Topological Antilaser in Photonic Lattices

• Topological antilaser is defined as the time-reversed counterpart of a topological laser, achieving robust coherent perfect absorption via chiral edge modes in photonic lattices.
• It uses a nonreciprocal Floquet scattering network and engineered dissipation to pin S-matrix zeros to real frequencies, ensuring disorder-resistant absorption.
• Experimental results show that even with strong disorder, topological CPA maintains absorption efficiency above 92%, highlighting its potential in energy harvesting and photonic circuit design.

A topological antilaser is the time-reversed counterpart of a topological laser, synthesizing coherent perfect absorption (CPA) with topological protection. In this paradigm, dissipation is orchestrated via chiral edge modes of a nonreciprocal photonic lattice, yielding perfect, disorder-immune absorption—namely, an S-matrix zero anchored by topological invariants. This construction directly addresses the fragility of conventional antilasers and establishes CPA with robustness under strong lattice disorder and arbitrary absorber placement (Shen et al., 25 Jan 2026).

1. Theoretical Foundations of Coherent Perfect Absorption

Coherent perfect absorption is defined as the condition where incident waves on a multi-port scattering system are entirely absorbed under a finely tuned configuration. Formally, for a generic $N$-port system, the scattering relation is:

$$a_{\text{out}}(\omega) = S(\omega) \, a_{\text{in}}(\omega)$$

where $a_{\text{in}}$, $a_{\text{out}}$ are $N$-component vectors of incoming/outgoing amplitudes, and $S(\omega)$ is the scattering matrix. CPA occurs at frequency $\omega_0$ when an eigenvector $a_{\text{in}}$ exists such that $a_{\text{out}} = 0$, which implies $S(\omega_0)$ has a zero eigenvalue. In the two-port case, this simplifies to:

$\det S(\omega_0) = 0$

(equation (CPA condition) in (Shen et al., 25 Jan 2026)), mirroring the lasing threshold under time reversal.

The transfer matrix $M$ formulation provides a complementary perspective:

$$\begin{pmatrix} b_{\rightarrow}\ a_{\leftarrow} \end{pmatrix} = M \begin{pmatrix} a_{\rightarrow}\ b_{\leftarrow} \end{pmatrix}$$

with the CPA condition corresponding to a real-frequency pole of $M$ (i.e., $t \to \infty$), which is the exact time-reverse of lasing at threshold.

2. Photonic Lattice Model and Topological Edge Modes

Experimentally, the topological antilaser platform utilizes a “Floquet scattering network” comprising three-port microwave circulators as nodes, interconnected by waveguide links (Fig. 1a). Each circulator is described by the matrix:

$$S_0 = \begin{pmatrix} R & D & T\ T & R & D\ D & T & R \end{pmatrix}$$

(Eq. (1)), effecting nonreciprocal transmission. Enforcing translational symmetry enables a Floquet–Bloch condition $\psi_{j+1} = e^{-i\phi}\psi_j$, with phase delay $$\phi(\omega) = 2\pi L f \sqrt{\epsilon_{\rm eff}} / c$$. The network Floquet unitary is:

$U(k_x) = S_0 \cdot e^{i k_x a}$

whose diagonalization yields the quasienergy bands $\phi_n(k_x)$ (Figs. 1b–d).

Nonreciprocity breaks time reversal, instating nontrivial Chern numbers for each bandgap:

$$C_n = \frac{1}{2\pi} \int_{\rm BZ} \Omega_n(k)\,d^2k,\quad \Omega_n \equiv \nabla_k \times \langle u_n | i \nabla_k | u_n \rangle$$

which guarantee the presence of chiral (unidirectional) edge modes. In a strip geometry, a single edge mode spans each gap, immune to backscattering and carrying energy around the sample perimeter.

3. Dissipation Engineering and Topological Pinning of CPA Zeros

CPA is enabled by introducing controlled dissipation through a 50 Ω antenna inserted along a boundary link, with insertion depth $\ell$ parameterizing extra loss $\alpha(\ell)$. Two edge ports are used to inject coherent signals, with amplitude and phase adjusted to selectively excite the edge mode such that observed reflection at both ports vanishes.

The topological chiral edge mode is spatially robust and insensitive to local disorder or port/absorber position, ensuring that any boundary-placed absorber uniformly drains the edge current. This manifests mathematically as a topologically protected zero of the S-matrix, which remains on the real axis for arbitrary onsite phase disorder. In contrast, CPA zeros for trivial (non-topological) modes drift off the real-frequency axis under disorder, precluding exact absorption for any real input configuration.

4. Experimental Implementation

The experimental setup (Fig. 2a) incorporates a four-port Vector Network Analyzer (VNA), dividing the microwave source into two frequency-matched arms with variable digital attenuators (0.1 dB resolution) and phase shifters (1° resolution). These arms inject signals into sample edge ports; reflection is simultaneously monitored at both ends.

The honeycomb photonic lattice is constructed from 3-port circulators (UIYSC9B55T6), each magnetically biased ($\sim$62.8 mT), coupled by Rogers RT/duroid 5880 microstrip lines (length $L \approx 29.8$ mm, $\epsilon_{\rm eff} \approx 2.2$). Fabrication tolerances are $\pm 20\,\mu$m. A “hollowed” reciprocal control network is made by replacing circulators with bare microstrip loops, thereby removing topological features.

5. Experimental Results: Observation and Robustness

Chiral Edge Modes

Driving a single-site edge at the topological gap frequency (6.765 GHz) launches a wave localized to the perimeter and propagating unidirectionally (Fig. 1e). At bulk frequencies or in the reciprocal control system, the signal either delocalizes or splits bidirectionally (Figs. 1f,g).

CPA and Antilasing Response

A multidimensional parameter sweep over frequency, phase, amplitude, and absorber loss identifies a unique CPA “critical point” (Fig. 2b–e) at which $S_{11} = S_{22} = 0$. Experimentally measured lineshapes exhibit Lorentzian profiles in frequency and correspondence with scattering-matrix numerics (Figs. 2f–h).

Disorder Immunity

Disorder is introduced by randomizing link lengths, producing phase noise characterized by $W = 2\pi A f \sqrt{\epsilon_{\rm eff}} / c$. For topological chiral edge CPA, absorption remains $>\!92\%$ even at $W \approx 1.5$—substantially stronger disorder than required to destroy conventional CPA, which degrades at $W \gtrsim 0.5$. Chiral CPA field profiles remain highly correlated under disorder: profile correlation $\rho(W) \approx 1$ for the edge mode, but collapses for trivial modes (Fig. 3f).

Super-universality and Arbitrary Port/Absorber Placement

At strong disorder ($W = 3$), randomized placement of ports and dissipation yields successful CPA for almost all (2520) port/loss boundary arrangements (Fig. 4a,b,d–f) in the topological network, while the hollowed control fails in $\approx$73% of cases (Fig. 4a,c,g–i).

6. Significance, Prospects, and Emerging Directions

The topological antilaser paradigm overcomes the extreme parameter sensitivity of conventional CPA by guaranteeing that the S-matrix zero—defining perfect absorption—is pinned by topology to a real input frequency, and persists for any boundary absorber or input configuration. This opens avenues for robust energy harvesting, disorder-immune sensing, and flexible photonic circuits.

Potential research extensions include silicon photonics implementations, topological phononic or acoustic lattices, PT-symmetric CPA–laser hybrids in topological settings, and utilization of non-Hermitian skin effect modes. The unifying principle is topological protection of absorption zeros, enabling technologically relevant control over light–matter interactions under real-world conditions (Shen et al., 25 Jan 2026).