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We report on spectrally resolved four-wave mixing experiments on In x Ga 1Ϫx As/GaAs quantum wires for a wide range of lateral sizes. Due to the polarization dependence of the four-wave mixing signal, beats in the decay of the signal and an additional emission line in the four-wave mixing spectrum can be clearly attributed to biexcitons. We find that the biexciton binding energy depends on both the vertical and lateral dimensions of the wires. For quantum wires with a large vertical confinement we observe an enhancement of the binding energy of about 40% as compared to a two-dimensional reference sample whereas the biexciton binding energy is found to be wire width independent in wires with shallow vertical confinement. ͓S0163-1829͑98͒52328-4͔ Biexcitonic features in low-dimensional semiconductor structures have been investigated experimentally and theoretically in a number of works in recent years. 1 In twodimensional ͑2D͒ systems, biexcitons first have been observed in 1982 by Miller et al. 2 Since then their existence has been confirmed by photoluminescence studies 3 and, more recently, in nonlinear spectroscopy, e.g., in four-wave mixing ͑FWM͒ experiments. [4][5][6][7] The FWM technique is well suited to study biexcitons. Previous studies on 2D structures have shown characteristic beats between excitonic and biexcitonic states in the timeintegrated FWM signal. 5,6 The heavy-hole ͑hh͒ as well as the light-hole ͑lh͒ biexciton have been observed in spectrally resolved ͑SR͒ FWM experiments. 4,7 Excitonic and biexcitonic contributions to the FWM signal can be separated by a proper choice of polarizations of the exciting beams. 8,9 In 2D structures the biexciton binding energy was found to be greatly enhanced compared to bulk semiconductors. In GaAs/Al x Ga 1Ϫx As quantum wells ͑QW's͒ biexciton binding energies as high as 3.5 meV have been reported 10 that have to be compared to a value of 0.13 meV observed in bulk GaAs. 11 This enhancement has been attributed to confinement effects, but lately the importance of polariton effects has also been pointed out. 12 Recently, Birkedal et al. systematically investigated the QW width dependence of the biexciton binding energy and found that the ratio of the biexciton and exciton binding energies is independent of the well width as long as the width of the QW is smaller than the exciton and biexciton diameters. 7 In zero-dimensional ͑0D͒ quantum dot structures biexcitons have been reported in a number of material systems. 13-15 The binding energy was found to be enhanced by more than an order of magnitude over the corresponding bulk values and increased with decreasing dot diameter in agreement with theoretical predictions.To the best of our knowledge, biexcitons in onedimensional ͑1D͒ structures have not been observed experimentally to this point. However, the existence of biexcitons in 1D quantum wire structures has been addressed in a few theoretical works. In an early study biexciton binding energies were calculated variationally for cylindrical GaAs/Al x Ga 1Ϫx As quan...

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