Quantitative phase imaging (QPI) is an optical microscopy technique that quantifies the morphology and refractive index distribution of a sample by measuring the phase shift of light waves passing through it, without the need for dyes or labels. With its non-invasive and highly sensitive nature, it has found wide applications in biomedicine, materials science, and other fields. Compared to traditional intensity imaging techniques, QPI provides more structural information and is particularly suitable for studying transparent or weakly scattering samples (such as live cells, tissue sections, two-dimensional materials, etc.). Although various implementation methods for QPI technology exist, traditional methods face several limitations: 1. Bulky instruments: Most methods rely on free-space optical systems, including complex optical elements (such as phase modulators and interferometers), which limits their portability and stability. 2. High phase noise: QPI systems using coherent light sources are susceptible to speckle noise caused by light spot interference, leading to reduced measurement accuracy. 3. High cost: The demand for complex optical elements makes the overall system cost high, hindering large-scale applications. Due to these limitations, achieving miniaturization, compactness, and high precision simultaneously in quantitative phase imaging technology remains a significant challenge in current research, but also points out future development directions.
To address these issues, Dr. Chupao Lin and Prof. Nicolas Le Thomas from the Ghent University-imec Photonics Research Group in Belgium have proposed a novel QPI technique improved by photonic integrated circuits (PICs). By using a photonic integrated chip to shape and phase-modulate the light beam, they overcome many limitations of traditional methods. Relevant research results were recently published in Photonics Research, Volume 13, Issue 1, 2025. [Chupao Lin, Yujie Guo, Nicolas Le Thomas, "Demonstration of a photonic integrated circuit for quantitative phase imaging," Photonics Res. 13, 1 (2025)]
The method proposed by the team is based on a photonic integrated chip dedicated to the implementation of the Kramers-Kronig (KK) relations. It allowed them to achieve low noise quantitative phase imaging with photonic integrated circuits for the first time. The principle is shown in Figure 1. The integrated photonic circuit (PIC) enables mechanical motion-free light switching and provides oblique illumination beams at very precise angles. The KK method is then used to retrieve phase information from the intensity images of the light transmitted by the sample. The intensity images are recorded with a standard optical microscope. By leveraging the stability and compactness of the integrated photonic chip, as well as its powerful light beam shaping capabilities, excellent imaging performance is achieved. The QPI approach, based on PIC, presents several notable features. First a significant reduction of phase noise: 1. a spatial phase noise of 5.5 mrad is achieved, which is 7 times lower than the traditional KK method based on bulk optics. 2. Compared to existing mini-QPI modules (such as those based on metasurface technology), the noise performance is improved by 15 times. Second, an excellent spatial resolution: A diffraction-limited resolution of 400 nm is achieved, which is about 2-3 times higher than the typical resolution (~1 µm) of other mini-QPI modules. Third, a compact and efficient design: The photonic integrated circuit measures only 50 mm × 50 mm and can be easily integrated into standard optical microscopes without changing existing acquisition systems. Fourth, a portability potential: Compactness and low power consumption make it suitable for on-site monitoring and industrial quality control.
These features allowed us to image high-resolution phase maps of bacterial cells and distinguish between live and dead cells, validating its potential in pathogen detection. The approach is also well suited for characterizing two-dimensional materials: The measured thickness of single-layer graphene (0.45 ± 0.15 nm) is in good agreement with AFM technology, while the imaging speed is increased by 20 times.
The validated technique is anticipated to be utilized in medical diagnostics for blood cell analysis and early cancer screening. Additionally, it is expected to be applied in materials science for non-contact measurement of two-dimensional materials (such as graphene and transition metal dichalcogenides), thereby supporting advancements in nanoelectronics and optoelectronics.
This research significantly improves the overall performance of QPI technology, pushing it towards a wider range of practical applications. The PIC-based design not only improves the portability and stability of the optical system but also reduces costs, bringing a breakthrough in the field of optical imaging.
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Figure 1: Working principle of the PIC-based QPI technique. (a) Schematic diagram of the device. (b) Illustration of the KK-based QPI technique in k-space. Purple disk, aperture of the microscope objective with kmax the maximum modulus of the transmitted transverse component of the wave vectors; left, standard normal illumination, with incident transverse k-vector kinc=0; right, oblique illumination with kinc=kmax, i.e., compatible with the KK relations. (c) Log-scale Fourier domain of the retrieved field image after merging the frequency bands of the four directions of illumination di=1 to 4. (d) Amplitude and phase images obtained by inverse Fourier transform of (c). (e) Schematic of the photonic integrated circuit used in (a) including a cross section of the aluminum oxide waveguide (green). The diffraction gratings provide the oblique illuminations that are switched on (current Ion) and off (current Ioff) with an integrated 1×4 switch made of 1×2 or 2×2 multimode interferometer (MMI) and thermal phase shifters (yellow). Inset: Optical image of the photonic chip mounted on a PCB board and electrically connected via gold wires.
