
- Opto-Electronic Advances
- Vol. 7, Issue 12, 240122 (2024)
Abstract
Introduction
Biological imaging techniques are indispensable for the exploration of biological processes, structures, and states. Advanced imaging techniques have been widely studied for various biological applications, such as diagnosis, biometrics, and fundamental biological research. As the importance of biological imaging has been highlighted, there has been an increased demand for advanced imaging techniques that are faster, wider, clearer, and more accurate. However, enhanced performance is inevitably accompanied by increased system complexity, resulting in limited system performance. Many efforts have been made to reduce the system complexity by utilizing versatile and compact components that serve multiple functions, thereby creating a compressed optical system
Metasurfaces, consisting of regularly aligned nanostructures, are considered promising optical components for imaging techniques. They can manipulate optical properties such as amplitude, phase, polarization, absorption, and reflection through the arrangement of subwavelength-sized meta-atoms
Figure 1.
Principle of electromagnetic phase modulation
The operating principle of a metasurface follows the generalized Snell's law
Figure 2.
To precisely refract the light path, it is necessary to carefully design the phase delay and transmission at each position on the metasurface. Designing a complex phase map capable of modulating light in the desired direction and intensity according to the generalized Snell’s law and diffraction optical theory enables the realization of diverse optical functionalities. In this section, we focus on the principles and design strategies for metasurfaces.
Propagation phase
Propagation phase modulation is an innovative method for manipulating the phase delay of electromagnetic waves propagating through meta-atoms. The propagation phase is the phase accumulated during the propagation of light within the material of the meta-atom, which depends on the physical parameters of the material and the wavelength. By altering the physical structure of the meta-atom, it is possible to control the 2π phase for the incident polarized light. The physical structure here refers to the shape or physical dimensions of the meta-atom (such as height, length, width).
Propagation phase metasurfaces can be classified into two theories: nano waveguides and medium equivalent refractive index. The approach based on the medium equivalent refractive index utilizes the variation in refractive index between two or more media, where typically one of the media is a high refractive index material. It is the same concept as the generally known effective refractive index, which explains the complex structure of the meta-atom as an effective medium. It refers to the effective refractive index that appears when a wave propagates through the composite structure, allowing control of the phase change of light propagated through the meta-atom. This is used to describe the property of the meta-atom acting as an effective medium. Propagation phase modulation involves using meta-atoms as nano waveguides, where the phase delay is influenced by the effective refractive index and the dimensions of the meta-atom. More detailed discussion on how it act
The relationship between the incident and transmitted electric fields can be expressed using the Jones matrix [
where
Geometric phase
The geometric phase
The Jones matrix of rotated meta-atom can be expressed using the rotation matrix
Practically, designing a metasurface using linearly polarized light is inconvenient because the metasurface must be precisely aligned in the polarization direction. Because of this alignment issue, circularly polarized light is commonly adopted for designing PB-phase metasurface. The transmission matrix of the linear polarization basis can be converted to that of the circular polarization basis using conversion matrix
where, the subscripts R and L in
According to
Geometric phase metasurfaces effectively utilize their degrees of freedom to manipulate various electromagnetic events, including unusual refraction or diffraction
Scanning electron microscopy (SEM) images of the meta-atoms in the propagation and geometric phases are shown in
Optimization of unit cell period
The periodicity of the meta-atom is one of a key design factor. If the periodicity is too large or small, unwanted coupling between meta-atoms may occur, owing to the emergence of additional propagation modes and the occurrence of higher-order diffraction orders [
where,
Additionally, the
Dispersion modulation
The dispersion is the deviation of light from the original path in a wavelength-dependent manner as it passes through a dispersive medium. The dispersion modulation is a common issue in the field of metasurfaces for wavelength dependent designing, such as achromatic metasurfaces
The sophisticated engineering of group delay (GD) and group delay dispersion (GDD) is being revisited as an effective solution for controlling the dispersion
where
where
Materials for metasurface
Proper selection of materials properly is critical for biological imaging in terms of imaging quality, fabrication and biocompatibility. The optical properties of materials, such as refractive index and optical extinction coefficient, greatly influence the phase design strategy and degree of freedom, as they govern the phase delay, transmission and polarization of matesurfaces in a wavelength dependent manner. The material should be compatible with nanofabrication for building meta-atoms. For biological applications, another crucial condition is biocompatibility. Metasurfaces that do not directly contact object being imaged are relatively free from biocompatibility concerns. However, in applications where direct contact occurs such as hyperlenses and local surface plasmonic resonance (LSPR), material selection should be carefully considered. It is worth noting that biocompatibility of designed metasurfaces should be tested before in vivo and in vitro applications since the biocompatibility depends not only on the materials, but also on the structure, size and species of subject. For example, although gold is widely known for its biocompatibility and used in plasmonic applications, gold nanoparticles can exhibit cytotoxic effect at certain sizes and concentrations
Plasmonic material
Surface plasmons (SPs)
Plasmonic resonators can control the phase, amplitude, polarization, and dispersion of light by adjusting the material properties, geometry, and vibrational frequency. Surface plasmonic resonators (SPR) have been adopted as unit cells for metasurfaces owing to their versatility. However, SPR has limitations arising from a phenomenon known as momentum mismatch
Figure 3.
