In this work we demonstrate that a Bragg Stack with a periodic gain-loss modulation can function as an Omnidirectional Mirror (OM) with complete reflection at any angle of incidence irrespective of the light polarization. The Bragg Stack is composed by the periodic variation of two layers with the same value of the real part of the refractive index (nr) and a periodic modulation in the imaginary part (ni). The origin of the band gaps is due to the interference of complex waves with propagating and evanescent fields in each layer. It is found that the band gaps are wider as the contrast ni/nr increases. We have found the ambient conditions to obtain an OM considering an auxiliary medium n′ external to the Bragg Stack.
In this work we present experimental evidence of the enlargement of the non-transmission range in one-dimensional phononic crystal heterostructures. Heterostructures are composed by a tandem of different phononic crystal lattices. The constituent phononic crystal lattices have been properly chosen so that their band gaps overlap each other to obtain a giant stop band. Heterostructures consisting of a periodic arrangement of aluminum and epoxy layers were fabricated and characterized. We have designed giant stop bands in the range of MHz obtaining a good agreement between theoretical and experimental results.
In this work, we demonstrate that acoustic ultra-short pulses can be used to characterize multiple guided modes in a solid-liquid-solid planar waveguide via the determination of the time of flight (τ) using the Short Time Fourier Transform (STFT). To obtain experimentally τ, we introduce a time dependent ultra-short acoustic signal s(t) at one side of a finite waveguide and we perform the STFT of the outgoing acoustic signal s′(t) from the other side. We have found that this technique is able to discriminate the signature of multiple even and odd modes at the same experimental run.
In this work, we present a study to determine the transit times and frequencies of pulses by usingthe Short-Time Fourier Transform (STFT). We consider the case of an acoustic signal composed offive Gaussian pulses which have a high overlapping in time but oscillate at different frequencies. Weproceeded in three steps. First, we illustrate how the STFT calculated through a sliding windowproduces a spectrogram where transit time is on one axis and frequency on the other. Second, wederive an exact analytical solution of the STFT to develop an intuitive vision of the mathematicaltechnique. Finally, in a third step, we present an experiment to demonstrate that the STFT is auseful technique to characterize a complex acoustical signal.
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