Abstract
A multiple quantum-well semiconductor saturable absorber mirror (MQW-SESAM) structure has been investigated by femtosecond pump-probe laser spectroscopy at a central wavelength of around 1050 nm. Coherent acoustic phonons are generated and detected over a wide frequency range from ~15 GHz to ~800 GHz. In the optical absorption region, i.e., in the multiple quantum wells (In0.27Ga0.73As), acoustic frequency combs centered at ~365 GHz, with a comb spacing of ~33 GHz, are generated. Most importantly, in the transparent region, i.e., in the distributed Bragg reflector, which is formed by a non-doped long-period semiconductor GaAs/Al0.95Ga0.05As superlattice, the mini-Brillouin-zone center, as well as zone-edge acoustic modes, are observed. The mini-zone-center modes with a fundamental frequency of 32 GHz can be attributed to the spatial modulation of the pump optical interference field with a period very close to that of the distributed Bragg reflector, in combination with the periodic spatial modulation of the electrostriction coefficient in the distributed Bragg reflector. The excitation of mini-zone-edge modes is attributed to the stimulated subharmonic decay of the fundamental center modes. Their subsequent back-folding to the mini-Brillouin-zone center makes them Raman active for the probe light.
Highlights
Periodic structures such as multiple quantum wells (MQWs) and superlattices (SLs) have been used as attractive tools for the observation of folded longitudinal coherent acoustic phonons (LCAPs) [1,2]
If light absorption is involved in semiconductors, coherent acoustic phonons are excited mainly due to the deformation potential interaction or the inverse piezoelectric effect related to electron-hole excitation via inter-band light absorption [7,8]
We report on the generation and detection of CLAPs in a MQW- semiconductor saturable absorber mirror (SESAM) structure performed by femtosecond time-domain spectroscopy in a reflection geometry at a wavelength of around
Summary
Periodic structures such as multiple quantum wells (MQWs) and superlattices (SLs) have been used as attractive tools for the observation of folded longitudinal coherent acoustic phonons (LCAPs) [1,2]. It is not common to find experimental demonstrations of the electrostriction excitation mechanism in transparent semiconductor heterostructures and SLs where the LCAPs spectrum exhibits mini-Brillouin-zone (MBZ) center and edge modes. In stationary or frequency-domain Raman experiments on thermal phonons, the observations of mini-zone-edge phonons are attributed to disorder-induced qz -non-conserving scattering, which can be caused, for example, by interface roughness [19] and interface defects [20] in GaAs/AlAs superlattices. The MQW-SESAM structure exhibits multiple QWs as embedded saturable absorber layers and a distributed Bragg mirror (DBR) which are spatially separated by GaAs and AlGaAs layers Both of the pump and probe photon energies are above the bandgap of the saturable absorber layers but below that of constituting materials of the DBR, whose reflectance is close to 100% for our laser wavelengths. The structure-related complexity of our experimental phonon spectrum will be discussed in Appendix A
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