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Mid‐infrared (MIR) pulsed lasers operating at 2–5 µm have important applications in communication, sensing, and medicine. However, the lack of robust MIR saturable absorbers (SAs) remains a major obstacle. Here, a semiconductor material cadmium oxide (CdO) film with high laser‐induced damage threshold (800 nm, 134 mJ cm−2) and broadband saturable absorption (2.0–3.9 µm) is investigated. The effective tuning of the nonlinear optical response of CdO is demonstrated by adjusting the carrier concentration via a tungsten (W)‐doping scheme. The saturable absorption is improved and carrier relaxation process is accelerated after increasing the W‐dopant level. Based on these findings, the robust CdO‐SAs with customizable parameters are realized and Q‐switched lasers with decreasing pulse width from 372 to 254.3 ns are obtained at 2 µm. The design flexibility provided by CdO opens up a large parameter space that enables the continuous improvement of compact and high‐performance MIR ultrafast lasers. [ABSTRACT FROM AUTHOR]

Indium tin oxide (ITO) has important application in photoelectronic and photonic devices, demanding effective approach and appropriate parameters to modulate the ITO properties. The effects of laser annealing at 1064 nm with different pulse width (875 ps, 10 ns, and 300 ns) on optical and nonlinear optical properties of ITO films were investigated. Compared with 875 ps and 10 ns, laser annealing at 300 ns significantly improved the near-infrared transmittance of ITO films. The change arises from the elimination of oxygen vacancies and reduction of tin oxides, resulting in the decrease of carrier concentration. Finite element simulations indicate that laser annealing with pulse width of 875 ps and 10 ns have high instantaneous peak temperatures of 1128.5 K and 783 K, respectively, while the temperature accumulation between pulses is negligible. On the contrary, the laser annealing with a pulse width of 300 nshas lower peak transient temperature of 343 K but high static residence temperature of 973 K. The results reveal that the modification of ITO films depends more on the static average temperature rather than the instantaneous peak temperature. The modulation of carrier dynamics in both amplitude and temporal domain was demonstrated after 300 ns laser annealing, achieving ultrafast transient response approximately 150 fs. This work provides theoretical and experimental basis for us to select suitable laser conditions for directional annealing of ITO film to make it suitable for the applications of broad optical spectrum photovoltaic devices or ultrafast optical exchange for high-speed data processing. • Laser annealing (LA) is shown for three pulse widths at approximately 0.2 J/cm2. • LA at 300 ns significantly improves the near-infrared transparency due to the reduction of carrier absorption. • Tailorable amplitude and time scale of carrier transient response are achieved by LA, with ultrafast response time 150 fs. • The modification of ITO properties depends on the static temperature rather than transient peak temperature. [ABSTRACT FROM AUTHOR]

During quasi-CW (QCW) 1064-nm laser annealing process, one excess oxygen could capture two free electrons, forming one O2− and then occupied one neighboring site of the oxygen vacancy, resulting in the annihilation of the oxygen vacancy. In addition, one Sn4+ ion was combined with two free electrons to create a Sn2+ ion. The two free electrons were transformed to form two bound electrons, indicating a reduction in free electrons. The oxygen vacancies annihilation and tin oxide reduction after QCW 1064-nm laser annealing reduced the number of free electrons, decreased the carrier density and enhanced the optical transmittance in the NIR region. • Quasi-CW (QCW) laser annealing (LA) notably prompts the transparency in NIR region. • Increase of transmittance after LA attributes to reduction of carrier absorption. • Oxygen vacancy elimination and tin oxide reduction result in carrier reduction. • More tin oxide reduction occurs in oxygen-poor zone than that in oxygen-rich zone. In this study, a 1064-nm quasi-continuous-wave laser was employed to anneal the indium tin oxide (ITO) films, thereby prominently prompting transparency in the near-infrared region with insignificant variation in resistivity. The decrease in carrier density due to laser annealing contributed to the improvement in transparency. Interestingly, the depth-resolved X-ray photoelectron spectroscopy distinguishes between two different mechanisms for tailoring the properties of the free carriers. Tin oxide reduction in the oxygen-deficient zone of the ITO films at a depth of approximately more than 80 nm was more significant than that in the oxygen-rich zone within the top 80-nm thin layer. In addition, the elimination of oxygen vacancies in the oxygen-rich environments did not indicate apparent advantage when compared with that in the oxygen-deficient environments owing to the oxygen supplied during the tin oxide reduction process. These results provide an alternative laser annealing technologies for tailoring the properties of the free carriers and elucidating the fundamental mechanisms in oxygen-rich and oxygen-deficient environments in an annealing process. [ABSTRACT FROM AUTHOR]