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Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi2Te3 heterojunction which combines monolayer graphene and a Bi2Te3 nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated by varying the thickness of the Bi2Te3 when synthesizing the heterojunctions. Furthermore, with the aid of the estimated junction electron escape time, a model of the photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from the Bi2Te3 bulk band. The increased modulation depth of the graphene/Bi2Te3 heterojunction can accordingly be realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi2Te3 thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work demonstrates the functionality of optical nonlinear engineering by tuning the thickness of the graphene/Bi2Te3 heterojunction and demonstrates its potential for applications.

In this study, strong two-photon absorption (TPA) in a layered bismuth telluride (Bi2Te3) topological insulator (TI) is observed and investigated by the Z-scan method under excitation with a femtosecond laser pulse at a wavelength of 1056 nm. Nonlinear transmission measurements reveal that Bi2Te3 possesses not only the property of single photon absorption (SPA) but also the effect of TPA. A saturation TPA coefficient as high as ∼2.29 × 106 cm GW−1 has been obtained, which is at least two orders of magnitude higher than that of other two dimensional (2D) materials, such as MoS2. The large TPA effect in the Bi2Te3 TI is attributed to the existence of high-lying bulk states located above the first bulk state. Meanwhile, it can also be used to explain the discontinuity of the optical nonlinear transmittance between the low and high excitation energy regions.

Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi

The successful demonstration of Q-switched and mode-locked fiber-laser operations using a topological insulator (TI) as saturable absorber (SA) has opened an application window besides TI's originally expected features. However, to date, all-solid-state mode-locked lasers base on TISAs are still unavailable and became a desired goal not only due to their application as light sources, but also because of providing a way for deeper investigation of the nature of ultrafast dynamics present in TISAs. In this paper, the realization of a continuous-wave mode-locked all-solid-state laser with a repetition rate of around 1 GHz is reported using a high-quality TI SA mirror (TI-SAM) with ultralow saturation intensity, fabricated by a spin coating-co-reduction approach. An output power of 180 mW and pulse duration of 8 ps are observed. In addition, a 61 dB pulse-train quality from the radio frequency spectrum of mode-locked operation prove the feasibility of the proposed laser. To the best of our knowledge, this is the first experimental demonstration of a mode-locked solid-state laser based on TIs. In addition, this paper shows that the use of TISAs is a promising option for the realization of scaling solid-state mode-locked lasers with higher repetition rates, reaching order of GHz.

This Letter compared the absorption characteristics of a homemade graphene-Bi

In this study, Bi2Te3 cluster was applied as a saturable absorber to investigate the mode-locking behavior of InGaAsP multiple quantum wells (MQWs) whispering gallery mode (WGM). Under optical excitation, the ultralow saturation absorption of Bi2Te3 from bulk state had modulated the intensity inside the microdisk, multi-lasing at different wavelengths with equal spectral spacing was realized. The achieved pulse repetition rate was as high as an estimated 4 THz. Additionally, a clear degenerate breaking was observed with a frequency shift of approximately 1 THz for each lasing mode. To the best of our knowledge, this is the first demonstration of a passively mode-locked WGM laser. These findings may have applications in dual-comb device.

In this paper, by using the prepared topological insulator saturable absorber mirror, a GHz, mode-locked Nd:YVO 4 bulk laser is realized for the first time with the output power of 180mW and pulse duration of 8ps.

Graphene has attracted plenty of attentions due to its amazing electric and optical properties. Owing to the unique electronic structure of Dirac cone and massless fermions, the strong optical nonlinearity has been realized and demonstrated with dependency of Fermi level [1]. In this work, Fermi-level tunable p-type graphene was prepared by controlling the relative humidity (RH). The Raman spectrum shows clear tendency of blueshift and the minimum conductance position of field effect transistor (FET) moves to higher applied voltage as increasing the RH. This is due to that electrons in graphene migrate to surface water molecules [2], leading to the downward movement of Fermi level to p-type further. Additionally, the Fermi-level dependent saturation absorption of p-type graphene was also investigated and discussed.

This Letter compared the absorption characteristics of a homemade graphene-Bi2Te3 (G-B) van der Waals heterostructure to a Bi2Te3 topological insulator (TI) with a similar preparation method and number of layers. The results indicate that the G-B heterostructure can tremendously enhance the modulation depth and saturable intensity. In addition, a passively Q-switched laser at 1.06 μm with a G-B heterostructure as a saturable absorber (SA) was demonstrated for the first time, to the best of our knowledge. Compared to Bi2Te3 TI, the G-B heterostructure Q-switched laser had better laser performance, indicating that a G-B heterostructure is a promising SA candidate for a 1 μm laser.