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Lattice deformation via substrate‐driven mechanical straining of 2D materials can profoundly modulate their bandgap by altering the electronic band structure. However, such bandgap modulation is typically short‐lived and weak due to substrate slippage, which restores lattice symmetry and limits strain transfer. Here, it is shown that a non‐volatile thermomechanical strain induced during hot‐press synthesis results in giant modulation of the inherent bandgap in quasi‐2D tellurium nanoflakes (TeNFs). By leveraging the thermal expansion coefficient (TEC) mismatch and maintaining a pressure‐enforced non‐slip condition between TeNFs and the substrate, a non‐volatile and anisotropic compressive strain is attained with ε = −4.01% along zigzag lattice orientation and average biaxial strain of −3.46%. This results in a massive permanent bandgap modulation of 2.3 eV at a rate S (ΔEg) of up to 815 meV/% (TeNF/ITO), exceeding the highest reported values by 200%. Furthermore, TeNFs display long‐term strain retention and exhibit robust band‐to‐band blue photoemission featuring an intrinsic quantum efficiency of 80%. The results show that non‐volatile thermomechanical straining is an efficient substrate‐based bandgap modulation technique scalable to other 2D semiconductors and van der Waals materials for on‐demand nano‐optoelectronic properties. 

Nanophotonic structures have shown promising routes to controlling and enhancing nonlinear optical processes at the nanoscale. However, most nonlinear nanostructures require a handling substrate, reducing their application scope. Due to the underwhelming heat dissipation, it has been a challenge to evaluate the nonlinear optical properties of free-standing nanostructures. Here, we overcome this challenge by performing shot-controlled fifth harmonic generation (FHG) measurements on a SiC meta-membrane – a free-standing transmission metasurface with pronounced optical resonances in the mid-infrared (λres≈ 4,000 nm). Back focal plane imaging of the FHG diffraction orders and rigorous finite-difference time-domain simulations reveal at least two orders of magnitude enhancement of the FHG from the meta-membrane, compared to the unstructured SiC film of the same thickness. Single-shot measurements of the meta-membrane with varying resonance positions reveal an unusual spectral behavior that we explain with Kerr-driven intensity-dependent resonance dynamics. This work paves the way for novel substrate-less nanophotonic architectures.