| Over the past fifty years.lasers have been widely developed and used in many fields, such as national defense, energy source, medical treament and scientific research. According to the operation mode,lasers are generally divided into continuous-wave (cw) lasers and pulse lasers. Due to the shorter pulse width, larger pulse energy and higher peak power, pulse lasers are more attractive in the transient process study and strong field light physics. Nowadays, the development direction of pulsed lasers, based on Q-switching and mode-locking technologies, is to be ultra-fast and ultra-strong, and broaden the application bands. Q-switched lasers are easy to obtain large pulse energies, hence have important applications in the fields of industrial process, and remote sensing detection. Mode-locked lasers are easy to get shorter pulse widths, and therefore, are of wide applications in the research fields of the transient process, such as, in-situ detection, complex dynamics analysis and ultra-fast spectral investigation.On account of the simple, compact and low-cost structure design, passive pulse modulation attracts more and more research attention in the field of low-power and medium-power pulse lasers. Passive optical switches are the important component in passive pulse modulation, and its performance directly influences the output quality of the pulse lasers. In recent years, the functional materials are developing toward low-dimensional, composite and integrated, and people go on exploring the optical switch materials, which are of the wide response wave band, high damage threshold, low optical loss and environmentally friendly property. For the composite and integrated development trends, the thesis first develops three-dimensional (3D) Cr4+:Y3Ga5O12 (YGG) and Cr4+:Y3Sc1.5Ga3.5O12 (YSGG) passive optical switches. Then, by exploiting the Nd:YGG and Yb:YSGG (0-100 at.%) gain media, the Nd,Cr4+:YGG self-Q-switched crystal, which plays the role of gain and loss at the same time, is developed. For the low-dimensional development trend, the two-dimensional (2D) MoS2 passive optical switch with broadband light response, environmentally friendly one-dimensional (1D) Au NRs passive optical switch and zero-dimensional (0D) Au NSs passive optical switch are developed. The main contents are as follows: 1.3D Cr4+:YGG and Cr4+:YSGG passive optical switchesCr4+:Y3Al5O12 (YAG) is widely used in passive pulse modulation around 1μm. However, the relatively small effective segregation coefficient of tetrahedral Cr4+ ions in Cr4+:YAG (0.02) is not beneficial for the integration and miniaturization of the devices. Compared to YAG, YGG and YSGG have bigger lattice constants and larger tetrahedral spaces, which are beneficial for doping Cr4+ ions. Moreover,YGG and YSGG show lower phonon energies than that of YAG, and hence the multiphonon processes are effectively suppressed. On the basis of developing Cr4+:YGG and Cr4+:YSGG crystals, this part also exploits the gain media, including Nd:YGG and Yb:YSGG (0-100 at.%) crystals, and Nd,Cr4+:YGG self-Q-switched crystal. The specific results are as follows:High-quality the Cr4+:YGG,Cr4+:YSGG,Nd:YGG,Yb:YSGG and Nd,Cr4+:YGG single crystals have been successfully grown by the optical floating zone (OFZ) method. The XRD analyses indicate that the crystal structure of all crystals belongs to the space group of laid. Besides, through the YSGG structure refinement, it is worth noting that introducing the Sc3+ ions could reduce the dodecahedra distortion and chemical stress from the doping of active ions.For the Nd:YGG crystal, Nd3+ ions shows a larger effective segregation coefficient (0.56) than that of Nd:YAG (0.18). For the Yb:YSGG crystals, the effective segregation coefficients of Yb3+ and Sc3+ions are close to 1. In the OFZ crystal growth, the effective segregation coefficient of doped ions near 1 is capable of maintaining the solid-liquid interface stability and reducing the influence of supercooling degree in the melting zone from the elemental precipitate.The room-temperature (RT) thermal conductivity of Nd:YGG is 8.53 W/mK, which is near to that of pure YGG (9 W/mK). For the Yb:YSGG crystals, the RT thermal conductivities of YSGG and Yb3Sc1.5Ga35O12(YbSGG) are 6.543 W/mK and 4.425 W/mK, respectively. Due to the serious structural disorder, the 50 at.