Gold Nanoparticle Based Modulation Of Freely Propagating Terahertz Wave

Terahertz(THz)wave is the electromagnetic wave that bridges microwave and infrared light.Many superior properties had been found,such as low photon energy,transparency to nonpolar materials,excellent temporal and spatial resolution and spectral fingerprint.These characteristics make THz technology applied broadly,such as THz communication,imaging and spectroscopy.To implement them,efficient modulation to freely propagating THz wave is essential.Various THz modulators have been developed,however a series of problems have not been solved,such as high cost,low modulation depth,slow modulation speed and/or narrow modulation bandwidth.To address the above issues,this dissertation studied the modulation of freely propagating terahertz wave by using gold nanoparticles.Through the unique physical properties of gold nanoparticles,THz wave phase and amplitude modulators with broad band,high performance and low cost were fabricated.THz phase modulators were realized via gold nanoparticle-toluene mixture solution,which show characteristics of broadband,large modulation and simple operation under optical excitation.All optical semiconductor amplitude modulators were greatly improved by modifying the interface by self-assembled monolayer of spherical and rod-shaped gold nanoparticles.Optical-electrical modulated Schottky type THz amplitude modulators were enhanced by understanding the modulation mechanism,optimizing the modulation mode,and coating gold nanoparticles.The main research works are summarized as follows:1.To address the difficulties of existing THz phase modulator to achieve optical,broadband,and large modulation,a THz phase modulator was obtained using gold nanoparticle toluene mixture solution.By taking advantage of the temperature sensitive refractive index of toluene and the photo thermal effect of spherical gold nanoparticles.And the effect of nanoparticle concentration,laser wavelength and power were analyzed with experiment to optimize the modulation properties.It realized both broadband(0.3-1.5 THz)and large THz phase modulation by irradiating laser.Phase change of 390°was achieved at 1.5 THz upon 2.5 W 520 nm laser irradiation.Besides liquid crystal based THz phase modulator,this is another way to modulate THz phase with broad bandwidth and large modulation depth,and it effectively expanded the THz phase modulation style.2.To solve the low modulation depth problem,the optical THz amplitude modulation by semiconductor substrate was enhanced by coating self-assembled monolayer of gold nanoparticles.(1)Without introduction of additional insertion loss,the modulation depth after monolayer gold nanoparticle coating could reach 70 % upon500 m W laser irradiation,which is almost 4 times higher than bare silicon.(2)To overcome the drawback of narrow band light absorption of spherical gold nanoparticles,gold nanorod monolayer were used instead,which show light absorption enhancement in broader band range.Experiment and simulation were then performed to analyze the mechanism of modulation enhancement,and the influence of gold nanoparticle morphology and arrangement under different wavelength laser.As a result,the THz modulation depth is enhanced by using gold nanorod modified silicon substrate upon638-915 nm laser irradiation.(3)To further broaden the excitation laser wavelength,the silicon substrate was replaced with germanium.Under 1550 nm continuous laser irradiation at 700 m W,the modulation depth reaches 86.6 %,which is better than the performance of the germanium(66 %)and graphene modified germanium modulator(77 %)under the same excitation condition.In addition,the modulation rate can reach up to 37 k Hz,which is more than 8 times that of above silicon-based modulators.The above researches could improve the modulation depth,decrease the stimuli laser power,expand the application scope,and make practical application of semiconductor substrates in the field of THz wave amplitude modulation.3.Upon the gold nanoparticle enhanced optical modulation depth of semiconductor modulators,the enhancement effect of gold nanoparticles on Schottky diode THz amplitude modulator was further studied.(1)For graphene/silicon Schottky diode THz modulator,by analyzing the band diagram of the diode at different bias conditions,the generation,recombination,transport and accumulation of photo-excited charge carrier in the diode was analyzed upon light and electric excitation to understand the underlying THz modulation mechanism.Based on the understanding of the mechanism,the best modulation protocol was established,and better modulation depth was achieved.Besides that,gold nanoparticle monolayer was enabled to further improve the modulation properties.Compared with the literature reported results,the modulation depth,after modulation protocol optimization and gold nanoparticle modification,can be enhanced from 83 %(100 m W,-5 V)up to 94 %(100 m W,-5 V),and the modulation speed can be enhanced from 1k Hz up to 4.6 k Hz respectively.(2)Gold nanoparticle modified metal/semiconductor Schottky junction was used for THz modulation instead of the graphene/silicon hybrid diode.The optical modulation depth can be enhanced from 40 % to 80 % by coating gold nanoparticles monolayer.Under the control of continuous laser and dynamic voltage,the modulation rate can reach up to 20 k Hz,which is 4 times higher than the optimized graphene /silicon Schottky diode modulators.In addition,the metal/semiconductor Schottky junction modulator can solve the problem of graphene of unstable performance and low compatibility with IC process.The above researches provide a new avenue to optimize Schottky type semiconductor device upon optical-electrical modulation simultaneously,and could decrease the laser power effectively.