Another promising plasmonic resonator is the gap surface plasmonic resonator (GSPR)
Although GSPR achieves high reflection efficiency, reducing optical loss remains a challenge in the design of highly efficient metasurfaces. This loss is particularly critical for visible light and communication wavelengths. Commonly used plasmonic materials such as gold and silver have limitations in the visible range owing to the inherent loss of metals and heat dissipation
Dielectric material
Owing to their low efficiency and high intrinsic ohmic loss
Dielectric meta-atoms with high refractive indices can effectively couple with various multipolar modes of Mie resonances
Dielectric materials with high refractive indices can effectively refract and confine light, functioning as waveguide modes that facilitate enhanced light-matter interactions. Operating as waveguide elements allows them to work over a broad bandwidth, demonstrating high transmittance and complete phase modulation. The energy of the incident light wave, which is limited inside the meta-atom, delays the propagation phase of the light, enabling phase accumulation
where,
The fill factor (FF), signifying the ratio of space filled by specific elements within a structure, allows for the manipulation of the phase shift
Tunable material
Light modulation is sometimes required during optical imaging in various purposes, such as switching between different imaging modes. Generally, spatial light modulator (SLM) has been employed for these purposes, but it is associated with drawbacks including high cost, complicated system requiring maintenance, and limited diffraction angles due to the large pixel sizes. As an alternative or complement, tunable matasurfaces are being explored for specific applications. Selecting appropriate materials with optical properties that can be modulated by external stimuli, such as electric fields or temperature, is crucial for implementing these devices. In this section, we will briefly discuss representative materials widely used for creating tunable metasurfaces.
One popular class is phase change materials (PCMs), which undergo transitions between states (e.g., amorphous and crystalline) depending on the external conditions, resulting in the significant change of optical properties. Chalcogenide compounds such as Ge2Sb2Te5 (GST)
Liquid crystal (LC)
Applications
To acquire high-quality images from biological samples, several parameters of optical system should be optimized, such as NA, field of view (FOV), depth of field (DOF), achromaticity, and imaging time. These parameters should be carefully tuned depending on the imaging modalities to build a high-performance optical system since they trade off against each other and typically cannot be optimized simultaneously. For example, while utilizing high NA metasurfaces enables high-resolution imaging, it reduces the FOV and DOF, restricting the large-field 3D bioimaging. Thus, many kinds of optimization techniques have been adopted to find optimal metasurfaces for bioimaging
The point spread function (PSF) engineering could be a good solution for optimizing the system for microscopic imaging. It is particularly worth noting the utilization of non-diffracting light, which preserves the size and intensity profile during propagation, in the field of bioimaging
Metasurfaces for cell imaging applications
Owing to its simplicity, cells have been used as a basic object for imaging in the field of biology. They are often used in a wide range of biological studies such as clinics, diagnostics, and drug screening. Due to their importance, a lot of imaging techniques adapted for cell imaging have been developed. In this section, various metasurfaces for advanced cell imaging techniques are discussed.
Super-resolution imaging
The optical resolution is defined as the minimum distinguishable distance between two points and is determined by the NA and wavelength of the optical systems. Even when enhanced using high-NA optics or short wavelengths, the resolution does not improve beyond a certain level (~250 nm) at visible wavelengths owing to the physical limit of diffraction, known as the ‘diffraction limit’
To achieve diffraction-unlimited resolution, the behavior of wave propagation should be considered. According to angular spectrum theory, waves are categorized into propagating and non-propagating waves, termed evanescent waves. Evanescent waves are exponentially decaying waves near a light source that convey structural information smaller than its wavelength
The initial versions of hyperlenses
Figure 4.
Metasurfaces also can be used for SIM, which achieves super-resolution by combining images illuminated by several patterns. The maximally achievable spatial frequency of conventional SIM is determined by the sum of the illumination and detection spatial frequencies
Typically, the metasurfaces for super-resolution imaging based on near field approaches suffer from optical loss and sample mounting
By sophisticated design, practical SOL metasurfaces have been fabricated by patterning a periodically aligned concentric nanostructure
Edge-detection metasurface
In the field of bioimaging, computational image processing has become indispensable for advancing optical imaging techniques. However, these techniques often require significant computational resources and processing time, which prohibit the real-time monitoring of biological activities. A promising solution to alleviate this burden is to employ an special optical elements for all-optical analog image processing, similar to hardware acceleration in the field of electronics
Figure 5.
