Last September, the Caribbean islands of Barbuda, St. Martin, Anguilla and others prepared to face the wrath of Mother Nature as Hurricane Irma, a powerful category 5 storm, barreled across the Atlantic, leaving a path of destruction in her wake. At the same time, 93 million miles from Earth, another storm raged—a solar storm.

Most of us look up at the sun and think of it as not much more than a glowing orb in the sky, bathing the Earth in light, but our star has a darker, violent side. As Hurricane Irma bulldozed her way through the Caribbean, a massive solar flare erupted on the surface of the sun, spewing X-rays and high-energy particles in all directions. Just eight minutes after the solar eruption, those same X-rays slammed into the Earth’s magnetic field. Most of the time, our protective shield keeps us protected from the sun, but it can be overwhelmed at times.

High energy particles from the sun run along magnetic field lines, like electric currents run through wires, looking for a release. Most often we see the lighter side of the sun’s wrath, in the form of auroras.

But really powerful ones, like in this instance, can cripple communication systems. Radio signals, like the ones we use to communicate, are able to travel great distances through a section of the atmosphere known as the ionosphere. But during powerful solar storms, those signals are cut off by the X-rays emitted by the sun, ultimately drowned out by crackling static. That’s exactly what happened last fall when islanders were preparing for the storm. Emergency communications throughout the Caribbean were silenced.

Bobby Graves, a ham radio operator and manager of the Hurricane Watch Net—a group of licensed amateur radio operators who help support the National Hurricane Center during emergencies—monitored the blackout (which lasted for several hours) from his home near Jackson, Mississippi.

“You can hear a solar flare on the air as it’s taking place. It’s like hearing bacon fry in a pan, it just all of a sudden gets real staticky, and then it’s like someone just turns the light completely off, you don’t hear anything. And that’s what happened this last year on two occasions,” Graves said in a statement recently released by the American Geophysical Union. “We had to wait ‘til the power of those solar flares weakened so that our signals could actually bounce back off the atmosphere. It was a helpless situation.”

Unfortunately, there wasn’t much anyone could do as we had very little warning that this solar storm was coming. But that’s all about to change as NASA embarks on its newest mission: sending a probe to study the sun close up.

Dubbed the Parker Solar Probe, scientists are hoping that the spacecraft may soon shed some light on space weather. On Saturday, August 11, the car-sized spacecraft will dare to go where no spacecraft has gone before as it flies into the sun’s searing hot outer atmosphere to answer some of heliophysicists’ most burning questions. (Like, how do these violent flares form?)

Solar flares are a common occurrence on the sun, and the most extreme of these belches, are known as coronal mass ejections (CMEs). It’s important to understand how and where on the sun these CMEs are born as they can do a lot of damage to vital systems such as power grids and communications satellites, as well as zap any astronauts (and even satellites) in space with harmful doses of radiation. Nicky Fox, a Parker Probe project scientist at the Johns Hopkins Applied Physics Laboratory, explained that the Parker Solar Probe is going to showcase how what happens on the sun translates into what we see and experience here on Earth.

Despite being the most prominent feature in our cosmic neighborhood, there’s still a lot that scientists don’t understand about the sun, making it difficult to predict with any certainty when and where it will spew material into space. This phenomena is known as space weather, and it comes in three different forms.

The first type is what we witnessed last September when solar flares caused radio blackouts throughout the Caribbean. When CMEs—the more powerful type of solar burp—and solar wind (a slew of charged particles speeding away from the sun) slam into the Earth’s magnetically charged upper atmosphere, they can create major disturbances (called geomagnetic storms) within the Earth’s atmosphere and the space surrounding our planet. This is the type of space weather that produces the beautiful auroras we see near the polar regions. Lastly, radiation storms are caused by rogue protons, whizzing by and slamming into orbiting satellites at nearly the speed of light. These can also affect people on the Earth as they penetrate the atmosphere and can also blast any astronauts in space with a dangerous gust of radiation.

The incident last fall isn’t the first time the sun has lashed out at us.

In 1859, an invisible wave of solar energy crashed over the Earth, knocking out communications systems in the process. During that event, dubbed the Carrington Event, so much solar energy was pumped into the Earth’s atmosphere that telegraph lines were fried and even caught on fire. If such an event would occur today, a national (and perhaps international) crisis would arise as widespread power outages would erupt. Imagine not being able to use your cell phone, or computer, or GPS, etc. Chaos would ensue.  

Residents of Quebec had a small taste of that on March 13, 1989 when a solar storm knocked out the power in the entire province. According to NASA, a 12-hour blackout followed that left millions of people in the dark, closing schools, businesses and even shutting down the airports and metro lines.

