Phosphorene, a monolayer of black phosphorus, exhibits unique optical properties due to its puckered honeycomb structure and intrinsic anisotropy. Unlike other two-dimensional materials such as graphene or transition metal dichalcogenides (TMDCs), phosphorene demonstrates strong in-plane anisotropy in its optical response, making it highly attractive for polarization-sensitive optoelectronic applications. Its direct bandgap, tunable from approximately 0.3 eV in bulk to around 2 eV in monolayers, allows for broadband light-matter interactions spanning the visible to mid-infrared spectrum.

One of the most distinctive features of phosphorene is its anisotropic light absorption. The absorption coefficient varies significantly along the armchair and zigzag crystallographic directions due to differences in electron-phonon coupling and electronic band dispersion. For instance, along the armchair direction, phosphorene exhibits higher absorption in the visible range, while the zigzag direction shows stronger absorption in the near-infrared. This anisotropy enables polarization-dependent photodetection, where incident light with different polarizations can be selectively absorbed, enhancing device versatility.

Photoluminescence (PL) studies reveal layer-dependent emission characteristics in phosphorene. Monolayers exhibit strong PL peaks in the visible range, while thicker layers show redshifted emission due to reduced quantum confinement. The PL quantum yield is lower than that of TMDCs like MoS2, but phosphorene’s broader spectral coverage compensates for this limitation. Additionally, environmental sensitivity affects PL stability, as oxidation under ambient conditions can quench emission. Encapsulation with hexagonal boron nitride (hBN) has been shown to mitigate degradation, preserving optical performance.

Nonlinear optical effects in phosphorene are particularly noteworthy. The material exhibits strong third-order nonlinear susceptibility, enabling applications in optical switching and frequency conversion. Saturable absorption has been observed at high fluences, making phosphorene suitable for ultrafast photonics, such as mode-locked lasers and optical limiters. The anisotropic nature further influences nonlinear responses, with polarization-dependent modulation depths reported in pump-probe experiments.

Phosphorene’s optical properties translate into promising applications in photodetectors, modulators, and waveguides. Broadband photodetection is achievable due to its tunable bandgap, with responsivities reported up to 10^5 A/W in the near-infrared. The anisotropic absorption allows for polarization-resolved detection without external filters, a feature absent in isotropic materials like graphene. In modulators, the strong light-matter interaction and electro-optic tuning of the bandgap enable high modulation depths at low voltages. Waveguides leveraging phosphorene’s anisotropy demonstrate directional light propagation, with lower losses along the armchair direction compared to the zigzag axis.

Wavelength-dependent performance is a critical consideration. In the visible range, phosphorene competes with TMDCs but offers superior flexibility in polarization handling. For near-infrared applications, its performance surpasses that of most TMDCs, which typically have larger bandgaps. In the mid-infrared, phosphorene outperforms graphene in terms of on-off ratios due to its semiconducting nature, though graphene retains advantages in ultrafast response times.

Comparisons with other 2D materials highlight phosphorene’s unique position. Unlike graphene, which lacks a bandgap, phosphorene provides semiconducting behavior with tunable optoelectronic properties. Compared to TMDCs, phosphorene’s anisotropy and broader spectral response offer distinct advantages, though its environmental instability remains a challenge. Hexagonal boron nitride, while optically inactive, serves as an ideal substrate or encapsulation layer to enhance phosphorene’s performance.

In summary, phosphorene’s anisotropic optical properties, combined with its tunable bandgap and nonlinear responses, make it a compelling material for next-generation optoelectronic devices. Its applications in photodetection, modulation, and waveguiding benefit from wavelength- and polarization-dependent functionalities, setting it apart from conventional 2D materials. While stability issues persist, advances in encapsulation and passivation techniques continue to improve its viability for practical implementations.