Multispectral Imaging via a Thermally Tunable Reflective Planar Lens

Abstract

Multispectral imaging plays a crucial role in simultaneously capturing detailed spatial and spectral information, which is fundamental for understanding complex phenomena across various domains. Traditional systems face significant challenges, such as large volume, static function, and limited wavelength selectivity. Here, we propose an innovative dynamic reflective multispectral imaging system via a thermally responsive cholesteric liquid crystal based planar lens. By employing advanced photoalignment technology, the phase distribution of a lens is imprinted to the liquid crystal director. The reflection band is reversibly tuned from 450 nm to 750 nm by thermally controlling the helical pitch of the cholesteric liquid crystal, allowing selectively capturing images in different colors. This capability increases imaging versatility, showing great potential in precision agriculture for assessing crop health, noninvasive diagnostics in healthcare, and advanced remote sensing for environmental monitoring.

Keywords

liquid crystal / planar lens / multispectral imaging / photopatterning

1 Introduction

Multispectral imaging (MSI) has kept being a vital technique across various fields, including agriculture, environmental monitoring, and biomedical applications [ 1 - 4 ]. By capturing images at multiple specific bands of the electromagnetic spectrum, MSI enables the simultaneous detection of spectral features and spatial information, facilitating comprehensive analysis of material compositions and real-time conditions that are often imperceptible to the naked eye. Its noninvasive nature ensures that samples remain intact, making it invaluable for delicate analyses. Traditional multispectral cameras typically utilize a series of discrete spectral filters combined with a lens array, thus suffering from limited spectral range and cumbersomeness \(\left\lbrack {5,6}\right\rbrack\) . Along with the developments of computational theories and optical components, snapshot imaging based on dispersive devices or coded apertures emerges. However, the spatial resolution is compromised and complex algorithms such as compressive sensing and deep learning are usually required for image reconstruction [ 7 - 9 ]. Thus, there is a pressing need for more streamlined, compact, and active solutions for advanced MSI systems.

As essential optical components in MSI systems, spectrometers have been improved via various strategies, such as filter arrays [ 10 ], quantum points [ 11 ], and metasurfaces [ 12 ]. In particular, liquid crystal (LC)-based tunable filters have been proved as a powerful tool thanks to their excellent electro-optical properties. Nematic LCs have been widely used as phase retarders, leveraging electrically controlled birefringence to modulate the spectral domain in spectral imaging systems [ 13 ]. A series of nematic LC wave-plates sandwiched between two polarizers were assembled together to obtain the continuous and arbitrary selection of wavelength in the visible band [ 14 ]. Moreover, a thin LC cell and a 3D neural network lattice were utilized to accelerate the speed and improve the efficiency of MSI [ 15 ]. However, extra glass lenses for imaging are still needed in the aforementioned systems, resulting in a bulky volume. An electrically tunable LC spectral singlet lens with high spectral fidelity and spatial resolution has been reported recently [ 16 ], however, complex recovery algorithms is required due to the oscillatory transmittance of nematic LCs. Cholesteric LC (CLC), characterized by periodic helical structures, presents a promising bandpass filter, arising from its circular polarization selective photonic bandgap (PBG)[ 17 ]. Diverse CLCs have been developed with a wide range of applications, including beam steering and circularly polarized luminescence [ 18 - 21 ]. Stimulated by various external fields, the PBG can be finely adjusted across the visible spectrum, enabling CLCs tunable color filters with excellent wavelength selectivity [ 22 , 23 ] . Since the discovery of the geometric phase based on CLC [ 24 - 26 ], CLC reflective planar lenses exhibiting spin selectivity have been broadly explored [ 27 - 30 ]. For instance, low \(f\) -numbers reflective CLC lenses have been fabricated by imprinting technology, exhibiting a reasonably high reflection efficiency and good image quality [ 27 ]. Such planar optics integrate the lens functions with a spectrometer, making the MSI systems compact and efficient.

