Sub-diffraction-limited THz Wave Ghost Microscopy

Terahertz(THz)wave refers to the electromagnetic radiation whose frequencies lie in the range of 0.1—10 THz.After 20-year development,THz science and technology brings a lot of applications in fields including physics,.chemistry,biology,medicine and material,and becomes one of the“Top 10 technologies that will change the world of tomorrow”.Due to the unique properties of THz wave,such as finger print,high transmittance,non-ionizing photon energy(1 THz?4 meV),broad bandwidth and so on,there are great demands for many applications,such as specific sensing,nondestructive inspection,biodetection,wireless communication at high speed and so on.Particularly in the terahertz wave imaging,it can be used to realize applications such as chemical mapping,security check,industrial inspection,biological tissues(e.g.tumour tissues)characterization/diagnosis and so on.However,limited by the well-known Rayleigh diffraction,the conventional THz wave imaging cannot achieve spatial resolution better than its wavelength scale(1 THz?300 ?m),and cannot meet the requirement for the studies within the micro field.Distinguishing from the far-field imaging,using near-field evanescent waves to sense the object is a feasible method to break the Rayleigh diffraction limit.Up to now,the developed near-field sub-diffraction terahertz wave microscopes are mainly categorized into two types,namely near-field mechanical scanning scheme and near-field electro-optic(EO)sampling scheme.Although both of them can achieve sub-diffraction-limit THz wave imaging,the former one suffers some drawbacks of invasive effects on sample,system instability induced by mechanical moving,long scanning time,inflexible system parameters(solid mechanical structures are designed for some specific imaging situation),and the latter one,which is based on optical rectification within LiNbO3,badly relies on a laser amplifier and requires a high level of system environment.Moreover,for ensuring the efficiency of EO sampling,the thickness of EO crystal cannot be too thin,which makes the spatial resolution limited at tens of microns.To realize a THz wave imaging system which can deeply break the diffraction limit and has advantages of non-invasive effects on sample,faster imaging speed,wider application scenarios,flexible system parameters,in this dissertation,we combined near-field evanescent wave theory with computational ghost imaging technology,and demonstrated a THz wave sub-diffraction ghost microscope.The relevant works are summarized as follow:(1)We proposed a spatial terahertz wave modulator(STWM)based on phase-transition nano-film material vanadium dioxide(VO2).Using spatial femtosecond(fs)laser induce the metal-to-insulator phase transition(MIT)in VO2,the THz waves are spatially coded in the near field.Associating with ghost imaging algorithm,the sub-diffraction THz image of an imaging target is reconstructed from the recorded THz signals.In experiment,a 180-nm-thick VO2 was used to perform THz ghost imaging(central frequency of 0.5 THz,and central wavelength of 600?m),and the resultant image achieves a spatial resolution of 4.5 ?m(better than ?0/100),which yields an improvement of over two orders of magnitude,compared to the conventional scheme.Besides,combing the ghost imaging with compressed sampling,it was demonstrated that compressed images with high fidelity could be obtained with sampling ratio much less than the Nyquist sampling limit,which allows us to make a tradeoff between image quality and imaging speed.(2)Based on the hysteresis of the MIT in VO2,we demonstrated an all-optically-driven reconfigurable THz wave memory device.Since the fs-laser-induced MIT is an athermal process,the VO2 memory states can be written by fs pulses in high speed.In experiment,we demonstrated a multi-state reconfigurable memory device by controlling the injected laser energy,and the resultant write time was measured as?22?s,yielding an improvement of over six orders of magnitude,compared to the existed thermally-driven VO2 memory device.Besides,fs-laser-written states are highly localized,since they barely suffer from thermal diffusion.Based on the localization,one can develop an optically-controlled THz wave spatial memory device with high spatial accuracy,which can release the synchronization requirement on the proposed THz wave ghost microscope and make our system available for more applications.(3)Based on the spintronic THz emitter(STE),we designed a THz emitter“array”(STEA),which can directly generate structured THz pulses with high accuracy under the spatial fs laser's excitation.In experiment,the spatial resolution was measured as 6?m(?0/100)and the potential spatial resolution was estimated as fine as the fs laser's diffraction limit.Besides,the polarization impacts on sub-wavelength structures were studied.After fusing images with mutually orthogonal polarizations,a polarization-free image was acquired.Moreover,combing with time-of-flight(TOF)measurements,THz tomography was explored using our system.And if one uses an oscillator with stable power to drive STE,the frame rate of the THz wave ghost microscope is believed to be improved significantly even up to real-time level(>24 frame/s),according to our reasonable calculations.