Chengang Ji

Wide-angle, polarization-independent ultrathin broadband visible absorbers

A capability of absorbing a broad range of visible lights is essential to boost the performance of various applications, such as photovoltaics (PV), photodetectors, and thermal emitters. Here, we present an angle-insensitive, polarization-independent ultrathin (<150 nm) broadband absorber in the visible regime exploiting strong interference behaviors in highly absorbing semiconductor materials. A proposed structure simply has four layers composed of two stacks of a metal and a semiconductor demonstrating a remarkably enhanced absorption property as compared with the device without a top semiconductor film. This is attributed to multi-cavity resonance effects in each cavity, which is obviously elucidated with phase calculations and electric field distributions. The maximum absorption efficiency of the device is 95.5% at a resonance and its absorption characteristic can be maintained over a wide angle of incidence up to ±70° regardless of the incident light polarization. Finally, we investigate how our approach can be utilized to achieve a tandem PV cell with high efficiency. Our strategy can be applied to other material systems and can be useful in diverse applications, including thermal emitters and PV.

Subwavelength nanocavity for flexible structural transmissive color generation with a wide viewing angle

We present a high angular tolerant structural coloration based on strong interference effects in a nanocavity patterned at a subwavelength scale on a flexible substrate. The structural colors, fabricated over a large area by nanoimprint lithography, feature a non-iridescent performance over a wide angle of incidence up to 60°, which is of great importance to various applications, such as imaging sensors and colored display systems. In addition to the non-iridescence, the transmissive colors of the proposed structure, simply consisting of triple layers, can be tuned with ease by altering a duty cycle of nanostructures, thus enabling the creation of individual RGB colors in a pixel unit via a facile one-step approach. Moreover, it is confirmed that their performances remain unchanged to the 10 mm bending radius condition, and the encapsulation effects of a polymer material on their optical properties are investigated for practical usage. The presented strategy could provide a new avenue for achieving improved efficiency and desired functionality, thereby opening the door to many potential applications, including anti-counterfeit tagging, imaging sensor systems, and color e-paper displays.

Robust Extraction of Hyperbolic Metamaterial Permittivity Using Total Internal Reflection Ellipsometry

Hyperbolic metamaterials are optical materials characterized by highly anisotropic effective permittivity tensor components having opposite signs along orthogonal directions. The techniques currently employed for characterizing the optical properties of hyperbolic metamaterials are limited in their capability for robust extraction of the complex permittivity tensor. Here we demonstrate how an ellipsometry technique based on total internal reflection can be leveraged to extract the permittivity of hyperbolic metamaterials with improved robustness and accuracy. By enhancing the interaction of light with the metamaterial stacks, improved ellipsometric sensitivity for subsequent permittivity extraction is obtained. The technique does not require any modification of the hyperbolic metamaterial sample or sophisticated ellipsometry set-up, and could therefore serve as a reliable and easy-to-adopt technique for characterization of a broad class of anisotropic metamaterials.

Subwavelength nanocavity for flexible structural transmissive color generation with wide viewing angle: publisher’s note

This Publisher’s Note corrects the author list of Optica3, 1489 (2016).10.1364/OPTICA.3.001489

Ultrabroadband and omnidirectional absorbers based on a tandem multilayer structure

We propose a novel scheme for ultrabroadband and omnidirectional absorbers featuring a tandem structure of various absorptive materials and graded index profile. They show an average absorption of 98% over 400-2000nm and >93% for 400-3500nm range.