band, this phenomenon does not have much practical significance.If computation works sensibly, within the PBG frequency range, transmission and reflection should add up to unity in the 2D case. Thus, during the optimization process (for example), it suffices to study the transmission power only. Because reflection is not studied, short input WG can be used, which limits the computation space. Also, the post-processing time is halved, because only one observation plane is used. Assuming that one is only interested in the transmission of the bend structure shown in Figure 7, one could approximately halve the computation space and use a single observation plane. Then, in the 2D case, one optimization step (simulation plus post-processing) with 4000 FDTD time steps, 2*27 E-points, 27 H-points, takes about 100 sec using a PC with 700 MHz/770-MB RAM (the speed of field computation is 1000 time steps/18 sec). CONCLUSIONModelling PBG waveguide components has been considered, with field computation using FDTD. As an example, a 120°b end has been studied more closely, and power transmission results have been shown for various cases. The true necessity of bend geometry optimization has become apparent. Considering optimization, an approximative result for bend transmission is enough, that is, the spectrum does not have to fully converge. Then, with a 700-MHz PC, one optimization step (field computation and post-processing) takes a few minutes when using a 2-D model in field computation. Taking the finite-PBG plate thickness into account by using a 3D model will increase the needed time to tens of minutes.With 2D bend structures, the convergence of the spectrum may be very slow in certain frequencies. With 3D structures this "ringing problem" is less severe, due to the radiation losses in the z direction. In general, the cell size ⌬ ϭ a/10 seems to be sufficient if studying these structures at frequencies fa/c Յ 0.3. This is an important point if one strives for quick optimization. However, there may be structures with special frequencies, which can be very sensitive to the cell size and the relative hole size d/a. Key words: wide tuning range; millimeter-wave VCO; P-HEMT technology INTRODUCTIONAs demand for higher-frequency systems such as high-speed optical communication networks and automotive radar systems has increased, millimeter-wave component technology has become very important. Tunable low-phase noise oscillators are key components in millimeter-wave systems. Several oscillators operating at millimeter-wave frequencies using HEMT or HBT technology have been reported [1][2][3][4][5]. Although HBT-based oscillators have exhibited better phase-noise performance than HEMT-based oscillators, the output power available from HBT-based oscillators cannot compete with that of HEMT-based oscillators. Also, the HBT process is not compatible with the P-HEMT process, which is the most favored millimeter-wave circuit technology. For highlevel integration of monolithic VCOs with other MMIC components using the mature GaAs P-HEMT...

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We report an analysis of the operation of a new type of laser resonator with two-dimensional distributed feedback from a photonic crystal. The gain medium consists of a 2-(4-biphenylyl)-5(4-tert-butylphenyl)-1,3,4-oxadiazole host doped with Coumarin 490 and DCM and is deposited on lithographically patterned Si/SiO2 structures. Bragg reflections caused by the grating diminish the group velocity of photons along some directions of crystallographic symmetry to zero, and the resulting feedback gives rise to laser oscillations. Dispersion relations for photons were calculated analytically and are used to interpret the laser emission spectra.

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We describe and demonstrate a new mechanism for low radiation losses in structures lacking a complete band gap, and show how resonant cavities with QϾ10 3 can be achieved without sacrificing strong localization in 3d. This involves a forced cancellation in the lowest-order term͑s͒ of the multipole far-field radiation expansion. We focus on the system of photonic-crystal slabs, one-to two-dimensionally periodic dielectric structures of finite height with vertical index guiding. Simulations and analytical results in 2d and 3d are presented.

We investigate the mechanism for the appearance of bound states in two-dimensional photonic crystal waveguides and contrast it with the corresponding mechanism for conventional guides. It is shown that the periodicity of the photonic crystal can give rise to frequency ranges above cutoff where no guided modes exist in the waveguides. Such mode gaps make possible the creation of bound states in constrictions and in bends. Bound states are found to correspond to analogous cavity modes and it is shown that their appearance strongly depends on the lattice geometry and cannot be described in a one-dimensional framework.

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