High-power mid-infrared lasers extensively apply in explosive monitoring, medical diagnosis, environmental monitoring, infrared countermeasures, and industrial control. However, limited by the upper-level electron injection efficiency and the energy level structure, the output power of the mid-infrared laser quantum cascade operating under fundamental transverse mode cannot exceed 3 W. Beam combining technology has been proven to be an effective way to further expand output power and brightness. Taking full advantage of good linear polarization characteristics of semiconductor lasers, polarization beam combining offers simple structure and high efficiency. Moreover, it can be synergistically combined with other beam combining technologies to further enhance output power and laser brightness. Traditional polarization beam combiners are Brewster plate, metal grating polarizer, and birefringent prism. For the Brewster plate, broadband (approximate 100 nm), high transmission coating for P-polarization is required. It is a big challenge to mid-infrared. Due to the presence of metal lines, the transmission of the metal grating polarizer is usually lower than 80%, which results in a low beam combing efficiency. Commonly used birefringent MgF2 prism in mid-infrared has a small separation angle of 1.2°, which makes the optical path arrangement difficult. Metasurfaces offer a high degree of freedom in optical wave amplitude, phase, and polarization state regulations. It has already been applied in polarization beam splitters, which inspires the design of a beam combiner. By anomalous reflection, two light beams with orthogonal polarizations and different incident angles can be reflected in the same direction. We propose a polarization beam combiner based on anomalous reflection metasurface, which shows a high efficiency and broad working spectral band.
The proposed metasurface consists of periodic supercells. Each supercell contains 10 discrete single cells, which comprise a metal substrate, a dielectric middle layer, and a top rectangular column. By changing the two side lengths of the rectangular column of single cells, desired phase responses can be achieved for two orthogonal polarized incident beams. When ten optimized single cells are arranged spatially, the X-direction linear polarization (X-LP) incident beams experience a positive linear phase response. Meanwhile, the Y-direction linear polarization (Y-LP) incident beam experiences a negative linear phase response. Therefore, both beams are reflected perpendicularly to the metasurface when the incident angles of X-LP and Y-LP beams are 11.54° and -11.54° respectively.
According to the purpose and methodology of this study, we design a metasurface polarization beam splitter optical path operating in the middle-wave infrared range [Fig. 1(a)] and a three-layer metasurface structure [Fig. 1(b)]. By modeling the individual unit cell of the metasurface [Fig. 1(c)], we calculate the phase and amplitude responses of X-LP and Y-LP incident beams as we vary the length and width of the rectangular antenna column from 0.2 to 1.6 μm [Fig. 2(a)-(b)]. The particle swarm optimization algorithm is employed to determine the dimensions of the rectangular antenna that satisfy our desired phase requirements (Table 1). The phase introduced by the designed single cell aligns well with expectations for both incident polarizations while maintaining consistently high reflectivity levels throughout [Fig. 2(c)]. When the collimated X-LP and Y-LP beams with a central wavelength of 4.0 μm and incident angles of 11.54° and -11.54° reach the metasurface, both beams are reflected anomalously in the normal direction of the metasurface [Fig. 3(a)-(b)]. Reversely, when the collimated X-LP and Y-LP beams incident perpendicularly on the metasurface beam combiner, the X-LP beam is reflected in 11.54° direction and the Y-LP beam is reflected in -11.54° direction [Fig. 3(c)-(d)]. When the incident beam has a divergence angle of 50 mrad, the reflected beam has a divergence angle of 48 mrad [Fig. 3(e)]. To study the anomalous reflection of the metasurface for a light source with broad spectral bandwidth, we scan the incident light central wavelength from 3.9 to 4.1 μm. The resulting combined beam exhibits a divergent angle of 10 mrad while the reflectivity is maintained as high as 95% (Fig. 4). Based on the above theoretical simulation results, the reflected beam propagation properties in the free space are investigated by Zemax’s physical optical propagation (POP) tool. Assuming both X-LP and Y-LP incident beams are fundamental Gaussian beams, the beam quality factor of the reflected X-LP beam is 1.11, and that of the reflected Y-LP beam is 1.12 (Fig. 5). Therefore, when X-LP and Y-LP beams are combined to propagate in the same direction, the beam quality factor of the combined beam is 1.12 [Fig. 6(a)]. The spectrum of the combined beam coincides well with the overlap of the two individual incident laser spectra [Fig. 6(b)]. Consequently, this study demonstrates that a polarization beam combiner based on the anomalous reflective metasurface has not only high combination efficiency but also broad operation bandwidth. It is suitable to be used for the mid-infrared QCL power combining.
We study a polarization beam combiner based on anomalous reflective metasurface, which is utilized to combine two incident beams with orthogonal linear polarizations. The beam combiner consists of periodic supercells with a dimension of 2 μm×20 μm. Each supercell contains 10 single cells of 2 μm×2 μm. The single cell comprises a metal substrate, a dielectric middle layer, and a top rectangular column. When the collimated X-LP and Y-LP beams with a central wavelength of 4.0 μm and incident angles of 11.54° and -11.54° reach the metasurface, both beams are reflected anomalously in the normal direction of the metasurface. The polarization beam combination is realized. Within a broad spectral band of 3.9 to 4.1 μm, high average anomalous reflectivity of 96.6% and 97.7% are obtained for X-LP and Y-LP incident beams, respectively. Based on the near field reflective intensity and phase distribution, the propagation of the combined beam in free space is simulated by the periodic field splicing method and Gaussian beam propagation law. Assuming both X-LP and Y-LP incident beams are fundamental Gaussian beams, the beam quality factor of the combined beam is 1.12. The metasurface beam combiner shows high design flexibility and can be prepared by mature MEMS technology. It has a good potential to solve the problems in mid- and long-infrared power beam combinations.
