Photoconductivity of individual aluminum nitride (AlN) nanowires has been characterized using different subband gap excitation sources. It is interesting that both positive (under 1.53 and 2.33 eV excitations) and negative (under 3.06 and 3.81 eV excitations) photocurrent responses are observed from the wide band gap nitride nanowires. The negative photoconductivity, which is attributed to the presence of electron trap and recombination center in the bulk of AlN, is capable to be inversed by a strong positive photoconductive mechanism of surface while changes the ambience from the atmosphere to the vacuum. An oxygen molecular sensitization effect is proposed to be the reason resulting in the enhancement of positive photocurrent and the inversion of negative photoresponse in the vacuum. Understanding of the diverse photoconductivity and its molecular effect is of great importance in the development of energy-selective and highly sensitive nanowire photodetector of AlN in the visible and ultraviolet ranges.
wavelength of the optical signal, considering that the practical FBG has a finite reflection bandwidth, unlike the ideal one mentioned above. Here, it has been expected that the optimum time delay may be achieved near the wavelength corresponding to the 3-dB bandwidth position in the long-wavelength side of the reflection spectrum with no voltage applied to the electrode, without significant variations in the output microwave-signal power. The measured time delay, as a function of the control voltage, is shown in Figure 3 for an optical-signal wavelength of ϳ1550.24 nm and a microwave-signal frequency of 2 GHz. The achieved time delay is about 100 ps for the control-voltage range from 5 to 11 V, which corresponds to electrical power consumption of about 280 mW. The measured time delay for the voltage rage below 5 V is relatively small and exhibits an irregular relationship with the applied voltage, which is not suitable for practical use. This may be due to the fact that the FBG used is not sufficiently chirped for voltage below 5 V, and thus the reflection positions of the optical signal are not clearly defined. Surely, the achievable time delay can be readily increased by lengthening the tapered FBG. For the achieved time delay of 100 ps, the maximum distance between the reflection positions of the optical signal is about 10 mm. And the variation in the time-delayed microwave signal power is less than 2 dB. Finally, our TTD has been successfully operated for the broad microwave frequency range from 1 to 10 GHz. CONCLUSIONIn summary we have presented a photonic TTD based on sidepolished fiber Bragg grating with resistive heater. It features voltage-controlled continuous operation involving no moving parts and no mechanical perturbation. FDTD ANALYSIS WITH MODIFIED MATRIX PENCIL METHOD FOR THE UC-EBG LOW-PASS FILTERS
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