Qiangqiang Hu

Femtosecond solid-state laser based on a few-layered black phosphorus saturable absorber.

In this Letter, a high-quality, few-layered black phosphorus (BP) saturable absorber (SA) was fabricated successfully, and a femtosecond solid-state laser modulated by BP-SA was experimentally demonstrated for the first time, to the best of our knowledge. Pulses as short as 272 fs were achieved with an average output power of 0.82 W, corresponding to the pulse energy of 6.48 nJ and peak power of 23.8 MW. So far, these represent the shortest pulse duration and highest output power ever obtained with a BP-based mode-locked solid-state laser. The results indicate the promising potential of few-layered BP-SA for applications in solid-state femtosecond mode-locked lasers.

Bulk growth, structure, and characterization of the new monoclinic TbCa4O(BO3)3 crystal

Bulk TbCa4O(BO3)3 single crystals with diameter of 25 mm and length of 70 mm were successfully grown by the Czochralski (Cz) method for the first time to the best of our knowledge. The main problems of crystal growth were discussed and solved resulting in a high quality crystal. The crystalline perfection of the as-grown crystal was evaluated with full-width at half-maximum (FWHM) of the rocking curve being 35.05′′, and the optical homogeneity being 7.76 × 10−5. TbCa4O(BO3)3 crystallizes in the monoclinic space group Cm with unit cell parameters of a = 8.0715(7) A, b = 16.0000(15) A, c = 3.5454(3) A, and β = 101.2550(10)°. Several interesting phenomena of atom occupancy and structure distortion were observed and analysed. Furthermore, the hardness, density, absorption and transmission spectra were measured. The TbCOB crystal has a Mohs hardness value of 5.58, and it exhibits a high transparency ranging from 490 nm to 1500 nm.

The origin of coloration of CaGdAlO4 crystals and its effect on their physical properties

CaGdAlO4 (CGA) single crystals have been grown in different oxygen concentration atmospheres by the Czochralski method. Crystals grown under an oxygen-containing atmosphere presented an inconsistent brown coloration. The origin of coloration was studied by density functional theory, and the simulated results show the oxygen defects and the absorption matched well with the experimental results. The brown coloration of CGA is due to the interstitial oxygen atoms. The fundamental properties of CGA crystals with different colors, including the crystal density, hardness, thermal and optical properties, were systematically investigated and compared when taking into account the effect of color center inside. The colorless CGA was grown under an inert gas atmosphere and exhibits a wide transparency range of 227–7140 nm. The maximum phonon energy was determined to be as low as 618 cm−1. The colorless CGA host crystal with a disordered structure, good thermal conductivity, low maximum phonon energy and wide transparency is promising for ultrafast ultraviolet and visible laser applications.

An extended application of β-Ga2O3 single crystals to the laser field: Cr4+:β-Ga2O3 utilized as a new promising saturable absorber

Considering their easy growth and doping in bulk crystal growth, and good crystalline quality, β-Ga2O3 single crystals, a very important wide-bandgap semiconductor, are now also considered to be a promising optical crystal candidate. In this work, a Cr4+:β-Ga2O3 single crystal has been grown successfully by the edge-defined film-fed growth method. The thermal conductivity of Cr4+:β-Ga2O3 has been measured along the a* direction at room temperature obtaining 16.2 W m−1 K−1, and it was much larger than that of the usually used Cr4+-doped crystals, such as Y3Al5O12 (YAG) or YVO4. The Raman spectrum indicated that the cutoff phonon energy of the β-Ga2O3 crystal was 767.8 cm−1. A passively Q-switched nanosecond pulsed Nd:YAG laser based on a Cr4+:β-Ga2O3 saturable absorber was experimentally demonstrated for the first time to our knowledge and its mechanism is explained by first-principles calculations. By inserting the Cr4+:β-Ga2O3 crystal into the Nd:YAG laser cavity, a Q-switched laser operation was obtained with a maximum average output power of 50 mW. The corresponding pulse repetition rate, pulse width, and pulse energy were determined to be of 421.5 kHz, 235.2 ns and 0.12 μJ, respectively.

Crystal growth and optimization of Pr:CaGdAlO4 by the flux-Czochralski method

Pr:CaGdAlO4 (Pr:CGA) crystals have been grown and investigated for the first time. A Na+ flux was designed to improve the crystal quality in the Czochralski growth process. The effect of the addition of the Na+ flux on crystal quality has been investigated, and its beneficial impact on the crystallinity, optical homogeneity and thermal properties of the grown crystals has been demonstrated. The optical spectra of Pr:CGA have been measured in order to assess its potential for photonics applications. The intensities of the absorption transitions have been analyzed by means of the Judd–Ofelt approach and the intensity parameters, the branching ratios and the radiative lifetimes of the emitting states have been evaluated. These have been compared with the experimental values obtained from pulsed light measurements. All the properties indicate that the Pr:CGA crystal has great prospects in the applications of visible lasers.

Crystal growth and spectral broadening of a promising Yb:CaLuxGd1−xAlO4 disordered crystal for ultrafast laser application

Yb:CaLuxGd1−xAlO4 (Yb:CLGA) crystals have been grown and studied for the first time. A more disordered structure than that of the Yb:CaGdAlO4 (Yb:CGA) crystal was designed by introducing Lu3+ ions into the crystal. The absorption spectra at 9.6 K provided clear evidence for the disordered structure of the Yb:CLGA crystal. Moreover, further spectral broadening was achieved compared to that of the Yb:CGA crystal. The crystal quality and spectroscopic properties of the Yb:CLGA crystal with the highest degree of disorder were investigated in detail. Improved ultrafast laser oscillators, in terms of narrower pulse width and higher peak power, are expected with this modified crystal.

One-step exfoliation of ultra-smooth β-Ga2O3 wafers from bulk crystal for photodetectors

High-quality bulk β-Ga2O3 single crystals have been grown by optimized edge-defined film-fed growth (EFG) method. The problems of cracking and polycrystals have been effectively solved by using a high-quality 1 inch-wide seed. The crystalline quality of the as-grown crystal has been confirmed by X-ray rocking curve with a full-width at half-maximum (FWHM) of 42.1 arcsec. Moreover, an economical and efficient one-step mechanical exfoliation method has been proposed to get epi-ready β-Ga2O3 wafers directly. The root mean square (RMS) roughness of the wafers is noted to be around 0.1 nm, which means they are comparable or even better than wafers processed by chemical mechanical polishing (CMP). The mechanism of the exfoliation method is analyzed from a crystallographic view. A photodetector is fabricated on the exfoliated wafer directly and the device is proved to show good performance in solar-blind band.