Hypergap materials reveal a second transparent region above the bandgap, overturning assumptions about optical limits and enabling low-loss light control, wavelength conversion, and new photonic device possibilities.
(Nanowerk Spotlight) Light interacts with matter through the movement of electrons. Every solid has an electronic bandgap, the energy difference between electrons bound to atoms and the higher energy states they can move into. When light has less energy than this bandgap, it passes through the material with almost no loss. When its energy is higher, the light is absorbed as electrons jump to those higher states, turning optical energy into heat. This rule defines the boundary between transparency and opacity in materials.
Throughout the history of optics, nearly all technologies have operated within the range below that boundary. Transparent solids such as glass and fluoride crystals transmit light efficiently but behave predictably. Their refractive index, which measures how much they bend light, rises steadily with frequency.
Semiconductors perform well in devices such as lasers and LEDs but still rely on energies where absorption remains weak. Metals behave very differently. Their electrons move freely, creating negative permittivity, a property that allows light to bind to the surface as plasmons, yet those same electrons also scatter energy rapidly, turning much of it into heat.
Nonlinear optical crystals can generate new colors of light, but they depend on precise crystal orientations or periodic structures to maintain phase matching, the condition that keeps interacting waves synchronized. Each of these material classes serves a purpose, but none can combine transparency, tunability, and low loss in a single form.
Progress in computational materials science is now making it possible to test new ideas about transparency. Large databases of crystal structures, combined with accurate quantum mechanical calculations, can predict how tens of thousands of solids interact with light. These tools let researchers search for compounds that might remain transparent at energies above the conventional bandgap.
Improved methods for crystal growth and optical measurement now make it possible to verify those predictions in the laboratory. One study that builds directly on these advances appears in Advanced Materials (“Hypergap Optical Materials”).
The paper proposes that certain solids can host a second transparent region at higher photon energies, called a hypergap. Within this region, light can travel with little absorption because there are no available electronic states for it to excite. The study shows that this second window can produce optical effects that are not possible in ordinary transparent materials, offering new ways to control light in photonic systems.
Hypergap material for optics. a) The electronic states of the hypergap material, where Eg is the bandgap. b), Conceptual illustration of the momentum-relaxed joint density of states (JDOS), dielectric constants, and group-velocity dispersions (GVD) of the hypergap material. The refractive index n = √𝜖1 when 𝜖2 = 0. The low-energy phonon absorption in the infrared is not illustrated in the plot. (Image: Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The research begins with a structural insight. In most solids, the electronic bands that define the bandgap lie close to other energy levels. Light with slightly higher energy can excite electrons into those levels, which causes absorption and makes the material opaque.
In a hypergap material, however, the electronic bands are isolated from other states. This isolation creates a narrow energy range above the bandgap where there are no available transitions. Light within that range is not absorbed, and the material becomes transparent again.
The study predicts that this hypergap region can lead to three important effects. First, it can produce negative permittivity with very little loss, allowing the behavior of metals without their high absorption. Second, it can make phase matching possible in materials that are isotropic or amorphous, avoiding the need for complex birefringent crystals or periodic structures. Third, it can create negative group velocity dispersion, which allows short pulses of light to compress rather than stretch as they move through the material.
To identify solids with these properties, the researchers performed a large computational search using two extensive materials databases, the Materials Project and AFLOW. Together these contain information on almost two hundred thousand inorganic compounds. The team selected experimentally verified crystal structures and removed metals with no bandgap. They then calculated the electronic and optical properties of about twenty-three thousand insulating band structures.
For each material, they computed the joint density of states, a measure of how many electronic transitions are available for light absorption, and included indirect transitions that involve crystal vibrations. If this density dropped to zero over a finite energy range above the bandgap, the material was labeled a hypergap candidate. After several refinement steps, including recalculations with experimental lattice constants and removal of narrow or uncertain gaps, the list narrowed to 221 promising materials.
These candidates show hypergap widths from about 0.1 to 10 electron volts, depending on their atomic composition. Although density functional theory often underestimates absolute energies, the trend suggests that some hypergaps extend into the ultraviolet, a range valuable for lithography and spectroscopy.
Many identified compounds contain fluorine or oxygen, which form strong ionic bonds and help isolate the key electronic bands. Some also include transition metals with narrow d or f orbitals or organic groups such as methyl and amine clusters that further confine electronic states.
