopic characterization of the fabricated samples was carried out. At visible wavelengths experimental Q-factors up to 5200 and 14000 have been ascertained in 2D PhC cavities and 1D nanobeam cavities, respectively, which represent the current state-of-the-art for both geometries at the time this dissertation was written. At IR frequencies the maximum Q which was measured amounts to 22500. Furthermore, it is important to estimate the fluctuations on both the Q-factor and the resonant wavelength of nominally identical replicas of the same cavity to account for the fabrication process reliability. This has been addressed by taking advantage of PhC cavities optimized by an effective genetic algorithm for Q-factor optimization (166000 at 1.3 µm in this case) [M.Minkov and V. Savona, Sci. Rep. 4, 5124 (2014)]. Thus, 20 groups of cavities were fabricated, to allow for lithographic tuning, that differ by 2 nm in the hole diameter from one group to another. Q-factor measurements were then carried out in a total of 60 cavities, namely 6 replicas of 10 different groups of cavities, demonstrating a high reproducibility and an average Q-factor ~17000. The second part of this thesis investigates the lasing characteristics of nanobeam cavities emitting in the violet-blue spectral range featuring a single InGaN quantum well (QW). In the high-absorption QW region lasing has been observed under continuous wave (cw) optical pumping at room temperature, demonstrating low threshold power densities ~740 W/cm2. The laser behavior was also analyzed by means of laser rate equations. The low cw lasing threshold is well accounted for by a large spontaneous emission coupling factor (>0.8), inherent to the nanobeam geometry. These results highlight the high potential of III-nitrides for the realization of nanophotonic devices and set a new step forward for the integration of wide bandgap semiconductors with silicon opening future prospects in biosensing and optogenetics.

EPFL2015

Piégeage optique résonant et analyse du mouvement de nanoparticules piégées dans des cristaux photoniques à cavités creuses

Mario Lucien Henri Tonin

Optical trapping was used for decades in biotechnology for a broad range of applications. It can be used for remote manipulation, application of pico-Newton forces and even study DNA processes. Nevertheless, optical trapping setups can be cumbersome. In order to reduce their footprint and be able to integrate them in lab-on-chips circuits, several techniques exist. This work explores the use of hollow cavities in two dimensional photonic crystals for integrated optical trapping in lab-on-chips. Photonic crystals are excellent candidates for this application thanks to their capacity to confine light below the diffraction limit. Photonic crystals sample were fabricated at EPFL, from silicon on insulator wafers. The crystals were made on the silicon membrane 220 nm thick. The holes of the photonic crystal lattice are 250 nm in diameter. The trapping occurs inside the hollow cavity, which is 700 nm in diameter. This hollow cavity introduced a localized mode with a hexagonal symmetry. Thanks to the gradient forces from the cavity mode, we are able to trap polystyrene nano spheres with sizes ranging from 500 nm down to 250 nm. The interaction between the trapped nano sphere and the mode gives rise to back action effects. Such resonant optical trapping differs greatly from classical optical tweezers. To understand this resonant trapping and find the most important parameters, we record the motion of single nano spheres trapped inside the photonic crystal cavity. The measurement is performed with back focal plane interferometry, which allows the measurement of the motion of the tracked object in three dimensions at large sampling frequency. To use this method of detection, a special sample and microfluidic membrane had to be designed and fabricated. The microfluidic membrane is made in polymer and glass to reproduces the thickness of a microscope coverslip. High-resolution imaging is performed through this microfluidic membrane, which is also used to carry the nano spheres in microchannels and control their flow with precision. The results show that ratio of the linewidth of the resonance of the cavity to the detuned resonance wavelength caused by the presence of the trapped object is a critical parameter. The analysis of 250 nm nano spheres displays a complex behaviour, where the trapped nano spheres are jumping from one lobe of the mode to another with a frequency depending of the power coupled to the cavity. Additionally, the measurement of the power transmitted through the photonic crystal after coupling to the cavity hold a considerable amount of information. It is possible to correlate the power coupled into the cavity with the trapping position. Moreover, the transmitted power can be used to identify the operation regime of the trapping cavity, as well as the size distribution of a solution of particles. It is also demonstrated that the trapping of several nano spheres is possible within a single cavity, while the nano spheres arrange themselves in complex structures.

EPFL2016