Singular optics has emerged as an important research area with diverse applications, yet controlling optical singularities in nanophotonic emitters is typically limited by fixed subwavelength geometries and diffraction-limited control. Here, we circumvent this limitation and demonstrate an all-optical mechanism for reconfiguring far-field singularities in a photonic crystal laser. The underlying principle involves optical pumping, which creates a mesoscopic potential landscape whose spatial variations are slow compared to the lattice period. Such a potential localizes a Bloch band into trapped states whose envelope functions, and thus far-field singularity textures, are defined by the pump geometry. Using a honeycomb photonic crystal that supports a symmetry-protected bound state in the continuum, we achieve room-temperature telecom-band lasing with real-space polarisation singularities that are reconfigurable in both number and position, while the intrinsic momentum-space singularity at the $Γ$-point is preserved. The experimental observations align quantitatively with an analytical framework that combines the Bloch mode of the structure and envelope function theory, establishing envelope engineering as a versatile route to programmable singular-light emission in active photonic lattices.
Optical singularities, points where the phase or polarization of light is undefined, play a central role in structured-light physics and carry quantized topological charges [1,2]. Their control underpins applications in robust communication [3,4], precision metrology [5], and vortex lasing [6][7][8][9]. In nanophotonics, such singularities may be imposed externally through phase engineering or arise intrinsically from the eigenmodes of resonant metasurfaces and photonic crystals (PhCs). Bound states in the continuum (BICs), whose radiation vanishes by symmetry or interference, provide a powerful route to generating singular beams directly from photonic resonances [7,[10][11][12].
Achieving dynamic control of these singularities remains challenging because the far-field texture of a Bloch mode is fixed by the unit-cell geometry, and conventional structures offer little reconfigurability. Thermal tuning in phase-change PhCs allows switching between BIC lasing modes with different topological charges [13], and photoisomerization in organic PhCs provides a similar modeswitching mechanism [14], but neither approach can reshape the singularity texture of a given Bloch resonance. Recent optical-control schemes modulate refractive in- * hai-son.nguyen@ec-lyon.fr dex at the unit-cell scale: ultrafast pumping can destroy BIC singularities by breaking in-plane symmetries [8], and gain-loss perturbations suppress vortex emission [9,15]. Sub-unit-cell pumping can even alter the response of individual elements [16], but remains limited to passive linewidth tuning and does not enable continuous control of far-field singularities. Fundamentally, all these schemes attempt to reconfigure the unit cell, an operation constrained by the diffraction limit. In this work, we introduce an alternative approach, instead of modifying the subwavelength Bloch function, we reconfigure the mesoscopic envelope of a PhC resonance using optical pumping (see Fig. 1). Carrier injection creates a smooth in-plane potential that localizes a Bloch band into trapped states whose envelope functions obey a twodimensional Schrödinger equation, which can be shaped arbitrarily by the pump profile. Because the radiated field is the product of the Bloch-mode far field and its envelope, this mechanism enables reconfigurable far-field singularities without altering the photonic crystal itself.
As a proof of concept, we use a honeycomb PhC supporting the lowest-energy band with isotropic negativemass dispersion and a monopolar BIC at the Γ-point (k ∥ = 0). Optical pumping forms tunable potentials that trap this band and yield lasing trapped states whose real-space far-field polarization singularities can be programmed by the pump geometry, whereas the momentum-space singularity remains fixed. Lasing occurs at room temperature and telecom wavelengths, and the measurements agree quantitatively with envelopefunction theory, establishing a general route to reconfigure singular beams in active photonic lattices.
We first consider the generic guided resonances of a PhC slab in the absence of optical pumping. For a fixed in-plane wave vector k ∥ = (k x , k y ), the corresponding Bloch mode inside the slab can be expanded over a finite set of basis consisting of guided modes of an effective homogeneous slab, each carrying a reciprocal lattice vector G n . The guided modes in the basis share the same out-of-plane profile u(z) and differ only in their in-plane Bloch harmonics. Factoring out the global Bloch phase e ik ∥ •r ∥ for compactness, the near field can be written as E near,k ∥ (r ∥ , z) = u(z) n A n (k ∥ ) e iGn•r ∥ p n , where r ∥ = (x, y) denotes the in-plane coordinate, {p n } are the polarization of the guided-mode basis. Here, A n (k ∥ ) are the expansion coefficients obtained by diagonalizing the effective non-Hermitian Hamiltonian of the PhC slab [17]. Radiation into the continuum is obtained by projecting this Bloch mode onto the outgoing Fabry-Pérot channel of the unpatterned slab (Fig. 1(a)). In the single-channel regime relevant here (subwavelength lattice, thus the zeroth diffraction order inside the light cone is the only leaky channel), the radiated field in direction k ∥ is proportional to the same guided-mode coefficients, but without the fast lattice harmonics e iGn•r ∥ [17]:
, where γ(k ∥ ) is the radiative loss rate of the Bloch resonance. The polarization texture and the existence of BICs are encoded in the vector n A n (k ∥ )p n , whereas γ(k ∥ ) sets the emitted intensity. The complex eigenfrequency of the guided resonance can be described as ω(k
, where m is the effective mass of the Bloch resonance. In general, the photonic band can exhibit anisotropy, but here, the guided resonances have been chosen such that the effective mass m is negative and isotropic. Under optical pumping, photogenerated carriers produce a carrierinduced optical nonlinearity that leads to a spatially varying refractive-index change ∆n(r ∥ ) < 0 [18]. This sm
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