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Modulation of interband-resonant light ͑ϳ800 nm͒ by intersubband-resonant light ͑5-7 m͒ was investigated in n-doped AlGaAs/GaAs multiple quantum wells by a two-color femtosecond pump-probe technique. Modulation with a recovery time of ϳ1 ps is observed in a plainer-type modulation device at room temperature. The modulation of interband absorption coefficient is ϳ1000 cm Ϫ1 when the energy density of the intersubband light pulse is ϳ4 fJ/m 2 . The modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ϳ1 pJ when a conventional waveguide-type device structure is utilized. The mechanism which determines the modulation speed is discussed in terms of carrier relaxation process. It is shown that the modulation speed is mainly determined by the inter-and intrasubband relaxation times, where the latter is influenced by hot phonon effects.The intersubband transition ͑ISB-T͒ in quantum well ͑QW͒ structures has interesting properties such as a large transition dipole moment in the midinfrared range 1 and an ultrafast energy relaxation time due to longitudinal optical ͑LO͒ phonon scattering ͑ϳ picoseconds͒. 2,3 The ultrafast relaxation process of the ISB-T is important for basic physics as well as for the development of high-speed optical devices. The authors previously proposed an ultrafast all-optical modulation scheme that utilizes the ISB-T. 4 This scheme uses three subbands formed in a n-doped QW ͑Fig. 1͒: the first valence subband ͑VB1͒ and the first and second conduction subbands ͑CB1 and CB2͒. Interband ͑IB͒ resonant light absorption (VB1→CB1) can be increased by exciting the electrons from CB1 to CB2 using ISB-resonant light. The increased absorption decreases as the electrons relax from CB2 to CB1, and the relaxation time appears to be as fast as ϳ1 ps. Thus, it is thought that an IB-resonant light signal can be modulated using an ISB-resonant light signal on the order of picoseconds.Theoretical and experimental investigations have revealed the static ͑not ultrafast͒ characteristics of the modulation. 4-7 However, the modulation speed was not determined in the previous reports. In this letter, we report femtosecond time-resolved measurement results for the alloptical modulation. A planar-type GaAs/AlGaAs QW modulation device was fabricated and investigated using a twocolor pump-probe technique. The modulation speed is shown to be ϳ1.3 ps. The relaxation process of the ISB-excited electrons is also discussed.The modulation device investigated consists of 150 periods (N well ) of GaAs(59 Å)/Al 0.35 Ga 0.65 As (150 Å) multiple QWs grown on a semi-insulating GaAs substrate by molecular beam epitaxy. The barrier layer was selectively doped at a Si density of 1.5ϫ10 18 cm Ϫ3 . A 0.5-m-thick Al 0.4 Ga 0.6 As etch stopping layer was grown between the QW layer and the GaAs substrate. The device was fabricated by fixing the sample on a thin transparent plate, and the GaAs substrate was removed by selective wet etching in order to make the device transparent to IB-resonant light. Th...

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