Nonlinear Microscopy in Solid-State Materials

Nonlinear optical microscopy techniques have become powerful tools for investigating solid-state materials, enabling high-resolution, label-free imaging and spectroscopy beyond the capabilities of linear methods. Multiphoton photoluminescence (MPPL) microscopy provides insight into carrier dynamics, defects, and recombination via nonlinear excitation of fluorescence. Second- and third-harmonic generation (SHG, THG) microscopy offers contrast based on structural symmetry and interfaces. Additional techniques like four-wave mixing (FWM) enable the study of ultrafast electronic interactions, while coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) are especially useful for imaging molecular vibrations in organic and hybrid materials, where chemical composition plays a key role.

Photoluminescence microscopy

Two-photon photoluminescence microscopy is based on a nonlinear optical effect, where two photons of lower energy are near-simultaneously absorbed to excite the material. Although widely used in biological imaging for its ability to visualize deep tissue with minimal damage, its application in solid-state materials is also expanding. In this context, multiphoton microscopy offers a valuable tool for studying layered structures and heterojunctions, providing spatially resolved insights into buried interfaces, defect states, or even charge carrier dynamics.

Harmonic microscopy

SHG and THG microscopy techniques are nonlinear optical phenomena that enable high-resolution, label-free imaging of crystal symmetry, interfaces, and ultrafast dynamics in solid-state materials. In terms of SHG, coherent light at frequency ω interacts with a non-centrosymmetric material, resulting in the emission of light at twice the original frequency (2ω). SHG microscopy offers high axial and lateral resolution due to its coherent nature, which depends on the square of the electric field intensity. Additionally, SHG microscopy exclusively occurs in materials with broken inversion symmetry structures. As for THG, light at frequency ω interacts with a material to produce a coherent signal at three times the original frequency (3ω), typically occurring at interfaces or in regions with variations in refractive index. Contrary to SHG, THG can be produced in both centrosymmetric and noncentrosymmetric materials. This makes THG signals more versatile for a broader range of 2D materials.

SHG and THG microscopy are increasingly applied to perovskites, van der Waals materials, and heterostructures to study buried interfaces, ferroelectric domains, strain-induced phases, excitonic resonances, and spin–polarization dynamics. For instance:

The SHG microscopy was used to visualize electrically induced ferroelectric domains and chiral states that exhibited switchable SHG circular dichroism in 2D Ruddlesden-Popper lead halide perovskites. This revealed field-tunable symmetry breaking in an achiral ferroelectric material (Yumoto et al., 2024).
The polarization-resolved SHG imaging was presented as an all-optical technique to map strain in monolayer WS₂ placed over cylindrical wells. The study emphasizes the method’s sensitivity to mechanical deformation in 2D materials (G. Kourmoulakis et al., 2024).
THG microscopy was used to study the in-plane anisotropy of ultrathin tin (II) sulfide (SnS) flakes, revealing orientation-dependent nonlinear optical responses. Polarization-resolved THG mapped crystal symmetry and highlighted its potential for optoelectronic applications (G. M. Maragkakis et al., 2024).

Four-wave mixing microscopy

FWM microscopy has become a powerful ultrafast imaging technique for studying excitons and other quasiparticles in solid-state materials. By directing multiple femtosecond laser pulses onto a sample and detecting the resulting coherent signal, FWM microscopy captures rapid processes with high spatial resolution. It is especially effective for investigating 2D semiconductors, heterostructures, and hybrid perovskites, where small local differences, such as uneven mechanical stress, defects, or weak bonding between layers, can significantly affect how the material interacts with light; see application note using CRONUS-2P.

Raman microscopy

Coherent Raman microscopy techniques like CARS and SRS enable chemically specific, label-free imaging of solid-state materials with sub-micron resolution. By exploiting nonlinear optical interactions, they reveal spatial distributions of vibrational modes, making them valuable tools for studying phase purity, molecular orientation, and ultrafast lattice dynamics in crystalline systems.

CARS and SRS microscopy have been applied to organic crystals, hybrid perovskites, and solid-state pharmaceuticals to visualize polymorphs, domain structures, and vibrational heterogeneity.

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