Realizing continuous-wave GW-class brightness in scalable double-lattice photonic-crystal surface-emitting lasers: from demonstrated 1 GW cm^-2 sr^-1 to the projected 10 GW cm^-2 sr^-1 regime

Double-lattice photonic-crystal surface-emitting lasers (PCSELs) were introduced by Yoshida et al. (Nature Materials, 2019) as a route to compact semiconductor lasers whose brightness (optical power per unit emission area per unit solid angle) could rival gas, fibre and disk lasers. The source paper demonstrated 10 W pulsed / ~7 W continuous-wave (c.w.) operation with narrow symmetric beams, projected a potential brightness up to 10 GW cm^-2 sr^-1, and identified the open experimental and technological issues on the way to that goal: realizing the designed balance of in-plane (180°) and orthogonal (90°) optical couplings with a $\pi$ phase difference in fabricated devices, patterning photonic crystals over apertures larger than a single lithography exposure field, managing thermal nonuniformity under c.w. operation, quantifying the intrinsic linewidth beyond spectrometer resolution, and validating GW-level brightness with reliable, manufacturable devices. Later work by the same and other groups has resolved several of these items: a general Hermitian/non-Hermitian coupling design recipe, experimentally realized 3-mm-diameter devices with 50 W c.w. single-mode output at 1 GW cm^-2 sr^-1 with thermal compensation, and measured 1-kHz-class intrinsic linewidths. The remaining open part of the original objective is: sustained c.w. single-mode operation at the full projected GW-to-10-GW cm^-2 sr^-1 brightness (kW-class output from roughly 10-mm-scale apertures), together with linewidth behaviour, reliability and device-to-device reproducibility at that scale.

Background

A photonic-crystal surface-emitting laser (PCSEL) is a semiconductor laser in which a two-dimensional periodic array of air holes embedded in the active region provides distributed in-plane feedback: light waves travelling along four symmetry-equivalent in-plane directions are coupled to one another by diffraction at the lattice, and 180°-diffracted light exits the surface vertically, so the laser emits a narrow, symmetric, circular beam from a broad area. The optical feedback is described by coupled-wave theory (the standard modal analysis of waves coupled to one another by the periodic lattice) through coupling coefficients: $\kappa_{1\mathrm{D}}$ (coupling between counter-propagating waves, i.e. 180° diffraction), $\kappa_{2\mathrm{D}}$ (coupling between orthogonally propagating waves, i.e. 90° diffraction) and a self-coupling term. The threshold gain margin $\Delta\alpha$ between the fundamental resonant mode and higher-order modes — the quantity that decides whether oscillation stays single-mode as the resonator is enlarged — is a function of these coefficients. In a conventional single-lattice square photonic crystal, $\kappa_{1\mathrm{D}}$ grows with lattice contrast faster than $\kappa_{2\mathrm{D}}$, so the margin shrinks as the resonator grows, limiting the emission area and hence the output power.

Yoshida et al. (Nature Materials 18, 121–128; 2019) proposed the 'double lattice': two lattice-point groups shifted by approximately a quarter wavelength in both x and y. Because waves diffracted by 180° and by 90° at the two lattice points interfere destructively, the in-plane (Hermitian) coupling is weakened without weakening the vertical emission, which enlarges $\Delta\alpha$. Their Supplementary Information shows that for the optimized structure ('structure IV'), the coefficients $\kappa_{1\mathrm{D}}^{+}$ and $\kappa_{2\mathrm{D}}^{+}$ become equal in magnitude and differ by about $\pi$ in phase at a lattice-group offset $\Delta d$ of about $0.010a$–$0.016a$ (optimum near $0.012a$, where $a$ is the lattice constant), yielding a threshold gain margin above 15 cm^-1 for a 1000-um resonator. Experiments gave 10 W pulsed output with a divergence angle below 0.3° from a 500-um-diameter circular aperture (brightness over 300 MW cm^-2 sr^-1) and up to about 7 W under continuous-wave (c.w.) conditions from 800-um devices; analysis of optimized designs projected a brightness of up to 10 GW cm^-2 sr^-1, which would let semiconductor lasers rival gas and fibre/disk lasers in direct laser processing and LiDAR (light detection and ranging). The paper identified what stood between the demonstrated devices and that projection: realizing the precise coupling balance in fabricated structures (requiring nanometre-level control of hole placement, shape and depth over large areas), electron-beam-lithography drawing accuracy when the aperture exceeds a single exposure field, thermally induced nonuniformity of the band-edge frequency under c.w. operation, linewidth measurements limited by spectrometer resolution, and the translation of the modelled designs into robust, reproducible, manufacturable devices.

