Incoherent X-ray Ghost Imaging With Ultra-low Radiation

Ghost imaging?GI?is an indirect imaging approach based on second-order correlation,which can utilize a single-pixel“bucket”detector to measure the total intensity,as long as the patterned field that illuminates an object is known by certain means.Compared with traditional imaging,ghost imaging can achieve super resolution even in poor illumination or a turbulent atmosphere.Although it has been realized with many wavelengths such as visible and infrared light,x-ray GI started late due to the difficulty in finding a suitable beamsplitter or modulator.X-ray ghost imaging has the potential to reduce radiation dose and ionization damage,which is significant in biomedicine,material science,and many other research fields.As a preparatory work,we first investigated traditional imaging at the single-photon level with an electron-multiplying charge coupled device?EMCCD?camera.Because of their semiconductor structure,computer cipher chips will emit photons when in operation,from which encrypted information can be retrieved.By measuring the photon emission from the static random access memory area of the AT89C52 cipher chip,we analyzed the relationship between the photonic emission location and the operations in the chip;the physical address can be determined to an accuracy of 8 bytes.This work presents a new type of optical side-channel attack based on visual single-photon detection.Ghost imaging with incoherent light and computational GI with visible light was then conducted experimentally.In addition,from the second-order intensity correlation,we analyzed the relationship between the statistical properties of the modulation speckles and the quality of the GI images.Under ideal conditions,GI images can be obtained as there is fluctuation of speckles,but in practice there is always noise,thus through numerical simulation we show that high-quality images can be retrieved by either gray scale or binary speckles,as long as their g^?2?is about 2.Therefore,either gray scale or binary speckles can be chosen,depending on the modulator.This work provided a theoretical foundation for how to select and optimize the x-ray GI modulators.We then prepared the GI experiment based on a tabletop x-ray source with incoherent thermal statistics.First we found that,due to the insufficient temporal and spatial coherence of the x-ray source and response of the detector,GI was not realizable with two ordinary x-ray imaging plates.Then,by pre-recording a series of modulation speckles generated by sandpaper,we successfully achieved pseudo-thermal x-ray GI of both planar and natural objects with our incoherent x-ray source.Already simple and inexpensive,our setup was further improved after optimizing the sandpaper,and retrieved a better image with higher resolution.According to the Rose criterion,in traditional imaging there is a minimum number of photons required to form an acceptable image,but we analyzed how ghost imaging may reduce this number and experimentally realized x-ray GI at single-photon level.Furthermore,through correspondence imaging,we reduced the number of frames and thus reduced the number of photons by one order of magnitude while maintaining the image quality.In addition,the number of frames can also be reduced by utilizing a controllable x-ray modulator and performing measurements directly in a sparse basis.This was realized experimentally by using specially designed modulation boards.We first used an etched copper printed circuit board,and achieved a resolution of 150?m.Then,with a micromachined gold plated mask a resolution of 10?m was achieved.Finally,we analyze how the quality of the modulator influences the image quality,and what further improvements can be made for future applications of x-ray GI in real practice.