In this work, a teepee-like photonic crystal (PC) structure on crystalline silicon (c-Si) is experimentally demonstrated, which fulfills two critical criteria in solar energy harvesting by (i) its Gaussian-type gradient-index profile for excellent antireflection and (ii) near-orthogonal energy flow and vortex-like field concentration via the parallel-to-interface refraction effect inside the structure for enhanced light trapping. For the PC structure on 500-μm-thick c-Si, the average reflection is only ∼0.7% for λ = 400-1000 nm. For the same structure on a much thinner c-Si ( t = 10 μm), the absorption is near unity (A ∼ 99%) for visible wavelengths, while the absorption in the weakly absorbing range (λ ∼ 1000 nm) is significantly increased to 79%, comparing to only 6% absorption for a 10-μm-thick planar c-Si. In addition, the average absorption (∼94.7%) of the PC structure on 10 μm c-Si for λ = 400-1000 nm is only ∼3.8% less than the average absorption (∼98.5%) of the PC structure on 500 μm c-Si, while the equivalent silicon solid content is reduced by 50 times. Furthermore, the angular dependence measurements show that the high absorption is sustained over a wide angle range (θinc = 0-60°) for teepee-like PC structure on both 500 and 10-μm-thick c-Si.

A key to the success of solid-state lighting is an ultraefficient light extraction, ∼90%. Recent advances in nanotechnology, particularly in creating nanorods, present an unprecedented opportunity to manipulate optical modes at nanometer scales. Here, we report an optically pumped nanorod light-emitting diode (LED) with an ultrahigh extraction efficiency of 79% at λ = 460 nm without the use of either a back reflector or thin film technology. We demonstrated experimentally three key mechanisms for achieving high efficiency: guided mode-reduction, embedded quantum wells, and ultraefficient light out-coupling by the fundamental HE(11) mode. Furthermore, we show that size reduction at nanoscale represents a new degree-of-freedom for alternating and achieving a more directed LED emission.

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The tantalizing possibility of 31% solar-to-electric power conversion efficiency in thin film crystalline silicon solar cell architectures relies essentially on solar absorption well beyond the Lambertian light trapping limit (Bhattacharya and John in Nat Sci Rep 9:12482, 2019). Up to now, no solar cell architecture has exhibited above-Lambertian solar absorption, integrated over the broad solar spectrum. In this work, we experimentally demonstrate two types of photonic crystal (PhC) solar cells architectures that exceed Lambertian light absorption, integrated over the entire 300-1,200 nm wavelength band. These measurements confirm theoretically predicted wave-interference-based optical resonances associated with long lifetime, slow-light modes and parallel-to-interface refraction. These phenomena are beyond the realm of ray optics. Using two types of 10-μm thick PhC's, first an Inverted Pyramid PhC with lattice constant a = 2,500 nm and second a Teepee PhC with a = 1,200 nm, we observe solar absorption well beyond the Lambertian limit over λ = 950-1,200 nm. Our absorption measurements correspond to the maximum-achievable-photocurrent-density (MAPD), under AM1.5G illumination at 4-degree incident angle, 41.29 and 41.52 mA/cm 2 for the Inverted Pyramid and Teepee PhC, respectively, in agreement with wave-optics, numerical simulations. Both of these values exceed the MAPD (= 39.63 mA/cm 2) corresponding to the Lambertian limit for a 10-μm thick silicon for solar absorption over the 300-1,200 nm band. The efficiency and cost of photovoltaics has steadily improved in recent years in the effort to create a competitive renewable energy resource. Silicon solar cells have been the dominant driving force in photovoltaics due to the abundance and environmentally friendly nature of silicon. The maximum possible power conversion efficiency of a single junction, crystalline silicon (c-Si) solar cell under one sun illumination at room temperature is 32.33% 2. The highest efficiency real-world n-type silicon solar cell to date, by Kaneka Corp 3,4 , exhibits 26.7% conversion efficiency, followed closely by the p-type silicon solar cell, by the Institute for Solar Energy Research Hamelin (ISFH), Germany with 26.1% efficiency 5,6. An analysis of the Kaneka, 165 μm thick, c-Si cell shows that in the absence of any extrinsic loss mechanism, the limiting efficiency of such a cell is 29.1% 3. The competing factors responsible for this limit of the conversion efficiency are ray-optics light trapping 7,8 and intrinsic loss due to Auger charge-carrier recombination. Essentially, the thicker the cell, the more light is absorbed. However, this is accompanied by increased bulk non-radiative recombination loss of charge carriers. In the case of ideal Lambertian light-trapping and a state-of-the-art Auger recombination model 9 , the optimal silicon thickness is reduced to 110 μm and a theoretical limit to conversion efficiency (assuming no surface recombination losses) is increased to 29.43% 8. In traditional ray-optics based light trapping s...

