Tian Jiang

Characterization of nonlinear properties of black phosphorus nanoplatelets with femtosecond pulsed Z-scan measurements.

The nonlinear properties of black phosphorus (BP) nanoplatelets (NPs) have been characterized with Z-scan measurements under 800-nm femtosecond pulsed laser excitation. A transition from saturable absorption (SA) to reverse saturable absorption (RSA) with the increase of laser intensity was observed in the open-aperture (OA) measurements. Simultaneously, closed-aperture (CA) measurements were carried out to investigate the nonlinear refractive index of BP NPs together, and a value of n(2) ≃(6.8±0.2)×10(-13) m2/W was obtained. The nonlinear absorption properties were analyzed according to the band structure of BP. A theoretical analysis based on SA and two-photon absorption (TPA) was used to determine the nonlinear absorption coefficients from the experimental results, and the TPA coefficient at 800 nm was estimated about (4.5±0.2)×10(-10) m/W.

Nanosecond passively Q-switched thulium/holmium-doped fiber laser based on black phosphorus nanoplatelets

The generation of nanosecond pulses in a thulium/holmium-doped fiber laser passively Q-switched by a black phosphorus saturable absorber (BP-SA) was experimentally demonstrated. The high quality BP-SA with a modulation depth of ~24% was fabricated by depositing ~23 nm thickness of BP nanoplatelets onto a fiber ferrule. By inserting the BP-SA into a thulium/holmium-doped fiber laser cavity, stable Q-switched operation was achieved with the maximum pulse energy up to 632.4 nJ, the shortest pulse width of 731 ns and pulse repetition rates varying from 69.4 to 113.3 kHz. These results suggested that BP could be developed as an effective SA for pulsed laser operation at 2 μm.

Z-scan measurement of the nonlinear refractive index of monolayer WS 2

Transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS(2)), are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Recent advances in nanoscale materials characterization and few layer TMDCs unique optical properties make them a research hot-spot in nonlinear optics. In this work, the nonlinear refractive index of monolayer WS(2) has been characterized with Z-scan measurement under 800nm femtosecond pulsed laser excitation, and a value of n2 ≃ (8.1 ± 0.41) × 10(-13)m(2)/W is obtained. A shift from saturable absorption to reverse saturable absorption was observed at higher input pump intensities in the experiments. The transition process was analyzed using a phenomenological model based on two photon absorption, and the two photon absorption coefficient was estimated about (3.7±0.28)×10(-6)m/W.

Thulium/holmium-doped fiber laser passively mode locked by black phosphorus nanoplatelets-based saturable absorber.

By coupling black phosphorus (BP) nanoplatelets (NPs) with a fiber-taper evanescent light field, a saturable absorber (SA) based on the BP NPs has been successfully fabricated and used in a thulium/holmium-doped fiber laser as the mode locker. The SA had a modulation depth of ∼9.8% measured at 1.93 μm. A stable mode-locking operation at 1898 nm was achieved with a pulse width of 1.58 ps and a fundamental mode-lock repetition rate of 19.2 MHz. By increasing the pump intensity, phenomena of multi-pulsing operations, including harmonic mode-locked states and soliton bunches, were obtained in the experiment, showing that the BP NPs possess an ultrafast optical response time. This work suggests that the BP NPs-based SA is potentially useful for ultrashort, pulsed laser operations in the eye-safe region of 2 μm.

Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications

Fiber based supercontinuum (SC) sources with output spectra covering the infrared atmospheric window are very useful in long-range atmospheric applications. It is proven that silica fibers can support the generation of broadband SC sources ranging from the visible to the short-wave infrared region. In this paper, we present the generation of an ultrahigh-brightness spectrally-flat 2-2.5 μm SC source in a cladding pumped thulium-doped fiber amplifier (TDFA) numerically and experimentally. The underlying physical mechanisms behind the SC generation process are investigated firstly with a numerical model which includes the fiber gain and loss, the dispersive and nonlinear effects. Simulation results show that abundant soliton pulses are generated in the TDFA, and they are shifted towards the long wavelength side very quickly with the nonlinearity of Raman soliton self-frequency shift (SSFS), and eventually the Raman SSFS process is halted due to the silica fibers infrared loss. A spectrally-flat 2-2.5 μm SC source could be generated as the result of the spectral superposition of these abundant soliton pulses. These simulation results correspond qualitatively well to the following experimental results. Then, in the experiment, a cladding pumped large-mode-area TDFA is built for pursuing a high-power 2-2.5 μm SC source. By enhancing the pump strength, the output SC spectrum broadens to the long wavelength side gradually. At the highest pump power, the obtained SC source has a maximum average power of 203.4 W with a power conversion efficiency of 38.7%. It has a 3 dB spectral bandwidth of 545 nm ranging from 1990 to 2535 nm, indicating a power spectral density in excess of 370 mW/nm. Meanwhile, the output SC source has a good beam profile. This SC source, to the best of our knowledge, is the brightest spectrally-flat 2-2.5 μm light source ever reported. It will be highly desirable in a lot of long-range atmospheric applications, such as broad-spectrum LIDAR, free space communication and hyper-spectral imaging.

