Ultrafast Saturable Absorption of Core/Shell Colloidal Quantum Dots

Abstract

Ultrafast saturable absorption (SA) materials that are capable of blocking the optical absorption under strong excitation have extensive applications in photonic devices. This work presents core/shell colloidal quantum dots (CQDs) which have the quantized energy levels, excellent band gap tunability, and possess significant SA performance. When the band gap is close to the pump pulse energy, the CQDs show significant resonant SA response. At the same excitation conditions, the core/shell CQDs dispersions show better SA response than graphene dispersions, and comparable to the recently reported molybdenum disulfide. The carrier dynamics of the SA of the CQDs is analyzed systematically. The research has also found that the two‐photon absorption of the CQDs show nearly cubic power law of the band gap, while the SA performance keeps almost the same in the nonresonant regime. Further, superior passive Q‐switched laser behavior is observed using the CQDs as a saturable absorber. The results directly reveal the physical processes of this basic problem and broaden the applications of CQDs in photonic devices.

Visible-wavelength pulsed lasers with low-dimensional saturable absorbers

“…With an open-aperture Z-scan system, the nonlinear absorption responses of MoS 2 dispersions were measured with the fluence increasing from 10 to 35 mJ cm −2 at 400 nm (see Figure 4D). The results showed that the modulation depth and saturable intensity of the [131]. The saturation intensity was estimated to be 9.01 and 81.0 GW cm −2 .…”

“…For a long time, the linear optical region of QDs and their applications have been widely studied [137]. As an example, the linear optics of CdTe/ CdS QDs was demonstrated by Li et al [131]. Utilizing spectroscopic techniques, the bandgaps of CdS QDs at different growing stages were measured to be 2.42, 2.26, 2.17, and 2.09 eV through controlling the CdS shell thickness, as shown in Figure 2E.…”

“…These results prove that TMDs have strong saturable absorption characteristics at visible wavelengths and can be used to achieve pulsed operation in visible laser wavelengths. Since 1996, the nonlinear saturable absorption properties of QDs including CdSe QDs, CdTe/CdS CQDs, and CsPbBr 3 QDs in the visible spectral range have been extensively studied and reported[131,150,[158][159][160]. For example, with the openaperture Z-scan system at 515 nm, the saturable absorption responses of CdTe/CdS CQDs with the diameters of 3.0 nm (bandgap of 2.42 eV) and 3.5 nm (bandgap of 2.17 eV) were measured by Li et al…”