Remote Ultrastrong Femtosecond Laser Fabrication Of Nano/Mircostructured Semiconductor And Metal Functional Materials And Device Applications

Micro-nano structures,which serve as one of the important structure components of micro-nano devices such as micro-optics and microelectronics,have attracted substantial research interest in the field of micro&nano technologies.The unique microscopic morphologies of these structures bring new functions to materials,thereby providing an effective way for the realization of newly conceptutized photonic and electronic devices.At present,common technologies for fabricating micro-nano structures include CVD,NIL,self-assembly technology,photolithography,and femtosecond laser processing technology etc.Although these fabrication technologies have made inspiring research progress,they are still limited in many aspects such as fabrication efficiency,environmental protection and processing costs.During the propagation of ultrahigh peak-power femtosecond laser pulses in transparent optical media,they can maintain a high intensity over a long distance without obvious divergence,and form a long,thin,bright and high laser-intensity plasma channel in the medium.This unique nonlinear optical phenomenon is called femtosecond laser filamentation.Based on the characteristics of constant clamped high intensity,long focal depth,and broadband spectrum generated by nonlinear processes,femtosecond laser filament have showed broad prospects in applications from terahertz radiation to combustion diagnostics,and micro-nano structure processing.In this thesis,we mainly focus on femtosecond laser filament micro-nano processing technology to realize remote and rapid fabrication of functional materials,to reveal the physical mechanisms behind the material modification,and to investigate the fabrication and application of devices made based on these noval materials.The main achievements are summarized as follows.1.We investigated by using femtosecond laser filament the fabrication and characterization of black silicon materials,as well as their applications in optoelectronic devices.Our results showed that a significant improvement of the absorptivity by 50%in the SWIR spectral range between 1.5 and 2.5?m is achieved for the silicon samples coated with an Al film,when compared to that without Al coating.Surface microscopic morphologies and elemental analyses suggested that the absorptivity enhancement can be ascribed to both the changes in the morphologies of surface microstructure and the modification in the energy-band structure of silicon due to aluminum implantation.To further improve the absorptivity of black silicon for practical application,tellurium-doped black silicon with the absorptance of>85%over the wide range of 240–2500 nm was fabricated by filament laser ablation of a tellurium-film-coated crystalline silicon wafer.The improved absorption of the filament-processed silicon was mainly attributed to the three mechanisms:surface morphology modification,structure defect-induced Urbach states,and energy-band structure modification due to tellurium implantation.We then fabricated an n⁺-n photodiode using the tellurium-doped black silicon and tested its optoelectric characteristics.The results showed that the photodiode exhibits good rectification and photosensitivity characteristics with a responsivity of 445 m A/W at 650 nm,56 m A/W at 808 nm,and 15 m A/W at 1550 nm.This work provides a new route for rapid fabrication of broadband black silicon materials and cost-effective infrared optoelectronic devices.2.In light of the lacking of cost-effective techniques for fabricating fuctional metal surfaces with irregular shapes,we developed a fabrication method,which is based on femtosecond laser filament,and achieved remote and rapid fabrication of irregularly shaped,multifunctional metal surfaces with super-hydrophobic and antireflective properties.In combination of femtosecond laser filament processing with chemical fluorination treatment,multi-functional metal surfaces with the water contact angle of>150°and diffused reflectance of<9%over the wavelength range from 240 to 2500nm have been realized for different types and shapes of metals.We ascribed the enhanced light-trapping ability of the ablated metal sample to the formed microstructures on the surface of the sample,and the super-hydrophobicity to the combined effects of the formed microstructures and the free-energy reduction of the processed surface by fluorination.Additionally,we explored the vast parameter space available for laser filament processing of metal surfaces.This work offers the possibility of rapid fabrication of irregularly-shaped multifunctional metal surfaces for applications in aerospace,healthcare,and food industries.3.In view of the broad applications of TiO₂ nanomaterials in the fields of optoelectronics and photochemistry,we carried out the experimental research on femtosecond laser filament fabrication of black TiO₂ nanomaterials.By irradiating TiO₂nanoparticles in solution and air environments with femtosecond laser pulses in the filamentation regime,we found that the filament-processed TiO₂ samples have a certain degree of absorption enhancement over a broad spectral range from visible to infrared,and even to microwave.The analyses of crystalline structure and elements showed that the absorption improvement is mainly attributed to the filament-induced disorder and dopant impurity in the surface layer of TiO₂.This work opens up a new way for engineering black TiO₂.4.We fabricated large-scale honeycomb-microstructured silicon using femtosecond laser filament processing,and assembled,with the processed silicon as a mold,highly sensitive flexible piezoresistive sensors.By replicating the micropattern from the mold to two-layers SWNTs/PDMS thin films,sandwich-structural electrode-type flexible piezoresistive sensors were assembled,which exhibit good performance with high sensitivity(0.54 k Pa^-1),fast response time(80 ms),and excellent stability,and can be used to monitor human hand movements and blood pressure pulses.We further optimized the device structure and assembled a new flexible pressure sensor with a desirable compression sensitivity(0.266 k Pa^-1),a wide pressure range up to 160k Pa,and excellent stability,which was used with success for the detection of human physiological signals,such as wrist blood pulse and throat muscle movement.Moreover,with the assistance of principal component analysis(PCA)algorithm,we showed that the pressure sensor can unambiguously distinguish the pronunciation of different words,making it high application potential in physiological analysis systems.This work provides a new way for rapidly and large-scale fabricating cost-effective and high-performance flexible sensors.