Pinghua Tang

Topological Insulator:

An in-band pumped 1.645-μm Er:YAG ceramic laser passively Q-switched by a topological insulator: Bi₂Te₃ saturable absorber is reported. The average output power could reach up to 210 mW, corresponding to a pulsewidth, a pulse repetition rate, and a per-pulse energy of 6.3 μs, 40.7 kHz, and 5.3 μJ, respectively. This result indicates that the topological insulator: Bi₂Te₃ could be a promising saturable absorber such as graphene, with potential applications for solid-state lasers.

\hbox{Bi}_{2}\hbox{Te}_{3}

We demonstrate a new type of optical saturable absorber based on the self-assembled topological insulator Bi2Se3 membrane fabricated by the drop-cast/evaporation approach. The strong viscosity of Bi2Se3 allows its attachment onto the optical fiber end-facet for practical optoelectronic applications. The balanced twin-detector technique was used to characterize the saturable absorption parameters of the device, which has a saturating intensity of 101.8 MW/cm2 and a modulation depth of 41.2% at the telecommunication band. By deploying this device into a fiber laser cavity, we had achieved stable Q-switched pulses with a repetition rate of 8 kHz and pulse duration of 14 μs. Through fine tuning the laser pump strength and/or cavity birefringence, we could widely change the Q-switched pulse repetition rate from 4.508 kHz to 12.88 kHz, pulse duration from 13.4 μs to 36 μs, and lasing wavelength from 1545.0 nm to 1565.1 nm. A dual-wavelength passive Q-switching operation was also obtained by enhancing the intra-cavity birefringence. Our results show the effectiveness of developing Bi2Se3 optical saturable absorber device by the drop-cast/evaporation method and applications for pulsed laser operation.

Saturable Absorber for the Passive Q-Switching Operation of an in-Band Pumped 1645-nm Er:YAG Ceramic Laser

We report on the generation of large energy, widely wavelength tunable pulses in an erbium-doped fiber laser passively Q-switched by a topological insulator saturable absorber (TI-SA). The TI-SA is prepared through an optical deposition method. Its saturating intensity and modulation depth are measured to be about 57 MW/cm2 and 22%, respectively. We show that the high modulation depth of TI-SA allows the generation of stable Q-switched pulses with per-pulse energy up to 1.5 μJ and its broadband saturable absorption favors the tunable Q-switching operation from 1510.9 nm to 1589.1 nm. Our study suggests that TI: Bi2Te3 could be a promising saturable absorber for both the high energy and broadband optical applications.

Self-Assembled Topological Insulator: Bi

We experimentally investigated the formation of various multi-soliton patterns and noise-like (NL) pulses in an erbium-doped fiber laser passively mode-locked by a new type of saturable absorber: topological insulator. With the increase of pump power, various multi-soliton operation states—ordered, chaotic and bunched multiple-soliton—were subsequently obtained. Once the pump power exceeds 401 mW, an NL pulse state emerged, with a maximum 3 dB bandwidth of about 9.3 nm. This systematic study clearly demonstrated that a topological insulator could be an effective saturable absorber for the formation of various soliton operation states in a fiber laser cavity.

_{2}

In a passively mode-locked Erbium-doped fiber laser with large anomalous-dispersion, we experimentally demonstrate the formation of noise-like square-wave pulse, which shows quite different features from conventional dissipative soliton resonance (DSR). The corresponding temporal and spectral characteristics of a variety of operation states, including Q-switched mode-locking, continuous-wave mode-locking and Raman-induced noise-like pulse near the lasing threshold, are also investigated. Stable noise-like square-wave mode-locked pulses can be obtained at a fundamental repetition frequency of 195 kHz, with pulse packet duration tunable from 15 ns to 306 ns and per-pulse energy up to 200 nJ. By reducing the linear cavity loss, stable higher-order harmonic mode-locking had also been observed, with pulse duration ranging from 37 ns at the 21st order harmonic wave to 320 ns at the fundamental order. After propagating along a piece of long telecom fiber, the generated square-wave pulses do not show any obvious change, indicating that the generated noise-like square-wave pulse can be considered as high-energy pulse packet for some promising applications. These experimental results should shed some light on the further understanding of the mechanism and characteristics of noise-like square-wave pulses.

