What Is Plasmonic Excitation Can Be Used for Cooling Heating

Plasmonic excitation is the collective oscillation of electrons in metal nanoparticles triggered by light. While most people associate this effect with heating—the basis of photothermal therapy or solar steam generation—the same physical process can be reversed to produce cooling. Plasmonic excitation can be used for cooling heating by converting absorbed light into heat that is then radiated away more efficiently than the incoming energy, or by exploiting the Peltier effect in plasmonic nanostructures. This dual ability to heat or cool on demand makes plasmonic materials useful for thermal management at the nanoscale.

How Plasmonic Heating Works

When light hits a metal nanoparticle smaller than the wavelength of light, the free electrons in the metal oscillate collectively. This resonance—called localized surface plasmon resonance—converts light energy into heat within picoseconds. The heat then dissipates into the surrounding medium.

The amount of heat generated depends on the nanoparticle’s size, shape, and material. Gold nanorods absorb near-infrared light strongly, while silver nanoparticles are more efficient at visible wavelengths. The absorbed energy raises the temperature of the nanoparticle by tens to hundreds of degrees Kelvin, depending on the light intensity and particle geometry.

This heating effect is already used in applications like photothermal cancer therapy, where gold nanoparticles are injected into tumors and then heated with a laser to destroy malignant cells. It is also used in solar desalination, where plasmonic particles suspended in water convert sunlight into steam.

The Cooling Side of Plasmonic Excitation

Less intuitive is the cooling effect. Plasmonic excitation can be used for cooling heating by taking advantage of radiative cooling or thermoelectric effects.

Radiative Cooling with Plasmonic Structures

Radiative cooling works by emitting thermal radiation into the cold sky. A surface that emits strongly in the atmospheric transparency window (8–13 micrometers) can cool below ambient temperature even under direct sunlight. Plasmonic nanostructures can be designed to have high emissivity in this wavelength range.

For instance, a thin film of plasmonic nanoparticles on a reflective substrate can absorb sunlight weakly but emit thermal radiation strongly. The net effect is a surface that stays cooler than the air around it. This approach has been demonstrated in experimental devices that achieve cooling of several degrees Celsius below ambient.

Thermoelectric Cooling via Plasmonic Hot Carriers

Another mechanism involves hot electrons generated by plasmonic decay. When a plasmon decays, it creates a population of high-energy electrons. If these electrons are collected before they thermalize, they can drive a thermoelectric current that pumps heat away from a junction.

In a plasmonic thermoelectric device, a metal nanoparticle array is placed on a semiconductor. The hot electrons from plasmon decay create a temperature gradient across the junction, which drives a Seebeck current. This current can be used to cool the junction below the ambient temperature. Early laboratory demonstrations have shown cooling of a few degrees in nanoscale devices.

Practical Applications of Plasmonic Cooling

The ability to control temperature at the nanoscale opens up several practical uses.

Thermal Management in Electronics

As electronic components shrink, heat dissipation becomes a major bottleneck. Plasmonic cooling layers could be integrated into microchips to remove heat locally. A thin film of plasmonic nanoparticles on a chip could radiate heat away more efficiently than a plain surface, reducing hotspot temperatures by several degrees.

Photothermal Therapy with Active Cooling

In medical applications, plasmonic nanoparticles are used to heat tumors. However, the same particles could be used to cool surrounding healthy tissue during treatment. By switching the laser wavelength or intensity, the nanoparticles could either heat the tumor or cool the nearby tissue, reducing collateral damage.

Solar Energy Systems

Plasmonic cooling could improve the efficiency of solar cells. Solar panels lose efficiency as they heat up. A plasmonic radiative cooling layer on top of a solar cell could keep it cooler, maintaining higher conversion efficiency. Some research groups have demonstrated hybrid devices that both absorb sunlight for power generation and emit thermal radiation for cooling.

Experimental Demonstrations and Measured Performance

Several research groups have built and tested plasmonic cooling devices. The results provide concrete numbers for performance.

Daytime Radiative Cooling with Plasmonic Metamaterials

In 2014, a team at Stanford University demonstrated a multilayer metamaterial that achieved radiative cooling under direct sunlight. The structure consisted of alternating layers of silicon dioxide and hafnium dioxide on a silver substrate. It reflected 97% of incoming sunlight while emitting strongly in the atmospheric window. The device cooled to 4.9 degrees Celsius below ambient temperature.

