Photon-sparse microscopy: visible light imaging using infrared illumination
Reuben S. Aspden, Nathan R. Gemmell, Peter A. Morris, D. S. Tasca, Lena Mertens, Michael G. Tanner, Robert A. Kirkwood, Alessandro Ruggeri, Alberto Tosi, Robert W. Boyd, Gerald S. Buller, Robert H. Hadfield, Miles J. Padgett
Conventional imaging systems rely upon illumination light that is scattered or transmitted by the object and subsequently imaged. Ghost-imaging systems based on parametric down-conversion use twin beams of position-correlated signal and idler photons. One beam illuminates an object while the image information is recovered from a second beam that has never interacted with the object. In this Letter, we report on a camera-based ghost imaging system where the correlated photons have significantly different wavelengths. Infrared photons at 1550 nm wavelength illuminate the object and are detected by an InGaAs/InP single-photon avalanche diode. The image data are recorded from the coincidently detected, position-correlated, visible photons at a wavelength of 460 nm using a highly efficient, low-noise, photon-counting camera. The efficient transfer of the image information from infrared illumination to visible detection wavelengths and the ability to count single photons allows the acquisition of an image while illuminating the object with an optical power density of only 100??pJ?cm-2?s-1. This wavelength-transforming ghost-imaging technique has potential for the imaging of light-sensitive specimens or where covert operation is desired.
Conventional imaging systems rely upon illumination light that is scattered or transmitted by the object and subsequently imaged. Ghost-imaging systems based on parametric down-conversion use twin beams of position-correlated signal and idler photons. One beam illuminates an object while the image information is recovered from a second beam that has never interacted with the object. In this Letter, we report on a camera-based ghost imaging system where the correlated photons have significantly different wavelengths. Infrared photons at 1550 nm wavelength illuminate the object and are detected by an InGaAs/InP single-photon avalanche diode. The image data are recorded from the coincidently detected, position-correlated, visible photons at a wavelength of 460 nm using a highly efficient, low-noise, photon-counting camera. The efficient transfer of the image information from infrared illumination to visible detection wavelengths and the ability to count single photons allows the acquisition of an image while illuminating the object with an optical power density of only 100??pJ?cm-2?s-1. This wavelength-transforming ghost-imaging technique has potential for the imaging of light-sensitive specimens or where covert operation is desired.
Correlated photon imaging, popularly known as ghost imaging, is a technique whereby an image is formed from light that has never interacted with the object. In ghost imaging experiments, two correlated light fields are produced. One of these fields illuminates the object, and the other field is measured by a spatially resolving detector. In the quantum regime, these correlated light fields are produced by entangled photons created by spontaneous parametric down-conversion. To date, all correlated photon ghost imaging experiments have scanned a single-pixel detector through the field of view to obtain spatial information. However, scanning leads to poor sampling efficiency, which scales inversely with the number of pixels, N , in the image. In this work, we overcome this limitation by using a time-gated camera to record the single-photon events across the full scene. We obtain high-contrast images, 90%, in either the image plane or the far field of the photon pair source, taking advantage of the Einstein–Podolsky–Rosen-like correlations in position and momentum of the photon pairs. Our images contain a large number of modes, >500, creating opportunities in low-light-level imaging and in quantum information processing.
Correlated-photon imaging, popularly known as ghost imaging, is a technique whereby an image is formed from light that has never interacted with the object. In ghost imaging experiments two correlated light fields are produced. One of these fields illuminates the object, and the other field is measured by a spatially resolving detector. In the quantum regime, these correlated light fields are produced by entangled photons created by spontaneous parametric down-conversion. To date, all correlated-photon ghost-imaging experiments have scanned a single-pixel detector through the field of view to obtain the spatial information. However, scanning leads to a poor sampling efficiency, which scales inversely with the number of pixels, N, in the image. In this work we overcome this limitation by using a time-gated camera to record the single-photon events across the full scene. We obtain high-contrast images, 90%, in either the image plane or the far-field of the photon pair source, taking advantage of the EPR-like correlations in position and momentum of the photon pairs. Our images contain a large number of modes, >500, creating opportunities in low-light-level imaging and in quantum information processing.
