Synthesis of CdTe Microstructures on p-Type Silicon (111) Using Light-Modulated Electrodeposition

Wilder Cardoso

DOI: 10.1364/JOSAB.543319 Received 25 Sep 2024; Accepted 10 Nov 2024; Posted 11 Nov 2024  View: PDF

Abstract: In this paper, we present the results of our electrodeposition experiments of cadmium telluride CdTe microstructures on p-type silicon (111) spatially modulated by the incidence of light. This technique has shown itself to be practical, inexpensive and of immediate results. The construction from an adequate microcell to the process and all the required methodology by such technique are also shown. The photoelectrondeposition allows the creation of localized microdeposits, since the type of substrate used is activated locally by focalized light incidence. The photoelectrodeposited samples were characterized by optical perfilometry and scanning electron microscope (SEM). The perfilometry analyses have shown that the diameter of the depositions tend to increase with the magnification of the laser power, not suffering, however, much influence from the exposition time. The SEM has revealed information regarding the size and the concentration of the agglomerates on deposited regions. Both the size of the agglomerates and their concentrations increase as the laser exposure time increases. The size of the agglomerates also suffer influence from the laser intensity, showing itself higher on the center of the deposits than on peripheral regions.

Multiring polygon super-geometric beam generation based on quasi-frequency- degeneracy state of multiaxial super- geometric mode laser

Fuyong Wang and Lu Liu

DOI: 10.1364/JOSAB.539198 Received 12 Aug 2024; Accepted 08 Nov 2024; Posted 11 Nov 2024  View: PDF

Abstract: Novel structured modes with multiring polygon shape are anticipated in the quasi-frequency-degenerate (QFD) cavity, and their counterpart modes are multiaxial super-geometric (MASG) modes in the frequency-degenerate (FD) cavity. The multiring polygonsuper-geometric modes are numerically constructed by extending their decomposed multiaxialgeometric (MAG) modes in the FD state to the modes in the QFD state, implying that they area kind of special transverse modes in QFD laser cavity. Then, the characteristics of themultiring polygon super-geometric modes are investigated under different parameters.Studying on the multiring polygonal-shaped optical modes may broaden the application areasof structured light, such as particle trapping and manipulation.

Optomechanical microgear cavity

Roberto Zurita, Cauê Moreno Kersul de Castro Carvalho, Nicolaas Schilder, Gustavo Wiederhecker, and Thiago Alegre

DOI: 10.1364/JOSAB.543899 Received 02 Oct 2024; Accepted 07 Nov 2024; Posted 07 Nov 2024  View: PDF

Abstract: We introduce a novel optomechanical microgear cavity for both optical and mechanical isotropic materials, featuring a single etch configuration. The design leverages a conjunction of phononic and photonic crystal-like structures to achieve remarkable confinement of both optical and mechanical fields. The microgear cavity we designed in amorphous silicon nitride exhibits a mechanical resonance at 4.5 GHz, and whispering gallery modes in the near-infrared, with scattering-limited quality factors of up to $10^7$. Notably, the optomechanical coupling falls within 40% of the ideal configuration seen in a floating ring structure.

High Q-factor multi-Fano resonances in all-dielectric metasurface for high-performance optical switching and sensors

Tao Zhang, Yiping Huo, Congmu Xu, Jiamin Li, and Zilong Zhang

DOI: 10.1364/JOSAB.537123 Received 23 Jul 2024; Accepted 07 Nov 2024; Posted 11 Nov 2024  View: PDF

Abstract: The bound state in the continuum (BIC) in photonics has received widespread attentions owing to its ultra-high Q-factors and promise in realizing field enhancement and sensing. In our study, we proposed and investigated an all-dielectric metasurfaces consist of two silicon nano-quadriprism. By introducing broken symmetry in the nanostructure, the BIC modes were transformed into the quasi-BIC modes, resulting in four Fano resonances with limited but extremely high Q values. Through multipole decompositions and electromagnetic fields analysis, we revealed that these resonances were dominated by magnetic dipoles (MDs) and magnetic quadrupoles (MQs) responses. The sensing characteristics of the structure were examined, revealing a maximum sensitivity and figure of merit of 383.3 nm/RIU and 958.3 , respectively. Owing to the dependence of the transmission spectrum on the polarization, the structure can implement an optical switch function. In addition, the phase variations and group indices at the four Fano resonances were analyzed, with the maximum group index reaching 3000. These characteristics make the matasurface suitable for applications in high-performance biosensors, optical switches, and slow-light effects.

