Terahertz Wave Modulation And Control Devices Based On Subwavelength Metal Slits

Due to the rapid development of sources and detectors, terahertz (THz) technologyhas attracted much attention in many applications, such as public security, medicine imag-ing, and nondestructive testing, but the functional devices for THz wave are quite scarce,especially for tiny devices of integrated THz systems. Therefore, it is very important todesign and fabricate THz devices for developing terahertz technology. Metal subwave-length slits based on surface plasmon polaritons have enhanced transmission characteris-tic. By designing the parameters of metal subwavelength slits, the enhanced transmissionin special frequencies and suppressed transmission in all other frequencies are realized.And it can also strongly restrict electromagnetic wave, which benefits to realize minia-turization devices. It has many important applications in subwavelength optics, super-resolution imaging, integrated device, and so on. By extending the researches of metalsubwavelength slits based surface plasmon polaritons into the THz band, the research anddevelopment of THz components as well as the development of micro-integrated THzsystems can be all greatly promoted.In this thesis, the amplitude and phase modulation of the THz wave through themetal subwavelength slit structures on a silicon substrate have been experimentally andnumerically investigated. The modulation of the THz wave through the metal annular slitarrays and V-shaped slits have been investigated by numerical simulation. The amplitudemodulation of the metal annular slit arrays have been measured by a THz time domainspectroscopy, The phase modulation of ultrathin THz planar elements have been demon-strated by a THz focal plane imaging system, and the dynamic modulation of the THzdevices have been validated by the optical pump-THz probe system. The main contentsare summarized as follows:The amplitude modulation of THz wave through the H-shaped annular slit arrayspunctured in a thin metal film on a silicon substrate has been investigated. In the case thatthe width of the slit is much smaller than the wavelength of incident light, the resonancefrequencies of the annular slit are not sensitive to the details of the shape, but intensely de-pend on the total circumference of the unit cell, which is interpreted by the standing-waveplasmonic resonance mechanism. Furthermore, a clear inverse proportion relationship between the resonance frequencies and the total circumference is revealed and confirmedexperimentally. It provides theory for accurate and rapid design of THz filter devices.The amplitude modulation of THz wave through the symmetrical and asymmetricalmetal annular slits arrays have been investigated both experimentally and numerically. Itis found that breaking the symmetry of annular slit by gradually changing the H-shapedannular slit to the U-shaped annular slit, the second-order resonance appears and the po-sition of the resonance peak is nearly unchanged, but the transmission of the resonancepeak is gradually enhanced. An intensity modulation depth of98.7%for the second-orderresonance can be achieved. it is found that the distribution of surface plasmon shouldsatisfy the condition of standing-wave resonance and the symmetry requirement of thesamples. Simulation results verify the experimental conclusions well. It provides a newfreedom for design of THz filter devices.The phase modulation of THz wave through metal V-shaped slits punctured in a thinmetal film on a silicon substrate has been investigated. Various ultrathin flat optical ele-ments, including a cylindrical lens, a spherical lens, and phase holograms, are designedbased on the interface phase modulation of V-shaped slits in the THz range. The focus-ing and imaging performance of the lenses and image reconstruction ability of the purephase holograms are demonstrated experimentally. On the contrary to the conventionalbulky optical elements where the curve surfaces are used to control the light propagation,the manipulations of light propagation for these thin planar optical elements are achievedthrough the designed arrays of V-shaped slits in the flat gold films with a thickness of100nm (1/4000th of the wavelength of the illuminating light). This research is a signif-icant step towards the reduction of the THz elements thickness, which benefits to realizeminiaturization and integration THz system. Meanwhile, the approach used here can beexpanded to multifarious optical elements in diferent wave bands.The dynamic modulation of the THz devices have been experimentally demonstrat-ed. Firstly, the dynamic modulation and of switching of the THz wave through the metalU-shaped annular slit arrays punctured in a thin metal film on a silicon substrate have beeninvestigated. The dielectric properties of the silicon are modulated by changing the op-tical pump power, and thus the dynamic modulation of the multi-wavelengths THz wavethrough the metal U-shaped annular slits can be realized. By changing the pump-THzdelay time, the dielectric properties of the silicon can also be modulated, and thus thedynamic of switching of the THz wave through the metal U-shaped annular slits can be realized. The experimental results indicate that the dynamic of switching can be achievedin the picoseconds time scale, as well as the dynamic of switching of multi-wavelengthsTHz wave through the metal U-shaped annular slits are simultaneously realized. Thedynamic modulation of optical intensity of ultrathin planar lenses have also been investi-gated by using of the optical pump-THz focal plane imaging system. The dielectric prop-erties of the silicon are modulated by changing the optical pump power, and thus phasechanges of the V-shaped slits are also modulated. Consequently, the dynamic modulationof the THz optical intensity on the focal plane are realized.