The traditional localized surface plasmon resonance (LSPR) stimulated in metal materials has been studied and applied in the ultraviolet, infrared, and even terahertz band. Unfortunately, it has limited application in the infrared LSPR because of the large absorption loss. Indium tin oxide (ITO), a highly doped semiconductor material, can be an ideal LSPR material in the near-infrared (NIR) band. This is due to the wavelength corresponding to the zero dielectric constant that is located in the NIR band, as well as its small absorption loss in the NIR band. In addition, the LSPR wavelength can be controlled by the tuning of the carrier concentration of the ITO material, and the ITO is unsusceptible to mutual diffusion when in contact with a semiconductor. Although there have been some reports on the promising applications of the LSPR effect in ITO materials, there have been relatively few studies on the relevant impact parameters of the LSPR bands in ITO materials. In this study, the ITO material is chosen to simulate and analyze the LSPR effect, and the effective modulation of the LSPR wavelength of the ITO nanorods in the infrared band is achieved by the tuning of relevant parameters.

In this paper, we systematically simulate and analyze the extinction characteristics and the field intensity profiles of cuboid ITO nanostructures in the 780-5000 nm band using the finite-difference time-domain (FDTD) method. The dielectric constants of ITO materials with different carrier concentrations are calculated according to Drude's model. The construction of LSPR simulations for ITO nanostructures involves the following steps: first, a cuboid ITO model is constructed on the SiO₂ substrate, where the X and Y directions are set as periodic boundaries, and the Z direction is set as the boundary of a perfectly matched layer; second, a 780-5000 nm linearly polarized plane light source with an incident angle of 0° is set directly above the ITO nano model; third, reflectance and transmittance monitors are set to detect the intensity of reflected and transmitted light, and the electric field monitors for the XY (Z=0) plane and XZ (Y=0) plane are set to obtain the changes in the electric field around ITO nanoparticles (Fig. 1). The corresponding extinction spectra and the electric field intensity profiles are obtained by the tuning of the carrier concentration, size, spacing, and substrate refractive index of the ITO nanorods to tune their infrared-band LSPR peaks effectively.

According to the calculations by Drude's model, the real part of the dielectric constant of the ITO gradually decreases while the imaginary part increases with the incident wavelength. In addition, the wavelength corresponding to the zero dielectric constant decreases with the increasing carrier concentration (Fig. 2). The influence of the carrier concentration, size, spacing, and substrate refractive index on the LSPR effect of cuboid ITO nanorods in the NIR band is investigated. It is shown that when the carrier concentration of cuboid ITO nanoparticles rises, the resonance peak position undergoes a blue shift, and the peak intensity is increased. Within a certain range, an increase in the carrier concentration leads to an enhancement of the local field, and the trends of the electric field intensity and the peak intensity are consistent (Fig. 3). As the length of cuboid ITO nanoparticles increases, the resonance peak position goes through a redshift, and the peak intensity increases (Fig. 4). As the height or width of the cuboid ITO nanoparticle grows, the resonance peak position undergoes a blue shift, and the peak intensity increases (Figs. 5 and 6). As the spacing of the cuboid ITO nanoparticles widens, the resonance wavelength changes slightly while the peak intensity declines significantly (Fig. 7). As the value of the refractive index of the substrate enlarges, the resonance peak position goes through a redshift while the intensity of the peak drops (Fig. 9). The adjustment to the above parameters enables the localized surface plasmon resonance wavelength of cuboid ITO nanostructures to be modulated effectively in the NIR band.

In this paper, the FDTD method is used to research the LSPR phenomenon of the ITO material, and the influence of the carrier concentration, size, spacing, and substrate refractive index on the LSPR effect of cuboid ITO nanorods is discussed. The effective modulation of the LSPR wavelength in the infrared band is achieved thanks to the carrier concentration tunability of the ITO and the advantages of the ITO nanorod structure. Under the same structure, the corresponding LSPR wavelength is tuned from 2850 nm to 1985 nm by the adjustment to the change in the carrier concentration. Under the same conditions, the corresponding LSPR wavelength is tuned from 2090 nm to 3710 nm by the adjustment to the length of ITO nanoparticles from 100 nm to 400 nm. This indicates that the LSPR effect stimulated by ITO materials with nanorod structure has a great effect on the tuning of the LSPR wavelengths. The above parameters can be adjusted to achieve an effective modulation of the LSPR wavelength of cuboid ITO nanostructures in the infrared band, which has important research implications for broadening the application of ITO nanostructures to LSPR effects in the infrared band.