In this study, we propose a lightweight and cost-effective MSI method based on a thermally stimulated CLC lens, that seamlessly integrates tunable filtering with imaging functions. By varying the temperature within a range of \({5.3}^{\circ }\mathrm{C}\) , the entire PBG of the device can be reversibly shifted by \({250}\mathrm{\;{nm}}\) due to the helical pitch variations. The wavelength selectivity afforded by the planar lens allows for the precise extraction of images with desired colors through meticulous temperature control. An MSI across the entire visible spectrum is demonstrated accordingly. The self-assembled chiral superstructure preserves high image quality throughout the whole adjustment process. This research provides an efficient framework for active MSI that can capture both intensity and color information simultaneously, and significantly promotes applications of soft matter photonics.

2 Design Principle

The one-dimensional self-assembled chiral superstructure endows the CLC with a spin-selective PBG that spans from \({n}_{\mathrm{o}}p\) to \({n}_{\mathrm{e}}p\) , where \(p\) represents the helical pitch and \({n}_{\mathrm{o}}/{n}_{\mathrm{e}}\) represents the ordinary/extraordinary refractive index, respectively. When light matches the handedness of the CLC and within the PBG incident, it will be reflected and carry a geometric phase \(\varphi\) that is twice the initial surface LC orientation \(\alpha\) of the CLC, expressed as \(\varphi =\pm {2\alpha }\) , where the signs depend on the chirality of the CLC. In contrast, the remaining components transmit CLCs with a uniform phase delay. This phenomenon allows for the realization of reflective imaging by presetting the initial LC orientation as a parabolic profile of a focusing lens, which can be calculated based on the optical path difference between the center and edge of the lens, described by the following equation

(1) \[\varphi =- \frac{2\pi }{\lambda }\left({\sqrt{{r}^{2}+ {f}^{2}}- f}\right)\]

where \(\lambda , r\) , and \(f\) are the incident wavelength, radius, and focal length of the lens, respectively. Thus, incident light with the same handedness as the CLC is reflected, converged, and form an image. In addition, owing to the rich field responsiveness of CLC, the helical configuration can be flexibly manipulated through light, heat, electric/magnetic fields, and so on. It provides a robust platform for active photonic devices. In particular, the introduction of thermally stimulated CLCs facilitates dynamically tunable working wavelengths. By adjusting the temperature, one can finely control the helical pitch to tune the wavelength range of the reflected light. This temperature modulation advantageously expands the operational versatility of CLC-based devices, enabling responsive adjustments to environmental conditions.

Fig. 1 schematically illustrates the proposed active MSI system. Fig. 1 (a) shows the vertical view of the designed CLC planar lens, which features a chiral superstructure where the LC undergoes a periodic rotation along the helical axis, and the helical pitch \(p\) is defined as the distance over which the LC directors complete a full \({360}^{\circ }\) rotation. The pink curves represent parabolic equiphasic surfaces. When red (R), green (G), and blue (B) light from an object is simultaneously incident on thelens, only the wavelengths located within the PBG can be reflected and imaged. As depicted in Fig. 1 (b), when the device is subjected to thermal variations (heating or cooling), the helical pitch adjusts accordingly, resulting in a wavelength shift of the PBG. This adjustment alters the operation band of the system, thereby enabling adaptive functionality tailored to specific imaging requirements. Consequently, by meticulously controlling the temperature of the hot stage, the system enables the selective imaging of RGB light components, facilitating the realization of advanced MSI capabilities ( Fig. 1 (c)). This approach enhances the versatility and responsiveness of the imaging system, making it an innovative solution for applications requiring detailed and accurate spectral analysis.

3 Materials and Methods

Here, the CLC material was mixed by a nematic LC host E7 (NCLCP, China) and the left-handed chiral dopant S811 (NCLCP, China) at a weight ratio of 68:32. To facilitate alignment, a UV photosensitive alignment agent, SD1, was dissolved in dimethylformamide at a concentration of \({0.3}\mathrm{{wt}}\%\) and subsequently spin-coated onto indium tin oxide (ITO) glass substrates [ 31 , 32 ] . The coated substrates were annealed on a hot stage at \({100}{}^{\circ }\mathrm{C}\) for \({10}\mathrm{\;{min}}\) to form a uniform photoalignment layer. An \({8\mu }\mathrm{m}\) -thickness empty cell was assembled and subsequently exposed under a digital micro-mirror device (DMD). Finally, the prepared CLC material was filled into the cell by capillary action, initially in its isotropic state at \({70}^{\circ }\mathrm{C}\) . The temperature was then rapidly decreased to \({30.1}{}^{\circ }\mathrm{C}\) at a rate of \({30.0}{}^{\circ }\mathrm{C}/\mathrm{{min}}\) using a Linkam LTS120 hot stage, resulting in the formation of uniform textures. Under the effective guidance of the SD1 layer, a helical configuration was developed into a planar lens measuring approximately \(2\mathrm{\;{mm}}\times 2\mathrm{\;{mm}}\) .

