Source of article: Laser Industry Observation compiled from the Internet A femtosecond laser is an "ultra-short pulse light" generating device that emits light for an ultra-short time of only about a trillionth of a second. Fei is the abbreviation of the prefix femto in the International System of Units, and 1 femtosecond = 1×10^-15 seconds. The so-called pulse light emits light only for a moment. The light emission time of a camera's flash is about 1 microsecond, so the femtosecond ultra-short pulse light only has about one billionth of its time to emit light. As we all know, the speed of light flies at an unparalleled speed of 300,000 kilometers per second (circling the earth seven and a half times in one second). However, in one femtosecond, the light only advances 0.3 microns.
Usually, we use flash photography to capture the instantaneous state of moving objects. Similarly, if you use a femtosecond laser to flash, it is possible to see every fragment of a chemical reaction that occurs at a violent speed. To do this, femtosecond lasers can be used to study the mysteries of chemical reactions.
General chemical reactions proceed after passing through an intermediate state with high energy, the so-called "activated state". The existence of the activated state was theoretically predicted by the chemist Arrhenius as early as 1889, but because it existed for a very short moment, it could not be directly observed. But its existence was directly demonstrated in the late 1980s by femtosecond lasers, an example of using femtosecond lasers to pinpoint chemical reactions. For example, the cyclopentanone molecule decomposes into carbon monoxide and 2 ethylene molecules in the activated state.
Nowadays, femtosecond lasers are also used in a wide range of fields such as physics, chemistry, life sciences, medicine, and engineering. In particular, the combination of light and electronics is expected to open up various new possibilities in the fields of communications, computers, and energy. This is because the intensity of light can transmit large amounts of information from one place to another with almost no loss, making optical communications even faster. In the field of nuclear physics, femtosecond lasers have made a huge impact. Because pulsed light has a very strong electric field, it is possible to accelerate electrons to close to the speed of light within 1 femtosecond, so it can be used as an "accelerator" to accelerate electrons.
Application in medicine As mentioned above, in the world within femtoseconds, even light is frozen and cannot move very far, but even on this time scale, atoms and molecules in matter and electrons inside computer chips are still moving within the circuit. If you use a femtosecond pulse you can stop it instantly and study what happens. In addition to flashing to stop time, femtosecond lasers can also drill microholes in metal with a diameter as small as 200 nanometers (two ten thousandths of a millimeter). This means that the ultra-short pulse light that is compressed and locked inside in a short period of time achieves an amazing effect of ultra-high output without causing additional damage to the surroundings. Furthermore, the pulsed light of femtosecond lasers can capture three-dimensional images of objects in extremely fine detail. Stereoscopic image photography is very useful in medical diagnosis, thus opening up a new research field called optical interference tomography. This is a three-dimensional image of living tissue and living cells captured using a femtosecond laser. For example, a very short pulse of light is directed at the skin. The pulse light is reflected on the surface of the skin, and part of the pulse light is emitted into the skin. The inside of the skin is composed of many layers. The pulse light that enters the skin is bounced back as a small pulse light. From the echoes of these various pulse lights in the reflected light, the internal structure of the skin can be known.
In addition, this technology has great practicality in ophthalmic medicine, capable of capturing three-dimensional images of the retina deep in the eye. This allows doctors to diagnose problems with their tissues. This kind of examination is not limited to the eyes. If a laser is sent into the body using optical fiber, it can examine all the tissues of various organs in the body. In the future, it may even be possible to detect whether it has turned into cancer.
Realizing ultra-precise clocks Scientists believe that if visible light is used to make a femtosecond laser clock, it will be able to measure time more precisely than an atomic clock, and will serve as the world's most accurate clock in the next few years. If the clock is accurate, it also greatly improves the accuracy of the GPS (Global Positioning System) used for car navigation.
Why can visible light make an accurate clock? All clocks and watches are indispensable for the movement of pendulums and gears. Through the swing of a pendulum with a precise vibration frequency, the gears rotate for seconds, and accurate clocks are no exception. Therefore, to make a more accurate clock, it is necessary to use a pendulum with a higher vibration frequency. Quartz clocks (clocks that use crystal oscillation instead of a pendulum) are more accurate than pendulum clocks because the quartz resonator oscillates more times per second.
The cesium atomic clock currently used as the time standard has an oscillation frequency of about 9.2 gigahertz (the prefix of the international unit of gigahertz, 1 gigahertz = 10^9). The atomic clock uses the natural oscillation frequency of cesium atoms and replaces the pendulum with microwaves whose oscillation frequency is consistent. Its accuracy is only one second in tens of millions of years. In contrast, visible light has an oscillation frequency that is 100,000 to 1,000,000 times higher than the microwave oscillation frequency. That is, visible light energy can be used to create precision clocks that are millions of times more accurate than atomic clocks. The world's most accurate clock that uses visible light has now been successfully built in a laboratory.
Einstein's theory of relativity can be verified with the help of this precise clock. We placed one such accurate clock in the laboratory and the other in the office downstairs, and considered possible situations. After one or two hours, the result was as predicted by Einstein's theory of relativity. Due to the two There are different "gravitational fields" between the floors, so the two clocks no longer point to the same time, and the clock downstairs runs slower than the clock upstairs. If a more accurate clock were used, perhaps even the watches worn on the wrist and ankle would tell different times that day. We can simply experience the charm of relativity with the help of accurate clocks.
Light speed slowing down technology In 1999, Professor Rainer Howe of Hubbard University in the United States successfully slowed down light to 17 meters per second, a speed that cars can catch up with, and then successfully slowed down light to a speed that even bicycles can catch up with. This experiment involves the most cutting-edge research in physics. This article only introduces two keys to the success of the experiment. One is to build a "cloud" of extremely low-temperature sodium atoms close to absolute zero (-273.15°C), a special gas state called a Bose-Einstein condensate. The other is a laser that adjusts the vibration frequency (control laser), and uses it to illuminate a cloud of sodium atoms, and something incredible happens.
Scientists first use a control laser to compress the pulse light in the cloud of atoms and slow it down extremely. Then they turn off the control laser and the pulse light disappears. The information carried on the pulse light is stored in the cloud of atoms. . Then it is irradiated with a controlled laser, and the pulse light is restored and walks out of the cloud of atoms. As a result, the originally compressed pulse is broadened again and the speed is restored. The entire process of inputting pulsed light information into the atomic cloud is very similar to reading, storing, and resetting in a computer. Therefore, this technology can help realize the realization of quantum computers.
From the world of "femtosecond" to "attosecond" Femtoseconds are beyond our imagination. Now we are venturing into the world of attoseconds, which are shorter than femtoseconds. Ah is the abbreviation of the prefix "atto" of the International System of Units. 1 attosecond = 1×10^-18 seconds = one thousandth of a femtosecond. Attosecond pulses cannot be made with visible light because shortening the pulses requires the use of shorter wavelength light. For example, if you want to create a pulse using red visible light, it is impossible to create a pulse shorter than that wavelength. Visible light has a limit of about 2 femtoseconds, so attosecond pulses use x-rays or gamma rays with shorter wavelengths. It's unclear what will be discovered in the future using attosecond X-ray pulses. For example, using attosecond flashes to visualize biomolecules allows us to observe their activities on a very short time scale and perhaps identify the structure of biomolecules.
