A new photonic approach of microwave waveform generator based on time-domain synthesis is proposed and experimentally demonstrated, in which two single-drive Mach-Zehnder modulators biased at quadrature point are severed as optical pulse carvers and various microwave waveforms can be generated by carving and overlapping optical field envelopes. The theoretical analysis and simulation are developed. In experiment, a square waveform with 50% duty cycle, triangular waveform with full duty cycle, and sawtooth (or reversed-sawtooth) waveform with 50% duty cycle are generated. Furthermore, a frequency doubling sawtooth (or reversed-sawtooth) waveform with full duty cycle is also obtained.
Nanoparticle-assembled octahedral Ag nanocages with sharp edges have been successfully synthesized through a Cu O-based template-assisted strategy. In the reaction system, Ag nanoparticles can be self-assembled on the surface of Cu O octahedrons, which is accomplished by the reduction of Ag by NaBH in the presence of sodium citrate as a capping agent. The hollow octahedral Ag nanocages are obtained after removing the inner Cu O cores with acetic acid. According to the scanning electron microscopy (SEM) and transmission electron microscopy characterization, the Ag nanocages are weaved by small nanoparticles, the rough surfaces are bestrewed with pores and sharp edges. It is found that the pack density of Ag nanoparticles strongly affects the surface enhanced Raman scattering (SERS) activities. The as-prepared 1.05-Ag cages with optimal pack density have suitable interparticle distance and suitable size of pores, which significantly enhance SERS signals. The SERS signals of rhodamine 6G (R6G) molecules can be detected at an ultralow concentration of 10 m when 1.05-Ag cages are used as substrates. In addition to sensitivity, 1.05-Ag cages also exhibit good reproducibility. It is expected that the ultrahigh sensitivity will endow the Ag nanocages to become a promising candidate as high-performance SERS-based chemical sensor.
We propose and demonstrate a new photonic approach for triangular waveform generation. Based on an optical signal with sinusoidal envelope, an injection-locking process in a distributed-feedback semiconductor laser diode is employed to carry out the desired harmonic signal generation, and then the tailored signal is synthesized by the superimposition of these signals with proper power ratio and time delay. In this method, the conventional spectral line manipulating is transferred to individual signal envelope control, which reduces the technical requirement while the flexibility and accuracy is improved. In the experiment demonstration, triangular waveforms with the repetition frequencies of 9, 10, and 12 GHz are successfully generated. The results well agree with the theoretical predication and show great potential to realize arbitrary waveform generation.
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