Analytical Z-scan model for multiphoton absorption in Inhomogeneous Media

Ferhat Kessi

DOI: 10.1364/JOSAB.538137 Received 29 Jul 2024; Accepted 07 Nov 2024; Posted 11 Nov 2024  View: PDF

Abstract: This work presents an analytical model for multiphoton absorption in inhomogeneous materials, developed within the framework of the Z-scan technique and using the weak nonlinearity approximation. The model addresses arbitrary order n of multiphoton absorption processes and considers linear variations in material properties. Three cases are investigated: purely axial, purely radial, and combined axial and radial inhomogeneities in the absorption coefficient. Simulations reveal distinct normalized transmittance profiles for each case, demonstrating the significant impact of linearly varying inhomogeneities on multiphoton absorption. The purely axial case shows symmetric, broad dips in transmittance centered at the focal point, while the purely radial case exhibits narrower, more localized absorption profiles. The combined case demonstrates a synergistic effect, resulting in enhanced absorption.

Modified Fresnel equations for the case of oblique incidence on an isotropic gyrotropic medium

Vladislav Diukov, Kirill Grigoriev, and Vladimir Makarov

DOI: 10.1364/JOSAB.538095 Received 29 Jul 2024; Accepted 05 Nov 2024; Posted 06 Nov 2024  View: PDF

Abstract: Modified Fresnel equations for the oblique incidence of plane elliptically polarized electromagnetic waves on a flat boundary of a non-absorbing isotropic gyrotropic medium have been obtained. We have taken into account the influence of near-surface inhomogeneity of the matter. The derived relationships allow determining the intensity, degree of ellipticity, and the angle defining the orientation of the polarization ellipse of the reflected wave, as well as the intensities of two circularly polarized waves propagating at different angles in the isotropic gyrotropic medium. It is shown that considering the optical activity of the medium and its near-surface layer leads to significant differences between the modified and classical Fresnel equations.

Two-photon absorption and negative nonlinear refraction in few-layer graphene nanosheets suspensions via liquid-phase exfoliation

R. González-Campuzano, Briseida G. Pérez-Hernández, A. Sainz-Vidal, Doroteo Mendoza, and Jesus Garduño-Mejia

DOI: 10.1364/JOSAB.540941 Received 03 Sep 2024; Accepted 04 Nov 2024; Posted 06 Nov 2024  View: PDF

Abstract: In this paper, the nonlinear optical properties of few-layer graphene nanosheets in solution were investigated by the Z-scan technique and femtosecond laser pulses @ 800 nm, using different laser intensities in the ~0.17–0.22 GW/cm2 range. Few-layer graphene nanosheets were synthesized by the liquid-phase exfoliation method using a mixture of deionized water and Triton X-100, obtaining average sizes of ~183 nm. The nonlinear absorption obtained by an open aperture Z-scan setup showed a two-photon absorption effect compared to those reported in the literature, where the saturable absorption effect is dominant, e.g., in graphene synthesized by chemical vapor deposition and graphene oxide. In contrast, the nonlinear refraction measured by a close aperture setup revealed a negative lens-like (self-defocusing) behavior in all cases. The calculated effective nonlinear absorption coefficient ranged from 2.41×10-9 to 7.83×10-9 cm/W, while the nonlinear refraction index was -6.52×10−13 to -5.01×10-13 cm2/W. We found that as the laser intensities increased, the effective nonlinear absorption coefficient and the absolute value of the nonlinear refraction index also increased. In accordance with the obtained results, we propose few-layer graphene nanosheets for applications where two-photon absorption has significant advantages over conventional one-photon absorption.

