Photoluminescence

Photoluminescence is a form of luminescence in which matter emits light following the absorption of electromagnetic radiation. The process begins when photons excite electrons to higher energy levels, after which the system undergoes relaxation and re-emits photons at longer wavelengths. This phenomenon is widespread across gases, solutions, molecular compounds and inorganic semiconductors, and is an essential tool for probing structural and electronic properties of materials.

Fundamental Principles

Photoluminescence arises when a material absorbs photons and electrons are promoted from lower to higher energy states. The excited system then relaxes through a series of radiative and non-radiative processes. Emission timescales vary widely, from ultrafast femtosecond events in inorganic semiconductors to delayed emissions lasting minutes or hours in specific phosphorescent materials.
Observation of a photoluminescence signal at a particular energy typically indicates that electrons have populated the corresponding excited state. While this interpretation holds for atoms and simple molecules, many-body effects play a significant role in the photoluminescence of semiconductors. Correlations between electrons, holes and lattice vibrations require theoretical frameworks such as the semiconductor luminescence equations to accurately describe the emission process.
In many materials, absorbed photons are re-emitted with lower energy than the excitation photons. This red shift occurs because part of the excitation energy dissipates through molecular vibrations or interactions within the material.

Types of Photoluminescence

Photoluminescence processes may be classified by the relationship between the excitation and emission energies:

• Resonant excitation: The material absorbs photons and re-emits them almost immediately with the same energy. This phenomenon, often termed resonance fluorescence, involves minimal internal energy transitions. In crystalline semiconductors, resonant excitation can produce both elastic emission such as resonant Rayleigh scattering and inelastic components resulting from recombination processes.
• Non-resonant excitation: The absorbed photons have higher energy than the emitted photons. This category encompasses fluorescence and phosphorescence, two processes commonly distinguished in chemical and optical sciences.

In fluorescence, electrons drop from an excited state to the ground state rapidly, but with energy loss that results in red-shifted emission. Phosphorescence involves intersystem crossing into a triplet state, where transitions back to the ground state are quantum mechanically forbidden. This leads to notably slow emission, forming the basis for glow-in-the-dark materials.
Photoluminescence may also incorporate quantum-optical effects, such as photon antibunching in resonance fluorescence, indicating non-classical light emission.

Applications in Materials Science

Photoluminescence is a powerful diagnostic technique for evaluating semiconductor quality. In materials such as gallium nitride (GaN) and indium phosphide (InP), photoluminescence spectra reveal information about crystalline integrity, impurity concentrations and disorder. Time-resolved photoluminescence is widely used to measure minority-carrier lifetimes in direct-gap semiconductors such as gallium arsenide (GaAs). By analysing decay profiles following pulsed excitation, researchers can assess recombination mechanisms and the efficiency of semiconductor devices.
The sensitivity of photoluminescence to internal electric fields and the surrounding dielectric environment makes it a useful tool for studying photonic crystal structures and other nanostructured materials.

Photoluminescence in Direct-Gap Semiconductors

Direct-gap semiconductors provide a straightforward platform for studying photoluminescence. When illuminated with radiation exceeding the bandgap energy, electrons are lifted into the conduction band, leaving holes in the valence band. These charged excitations carry finite momentum and subsequently relax through Coulomb scattering, interactions with phonons and other many-body processes. Radiative recombination of electrons and holes then produces the observed photoluminescence signal.
Theoretical descriptions of these interactions use the semiconductor Bloch equations to model the induced polarisation and carrier dynamics. A detailed account requires consideration of collective interactions, lattice vibrations and optical coupling, which collectively shape the emission spectrum.
Internal electric fields, interface quality and environmental dielectric properties significantly influence the emission energies and intensities. Photonic crystals, for example, can alter the density of optical states and modify radiative recombination pathways.

Quantum-Well Structures

Quantum wells provide an idealised model for analysing the fundamental processes involved in photoluminescence. These heterostructures consist of thin semiconductor layers that confine electrons and holes in quantised subbands. A typical model includes several electron and hole subbands, each contributing characteristic excitonic resonances observable in the linear absorption spectrum.
The main resonances correspond to transitions between subbands such as the first electron and hole subbands, as well as from the associated continuum states or from the higher subbands. The confined geometry enhances excitonic effects and allows detailed analysis of how carriers relax and recombine under different excitation conditions.

Photoexcitation Conditions

Three primary regimes of photoexcitation are used in photoluminescence studies:

• Resonant excitation: The excitation energy aligns with the lowest exciton resonance of the quantum well. Coherent processes dominate the emission, with excitons generated directly by polarisation decay. Discriminating the photoluminescence from stray light and Rayleigh scattering poses experimental challenges due to spectral overlap.
• Quasi-resonant excitation: The excitation energy lies above the ground-state exciton but below the barrier absorption edge. Although some excess energy is introduced, it is significantly less than in non-resonant cases. Coherent contributions are negligible and the carrier system begins at an elevated temperature relative to the lattice.
• Non-resonant excitation: The material is excited well above the bandgap or barrier energies, imparting substantial excess energy to the electron–hole pairs. This is the most common excitation mode, as the emitted spectrum can be readily isolated using optical filters or spectrometers. Carrier relaxation occurs mainly through phonon interactions before recombination produces the photoluminescence.