Trapped rainbow effect in visible light left-handed heterostructures
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Trapped rainbow inside a tapered LHH at visible frequencies. (a) Schematic of trapped rainbow inside a tapered LHH. The LHH is a negative refractive index waveguide, which is a composite structure of LHM/air/LHM. The waveguide goes from a wider width to a narrower one on going from left to right. When the light beam propagates in the air core, each frequency component of the wave packet stops at a position of different core thickness, leading to the spatial separation of its spectrum and the appearance of a trapped rainbow. (b) Partial enlarged detail of LHMs. The LHMs are composite structures of glass substrate/ITO film/PVA film/silver dendritic cells/PVA film/silver dendritic cells/PVA film/ITO film/glass substrate. (c) Scanning electron microscopy (SEM) images of silver dendritic cells. (d) Transmission spectrum of LHMs at visible frequencies.
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Impact of the angle formed by the two LHMs in LHH on the peak wavelength shift. [(a), (b), and (c)] Spectra of transmitted light from the upper surface of LHH along the light propagation direction measured by fiber spectrometer. The angles formed by the two LHMs in LHHs in (a), (b), and (c) are 0, , and , respectively. The -axis, which represents the position, is along the light propagation direction, and the origin is at the interface between the ordinary waveguide and the negative refractive index waveguide. We are concerned only about the two primary peaks in each spectrum, which correspond to the two peaks in the spectrum of xenon lamp. (a) and (c) show no shift in the peak wavelength of spectrum; however, (b) shows significant shift in the peak wavelength of spectrum. In (b) the long peak wavelengths of spectra at , 4, 8, and 12 mm are 596, 625, 665, and 703 nm, respectively; the short peak wavelengths of spectra at , 4, 8, and 12 mm are 533, 553, 600, and 636 nm, respectively. The peak wavelengths in measured spectra exhibit a significant red-shift property as the tapered fiber probe moves away from the origin of coordinate. (d) Fitted curve of the shift in short peak wavelength per unit length LHH vs the angle formed by the two LHMs in LHH. It is demonstrated that there exists an optimum angle for the LHH to yield the most significant shift in peak wavelength of spectrum. The inset is the focus curve of LHMs in tapered LHH for monochromatic light of 530 nm.
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