Observation of Feynman-path interference in high-order sideband generation

Jiangong Hu, Luxing Zhang, and Tao-Yuan Du

DOI: 10.1364/JOSAB.534038 Received 02 Jul 2024; Accepted 04 Nov 2024; Posted 05 Nov 2024  View: PDF

Abstract: The quantum paths that satisfy the stationary phase condition of the Dirac-Feynmann path integral provide an intuitive picture in understanding extreme nonlinear optical phenomena, such as high-order harmonic generation (HHG) and high-order terahertz sideband generation (HSG). Each order of high-order harmonics or sidebands can be essentially captured by a few of quantum paths. In contrast to HHG, the process of HSG can be tailored delicately via controlling the excitation of the first step, which paves a way to regulate the recollisions of quantum paths. We report a scheme that invokes two beams of exciting fields, and their frequency difference is the even multiple of the driving-field frequency. We find that the number of spectroscopic dips is determined by the ratio of frequency difference to the driven-field frequency. And the positions of spectroscopic dips could be tuned elaborately via changing the initial phase difference between two exciting laser fields. This finding reveals a methodology to directly measure the amplitude of Feynman paths that contribute the high-order sidebands in the extreme nonlinear optics regime.

Optimizing Chiral Optical Response in Nanostructures Using Plasmonic Fano Resonance

Feiyue Tian, Peng Lang, wan ting, Boyu Ji, Yang Xu, Xiaowei Song, and Jingquan Lin

DOI: 10.1364/JOSAB.542390 Received 16 Sep 2024; Accepted 03 Nov 2024; Posted 04 Nov 2024  View: PDF

Abstract: This study investigates the chiral enhancement effects of plasmonic Fano resonance modes in planar metallic nanostructures. The nanostructure consists of a central Z-shaped or 卍-shaped element surrounded by six clustered gold nanorods, focusing on the coupling between these doubly rotationally symmetric structures. This coupling induces plasmonic Fano resonance, which significantly enhances the chiral response. Under normal incidence of circularly polarized light, the maximum chiral response can reach up to 41%. Finite Difference Time Domain simulations and multipole expansion analysis reveal the fundamental origin of this enhanced chiral response: the selective excitation of electric dipoles and toroidal dipoles in polarization. The study demonstrates that rotationally symmetric structures and coupling effects play a crucial role in regulating the chiral response of nanostructures.

Coherent mid-infrared vortex generation in room temperature for manipulation of microparticles

Nawaz Mallick

DOI: 10.1364/JOSAB.533391 Received 26 Jun 2024; Accepted 01 Nov 2024; Posted 04 Nov 2024  View: PDF

Abstract: We investigate the generation of mid-infrared (mid-IR) vortex beams carrying orbital angular momentum (OAM) through nonlinear processes in an inhomogeneously broadened 85 Rb atomic ensemble. By employing a four-level atomic system featuring two strong control fields and a weak probe field, we generate non-degenerate four-wave mixing signal at a wavelength of 5. μm. Applying the density matrix formalism, we derive analytical expression for the nonlinear atomic coherence which facilitates the transfer of vortex characteristics such as topological charge, intensity and phase profiles from the probe field to the mid-IR signal.Numerical solutions of Maxwell's wave equation confirm the generation of mid-IR vortex beams with adjustable topological charges and beam widths at different spatial positions. This technique offers significant potential for applications in mid-IR communication, providing additional bandwidth and improved data transmission rates, as well as in fields like microfluidics, biophysics, and nanotechnology, where OAM-carrying beams can manipulate microparticles with precision.

Comprehensive interference characteristics of three-parameter Hong-Ou-Mandel interferometer

Baihong Li, Zhuo-zhuo Wang, Qi-qi Li, Penglong Wang, Changhua Chen, Boxin Yuan, and Xiao-fei Zhang

DOI: 10.1364/JOSAB.539402 Received 13 Aug 2024; Accepted 31 Oct 2024; Posted 07 Nov 2024  View: PDF

Abstract: We theoretically extend the original Hong-Ou-Mandel (HOM) interferometer to three parameters by concatenating 50:50 beam splitters with three independent and adjustable time delays. The coincidence probability of such an interferometer is obtained based on the linear transformation of the matrix of beam splitters. We present a comprehensive analysis of the interference characteristics of the interferometer at different time delays with various types of frequency-correlated resources and input states. It is found that by properly setting the time delays, one can obtain typical interferograms that include multiple sub-interferograms associated with one- and two-parameter HOM interferometers, revealing richer and more complicated two-photon interference phenomena. In particular, additional enhancements of the interference signal at a specific delay can be achieved, and the symmetry of the interferograms on both sides can also be changed. This work provides comprehensive insights into the interference characteristics of the three-parameter HOM interferometer and enables the simultaneous optimal estimation of multiple time-delay parameters within a single interferometer, which holds potential for applications in multiparameter estimation and quantum metrology.

