Electrically tunable planar liquid-crystal singlets for simultaneous spectrometry and imaging

The information we acquire directly influences our understanding and perspectives of the world. It has long been an endeavor in optics to perceive the multidimensional information around us using the “toolbox” of light. In the 17th century, Sir Isaac Newton put forward the formula for lens imaging and carried out the color spectrum experiment. Since then, lenses and spectrometers have been extensively studied as essential optical components for capturing information. Cascading these two components can allow us to acquire more information – both spatial and spectral data. However, such a configuration leads to tradeoffs among device footprint, spectral resolution, and imaging quality, impeding portability and miniaturization of hyperspectral cameras.

 

In a new paper published in Light: Science & Application, a collaborative team of scientists from Wuhan University and Nanjing University have introduced a planar spectral singlet lens that unifies two distinct functions – optical imaging and computational spectrometry – into a single, ultra-compact planar device. With this spectral lens, a standard camera can be easily upgraded to a hyperspectral camera by replacing the original lens with the spectral lens, without the need of changing other parts in the camera.

 

This study originates from the research on multidimensional light manipulation in the group of Prof. Guoxing Zheng, the leading scientist of this project. “The limitation of spectral imaging performance in traditional cascading setup is rooted in the light manipulation capability of the optical elements.” said Prof. Guoxing Zheng, “Spectrometry and imaging are two distinct information acquisition techniques, and they have to be implemented with separate light manipulation mechanisms. We need to explore new principles for light controls to overcome the performance constraints”.

 

The spectral lens was physically implemented using planar liquid crystal (LC) optics. “We revisited the light manipulation of planar LC devices, and found that phase modulation and spectral control can be customized independently by exploiting two geometrically separable parameters – LC directors’ azimuth orientation and polar orientation,” said Prof. Peng Chen, an expert in liquid crystal optics and coworker of this project. “Such characteristics position LCs as a perfect candidate to develop spectral lens and address the challenges in current miniaturized spectral imaging systems.” In their proposed LC spectral lens, each unit cell acts as both a phase modulator and an electrically tunable spectral filter. This allows the LC device to possess precise phase controls in a broad spectral range for high-quality imaging; and at the same time, the spectral lens exhibits diverse focusing characteristics across different wavelengths to allow spectral information extraction.

 

The researchers demonstrated a proof-of-concept millimeter-scale hyperspectral camera using the LC spectral lens. With their hyperspectral camera, high-quality spectral images with 500 × 500 pixels, mean spectral fidelity of 96.3%, and a spatial resolution of ~ 1.7 times the diffraction limit were obtained. They also demonstrated the versatility of their proposed hyperspectral camera by applying it in various scenarios, including hyperspectral imaging of posters and LED screens. Compared to traditional hyperspectral cameras, their proposed approach with spectral singlet lens could significantly reduce the size, weight, and complexity of hyperspectral imaging systems, making them more accessible for a wide range of emerging applications such as drones, smartphones, and portable healthcare devices. “Our study provides a new recipe for miniaturized hyperspectral imaging,” added co-author Prof. Zile Li from Wuhan University. “This technology not only simplifies the current system but also maintains excellent performance, making it ideal for scenarios where miniaturization and portability are crucial.”

 

As the team continues to extend their research, they envision exploring the spectral singlet implementation with other novel materials, as well as upgrading the spectral lens through the synergy with other versatile light manipulation platforms to unfold imaging systems with more advanced functionalities.

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