“We need to understand how the sun works and be prepared for another Carrington-level event,” Jim Bridenstine, NASA’s administrator, told the Observer. “Back in the 1800s, it was a big event, but today, because of how dependent we are on electronics and the power grid, it’s a much bigger event.”

Forecasting space weather is tricky as scientists can’t predict when a flare will erupt, what direction it will go, or how powerful it will be, which is a key factor in understanding how the flare will affect the Earth’s magnetic field. But, scientists are keeping a watchful eye on the sun thanks to a fleet of solar observers currently in orbit.

And when sunspots come into view (visible warning signs that the sun is about to hulk out), they can issue warnings of the sun’s impending wrath. These warnings go out to every power grid operator in North America, allowing then to prepare for a potential solar flare. This way, power grid operators can take precautions (such as setting up temporary transformers) to prevent a major outage.

The Parker Solar Probe won’t be making real-time forecasts, but it will be collecting data that will help scientists better understand the sun.

“If we can have a CME go off when we’re near to it, that would be pretty awesome, and we’d learn a lot,” explained Nicki Viall, an astrophysicist at NASA’s Goddard Space Flight Center. That knowledge will give scientists the ability to make better predictions that could, in turn, help emergency crews and authorities anticipate blackouts, and perhaps put backup communication systems in place.

“Parker Solar Probe is a critical piece of science that has been developed by NASA for the better understanding and preparedness of humanity,” Bridenstine said. “I can’t wait to see it launch.”

INTRODUCTION

Energy and water shortages are two of the most serious global challenges, and many researchers have devoted themselves to search materials for solar energy harvesting because solar energy is theoretically inexhaustible (1–4). It is well known that the spectrum of solar radiation is located in the range from 295 to 2500 nm. To make the most of solar radiation, optical materials for efficient solar energy harvesting should exhibit strong and broadband absorption. Among a large number of materials, plasmonic and high-index all-dielectric nanostructures have been extensively investigated for solar energy conversion (5–10). On the one hand, the optical absorption can be largely enhanced by localized surface plasmon resonances, which are a collective oscillation of free electrons in nanostructures made of metals or highly doped semiconductors, such as Au (5), Ag (6), Al (7), and TiO_1.67 (8). Local heating through the internal decay of hot electrons inside metallic nanoparticles (11) has been applied in photothermal therapy (12), steam generation (5, 7, 13, 14), and photovoltaic device design (15, 16). On the other hand, recently, it has been experimentally and theoretically demonstrated that high-index all-dielectric nanoparticles can be heated as effective as plasmonic ones depending on Mie-type resonances based on the phase retardation effect, although their absorption coefficients are much smaller than those of metals (9, 10, 17, 18). High-index all-dielectric nanoparticles, such as Si and Ge nanoparticles, have been used for photovoltaics (19, 20) and water evaporation (9), respectively. Although the aforementioned plasmonic and all-dielectric nanostructures have their respective advantages in solar photothermal conversion, to our best knowledge, there are no nanophotonic materials that can combine the advantages of these two materials to more efficiently harvest solar energy.

In this contribution, we design a novel broadband perfect absorber based on self-assembly of tellurium (Te) nanoparticles. Te nanoparticles with a size distribution from 10 to 300 nm were prepared by nanosecond laser ablation in liquid (ns-LAL) (21). Perfect absorption (more than 85{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974}) can be achieved in the entire spectrum of solar radiation (300 to 2000 nm). Especially in the ultraviolet (UV) region (300 to 400 nm), the absorptivity reaches a value of more than 95{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974}. We establish that this good performance is attributed to the unique optical response of Te nanoparticles. Within the spectrum of solar radiation, the real part of permittivity of Te changes from negative to positive, which endows it with optical duality, enabling it to convert from a plasmonic-like material to a high-index all-dielectric material with increasing size. When a Te nanoparticle is smaller than 120 nm, it performs like a plasmonic nanoparticle whose resonance dominated by interband transition is located around 300 nm. As the size increases from 10 to 120 nm, the plasmonic-like resonance can be adjusted from 300 to 400 nm. It is worth noting that ground-state free carriers are absent in Te nanoparticles and that the negative permittivity is caused by interband transitions. Hence, we called this quasi-static resonance in Te nanoparticles plasmonic-like resonance to distinguish it from plasmonic resonance based on metal materials or doped semiconductors. When the Te nanoparticle is larger than 120 nm, both electric and magnetic Mie-type resonances are excited, which demonstrates that it converts to an all-dielectric material. Te has one of the highest refractive indices (6.3 to 4.8) in the visible (vis)–near-infrared (NIR) region so that Te slab or thin films can achieve large surface reflectance (19), making it promising for application as an efficient broadband backreflector (mirror) or the back surface of a solar cell. In addition, Te nanoparticles can support spectrally separated Mie-type resonances due to its high refractive index. Electromagnetic calculations based on Mie theory show that both plasmonic-like and Mie-type resonances can enhance the absorption. As the sizes of Te nanoparticles increase from 10 to 300 nm, the enhanced absorption covering the whole solar radiation spectrum can be realized. Hence, the wide size distribution of Te nanoparticles enables their aggregation to efficiently harvest solar radiation. Moreover, it is important that Te has a smaller bandgap (0.35 eV) (22) than Si (1.12 eV) and Ge (0.66 eV), which enables broader optical absorption up to 3500 nm.