Se

Ultrathin topological insulator bismuth telluride (Bi2Te3) nanosheets with uniform hexagonal nanostructure have been synthesized by cost-effective solvothermal method. Broadband spatial self-phase modulation phenomena of these topological insulator nanosheets have been observed with 400 nm, 800 nm, and 1070 nm ultrafast lasers. The experimental results suggest that this coherent light scattering is due to the broadband, ultrafast, and large third-order optical nonlinearity of Bi2Te3. With the pulsed laser excitation, the nonlinear refractive index (n2) of Bi2Te3 dispersion solution was measured to be ∼10−12 m2/W, and the third-order nonlinear susceptibility ∼10−7 esu. Our work may provide an inroad for developing the nonlinear optical applications based on topological insulators.

_{3}

Nonlinear transmission parameters of monolayer graphene at 1645 nm were obtained. Based on the monolayer graphene saturable absorber, a 1532 nm LD pumped 1645 nm passively Q-switched Er:YAG laser was demonstrated. Under the pump power of 20.8 W, a 1645 nm Q-switched pulse with FWHM of 0.13 nm (without the use of etalon) and energy of 13.5 μJ per pulse can be obtained. To the best of our knowledge, this is the highest pulse energy for graphene-based passively Q-switched Er:YAG laseroperating at 1645 nm, suggesting the potentials of graphene materials for high-energy solid-state laser applications.

Membrane as a Passive Q-Switcher in an Erbium-Doped Fiber Laser

We experimentally demonstrated a stable, high-average-power, continuous-wave (CW) passively mode-locked Er(3+)-doped ZBLAN fiber laser at 2.8 μm based on a semiconductor saturable absorber mirror. A stable mode-locked laser with a signal-to-noise ratio of 52 dB and a slope efficiency of 14% was obtained. The highest average output power in excess of 1 W was generated at the incident pump power of 8.2 W, with a pulse repetition rate of 22.56 MHz and pulse duration of 25 ps. To the best of our knowledge, this is the highest average output power of a CW mode-locked ZBLAN fiber laser in the mid-infrared wavelength regime up to now.

Large Energy, Wavelength Widely Tunable, Topological Insulator Q-Switched Erbium-Doped Fiber Laser

We demonstrated a passively Q-switched 2.8-μm Er^3±-doped ZBLAN fiber laser with topological insulator Bi₂Te₃ nanosheets prepared by the solvothermal method. Enabled by the gold mirror with self-assembled Bi₂Te₃ nanosheets film sandwiched by polymethyl methacrylate polymer, the Q-switched fiber laser can deliver a maximum average power of 856 mW with corresponding pulse energy of 9.3 μJ and a pulse width of 1.3 μs at a repetition rate of 92 kHz under the pump power 5.9 W. To the best of our knowledge, this is the highest output power for the pulsed fluoride fiber laser around 2.8 μm modulated by the two-dimensional Dirac material. This letter reveals that the topological insulator can be a kind of promising nonlinear modulator for a mid-infrared laser.

The formation of various multi-soliton patterns and noise-like pulse in a fiber laser passively mode-locked by a topological insulator based saturable absorber

A highly efficient and stable mid-infrared optical parametric oscillator is demonstrated, pumped by an electro-optic Q-switched Er:YAG laser with operating wavelength locked at 1645 nm by a volume Bragg grating. The oscillator, based on MgO-doped periodically poled lithium niobate (MgO:PPLN) crystal, yields a maximum overall average output power in excess of 1 W, corresponding to a conversion efficiency of 35.5% and a slope efficiency of 43.6%. The signal and idler wavelengths of the OPO are around ~2.7 μm and ~4.3 μm, respectively, corresponding to the two peak absorption bands of CO(2). Lasing characteristics of the oscillator, including the time evolution of the pump, signal and idler pulses at different pump power levels, are also investigated. Temperature tuning of the MgO:PPLN crystal gives signal and idler ranges of 2.67 to 2.72 μm and 4.17 to 4.31 μm, respectively.