Coherence evolution of multi-pulse pumped super-continuum in multicomponent GeSe2-As2Se3-PbSe chalcogenide photonic crystal fiber with four zero-dispersion wavelengths

Hieu LeVan

DOI: 10.1364/JOSAB.539742 Received 20 Aug 2024; Accepted 30 Oct 2024; Posted 30 Oct 2024  View: PDF

Abstract: The flattened dispersion profile with multiple-zero dispersion wavelengths (ZDWs) over an ultra-wide wavelength range can be used to generate rich nonlinear effects, and one of the advantages of its use is the generation of supercontinuum (SC). In this work, we proposed a new solid hybrid PCF of GeSe2 - As2Se3-PbSe chalcogenide to achieve ultra-flat and low dispersion with four ZDWs. By tuning the structural parameters of the fiber, an ultra-flat dispersion between -2.1 and 0.6 ps/nm/km (a variation of 0.75 ps/nm/km) is achieved over a bandwidth of 5077 nm. An ultra-broadband coherent SC spectrum with a bandwidth of 8107 nm, extending from 2827 nm to 10934 nm wavelength at a level of 45.6 dB, was generated by injecting an input laser pulse with a pump wavelength of 5.3 μm, a pulse width of 150 fs, and a peak power of 4.5 kW into a 20 cm long fiber in the anomalous dispersion region. Meanwhile, high coherence of the SC spectrum from 3253 nm to 10850 nm (bandwidth of 7597 nm) wavelength at a level of 19.3 dB can be generated in the normal dispersion region with the same pump power at a pump wavelength of 8.0 μm. The results show that the proposed hybrid PCF has great potential for nonlinear applications in the mid-infrared range.

Optical frequency synthesizer for wide range frequency scan and its application to high-resolution molecular spectroscopy

Masatoshi Misono, Shunji Kasahara, Akiko Nishiyama, and Masaaki Baba

DOI: 10.1364/JOSAB.542324 Received 13 Sep 2024; Accepted 29 Oct 2024; Posted 30 Oct 2024  View: PDF

Abstract: In high-resolution molecular spectroscopy, detailed structures and dynamics appear as frequency shifts, line broadening, intensity anomalies, and Zeeman effects. To observe these minute effects, the relative uncertainty of the frequencies should be less than 10-9. However, to comprehensively analyze the rovibronic energy-level structure and dynamics in a single vibronic excited state, the spectrum over the entire single vibronic band, which extends to hundreds of GHz, must be measured. We developed an optical frequency synthesizer (OFS) employing a scanning single-mode Ti:Sapphire laser and an Er comb to satisfy these contradictory demands. The continuous scanning range was not limited in the developed OFS, and wide-range spectra extending across the entire scanning range of the single-mode laser in the OFS were obtained. For a detailed examination of the developed OFS, we measured the D2 transition of Rb atoms. Although continuous scanning generally has an adverse effect on precise frequency determination, the obtained transition frequencies agreed with those obtained in previous studies, where laser frequencies were locked to a specific transition for a long time. To demonstrate the power of the developed OFS in molecular spectroscopy, the rovibronic spectrum of 1,2-benzanthracene was observed over a single band. We successfully obtained spectra in which the rotational lines were well-resolved.

Research and fabrication of color filters based on porous anodic alumina

Ni Haibin, Yajie Wang, Yi Shen, Tong Cai, Wenjie Wu, Jiasheng Han, Bo Ni, Yixian Ge, Tingting Wang, and Jianhua Chang

DOI: 10.1364/JOSAB.541159 Received 03 Sep 2024; Accepted 28 Oct 2024; Posted 29 Oct 2024  View: PDF

Abstract: This study introduces a porous alumina composite structure, incorporating silver on a silicon substrate, to achieve a wide range of color displays. The structure utilizes the Bragg interference effect to reflect light between the surface and the pore bottoms. Fabrication involved magnetron sputtering, anodizing, and sodium hydroxide etching to produce different pore sizes, depths, and silver film thicknesses. This method shifts reflection valleys in the spectrum, enabling light emission or absorption across various wavelengths, resulting in diverse color displays. Experiments reveal that hue is influenced by pore depth, pore size, and silver film thickness, whereas brightness and saturation are regulated by pore size and silver film thickness. Unlike periodic structural color filters made through complex methods like photolithography and self-assembly, this filter is easily manufactured using simple electrochemical and coating techniques. This cost-effective, simple, and controllable method is ideal for large-scale production and the creation of high-resolution, high-saturation structural color display devices.

