Ping-Chun Li

Experimental realization and modeling of a subwavelength frequency-selective plasmonic metasurface

We have modeled, fabricated, and characterized a plasmonic metasurface with subwavelength features, whose dominant resonance is the independent of incident angle and polarization, and sensitive only to the material composition and geometry of a single element. Higher-order resonances, associated with surface plasmon polariton (SPP) coupling and higher diffraction orders, are sensitive to the incident angle and the array periodicity and less pronounced compared with the metasurface resonance. Numerical simulations and theoretical analysis highlight a clear physical difference between the SPP resonances and the dominant metasurface collective resonance, whose properties may be of great interest for plasmonic solar cells and subwavelength color filters.

Light trapping in thin-film solar cells via scattering by nanostructured antireflection coatings

The use of nanostructured TiO2 layers fabricated on thin-film solar cells to provide, simultaneously, both antireflection functionality and light trapping via scattering of long-wavelength photons into guided optical modes is demonstrated and analyzed in thin-film quantum-well solar cells. Nanosphere lithography is used for fabrication of periodic arrays of subwavelength-scale TiO2 structures, and separation of active device layers from their epitaxial growth substrate and integration with the nanostructured TiO2 layer enables increased optical absorption via coupling to both Fabry-Perot resonances and guided lateral propagation modes in the semiconductor. The nanostructured TiO2 layer is shown to act as a graded-index coating at optical wavelengths and simultaneously to scatter incident light into guided optical modes within the device. The dependence of these effects on angle of incidence is also analyzed.

Wide-angle wavelength-selective multilayer optical metasurfaces robust to interlayer misalignment

Multilayer plasmonic optical metasurfaces are demonstrated and analyzed that provide highly wavelength-selective reflectance that is robust to variation in angle of incidence and highly tolerant of misalignment of features between vertically stacked layers. Structures containing two layers of Ag nanostructure arrays separated by a dielectric layer are shown to provide reflectance >75% and transmittance <1% over a bandwidth of ∼100  nm, with minimal variation for angles of incidence varying from 0° to 30°. These characteristics are shown to be robust to variations in vertical alignment between layers comparable to the array period. An analysis of these characteristics in terms of plasmonic behavior of individual Ag nanostructures, interference effects between multiple layers of nanostructure arrays, and phase shifts produced at each array layer is presented.

Flexible, low-loss, large-area, wide-angle, wavelength-selective plasmonic multilayer metasurface

Flexible, low-loss, large-area multilayer plasmonic optical metasurfaces are demonstrated and analyzed that provide wavelength-selective reflectance >95% and transmittance <1% with low absorption and robustness to variation in angle of incidence and polarization. These characteristics are shown to be insensitive to vertical misalignment between layers, and defects within individual layers. Analysis based on analytical modeling and numerical simulations provides physical insights into reflectance, loss, and bandwidth of these multilayer metasurface structures. Fabry-Perot resonances associated with phase shifts from each individual metasurface are also examined, and evidence of m = 0 resonance due to the nonzero, wavelength dependent phase shift from the metasurface cavity is demonstrated and explained. Finally, fabrication on flexible substrates via rapid, large-area nanosphere lithography, and the robustness of optical properties of interlayer misalignment together enable the demonstration of wavelength-selective focusing at optical frequencies.

Angular dependence of light trapping in In0.3Ga0.7As/GaAs quantum-well solar cells

The dependence of light trapping effects in In0.3Ga0.7As/GaAs quantum-well solar cells on wavelength and incident angle is experimentally characterized and analyzed. Separation of active device layers from their epitaxial growth substrate enables integration of thin-film semiconductor device layers with nanostructured metal/dielectric rear contacts to increase optical absorption via coupling to both Fabry-Perot resonances and guided lateral propagation modes in the semiconductor. The roles of Fabry-Perot resonances and coupling to guided modes are analyzed via photocurrent response measurements and numerical modeling for light incident at angles of 0° (normal incidence) to 30° off normal. Light trapping enables external quantum efficiency at long wavelengths as high as 2.9% per quantum well to be achieved experimentally, substantially exceeding the ∼1% per quantum well level typically observed. Increased long wavelength quantum efficiency is shown in experimental measurements to persist with increasing angle of incidence and is explained as a consequence of the large number of guided modes available in the device structure.