Further, the strong and broadband absorption of Te nanoparticles has been used for solar-enabled photothermal conversion. The temperature of a perfect absorber made of Te nanoparticles can rapidly increase from 29° to 85°C within 100 s irradiated by a halogen tungsten lamp whose spectrum is similar to sunlight. In addition, the water evaporation rate of a Te nanoparticle solution (10 μg/ml) can be improved by three times that of pure water under the simulated solar radiation of 78.9 mW/cm². The above photothermal conversion performance has surpassed that of Al nanoparticles (2.4 times improvement) (7), Ge nanoparticles (2.5 times improvement) (9), and layered BiInSe₃@Nickel foam (2.5 times improvement) (23). Meanwhile, we have demonstrated that Te nanoparticles or species are almost nonexistent in the steam during long-term steam generation by the determination of Te in the evaporated water using inductively coupled plasma atomic emission spectrometry (ICP-AES). In other words, the Te nanoparticle acts only as a photothermal conversion material, and no pollution is produced during the process of water evaporation. These results suggest that the Te nanoparticle can be expected to be an advanced photothermal conversion material for efficient solar energy–driven water evaporation.

RESULTS

Figure 1 shows the typical morphology and structure characterization of Te nanoparticles prepared by ns-LAL. It can be seen that the synthesized nanoparticles have a wide size distribution, as shown in Fig. 1A. The top and tilted scanning electron microscopy (SEM) images in fig. S1 (A and B) show that they are quasi-spherical. Other detailed microscopic morphology and structure of Te nanoparticles are analyzed by transmission electron microscopy (TEM). Figure 1B is the TEM image of a Te nanoparticle (about 150 nm). Figure 1 (C to F) represents its corresponding high-resolution TEM (HRTEM) image, selected-area electron diffraction (SAED) pattern, and energy-dispersive x-ray spectrum (EDS). The HRTEM analysis of the lattice fringes indicates that there are several crystal orientations coexisting in one nanoparticle. Figure 1 (C and D) shows that the interplanar spacing of the nanocrystal is 0.235 and 0.206 nm, which are in good agreement with the value of (102) and (111) of the hexagonal Te structure (24). Besides, fig. S1 (C and F) displays the TEM of another two Te nanoparticles (about 200 and 25 nm, respectively). The interplanar spacing of the nanocrystal shown in fig. S1 (D and G) (0.176 and 0.198 nm, respectively) corresponds to crystal faces (103) and (003). The x-ray diffraction (XRD) pattern shown in Fig. 1G confirms these results. In addition, Fig. 1H depicts the Raman spectrum taken for the synthesized Te nanoparticles. The characteristic vibration peaks at 93.8, 118.9, and 138.5 cm⁻¹ were observed at room temperature, which are consistent with the previous studies of other research groups (25).

