Optical absorbers based on strong interference in ultra‐thin films

Abstract

Optical absorbers find uses in a wide array of applications across the electromagnetic spectrum, including photovoltaic and photochemical cells, photodetectors, optical filters, stealth technology, and thermal light sources. Recent efforts have sought to reduce the footprint of optical absorbers, conventionally based on graded structures or Fabry-Perot-type cavities, by using emerging concepts in plasmonics, metamaterials, and metasurfaces. Unfortunately, these new absorber designs require patterning on subwavelength length scales, and are therefore impractical for many large-scale optical and optoelectronic devices. In this article, we summarize recent progress in the development of optical absorbers based on lossy films with thicknesses significantly smaller than the incident optical wavelength. These structures have a small footprint and require no nanoscale patterning. We outline the theoretical foundation of these absorbers based on "ultra-thin-film interference", including the concepts of loss-induced phase shifts and critical coupling, and then review several applications, including ultra-thin color coatings, decorative photovoltaics, highefficiency photochemical cells, and infrared scene generators.

Film Flip and Transfer Process to Enhance Light Harvesting in Ultrathin Absorber Films on Specular Back‐Reflectors

“…[1][2][3][4][5][6] Significant absorption of broadband light can be achieved in layers only a few nanometers thick, with low sensitivity to the angle of incidence. [1][2][3][4][5][6] Significant absorption of broadband light can be achieved in layers only a few nanometers thick, with low sensitivity to the angle of incidence.…”

“…[6] Tarnishing was a key challenge in our previous work, [6] where strong interference in ultrathin hematite (α-Fe 2 O 3 ) films on silver-gold alloy (90 at% silver and 10 at% gold) back-reflectors was used for highly efficient photoanodes for photoelectrochemical water splitting. [1][2][3][4][5][6] Significant absorption of broadband light can be achieved in layers only a few nanometers thick, with low sensitivity to the angle of incidence. Formation of crystalline hematite with good photoelectrochemical properties require high temperature and oxygen atmosphere.…”

Coherent perfect absorbers: linear control of light with light

“…Single-port thin-film absorbers and their relationships with CPAs are discussed in detail in Refs. [8,9,51,52]. It is worth emphasizing that one-port devices cannot exhibit coherent control of absorption through beam modulation.…”

“…Here, we use the term absorption in a broad sense, meaning any process by which the energy of an electromagnetic wave is transferred to a medium and hence converted, for example, to heat, electrical current or fluorescence. Perfect absorption is of great interest for a range of optical applications [8,9], including radar cloaking [4,5,10], sensing and molecular detection [11,12], photovoltaics [13], and photodetection [14,15].…”

Color display and encryption with a plasmonic polarizing metamirror

“…Among the plentiful research subjects in the big family of plasmonic metamaterials, one of the intensively investigated topics in recent decades is the design of color filters [14][15][16][17][18][19][20][21][22][23][24][25], where a specific color is observed if the scattered light of a particular wavelength range comes into our eyes when an object is illuminated with white light. By exploiting the plasmonic subwavelength structures, such as nanohole or nanoparticle arrays [1,14], light transmission/reflection can be selectively enhanced or hindered at a resonant wavelength.…”