Large-area omnidirectional antireflection coating on low-index materials

Large-area subwavelength dielectric hexagonal lattices of cylindrical pillars on quartz substrates that provide high optical transmittance at all angles of incidence under different polarizations of light, and are fabricated using low-cost patterning techniques, are demonstrated and analyzed. Transmittance >85% for angles of incidence in excess of 70° is demonstrated at visible and near-infrared wavelengths, and the structures employed are shown to be superior at visible wavelengths to tapered “moth eye” surfaces for practically achievable dimensions. Detailed analytical calculations and numerical simulations elucidating the impact of feature size, height, periodicity, and refractive index are presented.

Fabrication of birefringent nanocylinders for single-molecule force and torque measurement

Optically anisotropic subwavelength scale dielectric particles have been shown to enable studies of the mechanical properties of bio-molecules via optical trapping and manipulation. However, techniques emphasized to date for fabrication of such particles generally suffer from limited uniformity and control over particle dimensions, or low throughput and high cost. Here, an approach for rapid, low-cost, fabrication of large quantities of birefringent quartz nanocylinders with dimensions optimized for optical torque wrench experiments is described. For a typical process, 10(8) or more quartz cylinders with diameters of 500 nm and heights of 800 nm, with uniformity of ±5% in each dimension, can be fabricated over ∼10 cm(2) areas, for binding to a single bio-molecule, and harvested for use in optical trapping experiments. Use of these structures to measure extensional and torsional dynamics of single DNA molecules is demonstrated with measured forces and torques shown to be in very good agreement with previously reported results.

Effects of rapid thermal oxidation on electrical characteristics of chemical‐vapor‐deposited SiO2 gate dielectrics

In this paper, effects of rapid thermal oxidation (RTO) on electrical characteristics of thin (200 A) chemical‐vapor‐deposited (CVD) SiO2 have been studied. Current density‐electric field (J‐E) characteristics, flat‐band voltage, and gate voltage shifts under constant‐current stressing were also examined. Results show that RTO improves the charge trapping property of as‐deposited CVD oxides. In addition, RTO of CVD oxides also increases the electron injection barrier height of the as‐deposited samples at the cathode and produces devices with lower leakage current and tighter breakdown distribution.

Minimized open-circuit voltage reduction in GaAs/InGaAs quantum well solar cells with bandgap-engineered graded quantum well depths

The use of InGaAs quantum wells with composition graded across the intrinsic region to increase open-circuit voltage in p-i-n GaAs/InGaAs quantum well solar cells is demonstrated and analyzed. By engineering the band-edge energy profile to reduce photo-generated carrier concentration in the quantum wells at high forward bias, simultaneous increases in both open-circuit voltage and short-circuit current density are achieved, compared to those for a structure with the same average In concentration, but constant rather than graded quantum well composition across the intrinsic region. This approach is combined with light trapping to further increase short-circuit current density.

Metal-oxide-semiconductor characteristics of rapid thermal processed chemical vapor deposited SiO2 gate dielectrics

Abstract Current conduction, interfacial integrity, and charge trapping characteristics of 200 A rapid thermal nitrided/reoxidized (RTN/RTO) chemical vapor deposited (CVD) SiO 2 are studied. Results show that RTN reduces the leakage current of CVD oxides, contrary to the effects of RTN on thermal oxides. The as-deposited CVD oxide after rapid thermal annealing (RTA) in O 2 has the lowest electron trapping level as compared with the other devices, while devices with RTN/RTO gate dielectrics have superior endurance against constant current stressing. The increased electron trapping induced by RTN can be minimized by pre- and post-nitridation annealing in oxygen.