Spatial mode filters based on multimode interference couplers (MMI's) that offer the possibility of splitting off antisymmetric from symmetric modes are presented, and realizations of these filters in InGaAsP/InP are demonstrated. Measured suppression of the antisymmetric first-order modes at the output for the symmetric mode is better than 18 dB. Such MMI's are useful for monolithically integrating mode filters with all-optical devices, which are controlled through an antisymmetric first-order mode. The filtering out of optical control signals is necessary for cascading all-optical devices. Another application is the improvement of on-off ratios in optical switches.

1 FEBRUARY 1965 gy for ZnS:Ag, Er than for ZnS:Cu, Er. This sensitivity of the rare-earth spectrum to the specific group-I6 element used implies that the two ions are closely associated.The previous discussion pertains only to ZnS and CdS. We have also found that the addition of Cu to ZnSe strongly enhances the intensity of rare-earth fluorescence. ZnSe:Cu,Er, for example, emits most strongly from the transition 4 S 3/2 -4 / 15/2 at 18 100 cm"" 1 . As in the case of ZnS and CdS, preparative conditions favoring the high-energy Cu band at 2.36 eV over the low-energy Cu band at 1.95 eV produce samples with the strongest rare-earth emission. Our preliminary data indicate, however, that the rare-earth excitation spectra of ZnSe are qualitatively different from Fig. 1, which may imply a somewhat different mechanism of transfer. On the other hand, all of the II-VI compounds show analogous emission This Letter reports on the effect of an electric field on interband transitions well above the fundamental absorption edge. The effect is observed as a fie Id-induced change in the reflectivity which peaks at approximately the photon energy of the particular transition. This pronounced structure of peaks should be profitable in the analysis of the band structure of semiconductors.Although the Franz-Keldysh effect was observed in transmission at the fundamental absorption edge of several semiconductors, difficulties are encountered in extending this transmission measurement into the regions of strong absorption above this edge. It could not be determined, therefore, whether an electric field affects the transitions at higher photon energies in a similar manner.We reported previously that the electric field in the potential barrier at the surface of germanium is strong enough to cause a Franz-Keldysh effect which is observed in reflection rather than in transmission. 1 " 3 The study could now be extended beyond the fundamental edge into the region of strong absorption.The experiment employs the familiar field-bands due to group-Ift dopants, 5 so we expect that the system described here can be extended to include all II-VI compounds and 16 dopants. The experiments discussed here have all been done on powder samples to simplify the control of the dopants. They were prepared from high-purity ZnS, CdS, and ZnSe, and fired in an atmosphere of purified H 2 S or H 2 + Se at temperatures from 950 to 1150°C. We are now in the process of extending these studies using single crystals. to be published. effect configuration, with a transparent field electrode in close proximity to the electropolished germanium surface 4 at which the reflection of monochromatic light takes place at near normal incidence. The surface potential at the interface is modulated at 250 cps by an electric field of approximately 10 5 V/cm peak amplitude. Modulation impressed on the reflected beam is detected by either a photomultiplier or a PbS cell, followed by a narrow-band phasesensitive amplifier. Applying this ac method, changes AR/R of the relative reflected...

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