Broadband light absorption with disordered gold nanostructures

Broadband light absorbers are a crucial part of many applications, including thermophotovoltaic cells, plasmonic scatterers for photovoltaic cells, broadband thermal emitters, cloaking devices, and optical interconnects.^1–5 Ideally, such broadband absorbers should behave like a black body, i.e., a dark material that absorbs radiation at all wavelengths, angles, and polarizations, and without exhibiting any transmission or reflection. In reality, however, it is difficult to create a perfect black body because practical materials are intrinsically non-ideal.

To date, there have been significant efforts—focused at the microscale—to improve broadband absorption.^6–10 The highest darkness that has been reported thus far is 99.95% absorption for a vertical array of single-walled carbon nanotubes with thicknesses of 800μm (and broadband absorption of 98–99% for thicknesses of 300–500μm).^{6, 9} The design of these media is typically guided by the principle of optimizing light–matter interactions in a suitable resonant system of finite size (which is sensitive to incident angle and/or polarization).

In our work we have explored a different approach to improving broadband absorption.¹¹ We use the concept of chaotic energy harvesting¹² to reverse the effect in a biomimetic material that is completely dark. The material that we have created exhibits an extremely strong darkness, even at microscopic concentrations. In addition, our material is totally insensitive to polarization and the angle of illumination.

The inspiration for our approach comes from a white beetle—Cyphochilus—that is native to Southeast Asia and has an ultra-brilliant shell. The ultra-white color of the shell is caused by thin disordered scales that chaotically scatter light in all directions.¹³ We can reverse this effect by developing a complex ‘porous’ system, which is composed of a cavity attached to a random network of pores. These pores are made of infinitely long, metallic waveguides, as shown in Figure 1(a). As light propagates within the cavity, it bounces around the system until it impinges on one of the pores. Some light is then coupled into the waveguide channel and becomes fully absorbed, whereas the remaining energy is re-scattered. The disordered distribution of the pores means that the light reflections are completely randomized, which leads to the chaotic light scattering. Under these conditions, there is the same probability of light interacting with any of the distribution of pores, regardless of the input conditions (i.e., wavelength, angle of incidence, and polarization). With our approach we can therefore trigger a process of broadband absorption that creates a completely dark material.

Figure 1. Schematic illustration of the gold nanostructure design. (a) The porous material is composed of a metallic cavity and a random network of pores, each of which is an infinitely long waveguide. (b) Cross section of the porous structure and the associated light dynamics (left). The transformed structure, obtained by applying conformal mapping, is shown on the right. Shaded area of the cross section describes the pore that is mapped into the curved area near the kissing point (K) of the nanosphere and nanorod. Ω: Transformation optics. (c) The structure of the final black body is composed of a collection of the random scatterers (i.e., nanostructures). This structure is fully equivalent to the porous material shown in (a).¹¹(Image courtesy of Nature Publishing Group.)

To develop our dark material, we first transformed the ideal design—Figure 1(a)—into a realistic structure. To do this, we mapped each pore element—Figure 1(b) left—into a finite geometry—Figure 1(b) right—with the use of transformation optics.¹⁴ We rest a nanosphere on a nanorod and thus mirror the pores of the space in the area around the kissing point of the rod and sphere. By creating a random collection of these nanostructures—Figure 1(c)—we are able to fully replicate the disordered distribution of the pores. As such, the two systems shown in Figure 1(a) and (c) are completely equivalent. Whenever light reaches a nanostructure, some of the light enters into the equivalent pore and cannot return. The remaining energy, however, continues its random walk into a second nanostructure. After a sufficiently large number of scattering events all the light is fully absorbed in the system.

We have also developed a new colloidal approach, which is based on ‘seeded growth,’ to fabricate our nanostructures. For this process, we used pre-synthesized gold nanorods as the seeds for further growth of the gold nanospheres, and a single nanosphere is grown on top of each nanorod. We can thus form our desired nanostructure (see Figure 2). This wet chemistry technique provides an excellent realization of our ideal design. It is also suited to the scalable production of the designed nanostructure, at low cost.

Figure 2. (a) Visual appearance of the realized gold nanostructures suspended in water. (b) Low-magnification transmission electron microscopy (TEM) image of the nanostructures. (c) High-magnification TEM image of the single nanostructure marked in (b).¹¹ (Image courtesy of Nature Publishing Group.)

To quantify the level of darkness that can be achieved with our nanostructure when it is dispersed in water, or deposited over a silicon substrate, we have used a UV-visible light-IR spectrum analyzer equipped with an integrating sphere. We find that with a microscopic volume filling fraction of 1.9 × 10⁻⁵ in water, our nanostructure has a flat absorption of 98–99% at wavelengths between 400 and 1200nm. Furthermore, with a thin (10μm) layer of the nanostructures on silicon we can achieve an almost flat, average absorption of 98.5% between 400 and 1400nm. We also find that the average broadband absorption can be maintained at up to 98%, even if the incident angles vary by up to 70°. As such, the high absorption levels are virtually insensitive to the illumination angle. Our results indicate that this is the first material with which geometry is used to achieve broadband absorption. We have also achieved the highest ever averaged absorption with a material thickness of 10μm, and at oblique incidence across a large optical wavelength window.

In summary, we have demonstrated a new way to achieve broadband absorption in practical materials. We use an energy harvesting approach with a specifically designed light-porous system. We use a novel colloidal approach to create the necessary nanostructures that act as light scatterers in the material. Black body materials have great potential for many applications (e.g., harvesting more energy in solar heaters and creating invisibility for military stealth technologies). Our nanostructure approach—with its simple fabrication at ambient conditions, flexible use, and high absorption levels—is therefore a candidate for these applications. In the next step of our research we will use our material in new types of solar thermal collection units and to produce solar fuels.

This work was supported by King Abdullah University of Science and Technology (KAUST) research program Optics and Plasmonics for Efficient Energy Harvesting (award CRG-1-2012-FRA-005) and by Yu Han's baseline support funds from KAUST.