In our experiment, the device was observed under a polarized optical microscope (POM) with a pair of crossed polarizers. As illustrated in Fig. 2 (b), the POM images exhibit clear patterns with minimal defects and impurities that could compromise the optical efficiency. Alternation of brightness and darkness in microstructure were observed, which are consistent with the designed lens phase. The helical pitch \(p\) of the CLC is determined by the following formula [ 17 ]

(2) \[ p =\frac{1}{c \times \mathrm{{HTP}}}\]

where \(c\) denotes the weight ratio and HTP signifies the helical twisting power of the chiral dopant. The center wavelength \({\lambda }_{\mathrm{c}}\) and bandwidth \({\Delta \lambda }\) of the PBG are calculated as

(3) \[{\lambda }_{\mathrm{c}}= \left({{n}_{\mathrm{e}}+ {n}_{\mathrm{o}}}\right)/2 \times p \]

and

(4) \[{\Delta \lambda }= \left({{n}_{\mathrm{e}}- {n}_{\mathrm{o}}}\right)\times p \]

Therefore, we can derive the following formula:

(5) \[{\lambda }_{\mathrm{c}}= \left({{n}_{\mathrm{e}}+ {n}_{\mathrm{o}}}\right)/\left({2 \times c \times \mathrm{{HTP}}}\right)\]

Upon heating the device from \({29.9}^{\circ }\mathrm{C}\) to \({35.2}^{\circ }\mathrm{C}\) , a notable blueshift of \({250}\mathrm{\;{nm}}\) for \({\lambda }_{\mathrm{c}}\) was observed, with the rate of change gradually decreasing, as depicted in Fig. 2 (c). The \({\lambda }_{\mathrm{c}}\) in the cooling process closely matches that in the heating process, confirming the device’s reversibility. The experimental results agree well with the theoretically fitted curve based on Keating theory [ 33 ]. The model can be described as the formula

(6) \[ p =\gamma \frac{{T}_{0}}{T}\left({1 +\frac{\beta }{T -{T}_{0}}}\right)\]

where \(p\) is helical pitch, \(T\) is temperature, and \(\beta\) , \(\gamma\) , and \({T}_{0}\) are three parameters.

Fig. 2 (d) presents recorded reflection spectra measured through a fiber spectrometer at a normal angle, demonstrating that the widely shifted PBG spans nearly the entire visible light spectrum from 450 to \({750}\mathrm{\;{nm}}\) . It is attributed to the temperature-dependent solubility of S811 and variance in \({n}_{\mathrm{e}},{n}_{\mathrm{o}}\left\lbrack {{34},{35}}\right\rbrack\) . As the temperature rises, the solubility of S811 in E7 increases, shortening the CLC pitch length and causing a blue shift in the reflection spectrum due to the higher dopant concentration. Meanwhile, the birefringence of E7 decreases slightly with wavelength, and the bandwidth is influenced by both the birefringence and the helical pitch. The reflectivity of the device remains over \({40}\%\) , nearing the theoretical maximum of \({50}\%\) . The brilliant structural color of the texture also undergoes corresponding changes, sequentially becoming red, orange, yellow, green, cyan, and blue ( Fig. 2 (b)). Notably, the structure maintains well after temperature stabilization, and the entire process is reversible. The developed device exhibits excellent tunable optical characteristics and high reflectivity, making it highly significant for applications in multispectral imaging, where efficient color modulation and imaging performance are essential.

