Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated for many years as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs -a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated. 289-296 (1992). 11. J. Kim, J. Chen, J. Cox, and F. X. Kärtner, "Attosecond-resolution timing jitter characterization of free-running mode-locked lasers using balanced optical cross-correlation," Opt. Lett. microwave signals at 40-GHz with higher than 7-ENOB resolution," Opt. ©2012 Optical Society of America

Recent optical heterodyne measurements with distributed-Bragg-reflector diode-laser pumps demonstrate that low-temperature-grown ͑LTG͒ GaAs photomixers will be useful in a compact all-solid-state terahertz source. Electrical 3 dB bandwidths as large as 650 GHz are measured in mixers with low electrode capacitance. These bandwidths appear to be independent of pump-laser wavelength over the range 780-850 nm. Shorter wavelength pumping results in a significant reduction of the bandwidth. The best LTG-GaAs photomixers are used to generate coherent continuous-wave output radiation at frequencies up to 5 THz.

An electron-emission mechanism for cold cathodes is described based on the enhancement of electric fields at metaldiamond-vacuum triple junctions. Unlike conventional mechanisms, in which electrons tunnel from a metal or semiconductor directly into vacuum, the electrons here tunnel from a metal into diamond surface states, where they are accelerated to energies sufficient to be ejected into vacuum. Diamond cathodes designed to optimize this mechanism exhibit some of the lowest operational voltages achieved so far.Conventional cathodes for applications from television to power transmitters use heat to boil electrons out of a metal into vacuum. However, these cathodes do not have the power efficiency or the dimensional stability to be used with micrometre-size structures, which are required for flat-panel displays and some power amplifiers. Cathodes that can be scaled to micrometre sizes use high electric fields instead of heat to pull electrons out of a solid into vacuum. The reliability and current density of these electric field emission cathodes depend upon both their geometry and the material used in their construction. Here we review field emission cathodes and show that a new cathode geometry which uses a novel material, diamond, has properties superior to those of previous cathodes.For the cold cathodes we discuss, emission is obtained with a large electric field that causes electrons to tunnel over a potential barrier out of a metal substrate into vacuum. Material and fabrication techniques have both been used to increase emission by enhancing the electric field and reducing the barrier over which the electrons must tunnel. Excellent low electric field electron emission has been reported from diamond and amorphous diamond-like films on metal substrates, but practical application of these cathodes is limited by a serious lack of reproducibility 1-3 and inconsistency (M. E. Kordesch, personal communications). Emission originates from a few localized sites, which were believed to be due to the inconsistent bulk properties of the cathode material. The enhanced emission at the interface between the diamond surface, a conductive region, and vacuum, a new emission mechanism, may explain the localization of emission sites. If so, a discontinuous diamond film that provides an abundance of interfaces should be a better electron emitter than a continuous diamond film 4,5 .We now describe two generally accepted emission mechanisms: geometric electric-field enhancement 6 , and Schottky diode with a negative-electron-affinity semiconductor 2,3 . Semiconductors and insulators have a negative electron affinity (NEA) if the minimum energy of electrons in the conduction band is above the minimum energy of electrons in vacuum. Experimental results are then described that cannot be explained by the previous emission mechanisms. A mechanism is proposed that combines the high electric fields that can be obtained at the intersection of a semiconductor surface, a metal substrate and vacuum (a so-called triple junction) 7,8 with t...