Triple Threshold Transitions and Strong Polariton Interaction in 2D Layered Metal-Organic Framework Microplates.

Room-temperature interaction between light-matter hybrid particles such as exciton-polaritons under extremely low-pump plays a crucial role in future coherent quantum light sources. However, the practical and scalable realization of coherent quantum light sources operating under low-pump remains a challenge because of the insufficient polariton interaction strength. Here, at room temperature, a very large polariton interaction strength is demonstrated, g ≈ 128 ± 21 µeV µm2 realized in a 2D nanolayered metal-organic framework (MOF). As a result, a polariton lasing at an extremely low pump fluence of P1  ≈ 0.01 ± 0.0015 µJ cm-2 (first threshold) is observed. Interestingly, as pump fluence increases to P2  ≈ 0.031 ± 0.003 µJ cm-2 (second threshold), a spontaneous transition to a polariton breakdown region occurs, which has not been reported before. Finally, an ordinary photon lasing occurs at P3  ≈ 0.11 ± 0.077 µJ cm-2 (third threshold), or above. These experiments and the theoretical model reveal new insights into the transition mechanisms characterized by three distinct optical regions. This work introduces MOF as a new type of quantum material, with naturally formed polariton cavities, that is a cost-effective and scalable solution to build microscale coherent quantum light sources and polaritonic devices.

Graphdiyne-deposited microfiber structure all-optical modulator at the telecommunication band.

All-optical modulator is a crucial device in next generation of all-optical communications, interconnects, and signal processing. Here, we demonstrate an all-optical phase modulator with graphdiyne (GDY)-deposited microfiber structure. The phase shift of the signal light can be readily controlled by pump light by thermo-optic effect. This all-optical modulator can achieve a phase shift slope of 0.0296 π·mW-1 and a rising time of 5.48 ms at 25 Hz (3 ms, 50 Hz). Modes distributions in GDY-deposited microfiber at different wavelength are numerical analyzed and the normalized phase conversion efficiency of GDY are calculated. The results show that GDY has a considerable normalized phase conversion efficiency of 0.1644 π·mW-1·mm-1, which is higher than that of graphene, MXene and WS2 based all-optical modulators. This work proves the potential of GDY in all-optical modulator device at telecommunication band and provides a support to all-optical signal processing systems.

Scalable Production of Boron Quantum Dots for Broadband Ultrafast Nonlinear Optical Performance.

A simple and effective approach based on the liquid phase exfoliation (LPE) method has been put forward for synthesizing boron quantum dots (BQDs). By adjusting the interactions between bulk boron and various solvents, the average diameter of produced BQDs is about 7 nm. The nonlinear absorption (NLA) responses of as-prepared BQDs have been systematically studied at 515 nm and 1030 nm. Experimental results prove that BQDs possess broadband saturable absorption (SA) and good third-order nonlinear optical susceptibility, which are comparable to graphene. The fast relaxation time and slow relaxation time of BQDs at 515 nm and 1030 nm are about 0.394-5.34 ps and 4.45-115 ps, respectively. The significant ultrafast nonlinear optical properties can be used in optical devices. Here, we successfully demonstrate all-optical diode application based on BQDs/ReS2 tandem structure. The findings are essential for understanding the nonlinear optical properties in BQDs and open a new pathway for their applications in optical devices.

Layered Hybrid Perovskites for Highly Efficient Three-Photon Absorbers: Theory and Experimental Observation.

Multiphoton absorption may find many technological applications, such as enhancing the conversion efficiency of solar cells by the utilization of sub-band-energy photons, below-bandgap photodetection through the simultaneous absorption of several infrared photons for photocurrent generation, or light frequency upconversion for high-resolution, 3D imaging. To enhance multiphoton absorption in semiconducting materials, one of the strategies is to explore low-dimensional excitons. Here, a quantum perturbation theory on a giant enhancement in three-photon absorption (3PA) arising from 2D excitons in multilayered crystals of organic-inorganic hybrid perovskites is presented. The maximal 3PA coefficient is predicted to be in the range of 2-7 cm3 GW-2 at 1100 nm, the largest values reported so far for any 2D and bulk semiconductors at room temperature. Excellent agreement between theory and the experimental findings unambiguously demonstrates a pivotal role in the enhancement of 3PA played by 2D excitons. The theory predicts that the resonant 3PA coefficient should be enhanced further by at least two orders of magnitude with very low temperature. The findings are essential for understanding giant 3PA arising from 2D excitons in layered hybrid perovskites and may open new pathways for highly efficient conversion from infrared light energy to either electrical energy or higher-frequency light emission/lasing.

Two-photon absorption arises from two-dimensional excitons.

By applying quantum perturbation theory to two-dimensional excitons in monolayer transition metal dichalcogenides (TMDCs), we develop a theoretical model for two-photon absorption in the near infrared spectral region. By assuming the bandwidth of the final excitonic state to be 0.15 eV, the two-photon absorption coefficients are as high as 50 cm/MW and selenium-based, monolayer TMDCs exhibit greater 2PA coefficients than sulfur-based, monolayer TMDCs. Our model is also compared to the experimental data obtained by Z-scans or nonlinear transmission measurements.

Tunable terahertz/infrared coherent perfect absorption in a monolayer black phosphorus.