4 Results and Discussions

The thermally tunable planar lens with wavelength selectivity across the visible band provides a possibility for the active MSI. Fig. 3 (a) illustrates the optical setup employed for the focusing and imaging characterization of the device. A supercontinuum laser (SuperK EVO, NKT Photonics, Denmark) combined with an acousto-optic filter (SuperK SELECT, NKT, Denmark) is utilized to output various monochromatic lasers. A polarizer is combined with a broadband quarter waveplate to convert incident light into circularly polarized light consistent with the chiral superstructure. Since the lens operates under an on-axis reflective mode, a charge-coupled device (CCD) was strategically positioned in front of a beam splitter (BS) to capture the resultant images. First, the focal length \(f\) of the lens at \({533}\mathrm{\;{nm}}\) was detected by incrementally adjusting the position of the CCD near the focal point. Through the collection and processing of multiple sets of cross-sectional intensity data, a longitudinal propagation path in the \(x - z\) plane was constructed, as depicted in Fig. 3 (b). The peak light intensity indicated a focal length of \(f ={72}\mathrm{\;{mm}}\) at \({533}\mathrm{\;{nm}}\) , matching well with our design. Characterization of the corresponding point spread function (PSF) revealed a full width at half-maximum (FWHM) of \({30\mu }\mathrm{m}\) at this wavelength ( Fig. 3 (c)). To verify the MSI capacity of the lens, a hollow mask of the letter “E” was inserted in front of the BS as an object. By regulating the temperature of the hot stage, the letter “E” at different wavelengths can be clearly imaged, as shown in Fig. 3 (d). According to Eq.(1), \(f\) is negatively correlated with \(\lambda\) . The Gaussian imaging formula \(1/v + 1/u = 1/f\) , where \(v\) and \(u\) denote the image distance and object distance, respectively, reveals that \(v\) is positively correlated with \(f\) when \(u\) is fixed. Thus, the size of images increased as the wavelength decreased. In the test, \(u ={10}\mathrm{\;{cm}}\) was fixed, and the distances \(v\) measured were \({15}\mathrm{\;{cm}}\left({{633}\mathrm{\;{nm}}}\right)\) , \({17.5}\mathrm{\;{cm}}\left({{600}\mathrm{\;{nm}}}\right),{19.5}\mathrm{\;{cm}}\left({{580}\mathrm{\;{nm}}}\right),{24}\mathrm{\;{cm}}\) (533nm), and \({36}\mathrm{\;{cm}}\left({{480}\mathrm{\;{nm}}}\right)\) . Notably, the edges of the images became sharper and clearer at shorter wavelengths because of the improved resolution when the incident angle remained unchanged. The focusing efficiencies of the lens are measured as \({89.2}\%\left({{633}\mathrm{\;{nm}}}\right),{87.4}\%\) \(\left({{600}\mathrm{\;{nm}}}\right),{88.6}\%\left({{580}\mathrm{\;{nm}}}\right),{84.6}\%\left({{533}\mathrm{\;{nm}}}\right)\) , and \({77.8}\%\left({{480}\mathrm{\;{nm}}}\right)\) . These results demonstrate the effective performance of the lens in a broad band.

The potential of MSI utilizing this tunable lens, with its working wavelength selectivity across the visible spectrum, is particularly compelling. An RGB color image, consisting of the red letter “R”(645 nm), green letter “G”

(525nm), and blue letter “B”( \({465}\mathrm{\;{nm}}\) ), is generated by an LC on silicon (LCoS) as the object. Fig. 3 (e) shows color-selective images formed by our CLC planar lens under different temperatures. The images were rescaled to restore the correct size ratio according to the imaging formula. The image quality at different wavelengths is almost equally good and closely matches that of the commercial lens ( Fig. 3 (f)). The color information captured can be easily discerned by the human eye, making it promising in environmental monitoring, biomedical imaging, and agricultural assessments, etc. These capabilities underscore the role of stimulus-responsive soft matter in advancing multispectral imaging technologies, and pave the way for innovative applications that leverage the rich data obtained from diverse wavelengths.

