Pressure-induced changes in the optical properties of semiconductors provide critical insights into their electronic and structural behavior under extreme conditions. High-pressure studies reveal modifications in absorption spectra, photoluminescence (PL) emission, and refractive indices, which are essential for applications in optoelectronics, photonics, and quantum technologies. In-situ characterization techniques enable real-time monitoring of these changes, offering a deeper understanding of material responses to compression.
Optical absorption in semiconductors is highly sensitive to pressure due to alterations in the band structure. Compression reduces interatomic distances, leading to increased orbital overlap and changes in the energy gap between valence and conduction bands. For direct bandgap materials like CdSe quantum dots, hydrostatic pressure typically induces a redshift in the absorption edge. This shift occurs because pressure increases the energy separation between the conduction band minima and valence band maxima, reducing the bandgap. Experimental studies on CdSe quantum dots show a pressure coefficient of approximately 30-40 meV/GPa, depending on particle size and surface passivation. In indirect bandgap semiconductors like silicon, pressure can induce a direct-to-indirect transition, altering absorption characteristics significantly.
Photoluminescence spectroscopy under pressure reveals shifts in emission peaks, changes in intensity, and the emergence of new radiative transitions. In CdSe quantum dots, pressure-induced PL redshift correlates with absorption changes, but additional effects such as strain-induced defect formation or phase transitions can modify emission behavior. At pressures exceeding 5 GPa, CdSe undergoes a structural transition from the wurtzite to the rock-salt phase, accompanied by a dramatic PL quenching due to the loss of direct bandgap character. In-situ PL measurements using diamond anvil cells (DACs) with high-pressure transmitting media like silicone oil or argon allow observation of these transitions without sample degradation. For III-V semiconductors like GaAs, pressure enhances excitonic emission intensity up to a critical pressure where non-radiative recombination pathways dominate.
The refractive index of semiconductors also responds to pressure, primarily due to changes in electronic polarizability and density. Compression increases the refractive index as the material becomes more optically dense. For example, in GaN, the refractive index rises linearly with pressure at a rate of approximately 0.01 GPa^-1, as measured by ellipsometry or interferometric techniques. Pressure-induced birefringence can occur in anisotropic crystals, providing additional information about strain distribution and crystal symmetry modifications.
In-situ characterization techniques are indispensable for studying these pressure-dependent optical properties. Diamond anvil cells coupled with micro-spectroscopy setups enable simultaneous optical absorption, PL, and Raman measurements under hydrostatic or non-hydrostatic conditions. Synchrotron-based X-ray diffraction integrated with optical probes allows correlation of structural transitions with optical changes. Advanced methods like time-resolved PL under pressure reveal carrier dynamics and non-radiative recombination pathways, while high-pressure ellipsometry provides precise refractive index determination.
Pressure-tuning of optical properties has practical implications. In quantum dots, controlled pressure application can tailor emission wavelengths for tunable light sources or sensors. Wide-bandgap semiconductors like GaN and ZnO exhibit pressure-stable luminescence, useful in high-power optoelectronic devices. The sensitivity of optical properties to pressure also makes semiconductors viable candidates for piezophotonic applications, where mechanical stress modulates light emission.
Understanding pressure effects requires consideration of material-specific factors. Nanostructured semiconductors exhibit different pressure responses compared to bulk due to quantum confinement and surface effects. Heterostructures and layered materials may show anisotropic compression, leading to direction-dependent optical changes. Temperature and pressure coupling further complicate the behavior, necessitating combined high-pressure, low-temperature studies for comprehensive analysis.
In summary, pressure-induced modifications in optical absorption, photoluminescence, and refractive index provide a powerful probe for semiconductor behavior under extreme conditions. In-situ techniques enable precise monitoring of these changes, revealing fundamental insights into material properties and guiding the development of advanced optoelectronic devices. The ability to controllably alter optical characteristics through pressure opens new avenues for tunable photonic and quantum technologies.