Wave morphing and flat-top ground states in photonics systems driven by artificial gauge fields

Abstract

In quantum physics, classical optics, and many other wave systems, wave confinement in a potential well is associated with discrete oscillatory states, and the ground state is typically assumed to vanish uniformly. An open question is whether the ground state can counterintuitively support a flat-top, nonzero envelope, offering new opportunities for quantum emitters, optical antennas, and lasers. Here, we show that by applying the Byers-Yang theorem with an artificial gauge field, energy levels can be continuously shifted, driving eigenstates to morph into a ground state with a uniform yet nontrivial wave envelope. We implement this concept in a photonic crystal slab where a central bulk region is surrounded by heterogeneous bandgaps that engineer reflective phases acting as an artificial local gauge field. By inducing lasing, we probe directly the evolution of the energy levels, demonstrating wave morphing toward a flat-top ground state via near- and far-field measurements.

Figures (5)

• Figure 1: Principle of energy level shifting and wave morphing enabled by artificial gauge field.
• Figure 2: A 2D heterogeneous PhC realization.
• Figure 3: Sample design, fabrication and experimental setup. (a) Schematic of the designed sample on a hetero-bonded wafer, with a SOI patterned as PhCs, and III-V epitaxial layers providing optical gain. (b) Top view of the fabricated PhC (left panel), whose total size is $58.8\times58.8~\mu$m$^2$, and the dashed box distinguishes the bulk region from the boundary region. The right panel shows the zoomed-in details. (c) Cross-sectional view of bulk PhC obtained by FIB cleaving (left), with its schematic provided for reference (right panel). The red line shows the cross-sectional profile of the TM$_B$ mode as our lasing candidate. (d) Calculated band structures (left) and $Q$ diagram (right panel) of the bulk region. Band TM$_B$ is highlighted in red, and the photoluminescence range is gray-shaded. (e) Schematic of measurements setup, with pumping light (green line) and lasing emission (orange lines). ND: Neutral density filter; BS: Beam splitter.
• Figure 4: Experimental demonstration of energy level shifting and wave morphing.
• Figure 5: Near-flat-top ground state in $C_2$-symmetry-broken system. (a) Schematic of the irregularly-shaped air hole without $C_2$ symmetry, designed as a circle of radius $r$ cut by an equilateral triangle of side length $l$. (b) SEM image of the fabricated sample with designed irregularly-shaped air holes. (c) Measured single-mode lasing spectrum at a wavelength of $1534$ nm. (d) Measured NFP (left) and FFP (right panel) of the lasing beam. A flat yet nonzero NFP is obtained, arising from the radiation induced by the $C_2$ symmetry breaking. A bright spot in the far field is observed, with a small divergence angle of $1.6^\circ$. (e) Measured $x$- (left) and $y$-polarized (right panel) components of the lasing beam. The $x$-polarized component nearly vanishes while the $y$-polarized component exhibits a flat distribution.