The research, building upon the foundational work in light imaging, addresses a crucial aspect often overlooked: light's phase. Light is not only characterized by its intensity but also by its phase, a property that becomes critical when imaging transparent objects. Traditional imaging, whether capturing everyday moments with a smartphone or examining cell cultures under a sophisticated microscope, typically involves measuring the brightness of light pixel by pixel. However, to visualize transparent objects, measuring the phase delay of light they introduce is essential.
Reflecting on the history of phase imaging, the researchers pointed to the groundbreaking work of Frits Zernike, who was awarded the Nobel Prize in 1953 for his invention of phase contrast microscopy. This technique was a revolution in biomedical imaging, enabling high-resolution images of various transparent and optically thin samples. "It enables label-free and quantitative characterization of living specimens, such as cell cultures, and can find applications in neurobiology or cancer research," explains Dr. Radek Lapkiewicz, head of the Quantum Imaging Laboratory at the University of Warsaw's Faculty of Physics.
Despite these advances, current imaging techniques face challenges, particularly in the realm of interferometry-a standard method for precise thickness measurements. "For example, interferometry, a standard measurement method for precise thickness measurements at any point of the examined object, only works when the system is stable, not subject to any shocks or disturbances. It is very challenging to carry out such a test, for example, in a moving car or on a shaking table," Jerzy Szuniewcz, a doctoral student at the University of Warsaw's Faculty of Physics, notes the limitations of existing methods.
To overcome these challenges, the team drew inspiration from the classic experiments of Leonard Mandel in the 1960s. Mandel and his group demonstrated that correlations in light intensity could reveal interference patterns even when they are not directly detectable. "Inspired by the classic experiments of Mandel, we wanted to investigate how intensity correlation measurements can be used for phase imaging," Dr. Lapkiewicz states, elaborating on the genesis of their approach. In a correlation measurement, pairs of pixels are observed to determine if they become brighter or darker simultaneously.
"We have shown that such measurements contain additional information that cannot be obtained using a single photo, i.e., intensity measurement. Using this fact, we demonstrated that in phase microscopy based on interference, observations are possible even when standard interferograms average out losing all the phase information and there are no fringes recorded in the intensity. With a standard approach, one would assume that there is no useful information in such an image. However, it turns out that the information is hidden in the correlations and can be recovered by analyzing multiple independent photos of an object allowing us to obtain perfect interferograms, even though the ordinary interference is undetectable due to the noise," adds Dr. Lapkiewicz, detailing the process.
In their experiment, the researchers superposed light passing through a phase object with reference light, introducing a random phase delay between the beams. This delay simulates disturbances that usually obstruct standard phase imaging methods. As a result, no interference was observed in direct intensity measurements, rendering them incapable of providing information about the phase object.
However, the scenario changes with the intensity-intensity correlation technique. Szuniewcz explains, "The spatially dependent intensity-intensity correlation displays a fringe pattern that contains the complete information about the phase object. This intensity-intensity correlation is unaffected by any temporal phase noise varying slower than the speed of the detector (~10 nanoseconds in the performed experiment) and can be measured by accruing data over an arbitrarily long period of time - which is a game changer - longer measurement means more photons, which translates to higher accuracy." This innovative approach allows for accurate phase imaging even in challenging conditions.
Elaborating on their methodology, Szuniewcz adds, "Therefore, first we recorded a whole series of such frames using a camera and then multiplied the measurement values at each pair of points from every frame. We averaged these correlations, and recorded a full image of our object. There are many possible ways to recover the phase profile of an observed object from a sequence of images. However, we proved that our method based on intensity-intensity correlation and a so-called off-axis holography technique provides an optimal reconstruction precision," highlighting the technical finesse involved in their research.
This new phase imaging approach, based on intensity correlation, holds promise for widespread application in environments where noise is prevalent. Its compatibility with both classical (laser and thermal) and quantum light, and its potential for implementation in photon counting regimes, such as using single photon avalanche diodes, make it a versatile tool. Szuniewcz underscores its utility: "We can use it in cases where there is little light available or when we cannot use high light intensity so as not to damage the object, for example, a delicate biological sample or a work of art."
Concluding the discussion, Dr. Lapkiewicz envisions the broader impact of their work: "Our technique will broaden prospects in phase measurements, including emerging applications such as in infrared and X-ray imaging and quantum and matter-wave interferometry." This research not only represents a notable stride forward in imaging technology but also opens doors for new applications across various scientific and technological domains.
Research Report:Noise-resistant phase imaging with intensity correlation
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