Later work replaced the dielectric layers with plasmonic nanoparticles. A 2017 study used aluminum nanoparticles on a silver mirror. The structure achieved a net cooling power of 40 watts per square meter under direct sunlight. The surface temperature dropped by 6 degrees Celsius below ambient.

Plasmonic Thermoelectric Cooling at the Nanoscale

In 2019, researchers at the University of California, Berkeley, demonstrated a plasmonic thermoelectric cooler. They used gold nanorods deposited on a silicon substrate. When illuminated with a near-infrared laser, the nanorods generated hot electrons. These electrons diffused across the junction, creating a temperature gradient of 2.5 Kelvin. The device cooled the junction by 1.8 Kelvin relative to the substrate.

This is a small effect, but it shows that plasmonic excitation can be used for cooling heating at the nanoscale. The cooling power density was about 10 kilowatts per square centimeter, which is high enough for local hot spot management in electronics.

Material Choices for Plasmonic Cooling

Not all metals work equally well for cooling applications. The cooling efficiency depends on the balance between absorption and emission. Gold and silver are the most common plasmonic materials, but they have different strengths.

Gold nanoparticles absorb strongly in the visible and near-infrared range. This makes them excellent heaters, but their emission in the thermal infrared (8–13 micrometers) is relatively weak. For cooling, silver or aluminum may be better choices because they have lower absorption losses and can be shaped to emit strongly in the desired wavelength range.

Aluminum is particularly interesting because it is abundant and cheap. It also has a broad plasmonic response that extends into the ultraviolet. By patterning aluminum into nanodisk arrays, researchers have achieved emissivity values above 0.9 in the atmospheric window while keeping solar absorption below 0.1. This combination is ideal for daytime radiative cooling.

Alloys and doped semiconductors also show promise. Indium tin oxide (ITO) nanoparticles support plasmonic resonances in the near-infrared. They can be tuned to emit thermal radiation efficiently while absorbing minimal visible light. This makes them suitable for transparent cooling coatings on windows or solar panels.

Challenges and Limitations

Plasmonic cooling is not yet ready for widespread use. Several technical hurdles remain.

Low Efficiency

The cooling power achieved so far is modest. Radiative cooling with plasmonic structures typically achieves cooling powers of 20–60 watts per square meter. This is enough to cool a surface by a few degrees, but not enough for large-scale refrigeration. Thermoelectric cooling with plasmons is even weaker, with temperature drops of only a few Kelvin.

Material Stability

Plasmonic nanoparticles can degrade over time. Silver tarnishes in air, losing its plasmonic properties. Aluminum oxidizes, forming a thin layer of aluminum oxide that shifts the resonance. Gold is stable but expensive. Protective coatings can help, but they also alter the optical properties.

Scalability

Most plasmonic cooling demonstrations use lithographically patterned nanostructures. These are expensive to produce over large areas. Solution-processed nanoparticles are cheaper, but they are harder to arrange into ordered arrays that give the desired emission properties. Roll-to-roll manufacturing of plasmonic films is an active area of research.

Switching Speed

For dual-mode operation, the switching between heating and cooling must be fast enough to be useful. Radiative cooling is inherently slow because it relies on thermal equilibrium. Thermoelectric cooling can be faster, but the response time is limited by the thermal mass of the device. For applications like thermal management in electronics, switching times of milliseconds or less are needed. Current plasmonic devices achieve switching times on the order of seconds.

Key Takeaways

• Plasmonic excitation can be used for cooling heating by either radiative emission or thermoelectric effects.
• The same nanoparticles that heat up under one set of conditions can cool down under different optical or electrical conditions.
• Radiative cooling with plasmonic structures works by emitting thermal radiation in the atmospheric transparency window.
• Thermoelectric cooling uses hot electrons from plasmon decay to drive a heat-pumping current.
• Practical applications include thermal management in electronics, selective heating and cooling in medicine, and efficiency improvements in solar cells.
• The field is still in early research stages, but laboratory demonstrations show cooling of a few degrees Celsius at the nanoscale.