One major predicted feature of hypergap materials is the presence of negative permittivity with low optical loss. Permittivity describes how a material responds to an electric field. When it becomes negative, the induced electric polarization opposes the field, similar to metals that reflect most light.
Unlike metals, hypergap insulators can show this property without significant absorption because the electrons responsible for polarization are not free to move and dissipate energy. Calculations found that thirty-nine materials display negative permittivity within their hypergaps.
A specific example is rubidium hexafluoroniccolate (Rb₂NiF₆), where the interaction between nickel and fluorine orbitals produces a region of negative permittivity suitable for surface plasmon polaritons, surface-bound waves of light that could propagate with minimal damping.
Another predicted effect is phase matching through anomalous dispersion. Dispersion describes how the refractive index changes with light frequency. Normally, the index increases with energy, a pattern known as normal dispersion. Near electronic resonances, however, it can reverse, creating anomalous dispersion.
If the refractive index in the hypergap becomes similar to that within the bandgap, light waves at fundamental and harmonic frequencies can stay synchronized, enabling efficient nonlinear conversion. The study identifies 74 materials with this condition, thirteen of which lack a center of symmetry and can therefore generate second harmonics directly. 73 materials support third harmonic generation. This means that even isotropic or amorphous hypergap materials could double or triple optical frequencies without the complex engineering required by traditional nonlinear crystals.
The third significant outcome concerns negative group velocity dispersion, which determines how a light pulse spreads or compresses as it moves. In most transparent materials, shorter wavelengths travel more slowly than longer ones, which stretches the pulse. When dispersion becomes negative, the opposite happens and pulses can compress or even form self-stabilizing packets of light known as solitons.
In the predicted hypergap regime, negative dispersion arises naturally because the dielectric constant crosses zero, causing a sharp reversal in how the refractive index changes with frequency. Out of the candidate list, 155 materials show this effect within their hypergaps, often with magnitudes much larger than those achievable in standard media.
To illustrate these results, the paper discusses three representative materials. Rb₂NiF₆ supports a near-lossless surface plasmon polariton due to its negative permittivity. Calcium hexafluoroplumbate (CaPbF₆) exhibits strong negative group velocity dispersion within its hypergap. The organic–inorganic crystal [C(NH₂)₃][N(CH₃)₄]CrO₄ (tetramethylammonium guanidinium chromate), combines both properties. It shows anomalous dispersion phase matching and negative group velocity dispersion between about 3.8 and 4.6 electron volts, corresponding to violet and near-ultraviolet light.
The researchers synthesized this chromate salt and tested its optical response using spectroscopic ellipsometry, a technique that measures how polarized light reflects from a surface. The data revealed two distinct low-loss regions. One lies below 2.45 electron volts, matching the ordinary bandgap. The other, between 3.77 and 4.04 electron volts, aligns with the predicted hypergap.
Within that higher-energy window, the refractive index nearly equals the value inside the bandgap at twice the photon energy, showing that phase matching occurs through dispersion rather than crystal anisotropy. The same measurements also showed negative group velocity dispersion in the hypergap, confirming the theoretical prediction.
The results validate the concept. A well-isolated electronic structure can sustain a transparent window above the bandgap with optical behaviors that ordinary dielectrics cannot achieve. The study also acknowledges limitations. Computational models based on density functional theory tend to underestimate energy levels, so measured hypergap positions may appear at higher photon energies.
Some predicted materials might fail to produce strong dispersion if their electronic transitions are too weak. The screening also excluded magnetic systems and most organic compounds, suggesting that many unexplored candidates remain.
What the paper establishes is a practical framework for discovering new optical materials that break the usual link between transparency and electronic excitation. A hypergap provides a second window where light travels with little loss yet experiences dispersion and polarization effects opposite to those below the bandgap. This could enable near-lossless plasmonic devices, simpler wavelength converters, and compact pulse compressors built from bulk solids rather than complex optical assemblies. These materials could also be integrated into photonic circuits and light-based computing systems that depend on precise control of phase and dispersion.
By identifying more than two hundred existing compounds that meet the theoretical criteria and confirming one example experimentally, this study marks a clear step toward materials that operate beyond traditional optical limits. If further experiments confirm low absorption and stability across these hypergaps, they could provide a new foundation for photonics where both sides of the bandgap become useful terrain for manipulating light.
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Ling Lu (Beijing National Laboratory for Condensed Matter Physics)
, 0000-0002-8781-5979 corresponding author
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