We demonstrate high aperture (up to NA~0.64) three-dimensional focusing in free space based on wavefront-engineered diffraction gratings. The grating lens' optical response is tailored by spatially varying the grating ridge and groove width in two dimensions to achieve focal lengths of order 100 μm that are crucial for micro-optical applications. The phase profile of the lens includes multiple 2π phase jumps and was obtained by applying an algorithm for finding the optimal path for both phase and amplitude. Experimental measurements reveal a lateral spot size of 5 μm that is close to the size of a corresponding Airy disk.

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.

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.

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.

Continuous-wave diffuse reflectance or Near Infrared Spectroscopy (NIRS) offers the possibility to perform a preliminary screening of tissue for ischemia or other tissue anomalies. A tool for intracavity NIRS measurements during laparoscopic surgery, developed within the framework of the FP7-IP ARAKNES (Array of Robots Augmenting the KiNematics of Endoluminal Surgery) project, is described. It consists of a probe, that is located on the tip of an appropriately shaped laparoscopic manipulator and then applied to the tissue. Such a probe employs an array of incoherent semiconductor light sources (LEDs) frequency-multiplexed on a single detector using a lock-in technique. The resulting overall tool structure is simple and compact, and allows efficient coupling of the emitted light towards the tissue. The tool has high responsivity and enables fast and accurate measurements. A dataset gathered from in-vivo tissue is presented. The performance both indicates direct applicability of the tool to significant surgical issues (ischemia detection), and clearly indicates the possibility of further miniaturizing the probe head towards catheterized approaches.