Orbital Angular Momentum Carrying Mid-Infrared Bessel Beam generation at Room Temperature

Nawaz Mallick

DOI: 10.1364/JOSAB.541093 Received 03 Sep 2024; Accepted 26 Oct 2024; Posted 29 Oct 2024  View: PDF

Abstract: We explore the generation of orbital angular momentum carrying mid- infrared Bessel-Gaussian beams through the nonlinear interactions within a non-uniformly broadened 85 Rb atomic ensemble. Utilizing an efficient four-wave mixing scheme driven by two strong control fields and a weak probe field, we achieve coherent emission at a wavelength of 5. µm. By employing the density matrix approach, we obtain an analytical expression for the nonlinear atomic coherence involved in the four-wave mixingprocess, elucidating how the Bessel-Gaussian profile of the probe field is transferred into the mid-infrared signal. Numerical simulations of Maxwell’s wave equation ensure the generation of phase-matched, non-diffracting Bessel-Gaussian beams, which can be precisely controlled by manipulating the spatial susceptibility of the atomic medium. Furthermore, the study demonstrates the potential of higher-order Bessel-Gaussian beams with orbital angular momentum to significantly advance applications in high-speed communication, biomedical imaging, and optical manipulation, owing to their robust beam integrity and enhanced data transmission capabilities in the mid-infrared spectrum.

Research on Center-Assisted Ring-Core Few-Mode Fiber with Eccentric Circle for Mode Degeneracy Separation in Space Division Multiplexing

Xiao Ye, Tigang NIng, Li Pei, Lei Shen, Jingjing Zheng, Jing Li, bing bai, Ge Wu, and Shuyuan Zhang

DOI: 10.1364/JOSAB.534266 Received 05 Jul 2024; Accepted 24 Oct 2024; Posted 25 Oct 2024  View: PDF

Abstract: An eccentric-circle-assisted ring-core fiber (ECRF) structure is proposed, which can effectively eliminate the spatial degeneracy of the LPmn mode set of conventional few-mode fibers and maintain a low level of birefringence , and broaden the design idea of low-mode fiber for Space Division Multiplexing (SDM) system. Our simulation analysis demonstrates that the effective refractive index difference (Δneff) between the spatial degenerate modes in this fiber falls within the range of (2.44-6.42) ×10-4 at 1530-1565 nm, given the refractive index difference level typical of conventional fiber core and cladding. Additionally, the polarization separation level for each mode is on the order of 10-6 and below. The design requirements of significant separation of spatial degeneracy and basically no separation of polarization degeneracy is achieved. Based on the characteristics of spatial degenerate mode in this fiber, we extend the regulation law for spatial degenerate mode separation of the LPmn mode groups in center-assisted ring-core fibers. In comparison to the existing center-assisted ring-core fiber structure, the ECRF can further reduce the fabrication difficulty and increase the feasibility of preparation.

A Comparative Study of Generalized Sampling Theorem based Digital Superresolution for 2D Data.

NEETHU RAVI, Rakesh Kumar, and Bradley Ratliff

DOI: 10.1364/JOSAB.536473 Received 11 Jul 2024; Accepted 23 Oct 2024; Posted 25 Oct 2024  View: PDF

Abstract: The size of the pixels of the digital recording device, such as a CCD array, limits the resolution of images obtained by an optical imaging system. Consequently, digital super-resolution enhancement techniques are used to improve the quality of these low-resolution images. In this article, the authors formulate a mathematical framework for digital superresolution (DSR) based on the generalized sampling theorem (GST). The generalized sampling theorem-based digital superresolution (GST DSR) method's performance is evaluated by comparing it to existing enhancement methods, and the developed method exhibits an overall superior image reconstruction quality. The robustness of the proposed GST DSR method is further demonstrated in the presence of frame-to-frame shift error using percentage mean square error (%) MSE and structural similarity index measure (SSIM) for both the GST DSR and multi-frame interpolation approaches.