To evaluate the image quality of the thermally stimulated CLC lenses, we adopted a white LED as the light source and a 1951 USAF resolution target as the object. The measurement setup is illustrated in Fig. 4 (a), where the target was strategically positioned at less than one focal length to ensure optimal clarity during observation. Fig. 4 (b) shows the images of the 1951 USAF resolution target. Fig. 4 (c) represents the resolution test result of a commercial lens \(\left({f ={100}\mathrm{\;{mm}}}\right)\) . The entire Group No. 5 of the resolution target can be clearly observed. Owing to the unique polarization selectivity of our reflective CLC lens, only left-handed circularly polarized (LCP) light is reflected and focused, while right-handed circularly polarized (RCP) light passes through the lens, minimizing crosstalk in imaging compared with traditional geometric phase lenses. Furthermore, the thermally tunable PBG of the CLC lens allows for rich color variation in the images observed at different temperatures. Fig. 4 (d-f) show that the RGB resolution target images captured at \({30.1}^{\circ }\mathrm{C},{31.7}^{\circ }\mathrm{C}\) , and \({34.0}^{\circ }\mathrm{C}\) vividly demonstrate this capability. Notably, the entire Group No. 4 of the resolution target can be clearly discernible across these images, verifying our proposed lens’s excellent imaging performance and color selectivity. The resolution of our proposed method is slightly lower than that of commercial lenses. The slightly rounded edge of the image can be attributed to the simultaneous imaging of multiple wavelengths within the PBG. This thermal tunability enhances the versatility of the lens and enables the extraction of information corresponding to various colors from complex and colorful objects.

Compared with traditional MSI, our proposed MSI based on a CLC planar lens offers significant advantages, including low cost, lightweight, and compactness. The thin and flexible planar CLC lens weighs just \({1.1}\mathrm{\;g}\) , which reduces the overall weight of the imaging system, making it suitable for portable imaging devices. Furthermore, it features a dynamic and controllable working bandwidth across the visible range, surpassing the capabilities of conventional lenses. The phase transition sequence of the host LC \(5\mathrm{{CB}}\) is: Crystalline phase \(\left(\mathrm{{Cr}}\right)- {24}{}^{\circ }\mathrm{C}\) -Nematic phase (N) \(-{35.5}{}^{\circ }\mathrm{C}\) -Isotropic phase (I). Therefore, our CLC planar lens will not function during the crystalline or isotropic phase. Fortunately, thanks to the device’s reversibility and repeatability, its imaging capability can be recovered by applying the appropriate temperature. By capturing the reflected light and analyzing the data across different wavelengths, our proposed MSI unlocks significant potential in various fields. For instance, in precision agriculture, the proposed MSI may help monitor crop health by detecting early signs of stress, disease, or nutrient deficiencies. In non-invasive medical diagnostics, it can provide deeper insights without the need for invasive procedures. In environmental monitoring, it may improve remote sensing by enabling detailed monitoring of land use, vegetation health, and environmental changes. The working bandwidth can be extended to the infrared region by adjusting the concentration of the chiral dopant. Additionally, the spectral resolution and stability of the device can be improved by adopting nematic LCs with reduced birefringence or incorporating heliconical CLCs with narrower PBGs [ 36 ]. Moreover, to further increase the image quality, various strategies can be employed, including applying anti-reflective coatings on the substrates and enlarging the size of the lens zone through the use of lower amplified objective lenses during photopatterning. Beyond imaging applications, this innovative technology holds promise for various dynamic functionalities, such as tunable beam deflectors, vortex beam generators, and holographic displays, all achievable through careful predesign of exposure patterns.

5 Conclusion

We proposed a thermally tunable reflective CLC planar lens for MSI. It allows for the reversible and repeatable adjustment of the working wavelength range from \({450}\mathrm{\;{nm}}\) to \({750}\mathrm{\;{nm}}\) through temperature control. The planar lens has a focal length of \({72}\mathrm{\;{mm}}\) at \({533}\mathrm{\;{nm}}\) , and the FWHM of the corresponding PSF is \({30\mu }\mathrm{m}\) . Owing to the tunable bandpass PBG of CLC, MSI across the visible spectrum with good image quality and high efficiency is demonstrated. The device is compatible with both laser and non-coherent LEDs. Its polarization selectivity and reflective mode enable a reduction of optical components, thus minimizing the volume of the optical setup. The device suits to capture rich color information from targets, and paves the way for innovations across various fields, including telecommunications, medical diagnostics, and environmental monitoring.

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Funding

National Key Research and Development Program of China(2022YFA1203700)

National Natural Science Foundation of China(62405129)

National Natural Science Foundation of China(62035008)

University Research Project of Guangzhou Education Bureau(202235053)

Natural Science Foundation of Jiangsu Province(BK20241197)