Fig. 1. Schematic diagram of non-collinear phase matching process

Fig. 2. The relationship between phase matching wavelength and half-angle of the field of view with different polarization periods of lithium niobite (LN) crystal at 30 ℃ under the action of 1080 nm pump beam

Fig. 3. Schematic diagram of the experimental setup. (a) Experimental configuration diagram; (b) Schematic diagram of chirp polarization crystal structure

Fig. 4. Directly detecting the spectrum obtained from up-conversion beam

Fig. 5. Imaging results of mid-infrared illuminated scale plate

Fig. 6. Comparison of results between direct detection up-conversion detection of mid-infrared signal. (a) Imaging results of MCT thermal imager; (b) Imaging result of Si based EMCCD after up-conversion

Fig. 7. Single-photon level up-conversion imaging results of mid-infrared signal. (a) (b), (c), and (d) are the letters "U", "S", "T", and "C", respectively

Fig. 8. Imaging results of pump with different spatial modes. (a1)-(a4) Pump is in Gaussian mode, achieving imaging of the signal passing through the object; (b1)-(b4) Pump is in Laguerre-Gaussian mode, achieving edge enhanced imaging of mid-infrared signal

Fig. 9. Screenshot of real-time imaging video of candle flames. (a) When there is no external airflow disturbance, the candle flame burns vertically; (b), (c) The candle flame oscillates when disturbed by airflow

Fig. 10. Imaging results of soldering irons at different temperatures in a thermal background. (a) The temperature of the soldering iron is higher than the background; (b) The temperature of the soldering iron is approximately equal to the background temperature; (c) The temperature of the soldering iron is lower than the background temperature; (d)-(f) Imaging results of soldering iron under room temperature background, corresponding to temperatures of 333, 453, and 763 K, respectively