Daan Stellinga

Efficient Silicon Metasurfaces for Visible Light

Dielectric metasurfaces require high refractive index contrast materials for optimum performance. This requirement imposes a severe restraint; either devices have been demonstrated at wavelengths of 700 nm and above using high-index semiconductors such as silicon, or they use lower index dielectric materials such as TiO2 or Si3N4 and operate in the visible wavelength regime. Here, we show that the high refractive index of silicon can be exploited at wavelengths as short as 532 nm by demonstrating a crystalline silicon metasurface with a transmission efficiency of 71% at this wavelength and a diffraction efficiency of 95% into the desired diffraction order. The metasurfaces consist of a graded array of silicon posts arranged in a square lattice on a quartz substrate. We show full 2π phase control, and we experimentally demonstrate polarization-independent beam deflection at 532 nm wavelength. Our results open a new way for realizing efficient metasurfaces based on silicon for the technologically all-import...

Spatial Resolution and Refractive Index Contrast of Resonant Photonic Crystal Surfaces for Biosensing

By depositing a resolution test pattern on top of a Si3N4 photonic crystal resonant surface, we have measured the dependence of spatial resolution on refractive index contrast Δn. Our experimental results and finite-difference time-domain (FDTD) simulations at different refractive index contrasts show that the spatial resolution of our device reduces with reduced contrast, which is an important consideration in biosensing, where the contrast may be of order 10-2. We also compare 1-D and 2-D gratings, taking into account different incidence polarizations, leading to a better understanding of the excitation and propagation of the resonant modes in these structures, as well as how this contributes to the spatial resolution. At Δn 0.077, we observe resolutions of 2 and 6 μm parallel to and perpendicular to the grooves of a 1-D grating, respectively, and show that for polarized illumination of a 2-D grating, resolution remains asymmetrical. Illumination of a 2-D grating at 450 results in symmetric resolution. At very low index contrast, the resolution worsens dramatically, particularly for Δn <; 0.01, where we observe a resolution exceeding 10 μm for our device. In addition, we measure a reduction in the resonance linewidth as the index contrast becomes lower, corresponding to a longer resonant mode propagation length in the structure and contributing to the change in spatial resolution.

Tunable Optical Filters Based on Silicon Nitride High Contrast Gratings

We report the use of a 500-nm-thick silicon nitride membrane as a high-reflectivity mirror in the orange-red spectral range. High contrast gratings based on semiconductors have already been used as high-reflectivity mirrors in the near-IR spectral range, but their use in the visible, which is essential for many types of biosensors, is much less explored. Our membrane is patterned with a high contrast grating of 560-nm period and forms part of a tunable Fabry-Pérot cavity. The cavity is tuned electrostatically and functions as a tunable optical filter. Three different designs of the membrane suspension are investigated to establish the effect of the arm geometry on the surface stress and the displacement of the membrane. By applying 9 V to the device, we observe a 13-nm wavelength shift of the spectral peak centered at 630 nm.

Characterization of planar microlenses made of high contrast gratings

We report on the focusing performance of reflective 2D high contrast grating lenses based on silicon. The combination of their subwavelength nature and their high refractive index contrast make it possible to create highly tolerant and planar microlenses. We used a rigorous mathematical code to design the lenses and verified their performance with finite element simulations. We also investigated the effects of grating thickness, angle and wavelength of incidence in these simulations. Experimentally, we show the evolution of the beam profile along the optical axis for a lens with a high (0.37) numerical aperture. We have explored a wide range of numerical apertures (0.1 – 0.93) and focal lengths (5 μm – 140 μm) and show that the lenses behave as expected across the full range. Our analyses demonstrate the large design flexibility with which these lenses can be made along with ease of fabrication and potential for a number of applications in micro-optics.

Focusing with planar microlenses made of two-dimensionally varying high contrast gratings

Abstract. We report on the focusing performance of reflective two-dimensionally varying high contrast grating lenses based on silicon. The combination of their subwavelength nature and their high refractive index contrast makes it possible to create highly tolerant and planar microlenses. We used a rigorous mathematical code to design the lenses and verified their performance with finite element simulations. We also investigated the effects of grating thickness, angle, and wavelength of incidence in these simulations. Experimentally, we show the evolution of the beam profile along the optical axis for a lens with a high (0.37) numerical aperture. We have explored a wide range of numerical apertures (0.1–0.93) and show that the lenses behave as expected across the full range. Our analyses demonstrate the large design flexibility with which these lenses can be made along with ease of fabrication and potential for a number of applications in micro-optics.

An Organic Vortex Laser

Optical vortex beams are at the heart of a number of novel research directions, both as carriers of information and for the investigation of optical activity and chiral molecules. Optical vortex beams are beams of light with a helical wavefront and associated orbital angular momentum. They are typically generated using bulk optics methods or by a passive element such as a forked grating or a metasurface to imprint the required phase distribution onto an incident beam. Since many applications benefit from further miniaturization, a more integrated yet scalable method is highly desirable. Here, we demonstrate the generation of an azimuthally polarized vortex beam directly by an organic semiconductor laser that meets these requirements. The organic vortex laser uses a spiral grating as a feedback element that gives control over phase, handedness, and degree of helicity of the emitted beam. We demonstrate vortex beams up to an azimuthal index l = 3 that can be readily multiplexed into an array configuration.

Tunable optical filters based on silicon nitride mem-branes with subwavelength gratings

We report the use of 500 nm thick silicon nitride membrane as a high reflective broad-band filter in the orange-red spectral range. The membrane is patterned into a high contrast grating with 560 nm period to act as a high-reflectivity mirror. Using electrostatic tuning, we have achieved a tunable filter based on a Fabry-Pérot cavity architecture. By applying 9 V to the MEMs device, we observe a 13 nm wavelength shift of a peak centred at 630 nm.