β - Ga 2 O 3 used as a saturable absorber to realize passively Q - switched laser output

: β - Ga 2 O 3 crystal have attracted great attentions in the fields of photonics and photoelectronics because of its ultra wide - band gap and high thermal conductivity. Here, pure β - Ga 2 O 3 crystal was successfully grown by optical floating zone (OFZ) method, and used as saturable absorbers to realize a passively Q - switched all - solid - state 1μm laser for the first time. By placing the as - grown β - Ga 2 O 3 crystal into the resonator of Nd:GYAP solid - state laser, a Q - switched pulses at the center wavelength of 1080.4 nm are generated under a output coupling of 10%. The maximum output power is 191.5 mW while the shortest pulse width is 606.54 ns, and the maximum repetition frequency is 344.06 kHz. The maximum pulse energy and peak power are 0.567 μJ and 0.93 W, respectively. Our experimental results show that β - Ga 2 O 3 crystal has great potential in the development of all - solid - state 1μm pulsed laser.


Introduction
It is well known that saturable absorber plays an important role in Q-switching and mode locking operation [1][2][3][4]. Therefore, the development of different kinds of saturable absorbers as passive Q-switching devices to achieve high-quality pulsed laser output has always been a hot research field. At present, many studies on saturable absorbers are in full swing, such as dyes, bulk semiconductors, two-dimensional materials and transition metal ion doped crystals [5][6][7][8]. The pulsed laser realized by some of the materials has made important application prospects in industrial processing, high-energy laser, scientific research and so on [9][10][11]. Especially in ~1μm near-infrared laser, which has the advantages of high pulse energy and high peak power, can be widely used in space communication, nonlinear spectroscopy, biomedicine, military and many other fields [12][13][14]. However, traditional materials often have their own shortcomings, such as limited types, single wavelength, and long-term operation stability to be improved. Therefore, how to develop a stable, reliable and efficient new saturable absorber for application in ~1μm near-infrared band is a problem worthy of further discussion.
Ga2O3 is a semiconductor material with ultra wide-band gap (~ 4.8ev), high conductivity [15][16]. Therefore, Ga2O3 is an electronic and optical material with great potential.
Because of its unique physical and chemical properties, it has received great attention from researchers in different aspects, so it has been applied in many fields, including photo-detectors, photo-catalysis, field effect transistors and so on [17][18][19][20]. Ga2O3 is polymorphic, similar to Al2O3, is particularly interesting in applications, β-Ga2O3 has monoclinic structure and is the most stable phase, both physically and chemically [21][22].
β-Ga2O3 also inherits the excellent physical and chemical properties common to all phases of Ga2O3 [23][24]. These excellent properties make us see that β-Ga2O3 has great potential for application in saturable absorbers. But as far as we know, there are still no reports on the application of pure β-Ga2O3 crystal to saturable absorbers, and even the application of other oxide materials to saturable absorbers is rarely reported.
In terms of β-Ga2O3 crystal growth method, the common large-size crystals growth method is melting method, including Czochralski method, EFG method, Bridgman method and so on [25][26][27]. We have successfully grown high-quality β-Ga2O3 by optical floating zone (OFZ) method. This method has the advantages of simple operation, low equipment requirements, and can grow even in the air environment. It solves many technical problems such as complex equipment, difficult operation, easy introduction of impurities, unable to guarantee the growth quality and so on [28][29]. We systematically characterized the chemical and optical properties of the synthesized β-Ga2O3. The crystal is of good quality which is pure and crack free. At the same time, we realized the optical modulation of the β-Ga2O3 in the pulse laser generation for the first time on the laser device with b-cut Nd:GYAP(Nd:Gd0.1Y0.9AlO3) as the laser medium [30]. The maximum average output power is 195.1 mW which is obtained at 1080.4 nm. The corresponding shortest pulse duration is 606.54 ns and the maximum pulse repetition rate is 344.06 kHz.
The maximum single pulse energy is 0.567 μJ and the maximum peak power is 0.93 W.
From the experimental results, we have obtained a relatively stable pulsed laser with short pulse width and large repetition frequency, which shows that the β-Ga2O3 has good saturable absorption properties. We believe that our work will provide an important reference for the potential applications of nonlinear optical devices related to crystal growth and optical modulation.

The preparation and characterization of the β-Ga2O3
The β-Ga2O3 single crystal was grown by the optical floating zone (OFZ) method using a Quantum Design IRF01-001-00 infrared image furnace. Ga2O3 powder (purity 99.9999 %) was employed as the raw material. The raw material was pressed into rod using a cold isostatic press. The rod was subsequently sintered at 1400C for 10 h in air. And a <010> oriented crystal was used as the seed. Growth was carried out using Quantum Design IRF01-001-00 infrared image furnace. The sintered rod and seed were rotated at 10 rpm in opposite direction, and the crystal was grown in flowing air at the speed of 6 mm/h.  The X-ray rocking curve was measured using a Bruker D8 Discover X-ray diffractometer with a Cu Kα line at 40kV and 40mA. The optical transmittance spectrum was collected using a Lambda 1050+ UV/vis/NIR spectrometer (PerkinElmer). Figure.2 shows the X-ray rocking curve of the β-Ga2O3 (400) plane. The full-width at half-maximum (FWHM) is 100.8arcsec. It shows that the β-Ga2O3 crystal is the single crystal with good crystallization quality. Figure.

Experimental results and discussion
In order to study the saturable absorption characteristics of β-Ga2O3 SA in the 1μm wavelength region, a passively Q-switched laser composed of Nd:GYAP laser crystal and β-Ga2O3 SA was constructed as Figure. 4. The crystal was cut to 4 × 4 × 5 mm cuboid, along the b axis. The pump source is a 808 nm fiber-coupled semiconductor laser diode (LD), and the core diameter is 400 μm whose aperture is 0.22. Using an optical imaging system (1:1 imaging module), the spot radius of the pump laser beam focused on Nd: GYAP crystal is 200 μm. The resonator uses an input mirror M1 with high reflection from 1050 nm to 1100 nm and high transmittance from 800 nm to 820 nm, and an output mirror M2 with 10% transmittance from 1050 nm to 1100 nm. β-Ga2O3 SA is inserted between M2 and gain crystal. During normal operation, the Nd:GYAP crystal is wrapped in indium foil and maintained at 17 ° C by a chiller to minimize the thermal lens effect.    Based on the average output power, pulse width and repetition frequency, the corresponding single pulse energy and peak power of Q-switched laser are calculated.
The single pulse energy and peak power increase with the increase of pump power, which proves that our laser output is Q-switched mode rather than relaxation oscillation mode. When the pump power is 3.75 W, the maximum single pulse energy is 0.567 μJ.
The maximum peak power is 0.93 W. We also measured the central wavelength of laser emission. Figure.

4.Conclusion
In summary, we have successfully grown high-quality β-Ga2O3 by OFZ method, and firstly used them as saturable absorbers to realize the output of pulsed laser. The synthetic method is simple and practical, with low cost and low environmental requirements, and the grown crystal is pure and crack free. At the same time, the β-Ga2O3 crystal is applied to Nd:GYAP solid-state laser for the first time, and the pulsed laser output is realized. The maximum average output power of 195.1 mW is obtained at 1080.4 nm. The corresponding minimum pulse width is 606.54 ns and the maximum pulse repetition frequency is 344.06 kHz. Our results will promote the research of more Q-switched crystals and expand their potential applications in the field of ultra-fast photonics.