Fig. 1. Schematic of the 1D PTSPROWN, including three unit cells, one entrance, and one exit, where EI, ER, and EO are the input, reflective, and output EM waves, respectively, and the length for each waveguide segment is d. The thin black solid lines at the entrance and exit are all vacuum optical waveguide segments. The thick red solid lines in unit cells are PT-symmetric optical waveguide segments, where the refractive indices of the three subwaveguides are, respectively, n1, n2, and n3, and their length ratios are l1, l2, and l3, respectively.

Fig. 2. Dispersion curve of MgF2 defined by Eq. (2), where the thick red solid line expresses the range investigated in this paper; the thin black dashed lines are all scaling lines, and the values are, respectively, ν1=43  THz (λ1=7.00  μm), ν2=100  THz (λ2=3.0  μm), ν3=600  THz (λ3=0.5  μm), and ν4=1500  THz (λ4=0.2  μm).

Fig. 3. Distribution diagram of photonic modes, where white and red zones represent WPMs and SPMs, respectively. (a) Frequency range corresponds to the thick red solid line in Fig. 2. (b) Enlarged drawing of the zone for the first extremum spontaneous PT-symmetric breaking point, whose location is near the communication wavelength, λ=1550  nm.

Fig. 4. Transmission and reflection spectra at the first extremum spontaneous PT-symmetric breaking point of the 1D ring optical waveguide network with three unit cells, where red dashed and dotted lines and black dotted lines express left- and right-incident results of PT-symmetric network, respectively; thick blue dashed lines denote the results of pure gain-material network with the refractive index n=1.3705−ι2.181×10−8; thin green dotted lines indicate the results of pure loss-material network with the refractive index n=1.3705+ι2.181×10−8. (a) Transmission spectra; (b) reflection spectra.

Fig. 5. Intensity map of the photonic localization at the first extremum spontaneous PT-symmetric breaking point of the 1D PTSPROWN with three unit cells, where i–d–j (i,j=1,2,3) expresses the upper or lower arm waveguide segment between nodes i and j, and three frequencies are, respectively, ν1=193.415780  THz, ν2=193.415787  THz, and ν3=193.415793  THz. (a)–(c) Three-dimensional map; (d)–(f) two-dimensional map.