I have seen the light, IT BURNS. yeah don’t look at a LASER directly. But safety and environmental awareness will be covered in a later post (maybe). What is a LASER? Well, it’s an acronym used to describe an instrument which is able to generate light rays in a coherent manner. A couple weeks ago, my friend gave a short lesson how LASER’s work as part of my summer lab group’s technique demonstration rotations. Check it out here:https://www.youtube.com/watch?v=srOoyMmwaHo
Let’s just spend some time going over the basic principles that underlie LASER’s and how these different concepts converge in the overall operation. First of all, what is a LASER. LASER stands for Light Amplification By Stimulated Emission of Radiation. Well, by the name itself we can appreciate that there is quite a bit of stuff happening. There’s Light and Radiation so you can imagine that we will be playing around with light-emitting compounds, and this bit about “Stimulated Emission” which I will get to. What a LASER does is more around the corner. We’ve all seen lasers, they shine a very intense beam of a single color(monochromatic) of light. Contrast this with a light bulb, which radiates a mild light in a diffuse manner. Why is this? Don’t they both run on electricity, why are there such stark differences between the 2 techs. Let’s dive in, shall we?
Stimulated Emission
LASERs use a very sophisticated approach when it comes to creating light (emission) called stimulated emission. Jumping back to our quantum chemistry, we understand that light is produced (emitted) by the relaxation of a sub atomic particle (typically an electron) from an excited state to a ground state. For visible light, that energy difference must be within the visible frequency. This is the case in spontaneous emission, where in a population of atoms- electrons get excited to higher states by absorbing radiation(or other form of energy) and then emit radiation on the way back down. In stimulated emission, electrons still get excited to higher states-however, at this point, they are again irradiated by light whose energy matches the energy difference between the 2 energy states. As a result, the electron still “relaxes” to the ground state, but now releases double the emission(and energy). The energy doubling occurs because energy is conserved within the system.
It gets even better, as you stimulate some emission, the rest of the system follows in a chain reaction. (emitted light stimulates more emissions and on and on, and on, and on STRANGERS WAITING)
Also, this mechanism is inherently coherent. The emitted waves are in phase with one another, resulting in a constructive interference that serves to raise the intensity of the overall light. However, to accomplish this requires a degree of control. You have to ensure to a reasonable extent, that only 1 energy transition is taking place. Otherwise, you would have emitted light of a certain energy unable to stimulate emissions of a different energy. How is this accomplished in a laser? hmm
Population Inversion
An issue that poses a problem with the operation of a laser is how to guarantee emission. What I mean to say is that in order to have a net emission occurring in response to incident light, more electrons must exist in the excited state vs the ground state (otherwise, the light would just get absorbed to excite elections).
Here’s the catch, statistical mechanics states that for 2 energy levels- The fraction of particles in the excited state cannot exceed that of the ground state at thermal equilibrium(This is known as the Boltzman distribution of particles). So it seems the best we can achieve is transparency, which is not really helpful when we’re making lasers. How do we do it then.
In 1960, a brilliant physicist and engineer named Ted Maiman came up with an genius approach to making the first LASER using synthetic rubies. With this, he achieved what is known as a population inversion-where the number of particles in the excited state exceeded the number in the ground state. BUT didn’t I just say that’s not possible. Well, turns out it is… if you have a third energy level. (Obligatory goku going ssj3 gif: http://giphy.com/gifs/dragon-ball-z-dbz-tz7oXvXRRYSgU)
When you have 3 energy levels, you can achieve non-equilibrium states(Not possible in 2 state systems), where electrons are excited from a ground state to an excited state. At this point, electrons undergo a rapid radiation-less transition into a meta-stable state. From the meta-stable state, electrons can undergo a radiative transition back to the ground state. When you excite enough electrons really quickly from E1 to E3(known as pumping), you can achieve conditions where more electrons are in the meta-stable state than the ground state. So we’ve finally figured out how to stimulate emission, what’s next. Are we ready to make a LASER?
Optical Resonator
While one is able to achieve stable coherent light as a product of stimulated emissions, we cannot really control the direction nor the intensity. (The emissions could radiate in any direction, right). So we need to set up a system that produces a directional beam of light. This is accomplished by the optical resonator, which is 2 parallel mirrors (1 is an oscillator) placed apart from each other with the lasing medium in the middle. When emission occurs, the light is reflected between the oscillator and the mirror, essentially “trapping” it inside the optical cavity. Meanwhile, more emission is occurring, causing the light to become more intense. Eventually, the light reaches a certain intensity and exits the resonator as a very intense, coherent waveform. Naturally, only light which strikes the mirrors at a specific angle are able to take advantage of this mechanism with the remaining light scattering away. An aperture at the end of the oscillator controls the size of the beam. The end result is something like this http://s258.photobucket.com/user/jimifunguzz/media/yoda-lightsaber-gif.gif.html
Working Mechanism
Here’s a picture of a ruby LASER’s interior. It’s connected to a circuit to generate the electrical discharges that pump the electrons into the excited state.
The optical cavity is visualized between the coil structure, with 2 parallel mirrors amplifying the emitted radiation until BAM!
LASERs have widespread uses in all walks of the earth. They can be found in information technology, surgery, manufacturing, welding and cutting just to name a few. Rarely does one witness such a groundbreaking device that can play all positions on the field.Truly a game changing device of limitless potential.
shankarpro1 