Second-order spatial differentiation is a sophisticated mathematical procedure employed to ascertain the rate of change in the spatial gradient of a function
Edge-detection can also be implemented using spiral phase metasurfaces that incorporate a hyperbolic phase with a topological charge of 1
One promising approach for edge detection is nonlocal metasurface, which manipulate light in momentum domain. The term “nonlocal” indicates that meta-atoms of the metasurface interacts with the incident light in a collective manner rather individual elements
Differential interference contrast (DIC) microscopy is another example of label-free imaging of transparent samples
Tunable metasurfaces
Correlative imaging across various imaging modalities (i.e., combining bright field and edge-detection mode) is often required to unveil complex biological processes. However, complexity of optical setup restricts the implementation of combining different imaging modalities. Thus, tunable optics are preferred for constructing correlative imaging systems, as they can minimize the physical vibration and perturbations during mode switching. Metasurfaces also have emerged as strong candidates for tunable imaging system owing to their thin nature and abundant design flexibility. Switching modes of metasurfaces can be achieved in several ways, including electrical, mechanical (e.g., stretching), and stimuli-responsive (e.g., thermal) methods. In this section, tunable metasurfaces are comprehensively discussed.
Liquid crystal (LC) cell is the most representative tools for realization of electrically tunable metasurfaces. Their anisotropic refractive index and responsiveness to electric fields enable effective modulation of optical phases
Figure 6.
Metasurfaces for tissue and animal imaging applications
Optical microscopy is an indispensable tool for imaging biological sampels
Although these techniques exhibit superior performance, they require expensive and bulky hardware and careful maintenance. Integrating multiple functions into a limited space requires considerable cost and effort to precisely maintain and align the systems. Metasurfaces have received increasing attention as alternative solutions to address these issues. The multifunctionality and thin nature of metasurfaces enable the combination of multiple optical elements into a single flat component, resulting in significant system simplification. In this section, conventional optical imaging methods specialized for animal and tissue imaging are introduced and explored to understand their potential replacement by metalsurfaces.
Fluorescent microscopy
As the demand for fast and large-field imaging has increased in the field of biology, various types of advanced fluorescent microscopies are under development. Among them, light-sheet microscopy is particularly promising owing to its efficiency and fast imaging speed
Figure 7.
Another important microscopic technique in the biological field is multiphoton microscopy for deep-tissue imaging using a femtosecond laser in the near-infrared range. However, designing a metasurfaces for multiphoton fluorescence microscopy is challenging because of the larger difference between the excitation and emission wavelengths compared with single-photon microscopy. By utilizing the multifunctionality of metasurfaces, Arbabi et al.
Fluorescence is the process of emitting light with a longer wavelength after absorbing light with a shorter wavelength and is known as the Stokes shift. Because of its broadband spectrum, fluorescence imaging typically involves optical components designed for wide spectrum. While currently available conventional achromatic lenses exhibit superior performance in correcting chromatic aberrations, compensating for chromatic aberrations remains a challenge for metasurfaces
Photoacoustic microscopy (PAM)
Photoacoustic microscopy (PAM) is an innovative imaging approach, overcoming the limitation of the penetration depth of light in scattering media by harnessing acoustic signals
Extending DOF significantly increases the clear imaging volume, as the PAM acquires an ultrasound signal in the vertical direction. Although Bessel beams are commonly employed to increase the DOF, their low efficiency and the need for additional optical elements are obstacles to high-quality imaging. Song et al.
Figure 8.
Metasurfaces for human applications
This chapter discusses innovative idea of metasurfaces for human applications. From medical devices and biometrics, metasurfaces have enhanced the resolution, sensitivity, and diversity of devices for human applications, such as endoscopic imaging, MRI, 3D facial recognition and fingerprint imaging. This chapter explores into the innovative potential of metasurfaces on devices for human applications.
Endoscopic imaging
Endoscopy is an optical imaging technique used to visually inspect internal organs. This technology has become an essential tool for acquiring minimally invasive organ images, particularly by using high-resolution fiber-optic systems
Pahlevaninezhad et al.
Figure 9.
As mentioned previously, the ability of metasurfaces to control the properties of light enables more accurate diagnostics that are not possible using traditional fiber-optic catheters. Endoscopic imaging using a metasurface, which is sensitive to polarization, allows for a clearer distinction of the surrounding tissues compared to traditional fiber-optic endoscopes
Magnetic resonance imaging (MRI)
MRI is an advanced medical imaging technology that has evolved over nearly 50 years and can visualize a wide range of anatomical structures such as the brain, spine, muscles, joints, organs, and blood vessels. It offers exceptional contrast for soft tissues, enabling the diagnosis of neurological disorders, musculoskeletal conditions, cardiovascular diseases, and cancer
The use of metasurfaces has been proposed as a solution to enhance MRI resolution without a stronger magnetic field. A metasurface can resonantly enhance the strength of a magnetic field and spatially redistribute it by increasing the coupling between radio-frequency coils in specific areas
Schmidt et al.
