In materials science, achromatic optical elements have high transparency and low dispersion. Materials scientists have demonstrated that although metals are highly opaque, densely arrayed arrays of metal nanoparticles (more than 75% metal content by volume) are more transparent than media such as germanium under infrared radiation. This array can form an effective medium with almost no dispersion in the ultra-wideband wavelength range, thereby designing various next-generation metamaterial-based optical devices. Scientists can adjust the local refractive index of these materials by changing the size, shape, and spacing of nanoparticles, thereby designing gradient index lenses to guide and focus light on the microscopic scale.
The electric field can be strongly concentrated in the gaps between the metal nanoparticles in order to simultaneously focus and "squeeze" the medium field, thereby generating a powerful, double-enhanced hot spot. Scientists can use these hot spots to promote the measurement of infrared spectroscopy and other nonlinear processes over a wide frequency range. In a study published in the journal Nature Communications, Samuel J. Palmer and an interdisciplinary research team from the Department of Physics, Mathematics and Nanotechnology in the United Kingdom German research shows that artificial media can remain highly transparent to infrared radiation, even if the particles are nanoscale, this result can be observed.
• How metals, media, and effective media respond to slowly changing electric fields. In each system, the applied electric field is opposed to the induced electric field generated by surface charge accumulation. Photo: Nature Communications research has shown that electric fields penetrate particles (making them imperfect in conduction), causing them to interact strongly in close arrangements. The research results will enable materials scientists to design achromatic optical components for the mid-infrared wavelength region. The local refractive index of these components can be adjusted by changing the size, shape and spacing of the nanoparticles, while being sensitive to the local refractive index of the surrounding environment. Scientists have enhanced the electric field in the gaps of metal nanoparticles in the array, and at the same time designed the gradient index lens using its transparency, tunability and high metal filling rate. The study focused light on the micro-scale, compressing the electric field on the nano-scale, thereby generating double-enhanced electric field hot spots in the entire infrared (IR) region.
Scientists envision that this new study will facilitate the use of infrared spectroscopy and other nonlinear processes to measure over a wide range of frequencies. Materials scientists are currently able to develop new and advanced materials; however, no new material has the same structure. Most materials can be characterized by uniform microscopic properties, such as refractive index, where the heterogeneity of atoms is less than the average wavelength incident on the material. When the material contains enough sub-wavelength structure, the artificially constructed material called metamaterial is described by the effective index. Early metamaterials included artificial media consisting of arrays of metal particles a few centimeters in size, which can guide and focus radio waves like media.
• Effective permittivity of metal nanoparticle arrays. Early metal particles in artificial dielectric materials were very large. They had high transparency to radio waves and showed perfect conductor characteristics. The new research goal of materials science is to use nanometer metal particle arrays to establish effective visible and infrared spectral media. Subsequently, advances in the assembly technology of metal nanoparticles can achieve unprecedented complex engineering of the interaction between light and matter in the field of optics. In the current study, the transparency of nanoring arrays and nanospheres (although nanoparticles can have other shapes) and germanium was compared to prove that the array can guide and focus light. The nanowire array acts as an effective medium under the action of laterally polarized light; the lateral force on the electrons causes the surface charge to oscillate, simulating the oscillating dipole of the atoms in the real medium.
The cylinder's response to transverse magnetic polarized light is similar to that of bulk metal. Electrons move freely under the action of a longitudinal electric field without coming into contact with the surface of the cylinder. In this study, no matter what the incident polarization is, the nanosphere array behaves like an effective medium—focusing electrons in any direction, thereby generating surface charges that mimic the medium's oscillating dipole. Compared with real media such as germanium, this array shows a high degree of transparency-even if the metal content of the system is higher than 75%. To verify the accuracy of the theory, the researchers used gold nanoparticles with a diameter of 60 nanometers to make a highly ordered colloidal supercrystal. Supercrystals were deposited on the germanium substrate, and the materials were characterized by ultraviolet-visible-near infrared spectrophotometer (physical properties were tested).
• Experiments and numerical simulations of transparent metal arrays Scientists have observed that this material has high transparency, which proves the feasibility of making metamaterials experimentally. Using the near-field magnetic field, it was found that the effective medium is transparent enough to act as a micron lens for infrared radiation. Despite being 82% metal by volume, scientists have observed that decomposing pure gold into a series of gold nanocapsules can produce a transparent lens that can focus light, which is very similar to the behavior of a uniform dielectric lens. Then, the scientists compared different types of metals (aluminum, silver, gold, and titanium), and the results showed that the nanoparticle array produced by the material with the longer skin depth is the most transparent and the least dispersive. Studies have shown that at a fixed wavelength, the ratio of the particle diameter to the depth of the metal surface determines whether the particle behaves as a quasi-particle dipole or as a perfect conductor. In addition to high transparency, scientists can also adjust the system by controlling the size, shape and space of particles.
• Transparency is a function of the depth of the skin of the material. For example, by controlling the aspect ratio of the elliptical cylindrical array to indicate that the material anisotropic response is tunable. The numerical calculation results show that when the system rotates, the effective index can change by more than 50%. Therefore, scientists can adjust the effective index by fixing the particle position and adjusting their size. To highlight this potential for adjusting the local effective index, a triangular grid of golden cylinders was used to construct a gradient index (GRIN) lens, and the diameter of the cylinder was changed with position. Using GRIN lenses, scientists can simultaneously focus light on the micro-scale and then "squeeze" the light on the nano-scale, resulting in a strong "dual enhanced" electric field hot spot. Unlike plasma enhancement, this effect does not depend on loss resonance, showing broadband and low loss characteristics. The focal point of the GRIN lens must coincide with the closest accumulation area to maximize the compression of the electric field.
• The number (solid line) and Maxwell Garnett's mixed formula (dash) are different from the magnetic field that continuously exists on the air-metal interface in the study. As a result, compressing the 2μm wavelength to a 2nm gap produced a strong high-intensity hot spot in the study. In this way, a low-loss, effective medium is constructed using metal nanoparticle arrays. Scientists have obtained highly transparent arrays whose transparency exceeds that of real media such as germanium; it is known for its transparency to low-energy radiation. It is also possible to locally adjust and control the size, shape and space of new metamaterial particles. Scientists have shown that the effective refractive index is basically a constant greater than 2m for all wavelengths. This research will enable materials scientists to design and design precision optical devices with metamaterials that can guide or enhance light over a wide range of frequencies, with virtually no upper wavelength limit.