To understand the broadband perfect absorption of the self-assembled Te nanoparticles, we investigated the optical response of Te nanoparticles. First, we measured the scattering spectra of individual Te nanoparticles. Actually, optical response of the Te nanoparticle as a high-index all-dielectric material in the mid-infrared region has been reported by Ginn et al. (26). However, to date, the optical behavior of Te nanoparticles in the UV-vis–NIR region (300 to 2000 nm) is still unknown. As shown in Fig. 2A (solid curves), Te nanoparticles with diameters of 106.0, 120.6, 137.5, 156.1, and 178.4 nm were marked by SEM, and their backward scattering spectra were measured using a dark-field optical microscope. It can be seen that the peak located around 700 nm (orange arrow) has a distinct red shift as the particle size increases, while the peak located around 450 nm has a less obvious shift (green arrow). After exposure to air for about 2 months, the scattering spectra of the same Te nanoparticles are nearly unchanged, as shown in Fig. 2A (dashed curves). This result demonstrates that the Te nanoparticles have not been oxidized because if a Te nanoparticle is oxidized, then its scattering behavior will be changed because of the change of refractive index. For comparison, the scattering spectra of TeO₂ nanoparticles are also simulated, as shown in fig. S2. It can be seen that the scattering behaviors of Te and TeO₂ nanoparticles are totally different. Scattering results of Te nanoparticles are confirmed by finite-difference time-domain (FDTD) simulations shown in Fig. 2B. The discrepancies between actual experimental and simulated results are derived from imperfect geometry and the difference of permittivity between single crystal and polycrystal. In addition, Fig. 2C shows the corresponding simulated absorption spectra of Te nanoparticles. As the size increases, the absorption peaks located around 300 nm (green arrow) derived from plasmonic-like resonance remain quasi-static, while the peaks (orange arrow) derived from Mie-type resonances show red shifts. Further, when the Te nanoparticles are self-assembled to form oligomers, their scattering spectra are changed. As presented in fig. S3 (A to D), the scattering intensity of the oligomers are largely enhanced compared with the individual nanoparticles. The absorption cross section (C_abs) of Te nanoparticles with a size distribution shown in fig. S3E indicates that both plasmonic-like and Mie-type resonances can enhance the absorption. Although the spectrum of an individual Te nanoparticle is rather narrow, the overall absorption of particle aggregation becomes broadband due to the wide size distribution of the synthesized Te nanoparticles. When Te nanoparticles are combined to a dimer, the absorption mode becomes stronger and broadening (fig. S3F), which is beneficial to more efficient light harvesting.

To provide a better understanding of the unique optical properties of Te, we fundamentally investigated its dielectric function. Figure S4 shows the real part (n) and imaginary part (k) of the refractive index of Te, Au, and Si. It can be seen that Te is a low-loss, high-index all-dielectric material like Si in the NIR region, but it has a large extinction coefficient like Au in the UV-vis region. Figure 3A depicts a comparison of the real part of permittivity [that is, Re(ε)] of these three materials. As a plasmonic material, Au has a negative Re(ε) in the whole UV-vis–NIR region, and its plasmonic resonance is located at the wavelength (around 500 nm in the air) determined by Re(ε_Au) = −2ε_m, where ε_m is the dielectric function of the embedding medium (27). While as a high-index all-dielectric material, the Re(ε) of Si is positive, and Mie-type resonances can explain its optical behaviors. However, different from these two materials, the Re(ε) of Te is negative at 300 < λ < 490 nm (green area in Fig. 3A) while positive at λ > 490 nm. In other words, Te is of optical duality that exhibits as a plasmonic-like material at wavelength 300 < λ < 490 nm while a high-index all-dielectric material at λ > 490 nm. Its plasmonic-like resonance is located around 300 nm in the air.

The optical duality is completely reflected in the scattering spectra of Te nanoparticles. As depicted in Fig. 3B, when Te nanoparticles are smaller than 120 nm, the plasmonic-like resonance dominates their scattering spectra. The spectra have a slight shift and broadening as the size increases, which are similar to the case of Au nanoparticles shown in fig. S5A. The scattering behaviors of individual Te nanoparticles larger than 120 nm, as presented in Fig. 3C, are similar to that of Si nanoparticles (shown in fig. S5B). When the particle size increases from 120 to 340 nm, the Mie-type resonances of Te nanoparticles can be moved from the vis to the NIR region. Considering that Mie theory is a useful tool to illustrate the optical duality of Te nanoparticles, Fig. 3 (D to F) shows multipolar contributions for scattering efficiency of three Te nanoparticles (100, 200, and 300 nm, respectively) surrounded by air (n = 1) and water (n = 1.33). The electric dipole (ED) contribution dominates the optical response of the smaller Te nanoparticle (100 nm), and the ED resonance peak exhibits a red shift and broadening with increasing the ambient refractive index (Fig. 3D). This property makes it possible for applications in wavelength-based sensors (28). When the size increases to 200 nm in diameter, the Mie-type magnetic resonance begins to arise, although the magnetic dipole (MD) contribution is not big (Fig. 3E). As the size of Te nanoparticle further increases to 300 nm, the MD contribution surpasses ED contribution, and both of them govern the optical response (Fig. 3F). The difference between plasmonic and Mie-type resonances is that the former is wavelength-sensitive but the latter is wavelength-insensitive to the change of ambient refractive index [green and orange arrows in Fig. 3 (D to F)], which is consistent with our previous study (29).