Time-delayed reservoir computing based on spin-VCSEL: interplay between pump ellipticity and performance

Tao Wang, Qing Fang, Huiming Wang, and Yueyang Wang

DOI: 10.1364/JOSAB.540025 Received 20 Aug 2024; Accepted 14 Oct 2024; Posted 04 Nov 2024  View: PDF

Abstract: Reservoir computing, a simplified recurrent neural network, can be implemented using a nonlinear system with delay feedback, known as time-delayed reservoir computing. In this paper, we explore two time-delayed reservoir computing schemes based on the fast dynamics of two polarization channels of a Spin-VCSEL (Vertical-Cavity Surface-Emitting Laser), and investigate their prediction performance for the Mackey-Glass task. Our main focus is on understanding the impact of pump ellipticity on the prediction performance of the two reservoir computing systems, namely RC$_X$ ans RC$_Y$. Through numerical simulation, we find that when the pump ellipticity ($P$) is less than 0.73, the prediction performance of RC$_Y$ outperforms RC$_X$. However, beyond this threshold, the performance advantage shifts towards RC$_X$. This finding sheds light on the importance of considering pump ellipticity when designing and optimizing reservoir computing systems. Furthermore, we also investigate the influence of the ratio between delay time and input period on the memory capacity of these systems. Interestingly, we observe that using a delay time of 2.8 times the input cycle enables better prediction performance and memory capacity. This choice not only provides an optimal trade-off between memory capacity and computing speed but also avoids the computational slowdown caused by excessively long delay times. In general, our study emphasizes the flexibility and tunability of the spin-VCSEL-based reservoir computing system. By easily adjusting the ellipticity and delay-time parameters, we can optimize the memory properties, resulting in significantly improved prediction performance. Our findings offer valuable insights for enhancing the performance of reservoir computing systems based on the ultrafast dynamics of spin-VCSELs.

Quantum-induced Stochastic Optomechanical Dynamics

Pedro Paraguassú, Luca Abrahão, and Thiago Guerreiro

DOI: 10.1364/JOSAB.532994 Received 25 Jun 2024; Accepted 17 Sep 2024; Posted 20 Sep 2024  View: PDF

Abstract: We study the effective stochastic dynamics of a semiclassical probe induced by linear optomechanical interactions with a quantum oscillator. To do so, we introduce path integrals and the method of Feynman-Vernon influence functionals in quantum optics and analyse the semiclassical dynamics of a levitated nanoparticle interacting with quantum light, as well as with another quantum particle. In all cases, quantum fluctuations ubiquitously lead to state-dependent non-equilibrium noise. Notably, this noise can be exponentially enhanced by wavepacket delocalization, i.e. quantum squeezing, and displays both a stationary and a non-stationary contribution with intricate dependence on the squeezing angle. For the case of nanoparticles coupled by the Coulomb interaction such noise can imprint potentially measurable signatures in multiparticle levitation experiments. We also discuss the case in which the mechanical oscillators are coupled by gravity, and the relation of the quantum-induced noise to gravitational-induced entanglement. Quantum-induced optomechanical fluctuations also hold strong analogy to quantum gravitational wave noise and interconnect stochastic thermodynamics, graviton physics and the detection of gravity-mediated entanglement.

Cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using free-running quantum cascade lasers

Charles Markus, Jakob Hayden, Dan Herman, Philip Kocheril, Douglas Ober, Termeh Bashiri, Markus Mangold, and Mitchio Okumura

DOI: 10.1364/JOSAB.534286 Received 08 Jul 2024; Accepted 12 Sep 2024; Posted 16 Sep 2024  View: PDF

Abstract: Cavity-enhanced dual-comb spectroscopy promises broadband, high-resolution, and highly sensitive spectroscopic measurements on sub-millisecond timescales, making it highly attractive for trace gas monitoring.In this work, we demonstrate cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using two quantum cascade lasers (QCLs) operating as optical frequency combs centered at 1063 cm^-1 spanning 56 cm^-1. The high-finesse bow-tie cavity provided a 285 m effective pathlength, and the high power-per-mode of the QCL combs granted a strong multi-heterodyne signal of the swept-cavity transmission.This ultimately resulted in a noise equivalent absorption per spectral element of 1.8×10^-9 cm^-1 Hz^-1/2. Measurements of the ν₈ fundamental band of methanol determined concentrations as low as 1.3 ppm in a single shot, with a detection limit of 20 ppb after 30 averages. This demonstrates the potential of cavity-enhanced dual-comb spectroscopy in challenging applications such as free radical kinetics and environmental monitoring.