Why Light Going Backward Can Help us go Forward

Understanding Carbon Dioxide and Its Impact on Our Planet

Carbon dioxide (CO2) is a gas found in Earth’s atmosphere. While CO2 is naturally present in the air, human activities, like burning fossil fuels including coal, oil, and natural gas, have increased the amount of CO2. Scientists are very interested in tracking and predicting how much CO2 is in the atmosphere because it affects Earth’s temperature and weather patterns [1–3]. To do this, they use special satellites orbiting the Earth [3–6]. These satellites have advanced sensors that can detect and measure CO2 from space with amazing accuracy. This technology helps scientists see where CO2 comes from, how it is absorbed by plants and oceans, and how its levels change over time. By studying satellite data, they can better understand the carbon cycle: how carbon moves between the air, land, and oceans [3, 7].

This knowledge is important because it helps us predict future climate changes and find ways to reduce greenhouse gases. For example, satellite data can guide policies to cut emissions from factories and cars or can encourage planting more trees that naturally absorb CO2. Using satellite technology is a big step forward in protecting our environment. As climate challenges grow, satellites play an even greater role in providing the data needed for global cooperation and sustainable living.

But how can we see invisible gases from space? How do we measure something we cannot touch or even see? To find out, scientists turn to one of nature’s most powerful messengers: light. With the right tools, light can reveal secrets hidden in the air around us.

Detecting CO2

Gases are invisible, so it is hard to tell when they are in the air around us. But here is something cool: every gas has a favorite color! What is your favorite color? Just like you, gases have colors they “like” and colors they do not (Figure 1). When we shine light of a color that a gas does not like, the light passes straight through the gas. But when we shine a color it does like, the gas “keeps” that light—in other words, the light is absorbed.

Figure 1 - Every gas has a “favorite color” that it likes to absorb (hold on to). That means if we send lots of different colored lights into a container with unknown gases, we can figure out which gases are present by looking at which colors pass through the mixture and which are absorbed. Here, gases A, B and C took away red, yellow, and blue, respectively. So, we know that these three gases are in the box. CO2, however, likes infrared, an invisible color. If we do not send infrared into the box, we will not know if CO2 is there.

Scientists use this trick to find out if a certain gas is present in the air. They send out two beams of light: one beam has a color that the gas likes. This is called on-line with the gas. The other beam has a color the gas does not like. This is called off-line with the gas. After the light travels through the air, scientists measure how much of each color comes back. If the gas absorbed its favorite color, they know the gas is there, and even how much of it is present.

To make this work in real life, scientists use special machines and clever techniques, as shown in Figure 2. One of the most important tools is the laser. Lasers create bright light in many different colors, and they are everywhere in our daily lives: in cars, computers, phones, and even in laser pointers you might use to play with your cat! Laser light can travel very far, sometimes thousands of kilometers, without spreading out too much. In comparison, the light from a normal room lamp becomes too weak to see after only a few dozen meters. This makes lasers perfect for space experiments.

Figure 2 - Satellites are sent into space to measure how much CO2 is emitted on certain parts of the Earth, such as around industrial areas. A laser sends light of different colors toward Earth. One beam uses a color that CO2 absorbs strongly (on-line), and the other uses a color CO2 does not absorb much (off-line). The light reflects back to the satellite after bouncing off particles in Earth’s atmosphere. Scientists then check how much on-line and off-line light was collected, to calculate how much CO2 is in that area.

Powerful lasers are placed on satellites and sent toward areas where scientists think CO2 might be in the air. Some of the light is absorbed by the gas, and some of it continues on. The remaining light then bounces off tiny particles in the air, like little floating mirrors, and travels back to the satellite. There, special sensors catch the returning light and measure it, helping scientists learn more about our planet’s atmosphere.