% Yb3+-doped crystal shows the lowest RT thermal conductivity (3.708 W/mK).The absorption coefficient and full width at half-maximum (FWHM) of Nd:YGG around 808 nm are 2.8 cm-1 and 9 nm, respectively. The FWHM of Yb:YSGG crystals around 930 nm and 970 nm are 30 nm and 6-7 nm, respectively. The wide absorption range would reduce the influence of the instability of pump wavelength from semiconductor LDs. Besides, Cr4+:YGG,Cr4+:YSGG and Nd,Cr4+:YGG all have broadband absorption (300 nm) around 1 μm, which is suitable for the passive pulse modulation of Nd3+-doped and Yb3+-doped materials.Compared with Nd:YAG, Nd:YGG has the lower emission cross section (8.25× 10-20 cm2 at 1060 nm) and larger fluorescence lifetime (265μs). Yb:YSGG (5 at.%) also shows a lower emission cross section (1.5×10-20 cm2 at 1025 nm) and larger fluorescence lifetime (1.06μs) than those of Yb:YAG and Yb:YGG. Therefore, Nd:YGG and Yb:YSGG have the strong ability of energy storage, and are promising in the passively Q-switching lasers.The measured data show that Cr4+:YGG and Cr4+:YSGG have large nonlinear refractive indices (5.12×10-13 (esu) and 6.2×10-13 (esu), respectively), which is very favorable for the generation of ultrafast pulsed lasers by the Kerr-lens mode locking method. Moreover, in comparison to Cr4+:YAG, they show the bigger effective segregation coefficients of Cr4+ ions (0.02 and 0.23, respectively), comparable ground-state absorption cross sections(3.31×10-18 cm2 and 1.56×10-18 cm2, respectively) and larger excited-state absorption cross sections (2.31×10-18 cm2 and 6.30×10-19 cm2, respectively).The passively Q-switched laser at 1.06μm is performed with a Nd:YGG crystal as the gain medium and a Cr4+:YGG or Cr4+:YSGG crystal as the saturable absorber. For Cr4+:YGG and Cr4+:YSGG, the shortest pulse width is 5.3 ns and 3.5 ns, respectively, and the largest pulse energy is 55.3 μJ and 53.3 μJ, respectively. For the cw lasers of 5 at.% and 10 at.% Yb3+-doped YSGG crystals, the largest output powers are 7.9 W and 6.11 W, respectively, corresponding to the slope efficiencies of 64% and 80.1%, respectively, and the optical-to-optical efficiencies of 51% and 64.2%, respectively. The laser emission spectra of the cw laser of Yb:YSGG would shift to shorter wavelengths as the transmission of the output coupler is increased.By use of a Cr4+:YGG crystal as the saturable absorber, the passively Q-switched laser of Yb:YSGG is also investigated.The obtained shortest pulse width and largest pulse energy are 42.9 ns and 49.58 μJ, respectively. The shortest pulse width and largest pulse energy of the Nd,Cr4+:YGG self-Q-switched laser at 1.06 μm are 8.7 ns and 21.7 μJ, respectively.2.2D MoS2 passive optical switchLayered MoS2, a typical representative of the layered transition metal dichalcogenides family, was brought into various research fields,such as lubrication, hydrodesulfurization catalysis, supercapacitors, and field-effect transistors. Whether it is thin to atomic thickness (1.8 eV) or bulk (0.86-1.29 eV),layered MoS2 has a large bandgap. And therefore, it is impossible to obtain the broadband pulse modulation, as demonstrated in graphene or topological insulators. Based on the band-gap engineering, this part successfully fabricates the MoS2 optical switch with broadband optical optical response, and realizes the pulse modulation from 1.06 μm to 2.1 μm. The main contents are as follows:First, by virtue of the first-principle theoretical calculations, we systematically analyze the band-gap change of multilayer MoS2 in a theoretical analysis with the ratio (R) between Mo and S atoms slightly deviating from 1:2. The energy gap of MoS2 is 0.08 eV,0.23 eV,0.48 eV,0.63 eV and 0.26 eV for the R with a value of 2.09,2.04,1.97, 1.94, and 1.89, respectively. The calculated data indicate that the appropriate introduction of defects would reduce its bandgap, which is beneficial to expand its optical response.Then, the MoS2 sample (600 pulses) is fabricated by the pulse laser deposition (PLD) method. The Raman investigation shows the sample belongs to 2H-MoS2 crystal struture, which