Versatile digital micromirror device-based method for the recording of multilevel optical diffractive elements in photosensitive chalcogenide layers (AMTIR-1)

Alexandre Joerg 1 Mael Vigneaux 1 Julien Lumeau 1
1 RCMO - RCMO
FRESNEL - Institut FRESNEL
Abstract : A new alternative and versatile method for the production of Diffractive Optical Elements (DOEs) with up to 4 phase levels in AMTIR-1 (Ge33As12Se55) layers is demonstrated. The developed method proposes the use of the photosensitive properties of the layers and a specific in-situ optical monitoring coupled with a reverse engineering algorithm to control the trigger points of the writing of the different diffractive patterns. Examples of various volume DOEs are presented. Chalcogenide glasses (ChG) are composed from a chalcogen element such as Sulphur, Selenium, and Tellurium. To create a glass network, some other atoms are added such as As, Ge, Sb, Ga, Si or P. ChG are particularly interesting due to their specific optical properties. In fact they are transparent into the mid-infrared up to 20 µm depending on their chemical composition [1]. The refractive index is high n~2-3 and ChG possess also a high nonlinear refractive index n2 [2]. But one of the most striking property is their photosensitivity. Using light exposure with a wavelength below the optical bandgap, the chemical bonds can be rearranged. This effect directly modifies the optical properties of the ChG. This very specific property has led to different kinds of applications such as phase change memory [3]… As thin films, ChG were thoroughly studied and the impact of inserting a photosensitive layer into an optical filter to modify its spectral response was demonstrated [4]. As an example, it was shown, a few years ago, that it is possible to use the photo-bleaching or photo-darkening [5] effects to adjust the central wavelength of a Fabry-Perot filter and therefore generate ultra-uniform bandpass filters [6]. The ChG studied in this paper is AMTIR-1. It is composed in molar percent with 33% of germanium, 12% of arsenic and 55% of selenium and was provided by Amorphous Material Company. AMTIR-1 was already deposited as thin films by pulsed laser deposition [7], thermal or e-beam evaporation, and showed excellent optical properties [8]. Layers from AMTIR-1 were shown to be excellent candidates for the production of various optical elements such as ring resonators [9]… We recently showed that these layers are also a good candidate for the production of a new kind of optical elements [10]. Actually with the theoretical and experimental developments of diffractive optical elements, metamaterials or metasurfaces, there is a higher and higher demand for 3D micro-and nanostructuring of multilayer stacks. 1D-structures are easily obtained by the optical coating technology, while 3D-structures require micro-or nano-engineering the local thickness of the deposited structures using lithographic processes combined with etching techniques. We showed that for coarse structures, i.e. for structures having a pitch with size much larger than the wavelength (i.e. with size larger than 10 µm), it is possible to create volume phase structures by locally controlling the local refractive index of a photosensitive layer. Such an approach was applied to the fabrication of binary DOEs. However, it is well known that for many applications and in order to achieve higher diffraction efficiency, especially for the production of asymmetrical beams, the use of multi-level DOEs is mandatory. In this paper we demonstrate the possibility to photo-structure a photosensitive chalcogenide layer with 4 levels in order to produce a Volume Diffractive Optical Element (VDOE). Methods of production and monitoring are detailed. The chalcogenide layers and thin film stacks were produced using electron beam physical vapor deposition in a Balzers BAK600 chamber. The residual pressure inside the vacuum chamber was around ~10-6 mbar and the deposition rate was 10 ± 1 A/s. In a previous works [8] using those parameters, we first demonstrated that AMTIR-1 materials provide broad transparency range above 850 nm, high refractive index (n = 2.74 @ 1 µm) and photosensitive properties such as photo-bleaching effect. This effect occurs into the AMTIR-1 layers when exposed to a light with a wavelength whose energy is below the bandgap. This effect allows generating refractive index decrement down to-0.04 at 1 µm that was then applied to binary DOEs. Moreover, it was shown that this refractive index change is
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Alexandre Joerg, Mael Vigneaux, Julien Lumeau. Versatile digital micromirror device-based method for the recording of multilevel optical diffractive elements in photosensitive chalcogenide layers (AMTIR-1). Optics Letters, Optical Society of America, 2016, 41, pp.3415-3418. ⟨10.1364/OL.99.099999⟩. ⟨hal-01359464⟩

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