Sounds simple, right? Well, there is a catch. CO2’s favorite color is infrared, a color that our eyes cannot see. Lasers that make infrared light are heavy, expensive, and hard to use, especially in space. So, scientists often want to use smaller, cheaper lasers that shine the “wrong” color. But how can we change that color into the one CO2 likes? That is where a special crystal comes in…

The Crystal That Plays Tricks With Light

Imagine you have a fascinating portal. Every time you throw a blue ball at it, something strange happens: the ball comes out of the portal in a different color. One turns red, the next turns green, and another even turns invisible. Sometimes two balls bounce back instead of one! Scientists do something very similar—not with balls, but with light. And instead of a portal, they use a special crystal made of materials that do not behave like ordinary glass. Instead of just reflecting light or letting it pass through, the crystal can change light’s color entirely! When a blue laser shines into the crystal, light can come out as red, green, purple, or other colors. By slightly turning the crystal or gently heating it, scientists can choose the exact color they want. Small changes, like from red to orange, are easy; big jumps need a bit more “tinkering”.

Here is how scientists do this. First, they pick a material with a strong ability to change light, which is called high non-linearity. Then they decide which color goes in and which one they want to come out. Finally, they use a “trick” called periodic poling. Imagine cutting the crystal into many tiny slices, flipping every second one around, and gluing them all back together, thousands of times. Depending on how close these flips are, the crystal can change light into different colors.

Our crystal is especially cool. When you shine a laser into it, two colors come out: one from the front, and one from the back (Figure 3)! It is like throwing the blue ball at your portal and having a green ball bounce back while a red one slips through. To make this happen, the pattern inside the crystal, called the poling distance, must be smaller than one micrometer, which is about 100 times thinner than a human hair. That is incredibly hard to make.

Figure 3 - The non-linear crystal can convert light from a laser (blue) into other colors. One color goes forward (red) and one goes backward (green). The output colors can be chosen by changing the poling period, which means flipping the crystal repeatedly and gluing it together. The backward light is perfect for satellite missions because it has clean light (with one dominant color) and keeps its color over time. The forward light can be used for other purposes.

Helping Our Planet

Now, let us go back to our satellites. We know that Earth is warming, and one major reason is CO2 in the atmosphere. To understand and reduce this problem, scientists need to measure exactly how much CO2 is in the air and where it comes from. Our crystal can help [8]. When placed on satellites, it converts laser light into the infrared color that CO2 “likes”. This lets scientists “see” how much CO2 is in the atmosphere, almost like a thermometer for the Earth.

But life in space is not easy. Satellites face shaking rocket launches, freezing nights, blazing sunlight, and strong radiation. So, any tool we send up there must be extremely tough: like the perfect travel bottle that never cracks, leaks, or loses its shine. That is why our crystal is so special. It is not just smart; it is built like the world’s best water bottle for light! Here is why. First of all, our crystal is super sturdy. When a rocket launches, everything inside shakes like crazy. Many instruments can break, but our crystal is strong and compact. It is like choosing a sturdy water bottle instead of a fragile glass: perfect for a bumpy trip into space.

Second, it can handle radiation. Out in space, there is no air to protect things from powerful radiation: fast protons and high-energy light that can damage normal materials. But our crystal keeps working even after being hit by radiation [9, 10]. Think of it like a water bottle so tough you can leave it outside for a week and it is still as spotless as before.

Third, our crystal shines pure, clean light. Some lasers make light that is a bit messy, like water splashing everywhere when you pour it. Our crystal creates a steady, smooth beam as if, every time you open the bottle, a perfectly even stream of clear water flows out and never misses your mouth.

Finally, it always keeps the same color. Even if the input laser changes slightly, the crystal’s output color stays steady. It is like filling the same bottle from different fountains, but the water always tastes exactly the same.

Because of all these strengths, satellites equipped with our crystals can keep measuring CO2 with precision year after year, helping scientists track how our planet is changing. It is amazing to think that something so small and shiny could play such a big role in keeping Earth healthy and safe.

Looking Ahead With Backward Light

From orbiting satellites to fascinating crystals, scientists are finding creative ways to measure invisible gases and protect our planet. By understanding how CO2 moves through our atmosphere, we can make smarter choices for a cleaner, cooler future. So next time you see a clear blue sky, remember: far above you, satellites with tiny crystals are shining invisible backwards beams of light, helping scientists keep our Earth safe for generations to come.

Glossary

Satellite: ↑ A machine that orbits Earth and takes pictures or measurements from space, like Earth’s “weather spy” or “air checker”.

Sensor: ↑ A tool that notices or measures something, like light, temperature, or gas.

