A more efficient interaction with light is of central importance for applications such as sensing, energy harvesting and biological research among others. Materials and strategies to guide, confine, manipulate and absorb light are being continuously sought for. One of those new strategies is looking for novel materials to absorb the maximum amount of light with the smaller amount of material, the so-called light trapping scheme, a photonic design in which the light can be confined in nanometric thin films, achieving absorptions comparable to those of thicker layers. In this line of work, the challenge resides in the optical design and molding the original material into a nanostructure where light is confined and eventually absorbed in the active layer.
In this work, we transform an ultrathin slab of Germanium of less than 70 nanometers into a photonic architecture exhibiting an impressive 81 % total absorption from the visible to the near infrared. The strong absorption in such a thin film of Ge enables novel sensing and energy harvesting devices in flexible and portable substrates. Playing with a thin metallic substrate acting as a reflector and a system of a two height hole array in the semiconductor, the light finds several ways to resonate and remain within the structure. The photonic system shows almost full absorption in a broad frequency range, vastly exceeding the absorption of a flat film design. Interestingly, as the light confinement is given by the structural parameters of the architecture, by changing parameters such as height or pitch between holes, the absorption can be tuned spectrally, opening a wide range of engineering possibilities. Moreover, the whole nanometric structure is fabricated with a simple and scalable technique termed nanoimprinting lithography, which is similar to conventional pressure or temperature printing, allowing an easy implementation in large area surfaces.
This research provides the key design guidelines for super absorptive portable surfaces readily implementable in many optoelectronic devices.
Pau Molet, Juan Luis Garcia-Pomar, Cristiano Matricardi, Miquel Garriga, Maria Isabel Alonso and Agustín Mihi
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
Ultrathin Semiconductor Superabsorbers from the Visible to the Near Infrared.
Advanced Materials 30 (9), 1705876 (2018)
The fraction of light absorbed by a Germanium thin film on top of a mirror is compared to the absorption obtained by the same amount of Ge nanostructured as a metasurface (with and without antireflection coatings).
The authors acknowledge the Spanish Ministerio de Economía, Industria y Competitividad (MINECO) for its support through grant no. SEV-2015-0496 in the framework of the Spanish Severo Ochoa Centre of Excellence programme and also for its support through grant MAT2016-79053-P.
This project has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 637116, ENLIGHTMENT).