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Large amplitude time-domain oscillations are detected in InxGa1−xN/GaN structures via femtosecond differential reflectivity spectroscopy. The oscillation amplitude increases with increasing indium fraction and abruptly disappears at a critical time that depends on GaN thickness. We show that spatially localized, coherent acoustic phonon wave packets are generated via the photoexcited carriers and propagate into the samples modulating the reflectivity. Our results show that a system with strong built-in strain can be a very effective source for ultrafast acoustic phonon wave packets which can be used as a powerful probe for nanoscale structures.

We show that large amplitude, coherent acoustic phonon wavepackets can be generated and detected in InxGa1−xN/GaN epilayers and heterostructures in femtosecond pump-probe differential reflectivity experiments. The amplitude of the coherent phonon increases with increasing Indium fraction x and unlike other coherent phonon oscillations, both amplitude and period are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wavepacket reaches a GaN-sapphire interface below the surface indicating that the phonon wavepackets are useful for imaging below the surface. A theoretical model is proposed which fits the experiments well and helps to deduce the strength of the phonon wavepackets. Our model shows that localized coherent phonon wavepackets are generated by the femtosecond pump laser in the epilayer near the surface. The wavepackets then propagate through a GaN layer changing the local index of refraction, primarily through the Franz-Keldysh effect, and as a result, modulate the reflectivity of the probe beam. Our model correctly predicts the experimental dependence on probe-wavelength as well as epilayer thickness.

Femtosecond two beam and three beam (v 1 , v 1 ; v 2 ) four-wave mixing (FWM) experiments on GaAs quantum wells have been performed using two partially synchronized, independently tunable lasers with external jitter compensation. Heavy and light hole beatings are observed with these two mutually incoherent lasers. FWM signals are observed when v 2 is completely below the exciton energies, with no spectral overlap with the absorption profile. These off-resonant signals are stronger than the interband continuum signals for equivalent detunings. [S0031-9007 (98)06123-7] PACS numbers: 78.47. + p, 42.50.MdIn the past few years, femtosecond nonlinear spectroscopy of semiconductors and semiconductor quantum structures have provided new insights on incoherent and coherent dynamics. This was possible mainly through the proliferation of femtosecond sources such as the selfmodelocked Ti:Sapphire lasers, whose tuning range encompasses band gaps of GaAs, AlGaAs, InGaAs, CdTe, and other important semiconductors. One of the most widely used tools has been the two beam and three beam four wave mixing (FWM) [1]. There exists a wealth of information provided by this technique, which includes the relaxation of excitons, free carriers, and phonons, and interactions among them [2,3]. The wide tunability and the ease of second harmonic generation using femtosecond Ti:Sapphire lasers further extended the usage of FWM into wide band gap materials such as ZnSe [4] and GaN [5].Most of the femtosecond wave mixing experiments have essentially been "one color" in that one performs, say, degenerate wave mixing experiments and tune the photon energy. In picosecond and subpicosecond high power experiments, synchronously pumped two dye lasers or Raman shifted lasers were used to investigate phonon dynamics in semiconductors [6]. In the femtosecond regime, Cundiff et al. performed partially nondegenerate FWM experiments by taking advantage of the broad bandwidth of sub-100 fs laser [7]. To perform femtosecond two color wave mixing experiments with a wide range of tunability, one needs to synchronize two femtosecond lasers with timing jitters between them as short as the pulse width. Clever techniques to generate two color femtosecond beams from one laser [8] or to synchronize two lasers with cavity length detuning exist [9], but with limited tunability. Commercially available systems deliver easy synchronization over the entire tuning range of 700-1000 nm between two femtosecond Ti:Sapphire lasers. However, the timing jitter is in the range of 5-10 ps.In this Letter, we have performed the first two color femtosecond FWM experiments using two independently tunable, partially synchronized lasers. This was made possible by devising an easy way to externally compensate the inevitable jitters between the two lasers. We improved the effective time resolution from .5 ps to 150 fs, limited only by the pulse widths of the two lasers. Using this technique, we have performed femtosecond two beam and three beam FWM experiments on GaAs quantum wells. Our most...