Laser: ↑ A special light beam that is very strong and travels in a straight line. Used in scanners, pointers, and science experiments.

Infrared: ↑ A type of light we cannot see, but we can feel it as heat. Remote controls and satellites often use it.

Crystal: ↑ A special kind of solid material where atoms are arranged very neatly. Crystals can bend or change light in cool ways.

High Non-Linearity: ↑ A property of some materials that lets them strongly change light, such as turning one color into another.

Periodic Poling: ↑ A technique where a crystal is structured in repeating patterns to control how it changes light.

Poling Distance: ↑ The spacing between repeating patterns inside a crystal, which determines what new light colors are produced.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank Ginevra (age 16) for the valuable feedback she provided during the peer review of this manuscript. This work was funded by the European Union’s Horizon Europe Research and Innovation Programme under the Marie Skłodowska-Curie Grant Agreement No. 101072409 HOMTech.

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↑Vågberg, A., Brunzell, M., Widarsson, M., Mutter, P., Zukauskas, A., Laurell, F., et al. 2024. 2.7μm backward wave optical parametric oscillator source for CO2 spectroscopy. Opt. Lett. 49:4553–6. doi: 10.1364/OL.531038.

[1] ↑ Luo, B. 2004. Uncertainty analysis for distribution of greenhouse gases concentration in atmosphere. J. Environ. Inform. 3:89–94. doi: 10.3808/jei.200400030

[2] ↑ Meinshausen, M., Vogel, E., Nauels, A., Lorbacher, K., Meinshausen, N., and Etheridge, D. M., et al 2017. Historical greenhouse gas concentrations for climate modelling (CMIP6). Geosci. Model Dev. 10:2057–116. doi: 10.5194/gmd-10-2057-2017

[3] ↑ Lee, J., Jeong, S., Kim, Y. J., Roh, S., Kim, J., and Jin, H. 2025. Synergy of multiple-satellite measurements to fill the gap of global XCO2. J. Geophys. Res. Atmospheres 130. doi: 10.1029/2024JD042809

[4] ↑ Hammerling, D. M., Michalak, A. M., O’Dell, C., and Kawa, S. R. 2012. Global CO2 distributions over land from the Greenhouse Gases Observing Satellite (GOSAT). Geophys. Res. Lett. 39. doi: 10.1029/2012GL051203

[5] ↑ Abshire, J. B., Ramanathan, A. K., Riris, H., Allan, G. R., Sun, X., and Hasselbrack, W. E., et al 2018. Airborne measurements of CO2 column concentrations made with a pulsed IPDA lidar using a multiple-wavelength-locked laser and HgCdTe APD detector. Atmos. Meas. Tech. 11:2001–25. doi: 10.5194/amt-11-2001-2018

[6] ↑ Hu, K., Liu, Z., Shao, P., Ma, K., Xu, Y., and Wang, S., et al 2024. A review of satellite-based CO2 data reconstruction studies: methodologies, challenges, and advances. Remote Sens. 16:3818. doi: 10.3390/rs16203818

[7] ↑ He, J. and Huang, B. 2025. Estimating global anthropogenic CO2 emissions through satellite observations. Environ. Res. 279:121767. doi: 10.1016/j.envres.2025.121767

[8] ↑ Vågberg, A., Brunzell, M., Widarsson, M., Mutter, P., Zukauskas, A., and Laurell, F., et al 2024. 2.7 μm backward wave optical parametric oscillator source for CO2 spectroscopy. Opt. Lett. 49:4553–6. doi: 10.1364/OL.531038

[9] ↑ Lee, C., Laurell, F., Pasiskevicius, V., Mølster, K. M., Duzellier, S., and Zukauskas, A., et al 2023. Proton irradiation hardness of periodically poled Rb:KTP for spaceborne parametric frequency converters. Opt. Mater. Express 13:436–46. doi: 10.1364/OME.475442

[10] ↑ Coetzee, R. S., Duzellier, S., Dherbecourt, J. B., Zukauskas, A., Raybaut, M., and Pasiskevicius, V., et al 2017. Gamma irradiation-induced absorption in single-domain and periodically-poled KTiOPO4 and Rb:KTiOPO4. Opt. Mater. Express 7:4138–46. doi: 10.1364/OME.7.004138