| Since the invention of terahertz(THz)time-domain spectroscopy,THz technology has gradually become one of the most promising and valuable research directions in areas including science,economy and national security.However,at present,the development of THz technology is still restricted by the lack of functional devices.Moreover,the device is large,and there also exist problems such as phase mismatch,large space occupied by the free-space optical path,and so on.Through the interaction between electromagnetic waves and electrons in conductors,surface plasmon polaritons(SPPs)make it possible to localize and manipulate the THz field at the sub-wavelength level.The development of on-chip THz systems based on SPPs is considered an important way of pushing THz devices to smaller,more compact and multi-functional ones.To build the next generation of information and communication carriers,and realize on-chip transmission,switching,beam splitting,tuning,filtering,coupling,and integration of THz signals,there are many more complex functional devices to be studied.Therefore,a series of on-chip THz devices based on SPPs are investigated in this dissertation.The main contents are listed below.(1)Curved THz surface plasmonic waveguide devices.A series of THz plasmonic devices based on 90-degree curved spoof SPP waveguides composed of periodic metallic rectangular pillars are achieved,and the transmittance and bending loss of such 90-degree waveguide bends as a function of the radius of curvature are characterized.The experimentally measured bending loss of a curved waveguide with a radius of 2300μm at 0.56 THz is 1.55 d B.As the radius decreases,the bending loss gradually increases.The bending loss of a curved waveguide with a radius of 200μm at 0.56 THz is 6.45 d B.Based on this,a commutator is demonstrated to be able to accomplish wave transmission.A fiber-optic scanning near-field THz microscopy system is used to experimentally characterize the samples.In addition,coupling equations for bend-straight waveguides and bend-bend waveguides are derived and validated,which helps to optimize the spacing and radii of the two waveguides to control the output power ratio of the two waveguides.(2)High-performance and compact broadband THz plasmonic waveguide intersection.A novel plasmonic structure enabling suppressed crosstalk between intersecting THz spoof SPP waveguides is theoretically and experimentally demonstrated.This compact crossing has a footprint of less than 0.2×0.2 mm~2(about0.13λ~2 for f=0.55 THz)and can be fabricated with other waveguide components on the same platform.Experimental characterization of the crossings demonstrates that for a single crossing,the loss is as low as 0.89 d B/crossing and the crosstalk is less than-19.06 d B/crossing at 0.55 THz.The loss is less than 0.8 d B below 0.62 THz,and the crosstalk is less than-15 d B in the whole SPP transmission range.Minimum loss of 0.15 d B and crosstalk of-29.26 d B can be obtained at 0.61 and 0.64 THz,respectively.The structure shows low loss and good crosstalk suppression for both single-waveguide and multi-waveguide crossover systems within a wide frequency range.The relationship between crosstalk and the crossing angle is also investigated.The results show that more than 40%transmission can be obtained when using a larger-than-30°angle of intersection between the cross-waveguides.Because of its features in terms of loss,crosstalk,fabrication tolerance,device size,bandwidth,and so on,we believe that the waveguide intersection will facilitate large-scale interconnection and minimize the device footprint for future complex planar THz integrated systems.(3)THz spoof surface plasmonic logic gates.Logic gates are important components in integrated photonic circuitry.By changing such parameters as the length and period of the periodic pillars,the propagation phase of the THz SPPs can be engineered.Different logic gates can be achieved through the coherent interference between the SPPs propagating along two and more paths by adjusting their phase.A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions:AND,NOT,OR,and XOR.By cascading two such interferometers,NAND and NOR operations can also be achieved.Experimental investigations are supported by numerical simulations,and good agreement is obtained.The logic gates have compact sizes and high intensity contrasts for the output‘‘1’’and‘‘0’’states.(4)THz plasmonic wavelength diplexer.This part describes the design and actualization of an ultra-compact wavelength diplexer formed by THz spoof SPP waveguiding structures.By adding a certain number of periodic pillars in the coupling part of the directional coupler,the refractive index of the anti-symmetrically distributed odd modes can be engineered,thereby adjusting the coupling length of the waveguide modes at different frequencies.By adjusting the periodic pillar parameters properly,the SPP modes at two target frequencies will be coupled in the device for an odd or even number of times,so that the SPP modes at these two frequencies can be coupled out from different ports.The length of the wavelength diplexer is 1.6 mm,which is about 12.8%of its traditional counterpart.Minimum simulated transmittances of-24.34 d B and-26.27 d B can be obtained at 0.637 THz and 0.667THz,respectively.The insertion losses at the two operating frequencies are less than0.46 d B,and the extinction ratios are both better than 19 d B.By cascading the proposed diplexers,a compact wavelength demultiplexer with more channels can be obtained,which has important applications for future THz integrated communication systems.Based on spoof surface plasmon waveguides,this dissertation has accomplished a series of compact,superior and new devices for integrated on-chip THz surface plasmonic systems.The results presented here represent a key step for THz technology towards devices with smaller physical size,faster processing speed,and lower power consumption.They will also promote the development of THz science and technology. |
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