# Quantum Leap from Gold and Silver to Aluminum Nanoplasmonics for Enhanced Biomedical Applications
## 4 Enhancing Al Plasmonic Effects
### Quantum Enhancement  
One drawback of Al plasmonics is the strong plasmon damping relative to Au and Ag. As a result, near-field enhancement in Al NSs is weaker than that in Au and Ag NSs [216]. An approach to minimize losses by coupling plasmonic NSs with gain media based on surface plasmon amplification by stimulated emission of radiation (spaser) was proposed [232; 233] and experimentally realized in noble metal NPs [234; 235]. This stimulated generation of coherent plasmons leads to stronger near fields at the surface of NSs, which could be used for enhanced phototherapy. For example, a wound or tumor may be treated with Al NSs and irradiated with a weak UV light, serving as incoherent pump, g, to excite the three-level gain media, such as molecular dyes or quantum dots (Figure 9a). The zoomed-in picture of one of the Al NSs is shown in Figure 9b. It represents a spherical nanoshell filled with gain media particles with energy levels matching the NS SPRs. The localized UV near fields of Al NPs may be used for the enhanced treatment near the surface of the wound without affecting the surrounding healthy tissue. The role of Al NSs in this process is the nanoscale focusing of UV light at the surface of NS attached to the target tissue, lowering the required incident power, and increasing the treatment efficiency. Experimental realizations of Al nanolasers have been reported [236; 237; 238]. However, their practical applications require optimization of structural and optical properties [239].  
Figure 8: Time-dependent density functional theory (TDDFT) simulations of the optical response of pyridine adsorbed on Ag and Al NSs: Optical absorption spectra and transition density plots of the NP-pyridine systems with perpendicular (**A**,**B**) and parallel (**C**,**D**) orientations of pyridine, with respect to the NP surface. Red and green colors denote electron density accumulation (depletion) regions during the excitation, respectively. (Adapted from Mokkath [231]).  
The overall efficiency is limited by the near-field enhancement factors of the Al NSs and the maximum allowed UV dose for the target tissue. These limits may be overcome by using the quantum optical effects, proposed for the Ag [240] and recently extended to the Al NSs [216]. The efficiency of plasmon generation in Al NS spasers can be increased by generating quantum coherence between the states of the gain media using an additional drive laser field that can break the detailed balance and suppress the radiative damping (Figure 9c). Figure 9d shows a comparison of the number of plasmons generated in Al and Ag NS spasers as a function of the pump, g, for different drive fields, \(\Omega_{\text{a}}\). It shows more than an order of magnitude increase of the number of plasmons in Al NSs due to the quantum coherence generated by the drive. The inset of Figure 9d shows the corresponding decrease of the spasing threshold, that is, the required minimum pump strength to generate spaser action. That is another advantage of the quantum coherence as it reduces the UV dose absorbed by tissue due to the UV pump excitation. Other methods may be used to further enhance the performance of Al NS spasers to optimize energy transfer and field enhancement by the coherent control of field propagation in coupled plasmonic systems [241], as well as by transient [242] and feedback-controlled effects [243], leading to more efficient quantum medicine.