Direct Ink Writing Of Terahertz Photonic Crystals With External Field-Tunable Properties

Terahertz wave refers to the electromagnetic wave with frequency of 0.1 THz-10 THz and a wavelength range of 30 ?m-3 mm.Its wavelength is located between the microwave and infrared which have developed quite mature.Terahertz waves have a"dual characteristic" of microwave and light waves,that is,microwave-like penetrating power and directionality similar to light waves,and have very strong complementary characteristics with respect to electromagnetic waves of other bands.Compared with microwave and millimeter wave technologies,THz detection technology have higher resolution,better anti-interference ability and anti-stealth capability.Compared with laser technology,THz technology have a wider field of view and better search ability,and is more suitable for severe weather conditions.In the field of terahertz technology,the development of terahertz source and terahertz detection technology has made great progress,and many technical indicators are even close to the theoretical limit.However,the development of functional terahertz devices is relatively slow.Because of their capability to prohibit the propagation of terahertz irradiation for all directions in their band gaps,three dimensional terahertz photonic crystals with a periodic structure could create specific functionalities.In order to control and manipulate of THz wave transmission,the size of the photonic crystal have to be equivalent to the THz wavelength.The conventional processing method is difficult to fabricate photonic crystal devices to meet the requirement on both complex structure and precision in micro scale.Therefore,we chose direct ink writing technology to construct 3D photonic crystal structures with submicron to millimeter scale.Compared to other 3D printing technologies,the direct ink writing technology could be used for a variety of materials,such as biomaterials,ceramic materials,silicone materials,and print different materials at the same time.In this paper,we first conducted a systematic study on inks for the direct ink writing technology.Then,the flexible barium titanate/PDMS 3D photonic crystals with mechanically-tunable terahertz properties were fabricated.Next,we developed a new 4D printing technology based on the Fe/PDMS ink,and created 4D photonic crystals.Finally,we developed a dual-material printed photonic crystal technology that provides the possibility to prepare multi-material THz functional devices.First,a systematic analysis of various slurry ink systems for direct ink writing technology was conducted.The research status and hydrodynamic analysis model of direct-printing printing based on sol-gel slurry system were described.For the direct write ink printing,most developed slurry inks had shear thinning characteristics,which ensured the smooth ink extrusion from the needle.However,the change of the elastic modulus G' and the viscous modulus G"of inks with shear stress had not been considered comprehensively in most of these studies.We found that the most fundamental feature of inks for direct writing was that their elastic modulus G' and the viscous modulus G" would had a viscoelastical invertion after they were extruded from the needle.Three representative slurry ink forming mechanisms were studied.PDMS1700 slurry was shear stress-induced viscoelastic inversion ink,PL A slurry was solvent volatilization-induced viscoelastic inversion ink,and titanium dioxide sol gel slurry was chemical reaction-induced viscoelastic inversion ink.Secondly,based on polydimethylsiloxane?PDMS?silica gel and barium titanate?BaTiO₃?nanoparticles,BaTiO₃/PDMS composite ink was prepared in different proportions and various THz photonic crystal structures were printed.The band gap structures of the photonic crystals as the function of the structural parameters with the dielectric rod spacing d,the dielectric rod period height h and the refractive index difference ?n were studied by the plane wave expansion method.The THz transmission spectrum as the function of the structural parameters with the dielectric rod spacing d,the dielectric rod period height h and the refractive index difference ?n were measured,and the finite-difference time-domain method method was used to simulate the transmission spectrum.More interestingly,the THz properties of these 3D-TPCs could be tuned by the in-situ application of external force fields on them.When the tensile force field was applied to the photonic crystal,the photonic band gap position moves to the low frequency with the increase of the stretching rate.When the torsion was applied,the photonic band gap depth first increased with the increase of the torsion angle,then decreased,and finally faded away.Thirdly,a polydimethylsiloxane/Fe?PDMS/Fe?composite ink was developed for reversible four-dimensional?4D?printing to create magnetically responsive three-dimensional?3D?structures with fast response time for their structure evolvements under external magnetic field.These 3D structures could obtain or lose high magnetization immediately by the on or off of external magnetic field due to the low magnetic coercive force and high magnetic permittivity of soft magnetic Fe particles in this composite ink,while PDMS in this composite ink served as the flexible matrix component to ensure their shape recovery.As exampled by a 3D butterfly with the fast flapping of its wings under external magnetic field,complex 3D structures created by 4D printing with this PDMS/Fe ink could have the reversible magnetically-stimulated structure evolvement property and develop designed magnetically-induced motions with a fast response time for various magnetomechanical applications.Furthermore,structure evolvements of these 3D structures could also induce structure-related property changes as demonstrated by a 3D terahertz photonic crystal?3D-TPC?device with remotely-tunable terahertz?THz?properties,which could create novel functionalities for various functional devices created by 4D printing from this kind of ink through external magnetic field stimulation.Finally,an exploratory study was carried out to prepare multi-material THz functional devices by the direct ink writing technology.We developed a versatile method for fabricating complex and heterogeneous 3D terahertz woodpile photonic crystals using simultaneous direct-writing of double inks technology.Based on the direct writing of double inks technology,40 wt%and 10 wt%BaTiO₃ content BaTiO₃/PDMS composite inks were prepared and delivered layer-by-layer to pre-determined micrometer scale locations.The effect of double material coupling on the band gap of THz photonic crystals was studied.Our results suggested that the direct writing of double inks technology could provide a new way for the fabrication of hybrid structures for THz functional device applications.