We carried out the solar heating experiment to investigate plasmonic-like and Mie-type resonances, enhanced optical absorption, and broadband photothermal effect of the Te nanoparticles. The SEM image (Fig. 4A) and the size distribution diagram (Fig. 4B) show that Te nanoparticles synthesized by ns-LAL have a relatively wide size distribution from 10 to 300 nm. The self-assembled Te nanoparticle absorber deposited on an Si wafer is shown in Fig. 4C (right) compared with the bare Si wafer (left). The weight of the Te nanoparticle layer is 0.35 mg, as measured by an electronic balance. As presented in Fig. 4D, the measured absorptivity (red curve) of the absorber exceeds 85{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} from 300 to 2000 nm, covering the whole solar radiation spectrum (blue area). It is even larger than 95{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} at the UV region (300 to 400 nm). Since the UV radiation is harmful to humans, the Te nanoparticle absorber can be used as an outer coating to prevent it. This perfect absorption performance is due to the wide size distribution and the enhancement by both plasmonic-like and Mie-type resonances. To carry out the solar heating experiment, we radiated the Te nanoparticle absorber using a halogen tungsten lamp whose radiation spectrum is similar to solar radiation and recorded the thermal radiation using a Fluke thermal imager. We used the bare Si wafer as a reference. As shown in Fig. 4F, the temperature of the Si wafer increased from 29° to 47°C, and it remained constant after 100 s under radiation. However, perfect broadband absorption endows the Te nanoparticle layer with much more efficient heating. Its temperature increased rapidly from 29° to more than 85°C within 100 s and then reached a steady state. The heating performance of the Te nanoparticle layer exceeds that of the TiO_1.67 nanoparticle layer (a perfect absorber reported before) (10) whose temperature reached 80°C within the same time depending on plasmonic resonances. Figure 4 (G and H) shows thermal images of the bare Si wafer and Te nanoparticle absorber to give an intuitionistic description.

For nonmagnetic materials, the absorbed electromagnetic power can be simply calculated by the following equation (30)(1)where ω is the angular frequency, Im(ε) is the imaginary part of the permittivity, and |E| is the amplitude of the electric field. It is clear that strong field enhancements and the large imaginary part of the permittivity are the two important factors that contribute to strong absorption according to this equation. As depicted in Fig. 5A, Im(ε) of Te is compared with that of Si, Ge, and Au. Although Im(ε) of Si and Ge is larger at λ < 400 nm and Im(ε) of Au is larger at λ > 1200 nm, they are smaller than Te in the main solar radiation spectrum (blue area in Fig. 4D). In addition, Fig. 5 (C to H) shows the electric field enhancements at different wavelengths in a Te nanoparticle oligomer. The value reaches dozens of times from 300 to 2000 nm, which is comparable to the Au nanoparticle oligomer (fig. S6).

The Te nanoparticle is a material that converts sunlight to heat energy, and it can be used for photothermal water evaporation. Although Te has been considered to be mildly toxic, it plays an important role in biological systems (31, 32). According to the previous work, the content of Te in the human body is more than 0.5 g. It is inconceivable that Te is the fourth most abundant trace element after Fe, Zn, and Rb in the human body and is unusually abundant in human food and plants (31). Fränzle and Markert (32) even suggested that Te could be an essential nutrient according to certain criteria. Recently, a series of Te-containing polymers have been used as stimuli-responsive biomaterials (33). The field of biological Te chemistry has also become a major player in protein chemistry, imaging, and diagnostics, as well as in the search for new and more potent antibiotics and anticancer agents (34).