Devices for facial recognition
Traditional facial recognition has been studied for auxiliary communication, mental health and emotion monitoring, pain assessment, and the interpretation of facial nuances
Metasurfaces are considered promising components for dot projection-based 3D depth encoding devices owing to their simplicity and ability to generate an enormous number of point clouds
Figure 10.
Structured light-based facial depth encoding techniques have been adopted in smartphones for device unlocking and biometric payment systems. Conventional facial recognition modules often employ vertical cavity surface-emitting lasers (VCSEL) as structured light sources owing to their low power consumption. The integration of metasurfaces with VCSEL has been proposed for more power-efficient and arbitrary light modulation
Devices for fingerprint recognition
Biometric fingerprint recognition is a robust security mechanism that utilizes the unique fingerprint patterns of individuals for identity verification. The journey of fingerprint recognition commenced in England in 1684, when Grew discovered the uniqueness of people's fingerprints
Recently, there has been an increasing demand for fingerprint recognition, particularly for its integration into handheld devices such as smartphones, driving continuous advancements in technology. To satisfy these increasing demands, the use of metasurfaces has led to significant advancements over traditional methods. This enables the creation of compact, high-resolution, and sensitive optical systems, significantly enhancing their applicability in security and personal authentication. Metasurfaces are particularly suitable for large-field imaging over short distances, such as fingerprint imaging. Quadratic phase metasurface imaging systems, with their wide FOV (~100°) and ability to capture the fine patterns with high precision (~100 µm), can offer a compact and reliable method of fingerprint authentication [
Conclusions and future perspectives
In this review, we have explored the principles of metasurface design and its comprehensive applications in the field of biological imaging. The remarkable features of metasurface, such as their multifunctionality, thinness, and compatibility with conventional optical systems, render them promising optical elements. Particularly in bioimaging, their potential to replace bulky and complex components with simpler, more efficient ones is noteworthy. To comprehensively cover the metasurfaces from basic principles to applications, we first discussed the principes of phase modulation of metasurfaces. Then, we have explored the representative classes of materials for creating metasurfaces depending on their properties. Particularly, dielectric materials and tunable materials are most widely used for bioimaging due to its compatibility within visible range and versatility. Furthermore, we showcased the broad applications of metasurface in biological field depending on the subject, such as cell, tissue, and human. The detailed categories include promising techniques for bioimaging, such as super-resolution
Although metasurfaces appear to be promising tools, three major limitations and challenges need to be addressed for them to replace, or at least be used alongside, conventional optics. First, current metasurface-based imaging systems suffer from aberrations and efficiency issues. These problems degrade image quality and hinder the accurate collection of biological data. Additionally, the small metasurfaces require additional topcail system for magnification. Imaging through current metasurfaces has lower light efficiency and higher aberrations compared to traditional lenses. For instance, fluorescence imaging, which is gold-standard for bioimaging, requires a sensitive detection system due to its poor efficiency, and light collection efficiency of metasurfaces is not adequate. Efforts are being made to resolve these issues through computational methods. Recent image processing techniques based on deep learning offer impressive correcting performance. Additionally, advanced inverse design methods enables the development of high-efficiency metasurfaces
Despite these limitations, leveraging the advantages of metasurfaces can significantly enhance the performance in bioimaging, making them a promising technology. Among their various merits, we would like to highlight three key advantages of metasurface for bioimaging. First, metasurfaces are highly specialized for PSF engineering, which offers significant benefits for scanning type microscopes. If strong illumination, generally available for scanning type microscope, is given, it can compensate for the low efficiency of metasurfaces. Furthermore, data acquisition using scanning methods enlarges imaging field of view and reduces aberrations. Indeed, this is well demonstrated in previous studies such as PAM
Although still in the early stages, the integration of metasurfaces with biological imaging holds the potential to revolutionize the field. The synergy between metasurfaces and advanced imaging techniques promises to enhance the performance and capabilities of imaging systems, introducing novel functionalities that are unattainable with traditional optical components. In the future, we anticipate that metasurfaces will play a pivotal role in various imaging applications, including real-time diagnostics, high-resolution imaging, and non-invasive medical procedures. Continued research and development in optics, materials science, and image processing will expand the application boundaries of metasurfaces, establishing them as a cornerstone of next generation bioimaging technologies.
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