  The newly-emergent two-dimensional topological insulators (TIs) have shown
their unique electronic and optical properties, such as good thermal
management, high nonlinear refraction index and ultrafast relaxation time.
Their narrow energy band gaps predict their optical absorption ability further
into the mid-infrared region and their possibility to be very broadband light
modulators ranging from the visible to the mid-infrared region. In this paper,
a mid-infrared mode-locked fluoride fiber laser with TI Bi2Te3 nano-sheets as
the saturable absorber is presented. Continuous wave lasing, Q-switched and
continuous-wave mode-locking (CW-ML) operations of the laser are observed
sequentially by increasing the pump power. The observed CW-ML pulse train has a
pulse repetition rate of 10.4 MHz, a pulse width of ~6 ps, and a center
wavelength of 2830 nm. The maximum achievable pulse energy is 8.6 nJ with
average power up to 90 mW. This work forcefully demonstrates the promising
applications of two-dimensional TIs for ultra-short laser operation and
nonlinear optics in the mid-infrared region.