We radiated the Te nanoparticles dispersed in pure water using a solar simulator and recorded the vaporized amount of water. Before the radiation, we measured the UV-vis absorption spectrum of the Te nanoparticles colloid solution (5 μg/ml) with gray color (inset in Fig. 6A). The curve shown in Fig. 6A has two broad typical absorption peaks around 300 nm (green arrow) and 600 nm (orange arrow), respectively. According to the previous work (35), the absorption peak around 300 nm is due to the allowed direct transition from the valence band (p-bonding triplet) to the conduction band (p-antibonding triplet), and another broad absorption peak around 600 nm can be assigned to a forbidden direction transition. Note that the plasmonic-like resonance is located around 300 nm, which matches well to our absorption results. Meanwhile, we have demonstrated that Te nanoparticles or species are almost nonexistent in the steam during long-term steam generation by the determination of Te in the evaporated water using ICP-AES (detection limit, ~0.05 μg/ml), which is shown in Fig. 6B. The concentration of Te in the evaporated water is so low (under the detection limit) that it cannot be detected. Namely, the Te nanoparticle acts only as a photothermal conversion material, and it is pollution-free during the process of water evaporation. Te nanoparticles are insoluble in water, and of the chalcogens, Te has the highest melting and boiling points, at 722.66 and 1261.15 K, respectively (36). As presented in Fig. 6C, we measured four kinds of Te nanoparticle colloid solutions with different concentrations under the same irradiation at 78.9 mW/cm². The vaporized weight rises with increasing concentration, and the maximum evaporation rate reaches three times that of pure water, surpassing the performance of Al nanoparticles (2.4 times) (7), Ge nanoparticles (2.5 times) (9), and layered BiInSe₃@Nickel foam (2.5 times) (23) reported previously. Likewise, the evaporation rate increases with rising irradiation, which is shown in Fig. 6D. The water evaporation experiment demonstrates that the Te nanoparticle is a promising material for solar desalination.

To prove that the Te nanoparticles have not been oxidized during long-term steam generation, after working in steam generation for about 2 months, we again analyzed Te nanoparticles by SEM, XRD, TEM, and EDS, as shown in fig. S7. Compared with the “fresh” results shown in Fig. 1, there is little change between them. Besides, after long-term steam generation, the Te nanoparticles can still work efficiently. As shown in fig. S8, the same water evaporation experiment was taken for Te nanoparticles colloid solutions, which were exposed to steam generation for about 2 months. Compared with the results in Fig. 6, the performance is maintained. We think that the inoxidizability of Te nanoparticles is related to the synthetic method. Actually, ns-LAL is a preparation method based on high temperature (~1000 K) and high pressure (~GPa).

The materials used for efficient solar-enabled water evaporation should have (i) large optical absorption in the entire solar radiation spectrum, which we demonstrated as mentioned above, and (ii) high light-to-heat conversion efficiency. For plasmonic nanoparticles, the resonances can be damped radiatively by re-emission of a photon or nonradiatively through the creation of hot electron-hole pairs via Landau damping (11). The internal decay of hot electrons through the emission of phonons by interacting with the crystal lattice inside a metallic nanoparticle can lead to significant heating of the nanoparticle itself and its surroundings. However, for semiconductor nanopaticles, the mechanism for light-to-heat conversion was rarely discussed (37). In general, semiconductor nanoparticles with a large surface-to-volume ratio possess abundant dangling bonds and defects at their surface. This introduces surface states, resulting in the emergence of an electric level in the bandgap, which is similar to the intermediate band very recently discussed by Gaspari et al. (38). When the nanoparticle is excited, the excitation energy is transferred entirely to the carriers (electrons and holes), leading to the creation of nonequilibrium carrier densities with specific momentum states and elevated carrier temperatures initially. As the system evolves toward equilibrium, these carriers relax by intraband carrier-carrier scattering or interband recombining. The former results in Coulomb thermalization, forming a hot gas of thermalized carriers that couple with phonons and transfer their excess energy to the lattice (39). This results in the efficient heating of the nanoparticles. The dynamics of photoexcited electrons can be characterized by a pump-and-probe technique. Figure S9 shows an intuitive diagram illustrating the flow of energy in a photoexcited semiconductor.

NASA’s ambitious mission to go closer to the Sun than ever before is set to launch in the small hours between Friday and Saturday — at 3:53 AM Eastern from Kennedy Space Center in Florida, to be precise. The Parker Solar Probe, after a handful of gravity assists and preliminary orbits, will enter a stable orbit around the enormous nuclear fireball that gives us all life and sample its radiation from less than 4 million miles away. Believe me, you don’t want to get much closer than that.

If you’re up late tonight (technically tomorrow morning), you can watch the launch live on NASA’s stream.

This is the first mission named after a living researcher, in this case Eugene Parker, who in the ’50s made a number of proposals and theories about the way that stars give off energy. He’s the guy who gave us solar wind, and his research was hugely influential in the study of the sun and other stars — but it’s only now that some of his hypotheses can be tested directly. (Parker himself visited the craft during its construction, and will be at the launch. No doubt he is immensely proud and excited about this whole situation.)

“Directly” means going as close to the sun as technology allows — which leads us to the PSP’s first major innovation: its heat shield, or thermal protection system.

There’s one good thing to be said for the heat near the sun: it’s a dry heat. Because there’s no water vapor or gases in space to heat up, find some shade and you’ll be quite comfortable. So the probe is essentially carrying the most heavy-duty parasol ever created.

