Many anisotropic layered materials, despite their strong in-plane birefringence, exhibit substantial visible absorption, which severely restricts cavity lengths and hinders the observation of purely birefringence-governed optical phenomena. Here, we realize a birefringence-driven anisotropic optical cavity using $α$-MoO3 flakes, capitalizing on their ultralow optical loss and pronounced in-plane birefringence. Using angle-resolved polarized Raman (ARPR) spectroscopy, we observe a mode-sensitive enhancement of anisotropy, dependent on both flake thickness and Raman shift. A unified model that incorporates the intrinsic Raman tensor, birefringence, and chromatic dispersion accurately reproduces the experimental data, elucidating how cavity resonances at both excitation and scattered wavelengths interact. Within this framework, the intrinsic phonon anisotropy is quantified, providing invaluable insights for accurately predicting ARPR responses and identifying crystallographic orientation. This work provides fundamental insights into birefringence-governed cavities and opens avenues for high-performance birefringent optics and cavity-enhanced anisotropic phenomena.
Anisotropic layered materials (ALMs) exhibit exceptional in-plane optical anisotropy, [1][2][3] providing unprecedented control over light-matter interactions 2,4,5 through their anisotropic complex refractive index, characterized by birefringence and linear dichroism. 1,2,6,7 This fundamental property not only enables novel photonic and optoelectronic devices 4,8,9 but also offers unique spectroscopic fingerprints through Raman scattering. [10][11][12][13][14] The integration of ALMs into optical cavities, formed naturally on dielectric substrates, 2,4,15 creates hybrid systems where the material's intrinsic anisotropy is imprinted onto cavity modes, 2 establishing a powerful platform for anisotropic modulation on photonic processes in low-symmetry systems. This cavity-ALM synergy represents a significant advancement beyond conventional isotropic material systems, 16 offering new dimensions for controlling light-matter interactions.
A fundamental paradox confronts many prominent narrow-bandgap ALMs: despite their exceptionally strong birefringence and linear dichroism, yet suffering from significant optical dissipation in the visible spectrum. This limitation, observed in materials such as black phosphorus, 1,14,17 transition metal dichalcogenides (TMDs), 3,18,19 and quasi-1D materials, 20,21 severely curtails the effective cavity length and directly restricts achievable modulation in anisotropic Raman scattering. To overcome this absorption bottleneck, an ALM with ultralow optical absorption and thus negligible linear dichroism is required to unlock a distinct birefringence-driven cavity effect, where the cavity response is dominated purely by birefringence-an effect critical for designing cavity-enhanced low-loss birefringent nanophotonics and deciphering fundamental mechanisms anisotropic light-matter interactions. Accordingly, α-phase molybdenum trioxide (α-MoO 3 ) emerges as a promising ALM, 15,22 exhibiting exceptional phonon diversity, 15,22,23 and critically, drastically low optical absorption 24 with ultrabroadband birefringence across visible and infrared regions. [25][26][27] Leveraging this extreme in-plane birefringence, prior studies have demonstrated highly anisotropic polaritons 22,23 and subwavelength polarization/phase optical applications, 15 underscoring α-MoO 3 as an established materials platform for controlling low-loss anisotropic light-matter interactions. Furthermore, the wavelength-dependent optical anisotropy of α-MoO 3 and its wavelength dependence have been quantitatively revealed, 28,29 providing a solid foundation for the cavity engineering herein. This unique property portfolio establishes α-MoO 3 as an unparalleled platform to overcome cavity-length limitations and enable exploration of purely birefringence-driven cavity effects in anisotropic light-matter interactions such as Raman scattering.
In this work, we report a birefringence-driven anisotropic α-MoO 3 optical cavity, directly observed through angle-resolved polarized Raman (ARPR) spectroscopy. This cavity effect produces a Raman mode-sensitive enhancement of anisotropy that depends critically on both the α-MoO 3 thickness (d MoO 3 ) and the wavelength of the scattered light. Remarkably, this modulation of the ARPR intensity remains pronounced even in micron-thick crystals, starkly contrasting with the behaviour observed in strongly absorptive anisotropic materials. The entire suite of intriguing anisotropic Raman responses is well captured by a unified model incorporating the intrinsic Raman tensor (R int ), the photon wavelength of Raman scattering, and its chromatic dispersion in the birefringent α-MoO 3 crystal. This model not only enables quantitative prediction of ARPR intensities in ultra-thick α-MoO 3 flakes but also permits precise, unambiguous determination of its crystallographic orientation using the
Mode-sensitive anisotropic enhancement of ARPR intensity in α-
α-MoO 3 crystallizes as a centrosymmetric biaxial ALM with a layered orthorhombic structure (Figure 1a). 30 Each layer consists of MoO 6 octahedra that forms chain arrangements along the a-axis via corner-sharing and along the b-axis via edge-sharing. 30 The dielectric tensor in the crystallographic frame is diagonal, with principal components εa(b,c) = ϵ 1,a(b,c) + iϵ 2,a(b,c)
along the a-, b-, and c-axes confirming its strong optical anisotropy (Figure 1b,c). 31 Density functional theory (DFT) calculations reveal a giant indirect bandgap of ∼3.25 eV (Supplementary Figure S1) and pronounced anisotropic dispersion between the Γ-X and Γ-Y directions (Figure 1d). With minimal absorption in the visible regime, the optical anisotropy primarily manifests as birefringence (n a ̸ = n b ) dominated by the difference between n a and n b (Figure 1c).
The visible-range transparency of α-MoO 3 offers an ideal platform to investigate ARPR spectra modulated purely by birefringence. First-principles phonon dispersion calculations confirm 24 Raman-active
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