| Terahertz (THz) radiations, also known as far infrared waves or submillimeterelectromagnetic waves, are in the range between microwave and infrared, with richinformation, sub picosecond pulse width, high space time coherence, and low photonenergy. In recent years, with the development of terahertz technology, terahertzscience in wireless communications, imaging, radar, and defense security is graduallyshowing a very broad application prospects. Effective manipulation on THz waves isthe basis for terahertz technology in many applications. For example, temporallyshaped pulse trains will be needed for communications, signal processing andmaterials control, broadband pulses for remote sensing and materials characterization,and tunable narrow band radiation for enhanced imaging. Since terahertz science isstill under development, the modulation on THz waves is extremely limited, whichgreatly hindered the application of terahertz technology. In this thesis, the terahertzwave modulation techniques are proposed and studied, which is summarized asfollows:（1）Magnetic field modulation on terahertz radiationThe generation and detection of narrowband terahertz wave based on photorefractiveperiodically poled lithium niobate (PPLN) and periodically poled Mg: LiNb3(PP Mg:LN) crystal are realized by optical rectification. By changing the size and direction ofthe applied magnetic field, the effective modulation on the amplitude of thenarrowband terahertz wave is achienved in PPLN crystals. With the increase of theapplied magnetic field, the amplitude of narrowband THz wave in the PPLN crystal issupressed. When the applied magnetic field is sufficiently strong, narrowband THzwave in the PPLN crystal will be completely suppressed.（2）Optical excitation modulation on terahertz radiationUsing optical pump terahertz probe technology, changing the carrier concentration ofthe sample, we achieved the effective modulation of the THz transimition/reflectionby changing the intensity of pumping light. Two kinds of samples are involved, one is a bulk semiconductor, and the other is a semiconductor thin film. For bulksemiconductor, with the pump beam at800nm, we achieved the effective modulationon the ampiltude and phase of THz wave, and the secondary reflection peak of THz.With varing the pump intensity, the100%modulation of the broadband THztime domain spectroscopy can be achieved. For thin film semiconductor, with a pumpbeam of400nm, we successfully realized the modulation of THz transimitionspectroscopy. The modulation time is about5ps. And the modulation depth is morethan60%. This modulation opens a new idea in the application of ultrafast switch inTHz wave.（3）Terahertz wave coherent control of surface plasmon polariton (SPP)We demonstrate the terahertz coherent control of extraordinary transmission throughsubwavelength metallic hole arrays with double terahertz pulses. The interference ofexcitations of surface plasmon polaritons (SPPs) by two THz pulses sequence isemployed to control the propagation of SPPs in the periodical structure. The THzwave transmission is controllable by adjusting the delay time of the two THz pulses.When time interval of two THz beams is an integer multiple of the SPP cycle, SPPamplitude is strengthened, and its intensity is twice the original. When time interval oftwo THz beams is the odd multiples of the half cycle of the SPP, the SPP amplitude issuppressed, and its strength is substantially zero. By coherent control experiments caneffectively control the intensity of the terahertz SPP. It provides a universal ultrafastmeans to control the SPPs motion in various THz subwavelength structures. And alsofor the further realization of THz "0","1" encoding has laid a good foundation, whichis a new method for terahertz waves in communication and information processing. |
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