1. FEMTOSECOND LASER FABRICATION OF MICRO/NANO-STRUCTURES FOR CHEMICAL SENSING AND DETECTION Student: Yukun Han MAE Department Faculty Advisors: Dr. Hai-Lung Tsai MAE Department Dr. Hai Xiao ECE Department Introduction and Background ND Filter l/2 Wave Plate PolarizerShutter Dichroic mirror CCD DriverComputer Wavelength: 800 nm Pulse Width: 120 fs Repetition Rate: 1 kHz Coherent Legend Coherent OPerA Frequency conversion (300nm – 3000nm) Objective lens (NA 0.3 -0.9) Sample (glass, polymer, tissue……) Five-axis Stage Lamp Monitor Experimental Setup Major Parameters of Ti: Sapphire Femtosecond Laser System  Pulse Width: 120 fs  Wavelength: 300 nm–3000 nm  Pulse Energy: 1 mJ  Repetition Rate: 1 kHz  Average Power: 1 W  Minimum Spot Size: < 1 µm  Spatial Mode: TEM 00  5-axis CNC Operations  Coherent Legend Femtosecond Laser System Scanning Electron Microscopy Images of A Fiber Probe Periscope Optical Parametric Amplifier Illuminator Shutter Humidity meter Camera Turning mirror Half-wave Plate Polarizer Damper ND filters Turning mirror 90º Flipper 50-50 Beam Splitter One-Step Fabrication of Silicon SERS Substrate Results-SERS Substrate on Silicon SERS Spectra of R6G (10 -6 M solution) Conclusion  A fiber probe for SERS detection has been demonstrated by femtosecond laser machining with post chemical silver planting. The enhancement factor of the SERS substrate is up to 10 6.  we also present a way to ablate the silicon SERS substrate and reduce the silver ions simultaneously by femtosecond laser pulses. The process confirms the silicon SERS substrate can be completed with one step fabrication with EF of 5.4×10 5.  The high controllability and high efficient femtosecond laser fabrication make the miniaturized sensors attractive for many applications in chemical and biological sensing. Future Work  Working on investigating the laser-silicon interaction mechanisms that lead to the SERS enhancement.  Designing sensors for further chemical and bio applications. Acknowledgment The research work was supported by Intelligent Systems Center, Missouri S&T.  Minimal heat-affected-zone  High precision  Capable of processing any material  Improved 3D resolution  Surface modification Femtosecond Laser Micromachining Advantages: Miniaturization Trend:  MEMS (Microelectromechanical systems)  Biotechnology  Medical industry  Environmental technology  Information technology  Microelectronics industry  Microoptics technology Results- SERS Substrate on Fused Silica 500 µm a b c d (  10) Raman spectra of Rhodamine 6G (R6G) with a 1.7 mW He-Ne laser excitation power and 1 sec integration time.  (a) freshly cleaved fiber in a 10 -3 M solution,  (b) fs laser ablated, silver-coated (10 min) fiber SERS probe (1 m long) in a 10 -6 M solution,  (c) fs laser ablated, silver-coated (10 min) planar fused silica SERS substrate in a 10 -6 M solution with front excitation (shifted), and  (d) silver-coated (10 min) unroughened fiber (1 m long) in a 10 -3 M solution (multiplied by 10). Femtosecond Laser Micromachining Silver Chemical Planting (Tollen’s reaction) SERS Signals Detection  Raman scattering is an inelastic scattering of photons for materials analysis  SERS is a surface sensitive technique : The largest enhancements occur for metal (e.g., silver, gold, copper) surfaces which are rough on the nanoscale. Fiber SERS Probe Fabrication Surface Enhanced Roman Scattering (SERS) Background SERS enhancement factor (EF) calculation : Immersing in AgNO 3 for 10 minutes AgNO 3 film Objective lens Cleaning in acetone Silicon SERS substrate 2AgNO 3 → 2Ag + 2NO 2 ↑ + O 2 ↑ Laser ablation of silicon Periodic structures on silicon Silver particles are reduced from ions Silicon SERS substrate Platinum protection coating AgNO3 coating Silicon substrate Scanning Electron Microscopy Images of Silicon SERS Substrate a Raman shift (cm -1 ) Intensity (counts)  (a) Raman spectrum of R6G 10 -6 M solution on laser ablated SERS silicon substrate with an excitation laser power of 1.7 mW and integrated time of 2 sec.  (b) Raman spectrum of R6G 10 -3 M solution on the unablated silicon substrate with pre AgNO 3 soaking with an excitation power of 17 mW and integrated time of 2 sec. EF was estimated to be 5.4×10 5 500µm