Alfred Lam

Fabrication of hollow core carbon spheres with hierarchical nanoarchitecture for ultrahigh electrical charge storage

A simple and reproducible sol–gel synthesis strategy was developed to fabricate hollow core carbon spheres (HCCSs) with hierarchical nanoarchitecture through the hydrolysis, self-assembly and co-condensation of bis-[3-(triethoxysilyl)propyl]disulfide (TESPDS) and octadecyltrimethoxysilane (C18TMS). This synthesis route allows one to fabricate thioether-bridged organosilica (TBOS) with tailored spherical structure and particle size which can be further converted to HCCS upon calcination under N2 flow. It is assumed that hydrophobic octadecyl chains of hydrolyzed C18TMS first form a micelle-like self-assembly structure with hydrophilic trihydroxysilyl groups as heads, and a reactive core is then expanded by the base-catalyzed co-condensation of TESPDS and/or C18TMS over the C18TMS self-assembly structure. The organic moieties of TESPDS and C18TMS not only serve as a porogen during the formation of TBOS but also as a carbon precursor for transformation of TBOS into HCCS during the carbonization. Due to its unique hierarchical nanostructure composed of hollow macroporous core and meso/microporous shell, which facilitates fast mass transport, along with large surface area for electrical charge storage, the HCCS for the first time exhibits ultrahigh specific capacitance and energy, good cycling performance and rate capability.

Wide-band electrooptic intensity modulator frequency response measurement using an optical heterodyne down-conversion technique

This paper describes a novel system for measuring the frequency response of electrooptic intensity modulators at millimeter-wave frequencies. The system measures |S21| in decibels electrical. In it, the electrical input signal to the modulator is generated by heterodyning two tunable lasers. The output of the modulator is then down-converted to a microwave frequency at which the output is easily measured. Measurements have been made up to 78 GHz using a 55-GHz photodiode as the mixer for the two lasers. In addition, our system tracks any frequency variations in the outputs of the lasers. Our system can also measure the linear response of the modulator if it is operated in nonlinear regions since it can reject higher order harmonics, due to nonlinearities, generated in the modulator

Modified Optical Heterodyne Down-Conversion System for Measuring Frequency Responses of Wideband Wavelength-Sensitive Electrooptical Devices

This letter describes a modified optical heterodyne down-conversion system for measuring the frequency response of wavelength-sensitive electrooptical devices at millimeter-wave frequencies. The new system is presented, and the frequency response of an injection-locked vertical-cavity surface-emitting laser is measured to up to 35 GHz