Controlling the flux of photons is crucial in many areas of science and technology. Artificial materials with nano-scale modulation of the refractive index, such as photonic crystals, are able to exercise such control and have opened exciting new possibilities for light manipulation. An interesting alternative to such periodic structures is the class of materials known as quasi-crystals, which offer unique advantages such as richer Fourier spectra. Here we introduce a novel approach for designing such richer Fourier spectra, by using a periodic structure that allows us to control its Fourier components almost at will. Our approach is based on binary gratings, which makes the structures easy to replicate and to tailor towards specific applications. As an example, we show how these structures can be employed to achieve highly efficient broad-band light trapping in thin films that approach the theoretical (Lambertian) limit, a problem of crucial importance for photovoltaics.
The celebrated discovery of graphene has spurred tremendous research interest in two‐dimensional layered materials (2DLMs) with unique attributes in the quantum regime. In 2DLMs, each layer is composed of a covalently bonded lattice and is weakly coupled to its neighboring layers by van der Waals interactions. There are abundant members in this 2DLM family beyond graphene, such as transition metal dichalcogenides (MX2, M = Mo, W; X = S, Se, Te), semimetal chalcogenide (InSe), black phosphorus, etc. The 2DLMs afford rich and ideal material platforms for studying quantum effects and their corresponding applications in the two‐dimensional (2D) limit. In this review, the emerging quantum effects in 2DLMs are examined with particular focus on their band structure evolvement, valleytronics, and quantum Hall/quantum spin Hall effects. Based on the summary of quantum effects discovered in 2DLMs, the future research directions and prospective applications are also discussed.
A novel photoacoustic (PA) tomographic method, which is based on a time-resolved technique and an acoustic lens imaging system, is presented in this paper. A YAG laser operating at 532 nm with a 7 ns pulse width and 10 mJ optical pulse is employed as the excitation source to irradiate the tissue. PA signals generated from the tissue are imaged onto a multi-element linear array transducer with an acoustic lens. A 64 electronic switch is efficiently used for changing the parallel PA signals into a series. The proposed method directly provides PA images without any complex reconstruction algorithms. With the time-resolved technique, tomographic imaging can be achieved successfully. The results show that the images agree well with the original samples.
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