coincides well with the former theoretical calculation. The XPS data indicate R lies in the range 1.89-1.97 with moving from the center to the edge of the sample. Based on the theoretical analysis, the fabricated MoS2 sample would have a small bandgap, and the measured absorption spectrum further verifies the MoS2 sample has wide optical response. Saturable absorption of the as-grown MoS2 sample is measured at 1.06μm, The modulation depth (δT) and saturation intensity (Is) of the MoS2 sample are calculated to 27% and 2.45 GW/cm2, respectively.Using the MoS2 sample as the saturable absorber, passively Q-switched lasers at wavelengths of 1.06,1.42, and 2.1μm are successfully operated with Nd:GdVO4, Nd:YGG and Tm,Ho:YGG crystals as the gain material, respectively. The shortest pulse width is 970 ns,729 ns and 410 ns, respectively, and the largest pulse energy is 310 nJ,670 nJ and 1380 nJ, respectively. The obtained broadband pulse modulation of MoS2 shows that the bandgap engineering is significant in the exploration of new semiconductor materials’photonics and optoelectronics.3.1D Au NRs passive optical switch and 0D Au NSs passive optical switchSurface plasmon resonance of Au nanoparticles (NPs), such as Au NSs, Au NRs, Au nanocubes and Au nano-octahedra, is ascribed to the strong coupling between the free electrons in the conduction band and the external electromagnetic field. With strong optical irradiation, nonlinear optical response of the surface plasmon resonance would be generated. Saturable absorption (SA) and reverse saturable absorption (RSA) are two common types of nonlinear absorption, and they show the decreasing and increasing trends in the relationship between absorption and optical intensity, respectively. Au NPs not only have SA, but also are of RSA, and the physical mechanism is not clear so far. By surveying the nonlinear absorption of Au NSs at 1.06μm, it is worth noting that SA and RSA are related to the third and fifth-order nonlinearities, respectively. Along with an increase in optical intensity, compared to SA from the third-order nonlinearity, RSA from the fifth-order nonlinearity becomes more obvious. When the optical intensity is above 5 GW/cm2,this phenomenon eventually leads to a change in the major nonlinear absorption from SA to RSA.To avoid the RSA influence as much as possible, saturable absorption of Au NSs is investigated at the lower pulse energy of 2.7 μJ with a maximum optical intensity of 1.92 GW/cm2. δT and IS are determined to be 18.6%and 36.7 MW/cm2, respectively. The passively Q-switched laser at 1.06 μm is obtained with Au NSs as the saturable absorber and the Nd:GdVO4 crystal as the gain material. The shortest pulse width and largest pulse energy are is 491.5 ns and 66.7 nJ, respectively.With the maximum optical intensity below 3.5 GW/cm2, the saturable absorption properties of Au NRs (LSPR around 0.6μm) are investigated at the wavelengths of 605 nm,639 nm, and 721 nm, respectively. δT is determined to be 7.3% at 605 nm, 10.6% at 639 nm, and 5.9% at 721 nm, and IS is calculated to be 49.6 MW/cm2 at 605 nm,75.0 MW/cm2 at 639 nm, and 49.5 MW/cm2 at 721 nm. Then, using the Au NRs as the optical switch and the Pr:GdLiF4 crystal as the gain material, The passively Q-switched lasers are presented at the wavelengths of 605 nm,639 nm and 721 nm, respectively. The shortest pulse width is 237 ns,152 ns and 318 ns, respectively, and the largest pulse energy are is 81.5 nJ,64.0 nJ and 51 nJ, respectively. In addition, employing the Au NRs around 1 μm and 1.4 μm, the passively Q-switched lasers in Nd:LuVO4(1.06 μm) and Nd:YGG (1.42 μm) are realized, respectively. The shortest pulse width is 322.4 ns at 1.06 μm and 407 ns 1.42 μm, and the largest pulse energy is 180.2 nJ at 1.06 μm and 747.3 nJ at 1.42 μm.To sum up,3D Cr4+:YGG and Cr4+:YSGG passive optical switches have a good application prospect in the development of integrated,miniaturized and functional composite laser devices around 1μam.2D MoS2 and Au NPs passive optical switches have vital potential applications in the exploitation of broadband and environmentally friendly low-dimensional saturable absorption materials. |
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