| In 2004,graphene was first successfully prepared by British scientists via mechanical exfoliation.Because of its unique two-dimensional lattice structure,the movement of electrons is limited,leading to dramatic characteristics of light,electricity,heat,magnetism,which is quite different from that in graphite.Graphene has a special energy band structure with zero-band gap,whose energy and momentum satisfy the linear dispersion relationship in K space,and possesses ultra-high electron mobility and an ultra-wide optical-electrical interaction band,leading to potential applications in the wide-band light detection,absorber,sensor,saturable absorber mirror,and so on.In this dissertation,the graphene-based device model structure of photonic crystal,metamaterial absorber,sensor and modulator are designed and the photoelectric properties,such as transmission,absorbing,filtering,modulation,etc,of these graphene functional devices in the terahertz band were studied.On this basis,the absorber was applied to the terahertz band sensor for analysis and research.The modulating property of the modulator was measured and analyzed based on THz-TDS system.In addition,graphene was used as a saturable absorber in the 1.3 μm band laser modulation and compared with the modulation characteristics of the V3+:YAG saturable absorber.The transmission properties of graphene photonic crystals with one-dimensional quasi-periodic structures(Thue-Morse and Fibonacci)were investigated using transfer matrix method in detail.Graphene photonic crystals based on Thue-Morse structure have realized a frequency-tunable bandgap of graphene photonic crystals.Compared with Bragg gap,the photonic band gap induced by graphene was insensitive to polarization and incident angle.Tunable terahertz mirrors and terahertz filters were realized in graphene photonic crystals based on Fibonacci structure.The advantage of these two structures was that the Fermi level of graphene can be changed by applying bias voltage on graphene,thus realizing flexible tuning of the omnidirectional bandgap,Bragg bandgap and the working frequency of the filter.The tunable terahertz absorber based on graphene metamaterial and its sensing application were investigated.Firstly,a graphene-based tunable polarization-independent broadband terahertz metamaterial absorber was propsed.A numerical simulation was performed,the results show that the absorption of the absorber was over 95%,and the absorption bandwidth was 1～1.64 THz,and the bandwidth reaches 0.64 THz under normal incidence.In addition,the peak absorption of the absorber can be tuned from 30%to near 100%by changing the Fermi energy of graphene from 0.1 eV to 0.5 eV.Secondly,an independently tunable complementary graphene-based dual-band metamaterial terahertz absorber was proposed.The absorption of different interdigitated structures at different graphene Fermi levels was studied and the numerical simulation results show that both of the two absorption peaks can reach up to 75%at two resonance frequency.Finally,the sensing application of the structure of the independently tunable dual-band absorber was studied,and the influence of the refractive index of the sample to be tested on frequency sensitivity and amplitude sensitivity was discussed.When the thickness of the sample to be tested was 17 μm,the refractive index amplitude sensitivity of the sensor was 57.2%/RIU,and the refractive index frequency sensitivity was 28.43 GHz/RIU.A terahertz modulator with graphene-based split gold-ring metamaterial structure was investigated.The time-domain and frequency-domain spectra of terahertz waves transmitted through SiC-based graphene samples at different bias voltages in the THz-TDS system were analyzed.By changing the Fermi energy of graphene,the terahertz wave amplitude can be tuned,obtaining a high modulation depth of about 66%when the bias voltage increases from 0 to 20 V.Finally,the modulation rate response of the graphene metamaterial modulator were measured experimentally,which reaches up to 10 MHz.The polarization absorption,emission spectra,continuous-wave(CW)laser and passive Q-switched laser with V3+:YAG of Nd:ScYSiO5(Nd:SYSO)crystal were systematically studied.It was discovered that Nd:SYSO has the obvious characteristics of anisotropy as shown in the polarization spectrum results.A maximum output power of 1.28 W with a slope efficiency of 12.7%was realized in the Nd:SYSO CW laser operated at 1.3 μm.High pulse energy and high peak power Q-switched laser output were both achieved with laser diode(LD)and V3+:YAG crystal.The maximum Q-switched pulse envelope energy obtained was 1.63 mJ,corresponding to single pulse energy and peak power of 273 μJ and 39 kW,respectively.A graphene saturable absorber(SA)mirror with excellent properties was prepared and the characteristic of 1.3 μm Nd:SYSO Q-switched output pulse based on graphene SA was studied systematically.The maximum output power of Q-switched laser obtained was 0.84 W with an output coupler of Toc=1%.The shortest pulse width and the maximum repetition rate was 111 ns and 381 kHz with an output coupler of Toc=8%,corresponding to the maximum single pulse energy and the peak power of 1.54 μJ and 1.38 W,respectively.Obviously,the pulse repetition rate of the graphene Q-switched laser was significantly larger than that of the conventional V3+:YAG Q-switched,which mainly due to the lower saturation intensity and faster electron-hole relaxation of graphene. |