Black phosphorus (BP), a promising new two-dimensional (2D) material, has drawn a lot of attentions in academia and industry due to its extraordinary physical and chemical properties. In this paper, we theoretically demonstrate a monolayer BP that achieves coherent perfect absorption (CPA) at the THz/infrared band. It is found that quasi-CPA point does exist at the THz/infrared band. The CPA, which has a relative bandwidth of 141.3% and a coherent absorptivity of more than 90%, can be implemented at the quasi-CPA wavelength through a proper phase modulation. Moreover, the coherent absorptivity can be modulated with a high modulation depth by means of the phase difference between the two coherent counter-propagating beams. The angular selectivity of the monolayer BP is also investigated. The CPA wavelength is divided into two wavelength branches for TE and TM polarization at oblique incidence. In addition, the CPA wavelength can be tuned from THz to infrared band by adjusting the electron doping of the BP while maintaining the modulation depth of 104. Hence, our results may be potentially used for coherent modulations in terahertz/infrared detections and signal processing with 2D materials.

Few-layer black phosphorus based saturable absorber mirror for pulsed solid-state lasers.

We experimentally demonstrated that few-layer black phosphorus (BP) could be used as an optical modulator for solid-state lasers to generate short laser pulses. The BP flakes were fabricated by the liquid phase exfoliation method and drop-casted on a high-reflection mirror to form a BP-based saturable absorber mirror (BP-SAM). Stable Q-switched pulses with a pulse width of 620 ns at the wavelength of 1046 nm were obtained in a Yb:CaYAlO(4) (Yb:CYA) laser with the BP-SAM. The generated pulse train has a repetition rate of 113.6 kHz and an average output power of 37 mW. Our results show that the BP-SAMs could have excellent prospective for ultrafast photonics applications.

Z-scan measurement of the nonlinear refractive index of Nd(3+), Y(3+)-codoped CaF(2) and SrF(2) crystals.

By performing the Z-scan measurements at 800 nm using a femtosecond pulsed laser, we are able to characterize the nonlinear refractive indices of Nd, Y codoped CaF(2) and SrF(2) crystals. Based on our measured results, we conclude that the doped fluoride crystal possesses a small nonlinear refractive index and the doping of Nd(3+) and Y(3+) ions in CaF(2) can change its third-order nonlinear index, but the contribution is minor. The doped fluoride crystal may have large potential to be developed as the next generation of gain material for a high-energy laser system.

Improved transfer quality of CVD-grown graphene by ultrasonic processing of target substrates: applications for ultra-fast laser photonics.

In this paper, we experimentally found that the transfer quality of CVD-grown graphene could be improved by ultrasonic processing (UP) of target substrates thanks to the improved hydrophilicity. Atomic force micrograph and Raman spectroscopy revealed that the graphene films transferred onto the target substrate with UP possess less wrinkles and defects than that of the sample without UP. The improvement technique endows graphene more suitable for photonics applications because of its weaker optical loss, higher optical damage threshold and longer stability. By integrating a fiber pigtailed graphene (treated by UP) device into a fiber laser cavity, we could obtain narrower mode-locked pulse with higher optical-to-optical conversion efficiency and better optical spectral profile, in contrast with that without UP, which further verify the improved transfer quality of graphene by the UP technique. We anticipate that this transfer technique may be applicable to boost the performance of other graphene photonics devices, such as optical modulator, detector, polarizer, etc.

Third order nonlinear optical property of Bi2Se3.

The third order nonlinear optical property of Bi2Se3, a kind of topological insulator (TI), has been investigated under femto-second laser excitation. The open and closed aperture Z-scan measurements were used to unambiguously distinguish the real and imaginary part of the third order optical nonlinearity of the TI. When excited at 800 nm, the TI exhibits saturable absorption with a saturation intensity of 10.12 GW/cm² and a modulation depth of 61.2%, and a giant nonlinear refractive index of 10⁻¹4 m²/W, almost six orders of magnitude larger than that of bulk dielectrics. This finding suggests that the TI:Bi2Se3 is indeed a promising nonlinear optical material and thus can find potential applications from passive laser mode locker to optical Kerr effect based photonic devices.

Wavelength-tunable picosecond soliton fiber laser with Topological Insulator: Bi2Se3 as a mode locker.

Based on the open-aperture Z-scan measurement, we firstly uncovered the saturable absorption property of the topological insulator (TI): Bi2Se3. A high absolute modulation depth up to 98% and a saturation intensity of 0.49 GWcm(-2) were identified. By incorporating this novel saturable absorber material into an erbium-doped fiber laser, wavelength tunable soliton operation was experimentally demonstrated. Our result indicates that like the atomic layer graphene, the topological insulator Bi2Se3 could also operate as an effective saturable absorber for the passive mode locking of lasers at the telecommunication band.

Microwave and optical saturable absorption in graphene.

We report on the first experiments on saturable absorption in graphene at microwave frequency band. Almost independent of the incident frequency, microwave absorbance of graphene always decreases with increasing the power and reaches at a constant level for power larger than 80 µW, evidencing the microwave saturable absorption property of graphene. Optical saturable absorption of the same graphene sample was also experimentally confirmed by an open-aperture Z-scan technique by one laser at telecommunication band and another pico-second laser at 1053 nm, respectively. Herein, we are able to conclude that graphene is indeed a broadband saturable absorber that can operate at both microwave and optical band.