When designing a light source, one typically restricts their aim to certain correla­tion properties. For example, in a single-mode laser, corre­lations can be found across the entire wavefront. However, optical cavities that support only a single mode are usually relatively difficult to manu­facture. A cavity that supports multiple modes is easier to fabri­cate, but in such a laser the corre­lations vanish, depending on the number of modes present.

“Both of these source types have their uses and in the past 20 years, a lot of effort has been expended to attain the best of both worlds. A few years ago, the idea of switching correlation properties came up and it has gotten consi­derable attention ever since,” says Post­doctoral researcher Matias Koivurova from Tampere University. The team’s approach is more sophisti­cated than earlier attempts and achieves near perfect switching between high and low correlation states.

By inserting an enhanced epsilon-near-zero (eENZ) mirror as one of the cavity mirrors, the conven­tional multimode cavity is modified to support only one mode, regardless of the resonator para­meters. What makes this modi­fication possible is that the eENZ mirror behaves like an angular pinhole: when the incident light has a suitable polari­zation, it reflects light in a relatively narrow cone only. The eENZ mirror has a peculiar response to different polari­zations of light. When transverse electric (TE) polarized light impinges upon it, the meta­material behaves more like a conventional mirror. On the other hand, when the polari­zation is rotated 90 degrees to transverse magnetic (TM), the angular pinhole effect becomes prominent.

The TM-polarized light excites Ferrell-Berreman modes within the material, strongly atte­nuating light that hits the eENZ mirror at a large enough angle. “By controlling the polari­zation of the light inside the cavity, our team was able to switch the correlation properties of the light at will, going from nearly uncorre­lated to completely correlated. Now we just need to build the device!” Koivurova concludes.

The study is the first to demons­trate solid state switching of corre­lation properties and it is expected to find appli­cations in imaging, flow detection, free-space tele­communication, and even inertial confine­ment. Addi­tionally, the eENZ mirror allows for relatively easy fabri­cation of small and high-quality lasers. (Source: Tampere U.)

Reference: M. Koivurova et al.: Coherence Switching with Metamaterials, Phys. Rev. Lett. 127, 153902 (2021); DOI: 10.1103/PhysRevLett.127.153902

Link: Photonics, Faculty of Engineering and Natural Sciences, Tampere University, Tampere, Finland