Ultrathin Semiconductor Perfect Light Absorbers with High Spectral, Polarization, and Angle Selectivity for Arbitrary Wavelengths

Enabling perfect light absorption in ultrathin materials promises the development of exotic photonic devices. Here we demonstrate new strategies that can provide capabilities to rationally design ultrathin (thickness < {lambda}/10~{lambda}/5) semiconductor perfect absorbers for arbitrary wavelengths, including those at which the intrinsic absorption of the semiconductor is weak, e.g. Si for near-IR wavelengths. This is in stark contrast with the existing studies on ultrathin perfect absorbers, which have focused on metallic materials or highly-absorptive semiconductors. Our design strategies are built upon an intuitive model, coupled leaky mode theory that we recently developed and can turn the design for perfect absorbers to the design for leaky modes. The designed absorber is featured with extraordinary absorption enhancement, miniaturized dimension, and high selectivity for the wavelength, polarization, and angle of incident light. It can enable the development of flexible, light-weight, high-performance, cost-effective, and multifunctional optoelectronic devices that are difficult with current light absorbers.

Semiconductor solar superabsorber

Understanding the maximal enhancement of solar absorption in semiconductor materials by light trapping promises the development of affordable solar cells. However, the conventional Lambertian limit is only valid for idealized material systems with weak absorption, and cannot hold for the typical semiconductor materials used in solar cells due to the substantial absorption of these materials. Herein we theoretically demonstrate the maximal solar absorption enhancement for semiconductor materials and elucidate the general design principle for light trapping structures to approach the theoretical maximum. By following the principles, we design a practical light trapping structure that can enable an ultrathin layer of semiconductor materials,for instance, 10 nm thick a-Si, absorb > 90% sunlight above the bandgap. The design has active materials with one order of magnitude less volume than any of the existing solar light trapping designs in literature. This work points towards the development of ultimate solar light trapping techniques.

Solar Superabsorption of Semiconductor Materials

We theoretically demonstrate the fundamental limit in volume for given materials (e.g. Si, a-Si, CdTe) to fully absorb the solar radiation above bandgap, which we refer as solar superabsorption limit. We also point out the general principles for experimentally designing light trapping structures to approach the superabsorption. This study builds upon an intuitive model, coupled leaky mode theory (CLMT), for the analysis of light absorption in nanostructures. The CLMT provides a useful variable transformation. Unlike the existing methods that rely on information of physical features (e.g. morphology, dimensionality) to analyze light absorption, the CLMT can evaluate light absorption in given materials with only two variables, the radiative loss and the resonant wavelength, of leaky modes, regardless the physical features of the materials. This transformation allows for surveying the entire variable space to find out the solar superabsorption and provides physical insights to guide the design of solar superabsorbing structures.

General modal properties of optical resonances in subwavelength nonspherical dielectric structures

Subwavelength dielectric structures offer an attractive low loss alternative to plasmonic structures for the development of resonant optics functionality such as metamaterials. Nonspherical like rectangular structures are of most interest from the standpoint of device development due to fabrication convenience. However, no intuitive fundamental understanding of optical resonance in nonspherical structures is available, which has substantially delayed the device development with dielectric materials. Here we elucidate the general fundamentals of optical resonances in nonspherical subwavelength dielectric structures of different shapes (rectangular or triangular) and dimensionalities (1D nanowires and 0D nanoparticles). We demonstrate that the optical properties (i.e. light absorption) of nonspherical structures are dictated by the eigenvalue of the structures leaky modes. Leaky modes are defined as natural optical modes with propagating waves outside the structure. We also elucidate the dependence of the eigenvalue on physical features of the structures. The eigenvalue shows scaling invariance with the overall size, weakly relies on the refractive index, but linearly depends on the size ratio of different sizes of the structure. We propose a modified Fabry-Perot model to account for this linear dependence. Knowledge of the dominant role of leaky modes and the dependence of leaky mode on physical features can serve as a powerful guide for the rational design of devices with desired optical resonances. It opens up a pathway to design devices with functionality that has not been explored due to lack of intuitive understanding, for instance, imaging devices able to sense incident angle, or superabsorbing photodetectors.