Designing an Ultrathin Film Spectrometer Based on III-Nitride Light-Absorbing Nanostructures

Abstract

In this paper, a spectrometer design enabling an ultrathin form factor is proposed. Local strain engineering in group III-nitride semiconductor nanostructured light-absorbing elements enables the integration of a large number of photodetectors on the chip exhibiting different absorption cut-off wavelengths. The introduction of a simple cone-shaped back-reflector at the bottom side of the substrate enables a high light-harvesting efficiency design, which also improves the accuracy of spectral reconstruction. The cone-shaped back-reflector can be readily fabricated using mature patterned sapphire substrate processes. Our design was validated via numerical simulations with experimentally measured photodetector responsivities as the input. A light-harvesting efficiency as high as 60% was achieved with five InGaN/GaN multiple quantum wells for the visible wavelengths.

Ultrathin Optics-Free Spectrometer with Monolithically Integrated LED Excitation

“…The enhanced absorption can allow more photodetectors to participate in the reconstruction algorithm. This was shown in [ 22 ].…”

“…The photodetectors’ responses beyond 540 nm are not sufficiently random, which results in a large reconstruction error between 540 and 590 nm. This can be remedied by increasing the responsivities of all photodetectors [ 22 ]. In contrast, although all photodetectors absorb strongly between 450 and 480 nm, the correlation between their responsivities is weak enough to still lead to an accurate reconstruction.…”

“… ( a , b ) The responsivities of the 13 photodetectors measured at a zero bias using a tunable monochromatic light source (adapted from [ 22 ]). The optical power at the sample was characterized separately using an integration sphere.…”

“…Miniaturizing such a system is a nontrivial task involving the ability to integrate multiple material platforms and careful planning of performance tradeoffs, such as spectral resolution, sensitivity, system size, and cost. Among various approaches, spectrometers based on reconstructive algorithms [ 12 ] shift the complexity of processing spectral information from physical components to software computations [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ]. With the steady growth of computational power per watt-dollar, this approach has become increasingly promising in constructing an extremely compact spectroscopic system.…”

“…Recently, absorptive filters, either standalone or built into the photodetectors, have been successfully integrated into a reconstructive spectrometer [ 13 , 14 , 18 , 21 , 29 ]. Absorptive filters’ spectral responses have a weak dependence on the incident angle of light [ 22 ]. As a result, collimation optics either become optional or can be replaced by other imaging optics to enable a spectrometer array for hyperspectral imaging.…”