image: a Schematic diagram of the dual-color THz SLM. The upper and lower wings of a butterfly reflect THz waves at two frequencies, so their spectral responses, i.e., colors, are different. b Images of projection displays at two frequencies. Because of the distinct spectral responses of pixels at two frequencies, the obtained images are complementary. c Images of dispersive objects at two different frequencies were obtained using dual-color CS imaging. d Images of non-dispersive objects based on the frequency-switching method. The imaging quality based on the ACS algorithm is superior to the conventional CS algorithm for dual-color CS imaging and frequency-switching methods. view more
Credit: by Weili Li, Xuemei Hu, Jingbo Wu, Kebin Fan, Benwen Chen, Caihong Zhang, Wei Hu, Xun Cao, Biaobing Jin, Yanqing Lu, Jian Chen, Peiheng Wu.
Spectral fingerprints of materials in the THz range hold a myriad of captivating light-matter interactions for physics and material science, such as molecular rotation and spin waves. The noninvasive identification of these signatures with spectral and spatial information enables numerous potential applications in biomedical diagnostic, pharmaceutical industry, and security inspection. However, the current THz spectral imaging techniques, such as coherent receiver array and THz time-domain spectral imaging system, require complicated and expensive components, limiting their widespread application. Single-pixel CS imaging provides a route to achieve THz imaging at a low cost. However, few works have been made to attain multispectral imaging. The THz SLM, a critical component in CS imaging, still has shortcomings in modulation depth, uniformity, and speed.
In a new paper published in Light Science & Applications, a joint team of scientists, led by Professor Biaobing Jin, from Nanjing University and Purple Mountain Laboratories, China, have developed a THz SLM based on metasurface absorber and dual-frequency liquid crystal. Based on the THz SLM, they realized dual-color THz CS imaging. The imaging objects with dispersion characteristics could be imaged using the device. The auto-calibrated CS (ACS) algorithm was proposed to eliminate spatial nonuniformity of THz incident beam and pixel modulation. The ACS algorithm significantly improves the fidelity of the reconstructed images compared with conventional CS algorithms. Furthermore, the complementary modulation at two absorption frequencies enables Hadamard masks with negative element values to be realized by frequency switching. They proposed a frequency-switching method to realize the CS imaging of non-dispersive objects and shorten the imaging time to half. These scientists summarize the advantages of the proposed device and methodologies.
“Liquid crystal is an excellent tunable material for making THz programmable devices. Our work demonstrates the potential of liquid crystal programmable devices for multispectral THz CS imaging. Furthermore, the LC-based THz devices are compatible with the production line of the display and personal portable devices. Slow switching speed is a major limitation of liquid crystal devices. The frequency-switching method proposed in our work can effectively reduce the imaging time. The low-cost THz programmable devices may find vast applications in passive beamforming antenna, spectral imaging, and projection display.”
“The inhomogeneity of pixel performance and THz light source will cause the deterioration of image quality. Improving the uniformity of pixels in SLM is a challenging task for device fabrication, and eliminating the nonuniformity of the THz spot requires the precise adjustment of the optical path. The calibration process and image reconstruction are implemented simultaneously using the proposed ACS algorithm, and the above arduous work could be avoided.” The scientists added.
“In the future, training an end-to-end deep neural network capable of automatically calibrating these nonuniformities will be a promising direction for THz CS imaging. The liquid crystal THz SLM empowered by advanced CS algorithm provides a possible way for practical THz spectral imaging technology.” the scientists forecast.
Journal
Light Science & Applications