We have demonstrated highly efficient frequency doubling of femtosecond pulses in a thick, noncritically phase-matched KNbO(3) crystal under conditions of large group-velocity mismatch. At low power we observed a slope efficiency of ~300% nJ (-1) for harmonic conversion, and at higher powers we generated 170 mW of second-harmonic blue output for 300 mW of input light. Furthermore, we have shown that the focusing dependence for our conditions of large group-velocity mismatch is considerably different from that obtained for frequency doubling of continuous-wave light. We have also demonstrated that one can tune the spectral width of the generated blue light by varying the focusing conditions.

The JOSA Communications section is for short papers reporting new results that are deemed to be especially significant or of wide interest. A Communication must be no longer than four printed pages, including title and abstract.

Shaping, signal processing, and time-space conversion of femtosecond pulses can be achieved by linear and nonlinear manipulation of the spatially dispersed optical frequency spectrum within a grating and lens pulse shaper. In this paper, we first review our work on femtosecond pulse shaping and processing, with an emphasis on applications to high-speed communications and information processing. We then present a new concept for generalized time-space processing based on cascaded time-to-space and space-to-time conversions in conjunction with smart pixel optoelectronic arrays and provide a detailed discussion of our recent studies of time-to-space conversion based on second-harmonic generation (SHG) within a femtosecond pulse shaper.

The Bragg selectivity of volume holograms makes them not well suited for many Fourier imaging processing applications in the space domain because they perform the function of a spatial filter and limit the field of view. Similarly, for femtosecond pulse holography they reduce the spectral bandwidth of the diffracted signal. However, we show both theoretically and experimentally that it is much easier in the frequency domain than in the space domain to achieve a large enough diffraction bandwidth of volume holograms for the bandwidth of 100-fs pulses to be used for frequency-domain femtosecond pulse shaping. The experiments were performed by nondegenerate four-wave mixing in photorefractive InP:Fe with femtosecond readout at 1.5 m.

A solution of resonant tunneling problem in multilayered heterostructures is presented based on the quantum mechanical wave impedance concept. The transmission matrix is found using open and short circuit tests. By using the transmission matrix approach the transmissivity of the structure is determined as a function of the incident electron energy. The J-V curves are obtained by using this approach and the results compare to previous models with good agreement. The method was then applied to a new quantum well three barrier structure, where the inner barrier height is variable. It is found that increasing the inner barrier height causes the resonance energies to align, meaning that coupling between the wells increases. By using this method, the analysis and tunneling current calculations of any structure can be easily figured and new tunneling quantum well devices can be designed.