Subsequent work reframed and largely solved parts of this agenda. Katsuno et al. (Optics Express 29, 25118; 2021) developed a self-consistent photon–carrier–thermal analysis of PCSELs under c.w. operation. Inoue et al. (Nature Communications 13, 3262; 2022) derived a general recipe for 100-W-to-1-kW single-mode operation in 3–10 mm PCSELs that controls not only the Hermitian (radiation-free) couplings, of which the 2018 $\pi$-phase $\kappa_{1\mathrm{D}}$/$\kappa_{2\mathrm{D}}$ balance is a special case, but also the non-Hermitian couplings that accompany radiation loss; the relevant combinations are the real and imaginary parts $R$ and $I$ of the total in-plane feedback and the radiation-related coupling $\mu$. Yoshida et al. (Nature 618, 727; 2023) realized this experimentally in 3-mm-diameter double-lattice PCSELs: $R$ and $I$ were set through the lattice separation and the balance of air-hole sizes, $\mu$ through the cladding thickness and a backside distributed Bragg reflector (a stack of alternating refractive-index layers acting as a mirror), and a pre-installed spatial distribution of the lattice constant was used to cancel the temperature-induced band-edge distortion under c.w. drive. The result was purely single-mode c.w. output above 50 W with 0.05° divergence (beam-quality factor $M^2$, equal to 1 for an ideal diffraction-limited beam, about 2.36), corresponding to 1 GW cm^-2 sr^-1. Morita et al. (Optica 11, 333; 2024) then measured the intrinsic (Schawlow–Townes-type) linewidth — the linewidth that remains when technical noise is removed, set by the photon number and spontaneous-emission rate in the lasing mode — of 1-mm PCSELs under c.w. operation, demonstrating 1-kHz-class intrinsic linewidths at 5-W-class output. An independent route to scalable single-mode surface-emitting lasers via open-Dirac singularities (Contractor et al., Nature 608, 692; 2022) and triple-lattice PCSELs at 1550 nm (Wang et al., Light: Science & Applications 13, 44; 2024) address the same scalability problem by different means, with experimental areas so far much smaller. A 2024 review by Noda et al. (Nature Reviews Electrical Engineering 1, 802–814) records 1 GW cm^-2 sr^-1 as the c.w. state of the art, and kW-class peak powers have so far been reached only under pulsed operation; 10-mm-diameter devices were reported as under development in 2025. What remains is the upper end of the original 2018 projection.

Problem Statement

Demonstrate, in fabricated double-lattice photonic-crystal surface-emitting lasers, sustained continuous-wave single-mode operation at the brightness level projected for optimized double-lattice resonators by the source paper (Yoshida et al., Nature Materials 18, 121–128; 2019) — reaching the projected ~10 GW cm^-2 sr^-1 regime, that is, kilowatt-class single-mode output from roughly 10-mm-diameter apertures, beyond the 1 GW cm^-2 sr^-1 already demonstrated (50 W c.w. from 3-mm-diameter devices) — while satisfying the enabling conditions identified in that work: (a) realizing and verifying the designed Hermitian and non-Hermitian coupling balance (in the source paper's formulation, $|\kappa_{1\mathrm{D}}| = |\kappa_{2\mathrm{D}}|$ with a $\pi$ phase difference; in the generalized formulation of the later recipe, controlled $R$, $I$ and $\mu$) across the entire ultra-large aperture, with the required patterning accuracy of hole placement, shape and depth over areas exceeding many electron-beam-lithography exposure fields; and (b) thermal management under c.w. operation such that temperature-induced distortion of the band-edge frequency does not destroy single-mode behaviour and near-field uniformity at these power levels. The remaining objective also includes, for the target device(s) under the demonstrated c.w. operating range: (c) measurement of the intrinsic spectral linewidth, with the size dependence established by comparison against the already-characterized 1-mm devices (1-kHz-class linewidth at 5-W-class power); and (d) reproduction of the target brightness and single-mode operation on at least two independently fabricated devices, sustained across the reported c.w. characterization — the reliability, device-to-device reproducibility and manufacturability evidence needed for applications such as direct laser processing and LiDAR (light detection and ranging).

References

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