Introduction:
The purpose of this lab is to understand how the laser works by using Modern Physics, ActivPhysics Online. The general ideas of laser is contributed by three interaction processes between an atom and electromagnetic waves, as well as the energy carried by the photons. The energy difference between the ground state and the excited sate has to be considered, whether the gound state has higher energy or the excited state has higher energy will result in different interaction process between atoms and photons.
The first interaction process is Aborption, which is the case when the atoms are in their ground states. Therefore, when the photons are inciden to those atoms, the energy carried by the photon will be absorbed by the atoms hence being excited to the higher energy level.
The second interaction process is called Spontaneous emission, which basically interprets the oppocite concept on the transferring energy to the case of Aborption. In this case, all atoms are inicially in their excited states. Therefore, since the atoms carry higher energy than the ground states, the difference in energy would then emit photons in ramdon directions. Since the photons are emitted in random direction, it interprets that each photon is independent in this case, and it also shows that the emitted light in this process is incoherent.Spontaneous emission alwasy follow the process of Absorption, hence those emitted photons have the same frequency as the one originally absorbed.
The last interaction process is the Stimulated emission. In this process, the photons are incident to the atoms in the excited states, and the photons are also emitted from the excited states at the same time due to the energy difference between the inicial excited level and the ground level. However, one of the major differences in this case is that the photons are not emitted in random direction. Instead, the photons are emitted and perfectly matched to the incident phtons, with same frequency, direction, phase, and polarization. Therefore, the emitted light in this process in coherent. Basically speaking, the incident photons are jointed with the emitted photons hence creating a amplified radiation, also known as the light amplification.
If the three basics interaction processes are going to be analyzed further, the concept of pumping has also be considered. Here, the Maxwell-Boltzmann distribution would be discussed. Now the rario of energy for the ground state and the excited state is going to be analyzed by using the Maxwell-Boltzmann distribution. The ration for this two states is described by using the following equation:
Here, the Ni is denoted for the numbers of atom for the excited state, and the N is denoted for the numbers of atom for the ground state. After arranging the equation and calculating the ratio, the result shows that the fraction of atoms above a ground sate is extremely small. In other words, the rate at which energy is absorbed by the ground state atoms is far more than the rate at which energy is emitted by the excited atoms. However, as being illustrated earlier, the Stimulated emission happens only when the atoms are inicially in the excited states, hence in this case, the rate at which energy is emitted by the excited atoms should be increased. It also means that the numbers of atoms in higher-energy state should be increated to that it exceeds the numbers of atoms in lower-energy state. This situation when the rate of energy radiation by stimulating emission exceeds the rate of absorption is called the population inversion.
Since the spontaneous emission always happens when the atoms are in the excited level, the energy difference will cause the emission happen and make the atoms go to the ground state. However, the basics polulation inversion is to increase the number of atoms in excited state hence having more energy. In order to increase the energy and since the photons are the result of stimulated emission, the emitted light in this process is coherent as mentioned earlier, hence the resulting is much more coherent than the original sources.
Now we want to verify the concept with the Laser simulation provided by the ActivPhysics.
figure 1.-Absorption
figure 2.-Spontaneous Emission
figure 3.- Stimulated Emission
figure 4.-Laser
figure 5.-Laser
Conclusion:
For the process of Absorption, when the photons are incident to the atoms which are inicitally at ground state and excite then the higher-energy level. From figure 1., it shows that there are atoms being excited by the incident photons. The total input of the photons are six, one photon leaves without exciting any ground-level atom, and two atoms are about to exite atoms or leave without exciting atoms. Therefore, 3 excited atoms plus 2 incident photons plus 1 left photon equal to the total number of input photons. This is also proven by the process of Spontaneous emission, which instead of atoms receiving energy from the photons, those atoms emit energy and go to the ground level. In figure 2., the number for the atoms go to the ground state is 13, and it equals to the number of the photons output 9, plus the numbers of just emitted light 4. In figure 2., it also shows that the emitted lights go random directions. For the Stimulated Emission, in figure 3., the numbers of atom which are left in the excited state is 9, and the numbers for photons output is 2. It equals to the numbers of atoms which go to the ground state. In this process, even the light is emitted, the incident photons re-excite the atoms at the same time. Furthermore, figure 3. also shows that the light goes to the same direction. It happens becuase when the light emitted from the excited level, it is re-excited by the incident phtons at the same time, hence resulting in this coherent emitted light. From the three figures discussed above, it also shows that the only case there is no incident photons is the spontaneous emission, hence it is also the only case which atoms acutally go from higher-energy level to lower-energy level. For the concept of puming, how the photons are incident to the atoms are determined by the number of pumping number. In order to have incident photons to re-excite the atoms resuled from the emitted light, the number of incidents photons is much more when the pumping number is big than when the pummping number is small. It also shows in the figure 4. and figure 5. that larger pumping number is able to keep the number of atoms in excited state. However, the lower pumping number fails to keep the number of atoms in excited state. Hence, with larger pumping number, it would alow the laser to have larger number of atoms in the excited state than in the ground state, thus the laser can have higher energy.
