Femtosecond Four-Wave Mixing Experiments on GaAs Quantum Wells Using Two Independently Tunable Lasers

Abstract

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...

Modulation of exciton states through resonant excitation by continuous wave lasers in a GaAs/AlAs multiple quantum well

“…However, in the case of exciton overlap, interesting coherent oscillation phenomena occur. For excitons with different energies, such as heavy-hole (HH) and light-hole (LH) excitons, the overlap of simultaneously created excitons has been investigated based on quantum beats using ultrashort pulse lasers, as quantum beats are a typical temporal oscillation phenomena [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Moreover, Bloch oscillations also occur as coherent oscillations due to exciton overlap [19][20][21][22].…”

From Exciton Resonance to Frequency Mixing in GaAs Multiple Quantum Wells

“…Nevertheless, thus far femtosecond transient FWM has largely focused on the dynamics of the exciton resonance, both because the exciton plays a dominant role in the optical excitation [7] and because most femtosecond wave mixing has been performed with degenerate pulses. Only recently, some nondegenerate interactions have been reported in the femtosecond regime, including the partially nondegenerate FWM by Cundiff et al [8] and two color FWM using synchronized, independently tunable lasers by Kim et al [9].…”

“…The peak intensity of each pulse was about 10 MW͞cm 2 (I 0 ). At first glance, the broad tail arising below the heavy-hole exciton (HH) appears to be a transient diffraction at v 1 , that is, the instantaneous diffraction of the laser pulse [9,11]. However, there is a tail in the SR-FWM extending somewhat beyond the spectrum of v 1 .…”

“…The direction k 3 ͑v 1 ͒ 1 k 2 ͑v 1 ͒ 2 k 1 ͑v 2 ͒ is chosen in order to rule out the diffraction of v 2 at t 1 t 2 0 ps by the degenerate grating formed by the two beams at v 1 . We use two independently tunable, passively mode-locked Ti:sapphire lasers that are synchronized by a homemade electronic switch [9]. We also use a 10-20 fs Ti:sapphire laser and pick up two energies from the broad laser spectrum [10].…”