Physicists employ synthetic complex frequency waves to overcome optical loss in superlenses

Figure 1. Schematic of imaging under real-frequency and synthesised complex frequency excitation in a superlens. The same object, when imaged through a superlens under different real-frequency illumination, results in images with varying degrees of blurriness, and none of the real-frequency images can discern the true appearance of the object. By combining the field amplitudes and phases of multiple single-frequency images, a clear image can finally be obtained.

Professor Shuang Zhang, corresponding author of the paper, explained the research foci,"To solve the optical loss problem in some important applications, we have proposed a practical solution—using a novel synthetic complex wave excitation to obtain virtual gain, and then offset the intrinsic loss of the optical system. As a verification, we applied this approach to the superlens imaging mechanism and theoretically improved imaging resolution significantly.

What does complex frequency mean? Frequency of a wave refers to how fast it oscillates in time, as shown in Figure 2a. It is natural to consider frequency a real number. Interestingly, the concept of frequency can be extended into the complex domain, where the imaginary part of the frequency also has a well-defined physical meaning, i.e., how fast a wave amplifies or decays in time. Hence, for a complex frequency wave, both oscillation and amplification of the wave occurs simultaneously.

Figure 3. Imaging patterns in multiple real frequencies and complex frequency of the letter"H." Credit: HKU The team further extended the principle to optical frequencies, employing an optical superlens made of a phononic crystal called silicon carbide, which operates at the far-infrared wavelength of around 10 micrometers. In a phononic crystal, the lattice vibration can couple with light to create the superimaging effect. However, the loss is still a limiting factor in the spatial resolution.

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