/ OFC 2002 / TUESDAY AFTERNOONwavelengths between 1550 and 1620 nm, corresponding to a tuning range of 70 nm.The full width at half maximum (FWHM) of the devices was around 35 nm. The quantum efficiency measurements were done at 5 V reverse bias under 0.5 mW optical input power. When we increased the reverse bias beyond 3 V the active layer was fully depleted, and the quantum efficiency increased 6% with respect to zero bias. The responsivity of the PDs were also measured under various re~erse biases up to 6 mW optical power, which was the maximum power that could be abtained from the laser. Fig. l(b) shows the photocurrent versus input optical power at the resonance wavelength of 1572 nm. Under 3 V and higher re~erse biases, the PDs had a linear photoresponse up to 6 mW optical power. At 6 mW optical power, the device exhibited a 5 mA photocurrent. The saturation was mainly due to the electric field screening caused by photo-generated carriers?High-speed measurements were made with a picosecond fiber laser operating at 1550 nm. The 1 ps FWHM optical pulses from the laser were coupled to the active area of the p-i-n photadiodes by meam of a fiber probe. At zero bias, the responseofthephotadetectorshadalongtaildue to the diffusion of the carriers in the active layers. Measurements were done under bias to deplete the active layer completely and to get rid of the diffusion tail. Above 3 V reverse bias, we got a Gaussian response with a short tail. Fig. 2 shows the temporal response o f a Small area (5 x 5 hm') photodetector measured at 7 V bias by a 50 GHz sampling scope. The photodiode output had a 16 ps FWHM. The measured data was corrected by deconvolving the effect of the 40 GHr bias-tee. After the deconvolution, the device had a 3-dB bandwidth of 31 GHz. Larger area devices (80 pm2) also showed similar responses, which showed that the temporal responsewas limited by the transport of the photogenerated carriers.In conclusion, we have demonstrated highspeed, and high-efficiency resonant cavity enhanced (RCE) InGaAs based p-i-n photodetec~ tors. A peak quantum efficiency of 66% was measured along with 31 CHz bandwidth, which corresponds to 20 GHz bandwidth-efficiency product. The photoresponse was linear up to 6 mW optical power, where the devices exhibited 5 mA photocurrent. Time (psec)TuW6 Fig. 2. Temporal response of the photodetector with a 16 psec full width at half maximum. The inset shows the deconvolved frequency response obtained from the fast Fourier transform of the temporal dedetector response. No. INT-9906220, Turkish Department of Defense Grant No. KOBRA-001 and Thaler IP8.04 1. M.S. Unlu and S. Strite, "Resonant cavity enhanced (RCE) photonic devices:' 1. ,-Highspeed >90% quantum efficiency p-i-n photodiodes with a resonance wavelength adjustable in the 795-835 nm range:' Appl. power high-speed photodetectorbDesign, analysis, and experimental demonstration': IEEE Trans. Mimownve lkeory Tech., vol. 45, "0.8, l32&l33l,(1997). TUX 430 p m 4 0 0 pm 304 A-D Network Design 2 lane M. S h " n s , Corvir Cor...

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Short-pulse (100-psec) soft-x-ray excitation from a laser-produced plasma and fast optical detection have been used to study multielectron photoionization from the 3p neutral ground state to the ten terms of the 3p 4p configuration of Ar+. Good agreement between measured populations and those obtained from an excitation model based on energy-dependent cross sections calculated with a relativistic Hartree-Fock atomic physics code using configuration interaction techniques was obtained.A 100-psec duration soft-x-ray pulse from a laserproduced plasma combined with a subnanosecond optical detection system has been used to study electron correlations in Ar by observing multielectron photoiomzation from the 3ps ground state of neutral Ar to the 3p 4p configuration of Ar+. Measured population densities of the ten LS terms of this configuration produced by direct two-electron photoionization are in reasonably good agreement with those predicted by an excitation model based upon calculated photoionization cross sections to these ten terms and a measured soft-x-ray flux equivalent to an approximately 23-eV blackbody.The energy-dependent cross sections were computed with an atomic physics code using configuration interaction (CI) techniques. The sum of two of the calculated partial cross sections, with a peak value of 0.06 Mb at 40 eV (310 A), agrees within 30% over a wide range of energies with a cross section for those terms deduced from satellite intensities measured with photoelectron spectroscopy. When atoms are photoionized, observation of secondary processes involving excitation of more than one electron can be used to interpret the nature of the coupling or correlation between the electrons of the atom, not only in their initial and final states but also in the dynamics of the readjustment during the electron departure. Secondary photoionization can occur as a result of three processes The first is the "shake" type of process~dominant at higher energies. The second is the result of electron correlations in the initial and final ionic states giving rise to configuration interactions. The third is interchannel coupling which results from interactions between various final states by means of the continuum. The present investigation involves studies of the second process, the effect of electron correlations.Evidence of secondary processes were first observed in absorption spectroscopy3 but were often obscured by the primary one-electron processes. In many atoms secondary processes have been studied with photoelectron spectroscopy through the presence of sateIlite lines in the photoelectron spectrum. However, in cases w'here there are a large number of closely spaced excited states, identification is not possible either because of low resolution or because of the presence of other background emissions. Because the experimental technique described in this Rapid Communication monitors the emission from the various multielectron excited states immediately after the excitation terminates, before cascade or electron collision...

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