Mode-Locked Oscillation of Cr:ZnS Laser using a Single Walled Carbon Nanotube Film with Resonant Absorption at 2.4 μm

Ultrashort pulses in the mid-infrared (mid-IR) region have a great potential in the applications of advanced vibrational spectroscopy and strong field phenomena. Traditionally, mid-IR ultrashort pulses are generated by frequency conversion of near-infrared pulses from Ti:S or Yb-based lasers. Laser oscillators which directly emit mid-IR femtosecond pulses are advantageous from the perspectives of simplicity and energy extraction efficiency. Cr:ZnS is an attractive gain medium for mid-IR ultrafast lasers because it has a broad fluorescence spectrum, large stimulated-emission cross-section and high thermal conductivity [1]. So far, passive mode-locking has been realized for Cr:ZnS laser, by employing Kerr-lensing, or by using saturable absorbers based on semiconductors or graphenes [2]. A single walled carbon nanotube (SWCNT) film is also known as a good candidate for saturable absorbers, possessing tunable resonance and large modulation depth. However, SWCNTs have rarely been applied to this laser [3]. Here, we demonstrate self-starting mode-locked oscillation of Cr:ZnS laser by using a SWCNT film which has resonant absorption at the laser emission wavelength of 2.4 μm.