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The third-order nonlinear optical properties of water-soluble Cu2-xSe nanocrystals are studied in the near infrared range of 700–980 nm using a femtosecond pulsed laser by the Z-scan technique. It is observed that the nonlinear optical response of Cu2-xSe nanocrystals is sensitively dependent on the excitation wavelength and exhibits the enhanced nonlinearity compared with other selenides such as ZnSe and CdSe. The W-shaped Z-scan trace, a mixture of the reversed saturated absorption and saturated absorption, is observed near the plasmon resonance band of Cu2-xSe nanocrystals, which is attributed to the state-filling of free carriers generated by copper vacancies (self-doping effect) of Cu2-xSe nanocrystals as well as the hot carrier thermal effect upon intense femtosecond laser excitation. The large nonlinear optical response and tunable plasmonic band make Cu2-xSe nanocrystals promising materials for applications in ultra-fast all-optical switching devices as well as nonlinear nanosensors. [ABSTRACT FROM AUTHOR]

The carrier-density-dependent spin relaxation dynamics for modulation-doped GaAs/Al0.3Ga0.7 As quantum wells is studied using the time-resolved magneto-Kerr rotation measurements. The electron spin relaxation time and its in-plane anisotropy are studied as a function of the optically injected electron density. Moreover, the relative strength of the Rashba and the Dresselhaus spin—orbit coupling fields, and thus the observed spin relaxation time anisotropy, is further tuned by the additional excitation of a 532 nm continuous wave laser, demonstrating an effective spin relaxation manipulation via an optical gating method. [ABSTRACT FROM AUTHOR]

We report the observation of spectral broadening induced by 200 femtosecond laser pulses with the repetition rate of 1 kHz at the wavelength of 532 nm in semi-insulating 4H–SiC single crystals. It is demonstrated that the full width at half maximum of output spectrum increases linearly with the light propagation length and the peak power density, reaching a maximum 870 cm−1 on a crystal of 19 mm long under an incident laser with a peak power density of 60.1 GW/cm2. Such spectral broadening can be well explained by the self-phase modulation model which correlates time-dependent phase change of pulses to intensity-dependent refractive index. The nonlinear refractive index n2 is estimated to be 1.88×10−15 cm2/W. The intensity-dependent refractive index is probably due to both the nonlinear optical polarizability of the bound electrons and the increase of free electrons induced by the two-photon absorption process. Super continuum spectra could arise as crystals are long enough to induce the self-focusing effect. The results show that SiC crystals may find applications in spectral broadening of high power lasers. [ABSTRACT FROM AUTHOR]