Influence of Initial Gas Pressure on the Spatial Intensity Distribution of Double-Pass Amplified 69.8 nm and 72.6 nm Lasers

Light that is invisible to the human eye, sitting just beyond the violet end of the spectrum, holds a unique place in modern science. This is the realm of extreme ultraviolet, or EUV, a form of radiation so energetic that it can reveal the tiniest details of the atomic world. To create this light, scientists do not use traditional bulbs or lasers; instead, they generate it by superheating gas until it becomes a plasma, a state of matter where electrons are stripped from atoms, creating a seething, high-temperature soup. In this environment, specific ions can be coaxed into releasing bursts of light at very precise colors. While this light is incredibly useful for advanced manufacturing and microscopy, capturing and amplifying it is notoriously difficult. The very mirrors needed to guide and strengthen these light beams are often damaged or destroyed by the intense heat and bombardment of the plasma that creates them. This fragility has long forced researchers to use a "one-and-done" approach, letting the light pass through the plasma only once before collecting it, which limits the total energy they can extract.

A team of researchers has now found a way to get more out of this delicate process by successfully bouncing the light back through the plasma for a second pass, effectively doubling the amplification without destroying their equipment. They focused on two specific shades of extreme ultraviolet light, one at 69.8 nanometers and another at 72.6 nanometers, generated from neon-like argon ions. The key to their success was a mirror made of silicon carbide, a material so hard that it can withstand the harsh plasma environment where other mirrors would fail. By placing this mirror at the end of a narrow tube filled with argon gas, they were able to reflect the light back through the plasma column, giving it a second chance to grow stronger. This simple yet robust setup allowed them to study how the shape and spread of the light beam change as they tweak the initial conditions of the gas, revealing a surprising sensitivity to the pressure of the gas inside the tube.

The experiment began with a long, thin ceramic tube, known as a capillary, which was filled with argon gas. The researchers used a powerful electrical discharge to first create a small amount of plasma, and then sent a massive surge of current through it. This current acted like a magnetic squeeze, compressing the plasma into a tight, hot column. As the plasma reached its peak density, the atoms inside were excited and began to emit the specific ultraviolet light the team was hunting for. To see what was happening, they used a specialized camera and a grating that could separate the different colors of light, allowing them to map out exactly how the light was distributed across the beam. They compared two scenarios: one where the light traveled through the plasma just once, and another where the silicon carbide mirror sent it back for a second run. The results were immediate and dramatic. When the light made the round trip, the 69.8 nanometer beam became more than seven times brighter, while the 72.6 nanometer beam grew more than three times brighter. This confirmed that the mirror was working perfectly, reflecting the light back without being damaged, and that the plasma was ready to amplify the returning beam.

However, the most interesting discovery came when the researchers started changing the pressure of the argon gas inside the tube. They found that the shape of the laser beam was not fixed; it shifted dramatically depending on how much gas was present. At a lower pressure of 14 pascals, the amplified light remained focused near the center of the beam, creating a bright spot with a relatively narrow spread. As they increased the pressure to 16 pascals, the beam began to widen, and the light started to spread out more evenly. But when they pushed the pressure up to 18 pascals, the beam underwent a complete transformation. The bright center faded away, and the light rearranged itself into a hollow ring, or an annulus, with the strongest intensity now located at the edges rather than the middle. This shift happened for both colors of light, indicating a fundamental change in how the light was traveling through the plasma.

The reason for this transformation lies in the invisible structure of the plasma itself. As the gas pressure increases, the way the plasma is squeezed by the magnetic field changes, creating a steeper difference in the density of electrons from the center of the beam to its edges. This density gradient acts like a lens, bending the light as it travels. When the pressure is low, the gradient is gentle, and the light passes through with little deflection, staying near the center. When the pressure is high, the gradient becomes steep, acting like a strong lens that bends the light rays outward. The light that is reflected back by the mirror travels through this dense, gradient-rich environment and is bent so sharply that it is pushed away from the center and forced to the edges of the beam. This explains why the beam turns into a ring at higher pressures; the plasma is essentially refracting the light away from the middle and concentrating it on the periphery.

The researchers observed that even small changes in the gas pressure, just a few pascals, caused these large shifts in the beam's shape. This sensitivity suggests that the 69.8 and 72.6 nanometer lasers are highly responsive to the exact conditions of the plasma at the moment of generation. The team noted that while they had successfully amplified the 69.8 nanometer light to the point of saturation, meaning the plasma was giving up as much energy as it could, the 72.6 nanometer light still had room to grow. The fact that they could achieve this double-pass amplification for the 72.6 nanometer wavelength for the first time is a significant step forward. By understanding how the gas pressure controls the shape of the beam through refraction, scientists now have a clearer path to optimizing these lasers. This knowledge is essential for future attempts to build more complex systems, such as using a full optical cavity to bounce the light back and forth multiple times, which could lead to even more powerful and useful sources of extreme ultraviolet light.