Low-light-level imaging in the infrared regime could find applications in areas ranging from technological to biological. Still, its development is limited by the inefficiency of silicon-based detectors in this regime. Quantum Ghost Imaging (QGI) can address this limitation by allowing the separation of illumination and imaging [1]. This is accomplished by generating non-degenerate correlated photon pairs, whose infrared photon probes the object and is detected by an InGaAs bucket detector, while the spatial distribution of the visible one is collected by a single-photon sensitive silicon camera. Traditional QGI setups evaluate photon correlations through “heralding”, by triggering the camera upon idler detection and thus require an image-preserving delay-line, which limits their use for remote sensing [1]. In asynchronous QGI (aQGI) cameras are replaced with SPAD arrays [2] able to record photon timestamp with picoseconds precision. By identifying pairs through timestamps comparison, the mentioned limitations are resolved [3], enabling applications with reflective or diffusive objects where the timestamp delay depends on the object's position. In this work, we propose and demonstrate an experimental QGI setup for remote sensing in reflection configuration (Fig. 1). Collinear non-degenerate pairs of single-photons at 550 nm and 1550 nm are generated by pumping a 2 mm-long type-0 ppKTP crystal with a 405 nm laser. To enable imaging of objects at varying distances, we implemented a momentum-correlation-based setup. Infrared photons interacting with the object were collected by commercially available InGaAs SPAD detectors, while the visible photons were directly captured using a prototype silicon SPAD array developed by Politecnico di Milano [4]. It is based on a 32×32 SPAD, with in-pixel TDCs, and on-chip microlenses are employed to reach a fill factor of 78%. The time resolution is adjustable down to 250 ps, with 1 μs Full Scale Range (FSR). The measurement duty cycle (i.e. the ration of the TDC FSR and the frame time) can be increased with a multi-gate acquisition scheme. To validate the efficiency of our setup, we tested it using a proof-of-principle object, as shown in Fig. 1b. The reconstructed images, captured with the object placed at 30 cm intervals in an acquisition time of only 30 s, are shown in Fig. 1c. In summary, the shown results confirm the potential of the proposed setup for remote sensing applications, particularly for reflective or diffusive objects. Moreover, the proposed approach could offers the possibility of significantly reduce acquisition times compared to standard QGI setups.
Barreto Lemos et al. [Nature 512, 409-412 (2014)] reported an experiment in which a non-degenerate parametric downconverter and a non-degenerate optical parametric amplifier--used as a wavelength-converting phase conjugator--were employed to image object transparencies in a manner akin to ghost imaging. Their experiment, however, relied on single-photon detection, rather than the photon-coincidence measurements employed in ghost imaging with a parametric downconverter source. More importantly, their system formed images despite the photons that passed through the object never being detected. Barreto Lemos et al. interpreted their experiment as a quantum imager, as assuredly it is, owing to its downconverter's emitting entangled signal and idler beams. We show, however, that virtually all the features of their setup can be realized in a quantum-mimetic fashion using classical-state light, specifically a pair of bright pseudothermal beams possessing a phase-sensitive cross correlation. Owing to its much higher signal-to-noise ratio, our bright-source classical imager could greatly reduce image-acquisition time compared to that of Barreto Lemos et al.'s quantum system, while retaining the latter's ability to image with undetected photons.
Imaging systems play a crucial role in shaping understanding of our surroundings. Modern imaging systems enable the acquisition of images of objects at length-scales and resolutions previously deemed impossible. However, as the drive for better images and novel applications increases, several questions fundamental to our understanding of imaging are being raised. We are familiar with images containing many millions of photons, but how many photons does it take to form an image? In scenarios requiring covert imaging, or where high-light levels damage the sample being imaged, is it possible to image a sample with a very small number of very low-energy photons? This thesis will focus on answering these two questions. In order to answer these questions I developed a heralded quantum imaging system. The strong spatial correlations inherent between down-converted twin photons have been utilised in many quantum imaging techniques for the past 25 years. Thus far, standard quantum imaging techniques have relied on scanning a fibre detector across the field of view in order to measure the spatial information in the image. However, the scanning mechanism fundamentally limits the detection efficiency of the imaging system. Recent years have seen an increased interest in using the latest camera technology within quantum imaging systems. The advent of single-photon-sensitive cameras has opened up new possibilities within the quantum imaging field. It is now possible to obtain images of objects by detecting very low-light illumination across the full scene simultaneously. In the heralded quantum imaging system presented in this thesis, a combination of quantum mechanics and a high-sensitivity camera was employed to obtain high contrast images containing very few photons. This was the first such system built. Using the correlations between the twin photons generated by our down-conversion source, and our development of compressive sampling techniques to post-process the acquired data enabled the acquisition of an image containing an average of only 0.5 photons per pixel. This represents a significant reduction when compared with a conventional image that contains of order 105 photons per pixel. I also developed this imaging system to use twin photons at vastly different wave- lengths. The object was probed with low-energy infrared photons whilst the image was developed on a camera using the correlated visible photon. This disparity in energy between the two photons enabled the acquisition of images using only several thousand photons, corresponding to an energy deposition on the sample of only a few pJ.