It’s a sort of carbon sandwich, with superheated carbon composite on the outside and a carbon foam core. All together it’s less than a foot thick, but it reduces the temperature the probe’s instruments are subjected to from 2,500 degrees Fahrenheit to 85 — actually cooler than it is in much of the U.S. right now.

The car-sized Parker will orbit the sun and constantly rotate itself so the heat shield is facing inward and blocking the brunt of the solar radiation. The instruments mostly sit behind it in a big insulated bundle.

And such instruments! There are three major experiments or instrument sets on the probe.

WISPR (Wide-Field Imager for Parker Solar Probe) is a pair of wide-field telescopes that will watch and image the structure of the corona and solar wind. This is the kind of observation we’ve made before — but never from up close. We generally are seeing these phenomena from the neighborhood of the Earth, nearly 100 million miles away. You can imagine that cutting out 90 million miles of cosmic dust, interfering radiation and other nuisances will produce an amazingly clear picture.

SWEAP (Solar Wind Electrons Alphas and Protons investigation) looks out to the side of the craft to watch the flows of electrons as they are affected by solar wind and other factors. And on the front is the Solar Probe Cup (I suspect this is a reference to the Ray Bradbury story, “Golden Apples of the Sun”), which is exposed to the full strength of the sun’s radiation; a tiny opening allows charged particles in, and by tracking how they pass through a series of charged windows, they can sort them by type and energy.

FIELDS is another that gets the full heat of the sun. Its antennas are the ones sticking out from the sides — they need to in order to directly sample the electric field surrounding the craft. A set of “fluxgate magnetometers,” clearly a made-up name, measure the magnetic field at an incredibly high rate: two million samples per second.

They’re all powered by solar panels, which seems obvious, but actually it’s a difficult proposition to keep the panels from overloading that close to the sun. They hide behind the shield and just peek out at an oblique angle, so only a fraction of the radiation hits them.

Even then, they’ll get so hot that the team needed to implement the first-ever active water cooling system on a spacecraft. Water is pumped through the cells and back behind the shield, where it is cooled by, well, space.

The probe’s mission profile is a complicated one. After escaping the clutches of the Earth, it will swing by Venus, not to get a gravity boost, but “almost like doing a little handbrake turn,” as one official described it. It slows it down and sends it closer to the sun — and it’ll do that seven more times, each time bringing it closer and closer to the sun’s surface, ultimately arriving in a stable orbit 3.83 million miles above the surface — that’s 95 percent of the way from the Earth to the sun.

On the way it will hit a top speed of 430,000 miles per hour, which will make it the fastest spacecraft ever launched.

Parker will make 24 total passes through the corona, and during these times communication with Earth may be interrupted or impractical. If a solar cell is overheating, do you want to wait 20 minutes for a decision from NASA on whether to pull it back? No. This close to the sun even a slight miscalculation results in the reduction of the probe to a cinder, so the team has imbued it with more than the usual autonomy.

It’s covered in sensors in addition to its instruments, and an onboard AI will be empowered to make decisions to rectify anomalies. That sounds worryingly like a HAL 9000 situation, but there are no humans on board to kill, so it’s probably okay.

The mission is scheduled to last seven years, after which time the fuel used to correct the craft’s orbit and orientation is expected to run out. At that point it will continue as long as it can before drift causes it to break apart and, one rather hopes, become part of the sun’s corona itself.

The Parker Solar Probe is scheduled for launch early Saturday morning, and we’ll update this post when it takes off successfully or, as is possible, is delayed until a later date in the launch window.

A Star Is Born

Empire of the Sun

Sunset Boulevard

Wisconsin’s two largest public utilities are making bigger stakes in renewable energy and have pledged far deeper cuts in emissions of greenhouse gases than previous predictions.  

Madison-based Alliant Energy says it intends to stop burning carbon-intensive coal altogether in its electric power plants by 2050. 

Alliant and Milwaukee-based WEC Energy Group recently said they are setting new goals to reduce carbon emissions by 80{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} from 2005 levels by 2050. 

That’s a shift from 2016 pronouncements when the utilities envisioned carbon dioxide reductions of 40{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} by 2030. (WEC Energy Group, which operates We Energies, says it now expects to reach the 40{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} goal by about 2023.) 

The moves to renewables are driven by tumbling prices for wind and solar power at the same time power companies in Wisconsin and nationally are using more natural gas as an alternative to coal. 

WEC Energy Chairman Gale Klappa told analysts on July 29 that utility-scale solar has increased in efficiency and prices have dropped by nearly 70{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} in recent years. He called it “an option that also fits well with our summer peak demand curve and with our plan to significantly reduce carbon dioxide emissions.”

