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This program is focused on increasing light harvesting in thin film solar cells by exploiting the light trapping effects of surface plasmons and photonic crystals. Most of the research results have been or will be published in journal or conference papers. Accordingly, this report only contains milestone data of research.

1. Multilayer Metal-Insulator Stacks for Light Trapping

In this project, we investigated the multilayered metal-insulator (MMI) stack system by solving its fundamental super surface mode, and demonstrated the tuning of effective surface plasmon frequency (ESPF). By tuning the filling ratio and especially the coating material's refractive index, surface modes with arbitrary wave vectors and absorption coefficients are achieved. Figures 1(a) and (b) illustrate that the increased ESPF allows super surface modes at frequency beyond the conventional surface plasmon frequency. On the other hand, decreased ESPF from the multilayer stacks yields a decreased effective wavelength (Fig. 1(d), 0.26 λ₀) compared to Fig. 1(c) with 0.61 λ₀. The concept of ESPF would empower researchers to excite confined surface waves more freely from a limited pool of plasmonic materials, and to envision and demonstrate novel detecting/sensing scheme.

Based on the classical Kretschmann setup, the MMI system is experimentally tested as an anisotropic material to exhibit plasmonic behavior and a candidate of “metametal” to engineer the preset surface plasmon frequency of conventional metals for light trapping applications. The conditions to obtain artificial surface plasmon frequency are thoroughly studied, and the tuning of surface plasmon frequency is verified by electromagnetic modeling and experiments. As shown in Fig. 2, the ESPF and hence absorption peak can be either tuned down or up. The design rules drawn in this project would bring important insights into applications such as solar cells, optical lithography, nano-sensing and imaging.

2. Super Talbot Effect in Indefinite Metamaterial

Figure 3. (a) Schematic illustration of the investigated structure, the 1D grating is assumed to be infinite in y-axis. D=100nm, d=50nm, and the wavelength is 630nm. (b) No Talbot effect is seen in the air when the period of the grating is much smaller than the incident wavelength. (c) Periodic Talbot carpet pattern can be observed in an indefinite metamaterial (shown in H field distribution) with ε_z=-1 and ε_x=1.

In this project, we re-visited one classical optical phenomenon, named Talbot effect, and investigated this effect in an indefinite metamaterial without the paraxial approximation. In a conventional material, for example air, the information of the subwavelength features of the object carried by evanescent waves will decay exponentially. Therefore, there will be no Talbot effect observed as the period of the object is much smaller than the wavelength, as shown in Fig. 3(a, b). In contrast to the case in the air, self-imaging effect can be achieved in an indefinite metamaterial, where the evanescent waves are converted into propagating waves and conveyed far away, shown in Fig. 3(c).

Figure 4. (a) Super Talbot effect in an Ag/SiO₂ stack, shown in power. (b) Cross-sectional power profile along the white solid line shown in (a), where x=52nm.

The indefinite metamaterial can be approximated by a system of thin, alternating multilayer metal-insulator stack. Figure 4 shows the power distribution in an Ag/SiO₂ MMI stack, with each layer of thickness 5nm. The super Talbot effect may find various applications in the fields as nanolithography, optical storage, and solar cells. For solar cell applications, the indefinite materials enable a “power pulling” effect, which converts evanescent waves into propagating waves.

3. Designer Surface Plasmons

In this project, we performed the experimental demonstration of the remarkable advantages of using the designer surface plasmons (or spoof surface plasmons) to achieve deep subwavelength power squeezing. Simple metal gratings can function as designer surface plasmonic waveguides with suitable geometrical parameters. The dispersion of the guided modes is not sensitive to the widths of the metal gratings. This property enables the mode size along transverse direction could be squeezed into deep subwavelength scale, where the mode size gets smaller and smaller the intensity gradually increases. The mode size slightly varies with different frequencies and minimizes at 0.04λ-by-0.03λ. Currently, this concept is only experimentally demonstrated in the microwave regime, but we show that there is no difficulty to scale the structure into higher frequency regime. See Fig. 5. This will provide us a new means to squeeze and trap light power in thin film solar cells.

Figure 5. (a) Experimental demonstration of light power squeezing in the microwave regime. (b) Numerical modeling of the technique in the visible light regime.

4. Impact on PI's Career

The PI is an expert in nanophotonics and plasmonics, while solar cells are quite a new topic for the PI. This program gives the PI an opportunity to meet with one of the most critical issues facing human being. The outcome of this effort is to design and verify a prototype for further research. The PI is preparing proposals to submit to federal agencies, such as National Science Foundation and Department of Energy.

5. Impact on the Students

One PhD student and two master students have been supported by this program. The research results have been incorporated in two courses: Optoelectronics and Integrated Optics. Two invited lectures were given in the Introduction of Microsystems and Nanotechology based on the results of this program.

6. Plans for Next Year

We will keep on studying the enhanced light-trapping based on surface plasmons and photonic crystals. In addition, we will extend the exploration into the recently discovered graphene-based solar cells. In this case, graphene is treated as a new energy-harvesting material, whose absorption spectrum can be controlled by applied electric gating.