Ghost imaging in the time domain allows for reconstructing fast temporal objects using a slow photodetector. The technique involves correlating random or preprogrammed probing temporal intensity patterns with the integrated signal measured after modulation by the temporal object. However, the implementation of temporal ghost imaging necessitates ultrafast detectors or modulators for measuring or preprogramming the probing intensity patterns, which are not available in all spectral regions especially in the MIR region. A team of scientists, led by Sichuan University professor Houkun Liang and professor Goëry Genty from Tampere University, have developed a frequency downconversion temporal ghost imaging (TGI) scheme that enables to extend the operation regime to arbitrary wavelengths regions where fast modulators and detectors are not available. The approach modulates a signal with temporal intensity patterns in the near-infrared and transfers the patterns to an idler via difference-frequency generation in a nonlinear crystal at a wavelength where the temporal object can be retrieved. The absence of suitable instrumentation, such as ultrafast MIR electro-optic modulators for preprogramming temporal patterns at MIR light sources, has been a bottleneck in the direct implementation of computational TGI in the MIR. In the proposed new scheme, instead of directly preprogramming temporal patterns at MIR wavelengths, preprogrammed temporal patterns are modulated at NIR wavelengths using a conventional telecom modulator and, subsequently, these modulated patterns are transferred to a MIR idler via difference-frequency generation using a temporally stable continuous-wave NIR pump light source. The wide availability of tunable lasers in the NIR allows for flexible and versatile operation of the downconversion TGI scheme, enabling the extension of TGI to wavelength regimes where there is a lack of fast detectors and modulators. As a proof-of-concept, the researchers demonstrated a computational temporal ghost imaging in the MIR with operating wavelength that can be tuned from 3.2 to 4.3 μm. The scheme is flexible and can be extended to other regimes. According to the researchers, the results introduce new possibilities for scan-free pump-probe imaging and the study of ultrafast dynamics in spectral regions where ultrafast modulation or detection is challenging such as the MIR and THz regions. The team expects that it can also be applied in the spatial and spectral domains, which could open a new venue for single-pixel imaging and spectroscopy in the mid-infrared and THz regions The research was published in Light: Science & Application (www.doi.org/10.1038/s41377-024-01426-0).
