The „Common Rose“ butterfly (Pachliopta aristolochiae) is distributed across the South and South-East Asia. As its name suggests, this swallowtail butterfly is an abundant species. Counter-intuitively, „rose“ actually refers to its colored body rather than its wings, which are mainly (females) and even totally (males) black on the upside. This particular feature fulfills multiple functionalities ranging from visual communication to thermoregulation. Efficient sunlight harvesting, responsible for the black appearance of these butterflies under different viewing angles, is achieved by a combination of light absorbing pigments together with a sophisticated morphology of the wing scales. Remarkably, the latter exhibit a dense array of disordered nanoholes, with diameters just below the wavelengths of visible light.
Sunlight harvesting by disordered photonic biostructures
By conducting a number of optical spectroscopic measurements on the P. aristolochiae black wings, three main optical characteristics stand out: the strong dependence of light absorption with respect to the nanoholes array and their filling fraction/density, proving its major contribution to the light harvesting scheme; the high absorptance of photons over a broad spectral range, exceeding the sole visible spectrum; and the good robustness of the optical properties for varying illumination conditions (under normally incident or diffuse light).
Optical simulations of the actual nanostructures reveal in more details the mechanisms ruling these excellent light absorption properties. First, high energy photons that are easily absorbed by the pigments are efficiently channeled through the nanoholes, which optimize their capture. Second, the lower energy photons, which face a weaker absorption by the pigments, are scattered into the scales by the nanoholes array hence increasing their interaction with the pigments in horizontal plane and in turn, their absorption probability.
Interestingly, the disorder in the arrangements of nanoholes including size and position p provides unique advantages when it comes to sunlight harvesting. The size distribution and apparent „disordered“ arrangement of the nanoholes ensure that the previously described mechanisms are effective for both a broad spectral and angular range.
From the biological role model to real-world applications
Having described the wings of the „Common Rose“ butterfly as efficient sunlight collectors implies that they could straightforwardly inspire the design of novel solar cells. This consideration motivated the implementation of the black butterfly photonic structures into the absorbing layer of thin-film photovoltaic devices.
To this end, disordered nanoholes arrays were fabricated by the phase separation of a binary polymer blend. This scalable and potentially cost-effective technique is reminiscent of the micro-phase separation of lipid bilayer membranes of living cells and intracellular organelles which takes place in arthropods. As a proof-of-concept, the photonic structures thus produced were subsequently etched into a thin silicon-based photovoltaic absorber, resulting in an increase of the integrated light absorption of +90% under normal incidence and exceeding to 200% for high viewing angles compared to an unpatterned layout.
The range of applicability of these disordered photonic structures and of their associated fabrication process extends over the sole case of photovoltaics. In first prototypes, the black butterfly design was successfully used to improve the optical performance of optoelectronic and optomechanical devices as varied as light emitting diodes or biosensors.
Dr. Radwanul Hasan Siddique: Dr. Siddique has received his PhD with summa cum laude from Karlsruhe Institute of Technology (KIT), Germany in 2016 under the supervision of PD Dr. Hendrik Hölscher at the Institute of Microstructure Technology at Karlsruhe Institute of Technology (KIT). His PhD dissertation was focused on “Bio-inspired Nanophotonics”; that concentrates on understanding the underlying physics at nanoscale of fascinating biologically evolved optical structures, developing affordable nanofabrication techniques to implement them, and incorporating the discoveries into device-level biomimetic engineering solutions for opto-electronic and bio-medical applications. He had adopted interdisciplinary skills to answer his research questions; spanning across computational optics, imaging and spectroscopies of bio(-inspired) systems, and nanofabrication. He has collaborated with the group of Dr. Guillaume Gomard and Prof. Dr. Uli Lemmer (Light Technology Institute, KIT) on the topics related to optoelectronic devices and Dr. Silvia Vignolini (Department of Chemistry, University of Cambridge) on the development of the bio-inspired SERS substrate. Dr. Siddique initiated the black butterfly biomimetics project as a part of his doctoral thesis. He was involved in studying and replicating the black butterfly inspired nanostructure – design, fabrication and performing experiments for thin film photovoltaic absorber as well as for the SERS substrate for biosensing.
Yidenekechaw Donie: Mr. Donie is a third year PhD student in the sub-group “Nanophotonics” at the Institute of Microstructure Technology at Karlsruhe Institute of Technology (KIT) led by Dr. Gomard. His research is focused on light management using disordered nanostructures for optoelectronic devices, funded by DFG under the priority programme “Tailored Disorder”. Mr. Donie completed his master thesis with Dr. Siddique on studying and replicating the black butterfly inspired nanostructures including simulation and experiments. He continues the work as a PhD student where he is involved in thin film solar cells and organic light emitting diodes fabrication and characterization.
Dr. Guillaume Gomard: Dr. Gomard obtained a Humboldt Research Fellowship for Postdoctoral Researchers in 2013 and joined the Light Technology Institute (KIT) to lead the „Nanophotonics“ research activities, with a special emphasis on disordered photonics applied to optoelectronics. In 2015 he was granted a research project within the DFG priority programme “Tailored Disorder“, aiming at taking inspiration from nanostructures found in biological systems to engineer optical materials that purposely exploit structural irregularities. This topic has led him to analyze light harvesting structures found in Nature, both in the vegetal (epidermal cells of plants) and in the animal (wing scales of Glasswing and Black butterflies) kingdoms. He participated to the spectroscopic measurements and to the numerical analysis of the Pachliopta aristolochiae scales, and to the implementation of the black butterfly-inspired, self-assembled nanostructures in solar cells and organic light-emitting diodes demonstrators.