Ultrafast plasmonics in semiconductor nanostructures represents a cutting-edge field where light-matter interactions at femtosecond timescales enable novel applications in photodetection and photocatalysis. Plasmonic excitations in semiconductors such as InAs and CdTe facilitate the generation of hot carriers, which can be harnessed for efficient energy conversion and signal detection. The unique electronic and optical properties of these materials, combined with their nanoscale dimensions, create opportunities for manipulating light at unprecedented speeds and efficiencies.

Semiconductor plasmonics differs from traditional plasmonics in metals due to the tunable carrier densities and lower losses. In materials like InAs, the plasmon resonance can be adjusted via doping or nanostructuring, allowing precise control over light absorption and hot carrier generation. CdTe, with its direct bandgap and strong excitonic effects, exhibits enhanced light-matter coupling when structured at the nanoscale. These properties make them ideal for ultrafast optoelectronic applications.

Femtosecond dynamics in plasmonic semiconductor nanostructures involve several key processes. Upon excitation with a femtosecond laser pulse, surface plasmons are excited, leading to a collective oscillation of free carriers. This oscillation decays rapidly, within tens to hundreds of femtoseconds, into energetic electron-hole pairs known as hot carriers. The relaxation pathways of these hot carriers include thermalization through electron-electron scattering, phonon emission, and eventual recombination or extraction for useful work.

In InAs quantum dots or nanowires, plasmon-induced hot carrier generation has been demonstrated with efficiencies exceeding those of bulk materials due to quantum confinement and enhanced local fields. Studies show that hot electrons in InAs can reach energies up to 1 eV above the conduction band edge within 100 fs, making them suitable for high-speed photodetection. The short thermalization time, typically under 500 fs, ensures minimal energy loss before carrier extraction.

CdTe nanostructures exhibit similar advantages, with plasmon resonances tunable across the visible to near-infrared spectrum. The hot carriers generated in CdTe have been leveraged for photocatalytic reactions, including water splitting and CO2 reduction. The efficiency of these processes is enhanced by the prolonged lifetime of hot carriers, which can extend up to several picoseconds in carefully engineered nanostructures. This timescale allows for multiple redox reactions to occur before energy is dissipated as heat.

The role of nanostructuring in enhancing ultrafast plasmonic effects cannot be overstated. Nanoscale features such as sharp tips, gratings, or coupled resonators create localized electric fields that intensify light absorption and hot carrier generation. For example, InAs nanodisks with diameters below 50 nm exhibit plasmonic resonances that concentrate light into subwavelength volumes, increasing the probability of hot carrier generation per incident photon. Similarly, CdTe nanopillars with periodic arrangements show enhanced photocatalytic activity due to improved light trapping and carrier separation.

Experimental techniques for probing femtosecond plasmonics include pump-probe spectroscopy, time-resolved photoluminescence, and ultrafast electron microscopy. These methods reveal the temporal evolution of plasmon decay and hot carrier dynamics with sub-100 fs resolution. Pump-probe studies on InAs nanoparticles have quantified the hot carrier cooling rate to be approximately 1 eV/ps, while similar measurements on CdTe films show slower cooling due to stronger electron-phonon coupling.

Applications in photodetection benefit from the rapid response times of plasmonic semiconductors. InAs-based photodetectors with plasmonic nanostructures achieve responsivities exceeding 10 A/W at telecommunication wavelengths, with response times as short as 1 ps. The broadband absorption enabled by plasmon resonances allows these devices to operate efficiently across a wide spectral range. CdTe photodetectors, on the other hand, excel in the visible spectrum, with plasmon-enhanced devices showing detectivities rivaling traditional photodiodes but with faster response.

Photocatalysis leverages the hot carriers for driving chemical reactions. In plasmonic CdTe, hot electrons reduce water to hydrogen with incident photon-to-current efficiencies approaching 5% under visible light. The holes left behind oxidize water or organic molecules, completing the redox cycle. The ultrafast nature of carrier generation ensures that a significant fraction of the solar spectrum is utilized before thermalization losses dominate. InAs nanostructures have also been explored for photocatalytic nitrogen fixation, where hot electrons activate inert N2 molecules at room temperature.

Challenges remain in optimizing hot carrier extraction and minimizing losses. Defects and surface states in semiconductor nanostructures can trap carriers, reducing the available energy for useful work. Advanced passivation techniques and heterostructure designs, such as InAs/AlSb core-shell nanowires or CdTe/CdS heterojunctions, have shown promise in mitigating these losses. Additionally, controlling the plasmon resonance linewidth is critical for maximizing light absorption while maintaining narrow energy distributions of hot carriers.

Future directions include integrating ultrafast plasmonic semiconductors with other functional materials, such as 2D transition metal dichalcogenides or perovskites, to create hybrid systems with tailored energy landscapes. The use of machine learning for designing optimal nanostructures and predicting hot carrier dynamics is also gaining traction. Advances in fabrication, such as atomic-precision epitaxy and self-assembly, will further enhance the performance and scalability of these materials.

Ultrafast plasmonics in semiconductor nanostructures bridges the gap between photonics and electronics, enabling devices that operate at the fundamental limits of speed and efficiency. The unique properties of materials like InAs and CdTe, combined with precise nanoscale engineering, open new avenues for high-speed photodetection, efficient photocatalysis, and beyond. As understanding of femtosecond dynamics deepens, the potential for transformative technologies in energy, sensing, and computing continues to grow.