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Superabsorbing, artificial metal films constructed from Semiconductor Nanoantennas Soo Jin Kim, Junghyun Park, Majid Esfandyarpour, Emanuele Francesco Pecora, Pieter G. Kik, and Mark Brongersma Nano Lett., Just Accepted Manuscript • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 5, 2016
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Nano Letters
Superabsorbing, artificial metal films constructed from Semiconductor Nanoantennas Soo Jin Kim1, Junghyun Park1, Majid Esfandyarpour1, Emanuele F. Pecora1, Pieter G. Kik1.2 and Mark L. Brongersma1* 1
Geballe Laboratory for Advanced Materials, 476 Lomita Mall, Stanford, California 94305-4045
2
CREOL, The College of Optics and Photonics, University of Central Florida, 4000 Central
Florida Blvd, Orlando, Florida 32816 * To whom correspondence should be addressed. E-mail:
[email protected].
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Abstract In 1934 Wilhelm Woltersdorff demonstrated that the absorption of light in an ultrathin, freestanding film is fundamentally limited to 50%. He concluded that reaching this limit would require a film with a real-valued sheet resistance that is exactly equal to
,
where η = µ 0 ε 0 is the impedance of free space. This condition can be closely approximated over a wide frequency range in metals that feature a large imaginary relative permittivity ε″, i.e. a real-valued conductivity σ = ε0ε″ω. A thin, continuous sheet of semiconductor material does not facilitate such strong absorption as its complex-valued permittivity with both large real and imaginary components preclude effective impedance matching. In this work, we show how a semiconductor metafilm constructed from optically resonant semiconductor nanostructures can be created whose optical response mimics that of a metallic sheet. For this reason the fundamental absorption limit mentioned above can also be reached with semiconductor materials, opening up new opportunities for the design of ultrathin optoelectronic and light harvesting devices.
Keywords Metafilm, Mie resonance, Germanium nanobeam, Semiconductor nanoantenna, Light absorption
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The achievement of very strong light-matter interaction in ultrathin semiconductor layers is key to realizing next-generation optoelectronic applications. Thinner devices are more lightweight, flexible, and offer advantages in terms of reduced materials and processing cost. Shrinking device dimensions can also result in an improved performance. For example, achieving strong light absorption in increasingly thin semiconductor layers will naturally result in increases in the speed and efficiency of photocarrier extraction. This finds application in a wide variety of technologies, including solar energy harvesting1-4, photodetectors5,6 and thermal photovoltaics7,8. From the early 1900s, researchers have been eager to understand the ultimate limits to absorption of electromagnetic waves in layers of material that are much thinner than the wavelength λ of the incident radiation. Woltersdorf9, Dallenbach10, and Salisbury11,12,13 explored these limits for thin metal and lossy dielectric films with and without backreflectors. Here, we aim to understand the maximum absorption one can achieve in a subwavelength layer of semiconductor material deposited on a transparent substrate. As a starting, reference point, it is of value to note that the absorption limit of an ultrathin (t
