Editor's NoteFunded by the Shanghai Municipal Science and Technology Commission (Project No. 22DZ2304300), The Paper has collaborated with World Science to produce popular science reports on the achievements awarded national and Shanghai municipal science and technology prizes.

This article focuses on the first prize project of the 2022 Shanghai Natural Science Award, titled "Research on Air Laser Driven by Ultrafast Strong Light Fields." This award was completed through collaboration led by Professor Liu Yi from the School of Optoelectronic Information and Computer Engineering at the Shanghai University of Science and Technology, along with the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and other institutions.

Finding a low-cost, highly sensitive method to monitor the levels of atmospheric pollutants has always been a key focus in the research of optical remote sensing technology.

An ideal approach for atmospheric remote sensing applications is to use air as the gain medium, leveraging the remote excitation effects of short-pulse strong lasers to generate high-brightness lasers at any location in the atmosphere, which can then be used to detect trace pollutant molecules.

Previously, the project "Research on Air Laser Driven by Ultrafast Strong Light Fields," led by Professor Liu Yi at the Shanghai University of Science and Technology in collaboration with the Shanghai Institute of Optics and Fine Mechanics and other units, won the first prize of the 2022 Shanghai Natural Science Award.

The researchers discovered that exposing nitrogen gas in the air to strong laser light could produce nitrogen ion air lasers, and that the vibration direction (polarization) of the generated laser is consistent with that of the pump laser.

By precisely controlling the parameters of the pump laser pulses, the research team successfully achieved stable production of air lasers and could adjust the wavelength of the produced air lasers, making customized laser output possible. Additionally, this technology shows tremendous potential for the highly sensitive detection of atmospheric pollutants. It can precisely detect trace pollutants and isotopic molecules through stimulated Raman scattering.

This research links the fields of strong field laser physics and quantum optics, deepening the understanding of the behavior of molecules and ions under strong laser fields, which is crucial for applications in spectroscopy, laser technology, and atmospheric remote sensing.

Fortuitous Discovery Sparks a Wave of Air Laser Research

The concept of air lasers was proposed in 2003. Researchers discovered that using ultra-strong lasers to excite air molecules could potentially produce stimulated emission from the backward direction, fundamentally a laser phenomenon resulting from strong field excitation or ionization.

“In 2011, Professor Cheng Ya’s group at the Shanghai Institute of Optics first saw credible evidence of this in experiments, but the specific mechanisms and theories were not well understood at that time. Over the past decade, many have tried to theoretically explain this phenomenon,” Professor Liu Yi recounted.

In 2011, the Shanghai Institute of Optics and Fine Mechanics collaborated with Jilin University and others, unexpectedly discovering narrow-bandwidth, high-brightness, multi-wavelength coherent radiation produced by strong field-driven ionization of air molecules using tunable mid-infrared femtosecond lasers. This was subsequently widely referred to as air lasers.

After the phenomenon of air lasers induced by strong field ionization was reported, many premier international research teams from the United States, France, Canada, Japan, and Austria began their own investigations, sparking a surge in air laser research. Today, air lasers have evolved into a cutting-edge direction in the field of ultrahigh-speed photonics.

95% of the Time Spent on Experiment Preparation

“The preparation work for many experiments is quite complex, and it takes a significant amount of time and effort to find suitable experimental conditions. I often joke with my students that it takes years to complete a doctoral thesis, and if the experimental conditions are ready, we could obtain the experimental results in about a week. So why does it take years? Because 95% of the time is spent preparing and only 5% is spent on conducting the experiments,” Professor Liu Yi chuckled.

From 2014 to 2018, Professor Liu’s team collaborated with Professor Anne L'Huillier of Lund University, who later won the Nobel Prize in Physics in 2023.

“We published a collaborative paper in a top physics journal, and the experimental results were obtained during an all-night effort on a Thursday night.”

“Anne L'Huillier said that since the preparation was complete, no matter how late the night was, we had to finish the experiment.” Professor Liu recalled.

On that night, a total of six people participated in the experiment: two Chinese, one French, one German, one British, and two Swedish researchers.

The experiment finally concluded at 4 AM.

“The leader of the French group flew in from Paris that afternoon, saying to inform him regardless of how late the experiment ended. That night, I returned to the hotel, and instead of knocking on his door, I texted him to say the experiment was finished. To my surprise, he immediately called me, insisting on discussing the results right away. The next morning, he went back to Paris without resting,” Professor Liu said. From these scientists, he witnessed an invaluable passion for research.

Precursor Steps to Generating Air Lasers

Air lasers are a laser phenomenon generated through strong field ionization, with the first step involving focusing the strong laser into the air.

Professor Liu's team utilized advanced femtosecond laser technology to produce ultra-short laser pulses that range in duration on the order of 10^-15 seconds. These strong field pulses not only have an extremely short duration but also very high peak power, laying a foundation for subsequent experiments.

How intense are the pulses of ultrafast lasers?

Professor Liu explained, “We learned in high school about the composition of atoms and why electrons orbit around the nucleus. This is due to the Coulomb field of the nucleus preventing electrons from escaping. Strong light fields can approach or even exceed multiple orders of magnitude stronger than Coulomb fields, generally above 10^14 watts per square centimeter. Under such strong illumination, electrons become ionized into free electrons.”

“Previous studies have assumed that after ionization, the probability of ions being in the ground state is higher, while the probability of being in an excited state is lower. However, through experiments, we found that the probability of ions being in excited states may actually be greater, and the probability of being in the ground state is lower. This new effect of strong field ionization was previously unknown.”

Under the influence of strong light fields, molecules and atoms in the air undergo complex nonlinear effects such as tunneling ionization, multi-level coupling, vibrational excitation, and rotational excitation. These effects are crucial for generating air lasers.

Under specific conditions, the plasma induced by strong light fields can amplify light of specific wavelengths to form air lasers. This laser is achieved through optical amplification, ultimately resulting in stable laser output. By finely tuning the energy, phase, and spatial distribution of the laser pulses, the team successfully established stable air laser generation.

Controlling the Polarization of Nitrogen Ion Air Lasers

Polarization describes the direction of the electric field's vibration in lasers, which holds significant importance in controlling and applying lasers. In fields such as optical communication, materials processing, and biomedical imaging, the polarization state of a laser can significantly impact its effectiveness and accuracy.

In traditional laser research, the polarization state is usually determined by the design of the laser. However, the polarization characteristics of air lasers are largely influenced by the dynamic processes of laser interaction with gas molecules, rendering polarization control more complex.

Experimental results from Professor Liu's team indicate that the polarization state of nitrogen ion air lasers is related to the pump light's polarization, the polarization of the injected seed light, the intensity of the seed light, and the degree of optical amplification. Recent research has comprehensively understood the main reasons behind complex polarization effects, solving a long-standing mystery and providing a foundation for further controlling nitrogen ion air lasers across multiple dimensions.

By adjusting the parameters of the laser pulses, the wavelength and frequency of the generated air lasers can be controlled. This discovery opens up possibilities for customized laser output tailored to specific application needs. For instance, in communication, lasers of different wavelengths can be utilized for distinct communication channels, enhancing bandwidth and efficiency.