Lili Miao

Broadband nonlinear optical response in multi-layer black phosphorus: an emerging infrared and mid-infrared optical material.

Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, is a high-mobility layered semiconductor with direct band-gap determined by the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer). Therefore, BP is considered as a natural candidate for broadband optical applications, particularly in the infrared (IR) and mid-IR part of the spectrum. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for infrared and mid-infrared opto-electronics. Herein, we experimentally verified its broadband and enhanced saturable absorption of multi-layer BP (with a thickness of ~10 nm) by wide-band Z-scan measurement technique, and anticipated that multi-layer BPs could be developed as another new type of two-dimensional saturable absorber with operation bandwidth ranging from the visible (400 nm) towards mid-IR (at least 1930 nm). Our results might suggest that ultra-thin multi-layer BP films could be potentially developed as broadband ultra-fast photonics devices, such as passive Q-switcher, mode-locker, optical switcher etc.

Wide spectral and wavelength-tunable dissipative soliton fiber laser with topological insulator nano-sheets self-assembly films sandwiched by PMMA polymer

Topological insulators have been theoretically predicted as promising candidates for broadband photonics devices due to its large bulk band gap states in association with the spin-momentum-locked mass-less Dirac edge/surface states. Unlike the bulk counterpart, few-layer topological insulators possess some intrinsic optical advantages, such as low optical loss, low saturation intensity and high concentration of surface state. Herein, we use a solvothermal method to prepare few-layer Bi₂Te₃ flakes. By sandwiching few-layer Bi₂Te₃ flakes with polymethyl methacrylate (PMMA) polymer, a novel light modulation device had been successfully fabricated with high chemical and thermal stabilities as well as excellent mechanical durability, originating from the contribution of PMMA acting as buffer layers that counteract excessive mechanical bending within the fragile Bi₂Te₃ flakes. The incorporation of the as-fabricated PMMA-TI-PMMA as saturable absorber, which could bear long-term mechanical loadings, into the fiber laser cavity generated the stable dissipative soliton mode-locking with a 3-dB spectral bandwidth up to 51.62 nm and tunable wavelength range of 22 nm. Our work provides a new way of fabricating PMMA-TI-PMMA sandwiched composite structure as saturable absorber with promising applications for laser operation.

Broadband and enhanced nonlinear optical response of MoS2/graphene nanocomposites for ultrafast photonics applications.

Due to their relatively high compatibility with specific photonic structures, strong light-matter interactions and unique nonlinear optical response, two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, are attractive for ultrafast photonics applications. Here, we fabricate MoS2/graphene nanocomposites by a typical hydrothermal method. In addition, we systematically investigate their nonlinear optical responses. Our experiments indicate that the combined advantages of ultrafast relaxation, a broadband response from graphene, and the strong light-matter interaction from MoS2, can be integrated together by composition. The optical properties in terms of carrier relaxation dynamics, saturation intensity and modulation depth suggest great potential for the MoS2/graphene nanocomposites in photonics applications. We have further fabricated 2D nanocomposites based optical saturable absorbers and integrated them into a 1.5 μm Erbium-doped fiber laser to demonstrate Q-switched and mode-locked pulse generation. The fabrication of 2D nanocomposites assembled from different types of 2D materials, via this simple and scalable growth approach, paves the way for the formation and tuning of new 2D materials with desirable photonic properties and applications.

Broadband ultrafast spatial self-phase modulation for topological insulator Bi2Te3 dispersions

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.

Broadband ultrafast nonlinear optical response of few-layers graphene: toward the mid-infrared regime

Gapless linear energy dispersion of graphene endows it with unique nonlinear optical properties, including broadband nonlinear absorption and giant nonlinear refractive index. Herein, we experimentally observed that few-layers graphene has obvious nonlinear absorption and large nonlinear refraction, as investigated by the Z-scan technique in the mid-infrared (mid-IR) regime. Our study may not only, for the first time to our knowledge, verify the giant nonlinear refractive index of graphene (∼10−7  cm2/W) at the mid-IR, which is 7 orders of magnitude larger than other conventional bulk materials, but also provide some new insights for graphene-based mid-IR photonics, potentially leading to the emergence of several new conceptual mid-IR optoelectronics devices.

2.8-

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.

\mu \text{m}

We experimentally observed that a representative two-dimensional layered material, bismuth telluride (Bi2Te3) nanosheets, exhibit obvious broadband nonlinear absorption and large nonlinear refraction investigated by Z-scan technique. Our study may not only verify the giant nonlinear refractive index of Bi2Te3 (~10−8 cm2/W), but also provide some new insights for topological insulator-based photonics, potentially leading to the emergence of several new conceptual optoelectronics devices.

Pulsed Er3+: ZBLAN Fiber Laser Modulated by Topological Insulator

Through employing a cost-effective solvothermal method, ultrathin topological insulator (TI) bismuth telluride (Bi2Te3) nanosheets with uniform hexagonal nanostructures had been synthesized. Thanks to the uniformity of few-layer TI dispersion, we are able to adopt the drop-casting approach in order to directly transfer few-layer TI and, therefore, form a self-assembled uniform volatile TI membrane that is suitably deposited onto the end facet of an optical fiber as an effective optical saturable absorber. Its saturable absorption parameters could be deliberately tailored by thinning its thickness by mechanical exfoliation. The incorporation of the as-fabricated saturable absorber inside the fiber laser cavity allows for the operation of either a microsecond or a femtosecond pulse because different saturable absorption parameters can decide whether the fiber laser operates in the mode-locking or Q-switching state. Our work provides a convenient way of fabricating a high-quality TI membrane-based saturable absorber with promising applications for laser operation.

Broadband third order nonlinear optical responses of bismuth telluride nanosheets

We report a graphene Q-switched ytterbium-doped fiber laser with switchable cylindrical vector beam output. The 6-8 layers CVD-grown graphene films were transferred to the target substrate by an ultrasonic processing method, and its saturation intensity and modulation depth are measured to be about 0.61 MW/cm2 and 13.2% at 1072 nm by the Z-scan technique. We used a nanograting spatially variant waveplate as an intracavity polarization controlling element to convert pulsed Gaussian beam to radially or azimuthally polarized beam in the fiber laser cavity. Stable Q-switching operation can be achieved with the output power up to 253 mW and pulse energy 7.73 μJ at the maximum incident pump power. The polarization extinction ratio of radially and azimuthally polarized beam is 97.4% and 96.9%, respectively. The experimental results suggest that the graphene can act as a potential nonlinear optical material to modulate pulsed fiber lasers with spatially inhomogeneous polarizations.

Drop-Casted Self-Assembled Topological Insulator Membrane as an Effective Saturable Absorber for Ultrafast Laser Photonics

Transition metal dichalcogenides (TMDs), a family of two-dimensional layered materials exhibiting unusual electronic, optical, mechanical properties, have aroused much attention in recent years. Here, we focus on the nonlinear saturable absorption behavior of a new family member of TMDs, rhenium diselenide (ReSe2), and its application in pulsed laser generation. We have prepared the few-layer ReSe2 by the mechanical exfoliation method, and validated its ambient-stable nonlinear optical responses and all-optical tunable potential at 1.55 µm. In addition, the ambient-stable and parameter-tunable Q-switched fiber laser modulated by the ReSe2 has been realized.