We report what is to our knowledge the first observation of the effect of parallel-to-interface-refraction (PIR) in a three-dimensional, simple-cubic photonic-crystal. PIR is an acutely negative refraction of light inside a photoniccrystal, leading to light-bending by nearly 90 deg over broad wavelengths (λ). The consequence is a longer path length of light in the medium and an improved light absorption beyond the Lambertian limit. As an illustration of the effect, we show near-unity total absorption (≥98%) in λ 520-620 nm and an average absorption of ∼94% over λ 400-700 nm for our α-Si:H photonic-crystal sample of an equivalent bulk thickness oft 450 nm. Furthermore, we have achieved an ultra-wide angular acceptance of light over θ 0°-80°. This demonstration opens up a new door for light trapping and near-unity solar absorption over broad λs and wide angles. There is a great deal of interest in efficient light trapping in thin film solar cells for approaching the ShockleyQueisser limit [1] and for low cost photovoltaics. As solar radiation is broadband [2] and its incident angle on earth varies over time, solar light trapping must be effective over a broad range of wavelengths (λ) and wide angles (θ). A random surface-structure can be used to trap light by multiple scatterings and leads to a prolonged optical path. For a properly prepared random surface, geometric optics predicts an increased path length by as much as 4n 2[3]. However, the geometric optics approach is less effective where the sample thickness is comparable to the incident λ of light. Accordingly, sub-λ surface-structures, including plasmonic, nano-wire, and nano-cone, have been proposed as new routes for light trapping [4][5][6][7][8][9]. Periodic surface-structures, such as one-and twodimensional photonic-crystals [10-13], have also been studied due to their ability to localize and trap light near the band edge. Most recently, a three-dimensional (3D) simple-cubic photonic-crystal has been proposed to trap light by anomalous light refraction [14-16] parallel-to-theinterface (PIR) of a thin film sample [16][17][18]. This extreme light-bending by nearly 90 deg is expected to offer a two orders of magnitude absorption enhancement over that of single-path absorption. So far, there is no experimental demonstration of enhanced solar absorption over broad λs and wide θs using 3D photonic-crystal thin film architecture. Our 3D photonic-crystal possesses simple-cubic symmetry. For two decades, the most studied 3D photonic-crystal has a diamond symmetry that produces a large photonic bandgap [19]. Here, we choose simplecubic symmetry to enforce negative refraction in the photonic-allowed bands. One experimental realization of the lattice structure is to stack layers of 1D-gratings in a sequential fashion [19] as shown in Fig. 1(a). Two stacking layers form one unit-cell of the simple-cubic lattice. The vertical and horizontal red arrows indicate the directions of the incident and refracted light, respectively. In Fig. 1(c), we show the corresponding Bri...

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We report some striking results on thermal radiation properties of a resonantly coupled cavity photonic crystal (PhC) at elevated temperatures (T = 400-900 K). We experimentally found that at resonant wavelengths, λ = 1.1, 1.64, 2.85 μm, the PhC emission is spectrally selective, quasi-coherent, directional, and shows significant deviation from Planck's blackbody law at equilibrium. The presence of non-equilibrium effects, driven by strong thermal excitation and cavity resonance, may be the major cause for our experimental observation.

We present a study of the grating detuning effect on the transmission and reflection holographic memories recorded in a photopolymer material. By using the Bragg matching condition, we analyze the angular shift and the degradation of the diffraction efficiency of the reconstructed images. Based on these results, a method for precompensating the detuning effect has been proposed.