LASER

It’s the abbreviation of Light Amplification by Stimulated Emission of Radiation

Creating of light:

1.) system of atoms and molecules atomic energy levels, transitions, gain energy (drawing-away from the atomic nucleus), electrons lose energy (if they approach the atomic nucleus) ground- (they occupy the orbits with the least energies possible), excited-state electrons (they are on orbits that have more energies than orbits that they could occupy with the least energies) electrons have quantized (defined) energy levels molecular energy levels (figure 1)

figure 1 absorption: when the ground state electron absorbs a with a given energy, then it can get to a higher state, but only if between the excited state and the ground state the energy difference is exactly the photon’s energy (this is Bohr’s energy resonance condition)

ΔE = E2-E1 = h*f

The photon gets absorbed, it ceases to exist → gives it’s energy to the atom, which gets excited. These two processes are coinstantaneous!

2.) Fundamental radiation processes: : The excited state electron gets back to ground state after a certain time, while it gives away it’s extra energy emitting a photon with defined energy. This is spontaneous (direction, time) without outside impact.

figure 2 stimulated emission: in 1917 Einstein predicted that emission could have a version which is not spontaneous, but it happens on outside impact. This is called stimulated (induced) emission. It happens when a photon -with energy that satisfies the Bohr condition for excited state electrons- get by an excited-state electron.

At this time the electron gets back to the ground state while emitting a photon with the same energy. The outside photon and the emitted photon both have the same energy thus frequency, so the number of got doubled. The emitted photon also has the same moving direction, phase, and polarization as the outside electron. From this Einstein realized with this method it would be possible to create collimated light with high intensity. So in this case from one photon we get two so we amplify light. Laser operation’s most important condition is that in the interaction the number of photons should increase, so light amplification should happen. Σ:AMPLIFICATION! 1 → 2 photons The cause is an outside photon! same direction, time, phase, energy and wavelength The created photon: -has the same frequency as the original -travels in the same direction as the original -they have the same polarization -they have the same phase

Light amplification’s next step is a geometric optical solution: how to direct the light more than one time through the amplifier material (the gain medium) so we can use the stimulated emission in every case.

From the point of the gain medium we can speak about the following :

-solid-state (crystal+metal contamination)

-gas (CO2), He-Ne

-dye: organic dye solution -semiconductor lasers

figure 3

Processes between an atom’s two energy states: a)spontaneous emission b) absorption c) stimulated emission, E1,2-energy levels, photon (hv). Source: Kecik J.,2006). Source: http://www.szrfk.hu/rtk/kulonszamok/2005_cikkek/nanai_laszlo.pdf

3.) Population inversion: to get high intensity light we need numerous excited electrons, so we need to create lot of them in a given material. We can reach this with energy-investment, so we need to invest energy into the material from the outside, that is called laser pumping. If the number of electrons in the ground state is N1, in the excited state is N2 than basically we have N1>>N2. On the other hand after pumping we have N2>N1. This is called population inversion, which is depicted in the following figure. figure 4 if the number of excited state atoms is higher than ground-sate atoms we call this inverse population, or population inversion-than in this case induced emission has a higher possibility than absorption

We need at least a three energy level laser (we can’t make laser in a two energy level system) and from the higher levels at least one must have a long lifetime (laser energy level) so the possibility of spontaneous emission will be little.

Pumping can be achieved by

- thermal excitation (warming),

- optical excitation (with flash light) or by

- electric discharge

Laser is amplified light, that we create in an optical resonator. An optical resonator contains the gain material between two collateral mirrors, it ensures the positive feedback, and the proper frequency for the resonance.

4.) Optical resonator: two collateral plane or concave mirrors. It reflects back part of the departing light to the gain material. positive feedback, self-excitation, resonance

figure 5 Resonance condition in the laser: 2L = mλ (the double of the resonator length should be equal to one of the wavelength’s integral multiples, where L is the distance between the mirrors, λ is the wavelength in the given material, m: integer number) The resonator’s natural vibrations are standing waves (standing wave: it’s the superposition of two waves which have the same frequency, amplitude but travel in the opposite directions).

We make a direction more important by using mirrors. The photons that aren’t perpendicular to the mirrors escape from the resonator cavity. The photons that bounce back and forth between the two mirrors induce the emission of more and more photons. (figure 5). If we pump continuously (the occupation of the higher energy level) then more and more energy will be concentrated in the resonator cavity in the form of coherent photons. We can draw off from this energy by using a partially transparent mirror, so a given proportion of the created photons can leave the resonator cavity continuously. This leaving collimated, monochromatic, coherent radiation called laser radiation.

Laser light begins with a photon emitted spontaneously in axial direction. This gets multiplied up in the optical cavity by the induced emission. The photons travelling in the wrong direction get scattered out of the laser beam.

Σ For laser oscillation we need:

1-gain medium

2-intensive electron excitation (pumping)

3-positive feedback

4-optical resonator

5.) Attributes of the laser light:

- monochromatic: narrow bandwidth (the relative frequency bandwidth of a laser is Δf / f ~10-10)

- The gained light is coherent, it is able to create interference even in the case of big path- differences.

Time coherency: the photons emitted in different times have the same frequency

Spatial coherency: phase identity through the laser beam’s cross-section .

-The laser beam is a narrow beam with very little divergence, so it’s approximately collimated. - Laser energy is concentrated in a little space, in pulsed mode this happens during a very little time-interval, so laser light’s power density (E/At) can be much higher than ordinary light sources.

-polarized

-possibility of ultrashort-pulses (10-15 s)

Σ What is needed for lasers? •Pumping •Population inversion •Stimulated emission •Optical resonance •Mirrors with high reflectivity

6.) Comparison

Ordinary light sources vs LASER wide wavelength bandwidth monochromatic (narrow spectral bandwidth) divergent beam collimated beam not coherent coherent power density: for example: welding:~103 W/m2 ~1015 W/m2 non polarized polarized

7.) Laser types: lasers can be categorized by many aspects. For example considering the gain medium we can differentiate semiconductor, gas, solid-state and dye lasers. Considering the operation method we can speak about two big types: continuous and pulse lasers. Considering the energy we differentiate the laser types into four big classes.

8.) Application of lasers: -moulding, drilling, spot-welding, -surgical operation, retinal laser surgery, -gene surgery, -barcode reading, -CD-player laser playback head, - lengthiness and velocity measurement using interference, -direction setout - light source used to create holography (Gábor Dénes hologramme=whole picture)