Lasers
How does a laser work?
A laser is more than just an ordinary light source – the light it produces has very special qualities that are needed in many different parts of our everyday lives nowadays. Let’s take a closer look at what makes these devices so exceptional, how we use them in our labs, and how that differs from lasers you might use at home.
The word laser is actually an abbreviation for “Light Amplification by Stimulated Emission of Radiation”. As the name implies, it is a device that generates a (coherent) beam of light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. Let’s take a closer look at what that means.
Lasers consist of several basic components:
- A Gain Medium: The material in which light amplification occurs. It can be a gas, liquid, solid, or semiconductor.
- An Energy Source (often called “Pump”): Provides the energy required to excite the electrons in the gain medium. This can be electrical, optical, or chemical energy.
- An Optical Resonator (or “Cavity”): The cavity typically consists of two mirrors placed on either side of the gain medium. One mirror is fully reflective, while the other is partially reflective, allowing some light to escape as the laser beam.
How do these components work together and create the light coming from a laser?
The pump source energizes the electrons in the gain medium, raising them from a lower energy level (ground state) to a higher energy level (excited state). Electrons in this excited state are unstable and will eventually drop back to the ground state, releasing a photon (a particle of light) in the process. This photon is emitted in a random direction and phase. Light with these properties is not typical for a laser yet – we still need the cavity to create light with certain characteristics if we want to use it in different types of technology.
If a photon of a specific wavelength interacts with an electron in the excited state, it can stimulate the electron to drop back to the ground state while emitting a second photon of the same wavelength, phase, and direction as the first. This process amplifies the light within the gain medium. This is where the name laser comes from – because more light particles have been generated by stimulating the electrons to do so through sending in a light particle with exactly the right wavelength.
The photons that were emitted by the electrons in the gain medium can now bounce back and forth between the two mirrors of the optical resonator. As they pass through the gain medium multiple times, they stimulate the emission of more photons, leading to a chain reaction of light amplification.
In the cavity, a coherent beam of light builds up due to the position of the mirrors and the photons being reflected from one mirror to the other. The partially reflective mirror allows a portion of this amplified light to escape, forming the output laser beam. This beam is coherent, meaning the light waves are in phase in both time and space. It is also monochromatic, meaning it has a single wavelength (or color).
These are very specific characteristics of light. You might not see it with your naked eyes, but if you would compare the light waves from a laser with those of a light bulb with optical instruments, there is a clear difference. Light waves from a bulb come out in many different directions and have a variety of different wavelengths or colors. The specifics of laser beams make it possible to use them in many different applications today.
For example, they can be used in medical instruments for surgery or in an industrial setting to weld, cut, or engrave things. You can imagine that this means that some lasers have a lot of energy or could be focused on a very small area. Due to their properties, they are also used to transmit data in fiber optic communication because they have such a clear wavelength. If you modulate the laser light just a little bit, like changing the frequency of the light wave, you can use these changes to encode digital information and send it over long distances with very high speed and efficiency. Our internet infrastructure relies heavily on this kind of data transmission. And of course lasers can be used in consumer electronics, like DVD players or laser printers. These are just some of the many examples where lasers enrich our everyday lives.
The lasers that we use in our labs
In our labs, we use lasers for scientific research, especially in spectroscopy.
Oftentimes, our scientists have to build the lasers that we use in our laboratories themselves and cannot just buy them. Why is that the case? What is the difference between a cheaper, smaller laser you buy to point out something at a presentation and one that we use in our lab?
A big part of the difference is the components our researchers use. As mentioned before, the gain medium can be a different one depending on the laser. There are lasers that use a certain type of gas, solid-state lasers that use a crystalline or glass medium doped with certain ions, semiconductor lasers, or even lasers that use a liquid medium. Depending on the gain medium you use for a laser, you can have different effects. The electrons in the gain medium all need a very specific wavelength of light (depending on the material you are looking at) to produce the cascade of photons that make a laser.
Setup in one of our labs showing a chirped pulse amplifier, which is used to get a laser pulse to the desired power level.
That means it can become rather difficult to even generate this effect in the first place, since not all wavelengths of light are similarly easy to produce. Electrons also take a very different time at higher energy levels, depending on the material, before they produce a photon and drop back down to ground level. As you can imagine, that also has a big effect on the possibility to even create a laser.
The cavities or optical resonators in a research setting are also much more complex than just using a simple set of two mirrors. In a laboratory, the smallest changes in our light beam can have huge influences on an experiment. Scientists use a variety of optical lenses, filters, different types of mirrors, and other instruments to create light with just the right characteristics so they can use it for their experiments. The scales they have to work with so their experiments even show results are extremely small. If a part of the experiment is moved by just one micrometer (=0,000 001 meters) or an optical instrument gets a tiny scratch on that scale, you might not see any outcome from the laser anymore. To give you a comparison - an average human hair is between 40 and 120 micrometers thick.
Because of all these and many more reasons, the lasers that we use in our laboratories are often big experimental setups on special optical workbenches and do not look like a laser you can buy in a normal supermarket at all. These optical workbenches are hydraulically mounted, so the experiments stay in place even if a scientist touches the desk. The optical equipment we use has to be handled very carefully because the tiniest scratch on a lens can change the outcome of an experiment. Equipment is often very expensive because you need very specific material, like mirrors with surfaces made from gold, to generate the effect you are looking for. And scientists have to take certain precautions when working in a lab, like wearing hairnets, so nothing can interfere with the things they actually want to measure.
In our Special Research Programs we are looking specifically at two materials that have not been excited by a laser like we try to do it before. Since nobody did those specific experiments our scientists are looking at, there are no ready-made lasers from the industry that we could just buy, even in very special equipment stores. Our researches have to come up with their own mathematical theories and experimental setups to make the results they would like to observe even possible. A lot of people have to work together in a setting like our universities and research institutions to make their ideas possible. It is a laborious task all of our researchers are working on, but it is an extremely exciting one. Because in the end, when they reach all of their goals, they can see and explain something that nobody in the world has ever done before them.