The process of image recording is arguably one of the most prevalent technology in modern society and continues to inspire vast swathes of research due to its widespread applications spanning military, medical and consumer spheres. The danger present in a field so broad is that separate niches of research can become isolated with critical advancements struggling to traverse the gulfs between. Unifying the field is the omnipresent drive to acquire an ever increasing quality of images at the lowest possible cost, a goal which warrants continual fundamental research. Quantum entanglement exhibits many intriguing characteristics which make it a suitable tool for such fundamental investigations. The process of spontaneous parametric down- conversion has offered a yet unbeaten strength of photon correlations with the quantum nature of their production providing a reliable and controllable source of single-photons. Arising from these attributes is the technique known as ghost imaging. Though now known to be classically possible, the strength of entanglement generated correlations is yet to be surpassed. This thesis implemented this technique in tandem with the cutting edge detector technology in order to probe the fundamentals of image formation. This form of imaging allowed us to subject an object to a known number of photons whilst acquiring structural information from spatially separate, correlated photons which never interact with the object. The strength of the produced correlations allow us to acquire low background, high resolution images with far less light than traditional techniques and affords many novel benefits. The possibility of incorporating this technique with pre-existent regimes allowed me to draw from advancements made across the landscape of imaging research. Although referred to as “quantum ghost imaging” throughout this work, it should be noted that the intrinsic quantum nature of the correlations was not directly relied upon but provided an ideal source of strongly correlated photons. In order to determine the limits of a traditional imaging system this thesis first sought to answer the question: “can an image of an object be reconstructed from fewer photons than ii pixels in the image?” In chapter 3 I approached this from the perspective of compression, which minimises redundant information within a signal. This lead to the development of an imaging regime capable of imaging with far fewer photons than pixels in the image. By employing assumptions about the sparsity of natural images I was able to reconstruct an image of a biological sample containing an average of less than one photon per image pixel. Having reduced the number of photons necessary to form an image I then considered alternative methods for reducing the optical energy impinging on a sample. I sought to answer the question: “can non-degenerate ghost imaging reduce the optical energy impinged upon an object during imaging”. The photons produced in SPDC need not be of similar wavelengths, however may be chosen far from degeneracy, i.e. non-degenerate. In Chapter 4 I presented a ghost imaging system which illuminated the object with infrared light whilst recording the structural information via entangled visible photons. This allowed for objects opaque to visible light to be imaged in high quality without the need for a spatially resolving infrared detector, the state of the art of which lags behind their silicon based visible counterparts. I presented the systems capabilities by imaging objects which were etched into a gold substrate layered on to silicon, both of which are opaque to visible light. Not only did a reduction in energy deposition arise from the lower energy probe wavelength but applying the reconstruction techniques from the previous chapter brought that down to as low as ≈16 nJcm−2s−1. Seeking to expand the repertoire of applications, the low-light capabilities of my ghost imaging were applied to the technique of phase-contrast microscopy in chapter 5. Typically applied to translucent objects, phase-contrast imaging transfers phase information, i.e. the refractive index changed within the object, into an intensity distribution through the use of a phase-filter. In many of these applications the objects tend to be biological in nature, where high optical exposure can result in bleaching or damage. By applying the phase-filter non- locally, i.e. to the photons correlated to those probing the object, I acquired edge-enhanced images of a phase object whilst illuminating with significantly fewer photons than standard phase-contrast techniques. Having displayed the broad applicability of our low-light ghost imaging system, I then sought to determine the optical resolution in chapter 6. The resolution limits of ghost imaging are not clear at first glance owing to the resolutions dependence upon the strength of spatial correlations. As the length over which the spatial correlations are produced can be brought below the standard diffraction limit, it would seem the resolution of the system could be brought similarly low. To clarify this I artificially restricted the number of spatial modes in each of the correlated beams to uncover the physically realisable resolution. I show that although the resolution of a ghost imaging system to be fundamentally determined by the strength of the correlations, this can never be reached due to the inherent limitations of the intervening imaging system.