The over-saturation phenomenon of a Hg0.46Cd0.54Te photovoltaic detector irradiated by a CW laser

A Hg0.46Cd0.54Te (~0.91 eV bandgap) photovoltaic detector is exposed to 10.6 ?m and 1319 nm intense laser radiations. The open-circle voltage (Voc) of the detector is measured and found to decrease with laser power density above the saturation threshold. The characteristics of over-saturation phenomena are quite sensitive to the temperature. For the Hg0.46Cd0.54Te detector, the photon energy of a 10.6 ?m laser (sub-bandgap laser) is less than the bandgap, and the photon energy of a 1319 nm laser (above-bandgap laser) is above the bandgap. Both lasers can induce an over-saturation phenomenon, but the dominant mechanism is different. The results indicate that photovoltage and thermoelectric voltage are the dominant mechanisms for above-bandgap illumination, and thermovoltage and thermoelectric voltage are the dominant mechanisms for sub-bandgap illumination.

Effects of thermally generated carrier and temperature dependence mobility in InSb photoconductive detector under CW 10.6 µm laser irradiation

The response mechanism of n-type indium antimonide photoconductive detectors under intense continuous wave (CW) 10.6 µm laser irradiation is investigated. The magnitude of the Voc signals and the exact shape of the signals vary greatly with laser power density and irradiation time. It is found that the signals begin to decrease at a critical time when laser power density is held in a constant value. If the irradiation time is fixed, the signals begin to decrease in a critical laser power density. The Voc signals for both radiation process and the end of laser irradiation have two different response time scales. A two-dimensional model of the detector for CW laser irradiation is presented. The calculated response curves agree well with the experimental results. The two separate time scales are found to be due to two thermally resistive bonding layers. Temperature-dependent mobility is the domain mechanism for the increasing tendency of Voc signals, and the decreasing tendency results from thermally generated carrier effect.

Study of photovoltaic effect photovoltaic HgCdTe detector irradiated by spectral unrelated laser

A spectral unrelated laser was induced on the short wave and medium wave photovoltaic HgCdTe infrared detectors respectively, under various power density, the detectors had a serious of output. The results show that the two detectors all had response of spectral unrelated laser, but the response was in the opposite direction. Through the analysis of experimental phenomena, it is found that carrier concentration of detectors before laser irradiation is the reason why the response of detectors was in the opposite direction. It showed valuable clue for future research on photovoltaic HgCdTe irradiated by spectral unrelated laser.

Optically controlled terahertz modulator by liquid-exfoliated multilayer WS_2 nanosheets

Lack of efficient routes to modulate the propagation properties of the terahertz (THz) wave is a major barrier for the further development of THz technology. In recent years, two dimensional transition metal dichalcogenides (2D TMDCs) were applied to the design of effective THz modulator by forming heterostructure with Si. Here, we experimentally demonstrate an optical controlled THz modulator consisting of liquid-exfoliated WS2 nanosheets and a silicon substrate (WS2-Si). By innovatively depositing liquid-exfoliated WS2 nanosheets on the Si instead of growing by chemical vapor deposition (CVD) method, both of the size and the thickness of WS2 film is controlled. The WS2-Si sample presents a flat modulation depth from 0.2 THz to 1.6 THz. The modulation depth reaches 56.7% under a 50 mW pumping power, which is over 5 times enhanced compared with that of the Si substrate. With the increase of illumination power, the modulation depth continues to increase, finally reaching up to 94.8% under 470 mW. Besides, the WS2-Si sample also achieves ~80% modulation depth under 450 nm illumination, indicating its ability to operate under either of wavelength in visible spectra. Moreover, we compare the sample to the reported modulators including CVD growth TMDCs-Si ones and find our sample has comparable modulation effects while is much easy to be prepared. Therefore, we believe our work is meaningful to provide an alternative route to achieve effective modulation of THz waves by adopting liquid-exfoliated 2D materials.

The output of photovoltaic detector irradiated by spectral unrelated laser

The composition of open-circuit voltage of photovoltaic detector under irradiation of spectral unrelated laser is analyzed. The structure of detector, the energy band of PN junction, temperature gradient in the detector and the movement of carriers are investigated to explain the mechanism of thermovoltage. For illumination, the detector whose irradiation surface is n-type or p-type semiconductor is taken as an example to make an analysis of the composition of open-circuit voltage. Results show that, the open-circuit voltage of photovoltaic detector under irradiation of spectral unrelated laser is decided by thermovoltage and temperature difference-EMF (Electric Motive Force). Moreover, the voltage directions of them are opposite no matter what region (p or n) is taken as irradiation surface.