Natural gas, which is in abundant supply because of new drilling technologies, produces about half the carbon emissions as coal.

Critics worry about the effect on residential rates, or they think the companies should be moving faster to embrace green energy as the majority of climate scientists see a clear link between a buildup of carbon dioxide in the atmosphere and a changing climate.

Power plants are the largest source of carbon dioxide emissions in the country, according to the U.S. Environmental Protection Agency, and make up about one-third of greenhouse gas emissions.

“We’re glad to see that the utilities are recognizing that this is what people want,” said Elizabeth Katt-Reinders of the Wisconsin chapter of the Sierra Club. “But actually it’s very underwhelming. We need to move off coal completely — and sooner.”

Still, the actions of the companies are an about-face from a decade ago when Alliant was seeking regulatory approval to build a new coal plant in Wisconsin. The state Public Service Commission in 2008 rejected the plan with one commissioner then calling it the “wrong project at the wrong time.”

In 2010 and 2011, after a big battle with environmentalists, WEC Energy opened a pair of coal-fired units in Oak Creek on Lake Michigan, which are among the last of their kind constructed in the United States.

The latest moves by the companies are coming as pressure from regulators is easing. 

The administration of President Donald Trump is working to dismantle Obama-era regulations that would limit carbon emissions from power plants. The president also withdrew the U.S. from the Paris climate agreement. 

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Also, unlike neighboring states, Wisconsin under Republican Gov. Scott Walker has not raised mandates for renewable power above a requirement of 10{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} of retail sales.

“This is not being driven by a federal policy because there really isn’t any right now,” said Doug Scott, a vice president of the Great Plains Institute, a Minneapolis-based nonprofit energy research group that has worked with utilities, including WEC Energy, on strategies to cut carbon emissions. 

“It’s being driven by their own decisions and the thought that decarbonization is going to happen for a variety or reasons.”

Besides falling prices for wind, solar and natural gas, Scott said utilities see other factors: A push by some states and cities for carbon-free sources of power; technological improvements such as battery systems that will increase the efficiency of solar and wind; pressure from investors with an interest in socially responsible investing; and large customers seeking greener sources of energy.

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Klappa said in his meeting with analysts that WEC Energy is discussing plans with Foxconn Technology Group about a major solar installation at the Taiwan company’s sprawling campus in Racine County — either with Foxconn building it itself, or his company making an investment.

“Everything’s on the table and we’re looking to how this works best for both parties,” he said. 

Alliant and WEC Energy detailed their plans to cut carbon emissions at the start of the month in their annual reports on corporate responsibility practices. 

Both said they planned to exceed the carbon reduction goals under President Barack Obama’s Clean Power Plan of cutting emissions by 32{0b7da518931e2dc7f5435818fa9adcc81ac764ac1dff918ce2cdfc05099e9974} from 2005 levels by 2030.

Alliant is targeting the use of more wind power and says it plans to spend $2 billion in the renewable sector between 2016 and 2020. The company is doubling its wind fleet by adding six wind farms — all in wind-friendly Iowa where the company sells electricity.

Meanwhile, it is building a 730-megawatt natural gas plant in Beloit opening next year.

The company has not said when it plans to close its coal-power plants. But spokesman Scott Reigstad said they will all be 65 years or older by 2050. 

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WEC Energy said it is retiring more than 1,800 megawatts of coal generation between now and 2020, including the recently shuttered Pleasant Prairie power plant in Kenosha County; adding more than 400 megawatts of natural gas generation in the next four years; and investing in more than 350 megawatts of “zero-carbon generation” in the state, including two solar projects. 

WEC Energy has no near-term plan to shut down operations in Oak Creek — especially the newer units, because they run efficiently and churn out electricity relatively inexpensively, said Daniel Krueger, senior vice president.

“I think that it is fair to say, going forward, it’s going to be solar, wind, batteries and some natural gas,” Krueger said. 

One worry is the impact on consumers in a state where electricity rates rank higher than most Midwest states. Power companies are entitled to recover the cost of investments, even if plants are mothballed and replaced with cleaner sources of power. 

“The utilities are certainly seeing green in green energy,” said Thomas Content, executive director of the Madison-based Citizens Utility Board, a watchdog group.
  
“The concern is how does this go from plans to reality in a way that is economical and cost-effective for homeowners and renters?” he said. 

“It can be done right, but if there is a bumpy transition, and if you are taking plants out of service that are still efficient and still need many years to pay off, that’s a concern.”

 

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