Apertureless Near-Field Optical Microscope basedonSecond-Harmonic Generation

Apertureless Near-field Optical Microscopy (ANSOM)based onthedetection oftheSecond-Harmonic (SH)signal induced attheprobe tiprepresents apossible route toinvestigate nanoscale linear andnonlinear optical properties ofa surface. Byfocusing femtosecond light pulses onametal tipforatomic force microscopy, SHgeneration islocalized at its endduetofield enhancement caused bythesharp tipcurvature. Suchlight nanosource canbeemployed toperform ANSOM,freefromthebackground elastic scattering attheFundamental Wavelength (FW)generated byglobal illumination ofthetipand/or sample. Herewereport ontheachievement ofmaterial optical contrast onthenanoscale bymeansofANSOM withsecondharmonic generation (SH-ANSOM). We adopted agrazing incidence illumination configuration, withexcitation ofthe tipdipole along itsaxis, andbackscattering collection arrangement along themaximumemission direction. This optimized geometry resulted ina2÷3 orders ofmagnitude increase oftheefficiency (up to106photons/s) withrespect toprevious experiments, allowing standard scanrates forNSOM imaging. We developed anextended-cavity Ti:Sapphire oscillator that generates 25-fs pulses at800nmwith25MHz repetition rate, obtaining afourfold increase inpulse energy atgiven average power, minimizing thermal effects onthetip.