S ATURATED L ASER A BSORPTION S PECTROSCOPY By Melanie Pelcher and Theodore Rapach

I NTRODUCTION Laser Spectroscopy What is Hyperfine structure? Experimental Setup Apparatus Components Experimental Techniques Results Conclusion Discussion

L ASER S PECTROSCOPY Using Laser Spectroscopy, observe the 5 2 S 1/2 to 5 2 P 3/2 transition in Rubidium 87. Specifically, we hoped to observe the Hyperfine Structure of Rubidium 87. Hyperfine structures occurs because of the weak magnetic interaction between electrons and nuclei.

L ASER S PECTROSCOPY Atoms absorb and emit photons of quantized energies.  E = h Knowing this we can use our laser to see which energy level transitions are allowed The transition of state in our Rb sample is initiated by a laser beam of frequency GHz which corresponds to an energy transition of eV.

L ASER S PECTROSCOPY Scan over a range of frequencies. See absorption of photons at frequencies corresponding to specific hyperfine energy levels

D OPPLER S HIFT Due to the thermal motion of the gas molecules, taking a simple spectrum of the Rb gas will not show the hyperfine structure. Must account for the fact that there are random motions of atoms due to thermal energy. At room temperature v rms is about 293 m/s. Doppler shift changes the apparent energy of an incoming photon

D OPPLER S HIFT The difference between atom and laser is approximately 341 MHz. The difference between the two closely spaced hyperfine energy levels is 72.2 MHz. This means that we wouldn’t be able to resolve these energies since they are clouded by the thermal motion.

V ELOCITY OF THE R B G AS The atoms in the Rb gas have random motion due to thermal energy. The distribution of the particles with components of velocity in the direction of the laser beams looks like this.

L ASER S PECTROSCOPY So due to Doppler shifting of the light according to the moving atoms, our spectrum appears like this. This can be understood by recalling the previous slide showing the thermal velocities of the atoms. The more atoms with a given velocity correspond to a lower intensity of light detected due to the photons being absorbed and exciting the atoms instead of being transmitted.

D OPPLER -F REE S PECTROSCOPY Technique that allows one to observe only stationary atoms. Bloembergen and Schawlow won a 1981 Nobel Prize for the development of this technique.

O UR S ETUP

D OPPLER -F REE S PECTROSCOPY Pump beam from the laser excites the stationary atoms in the Rubidium cell. The probe beam is sent through the Rubidium but the stationary atoms are still excited so the laser is transmitted, not absorbed. This laser light reaches our detector. Transmitted!

D OPPLER F REE S PECTROSCOPY The pump beam of the laser will excite atoms with a +x velocity when the relative Doppler shifted frequency is equal to the frequency of an allowed transition. The probe beam of the laser will excite atoms with a -x velocity when the relative doppler shifted frequency is equal to the frequency of an allowed transition.

D OPPLER -F REE S PECTROSCOPY The laser frequency is just right to cause a transition to a hyperfine energy level. In this case, the Probe beam will be of higher observable intensity since already excited stationary atoms will allow the light to transmit.

D OPPLER -F REE S PECTROSCOPY We scan over all frequencies that could stimulate a transition in moving atoms. The moving atoms will absorb the pump and probe laser beam. The stationary atoms will allow transmission of the probe beam at the frequency that corresponds to allowed energy transitions.

D OPPLER -F REE S PECTROSCOPY This picture shows what is actually happening with everything put together. This makes for a beam leaving the gas at a higher intensity for the frequencies corresponding an allowed hyperfine transition in the stationary reference frame.

D OPPLER -F REE S PECTROSCOPY For photons with energy that do not correspond to a transition, this happens. This absorption of the laser beam lowers the intensity of the beam detected by our photo diode.

D OPPLER -F REE S PECTROSCOPY

O UR A PPARATUS

D IODE L ASER

E XPERIMENTAL S ETUP - D IODE L ASER Our diode laser emits in the Infrared region and is difficult to see with our eyes. In order to detect our beam, we used a photo card and a security camera. The goal was to tune the laser to Gigahertz which corresponds to a wave length of approximately 780 nanometers. This corresponds to the energy needed for exciting an electron in Rb from 5 2 S 1/2 to 5 2 P 3/2.

E XPERIMENTAL S ETUP - C OLLIMATION Collimation occurs when all the rays of light are essentially parallel and have minimally spreading. Collimation is checked by looking at the laser beam on a photo card or security camera. The beam should stay the same size and be uniform looking a long ways from the experiment, or at least for the distance that the beam will have to travel in the experiment. Collimation can be adjusted by small lens in the laser mount.

E XPERIMENTAL S ETUP -L ASING T HRESHOLD Laser Threshold: The threshold voltage is when enough power is put into the laser that the optical gain of the lasing medium in equal to the losses of the light experienced in the lasing cavity. This is a large marked increase in intensity of the laser when one crosses the Threshold.

E XPERIMENTAL S ETUP - F EEDBACK The laser needs to hit the diffraction grating and it must be positioned just right so that the first order refracted beam must be projected back into the laser. This allows us to greatly lengthen the lasing cavity giving us much tighter control of our output frequency. Turning the laser down just below threshold, we adjust the diffraction grating that is mounted on a dual axis adjustment first adjusting the "x“ component to the optimum position, and then the "y". The idea is that the intensity will increase and the laser jump above threshold when the first order diffracted beam is reflected back into the laser. We monitored the brightness by shining the laser on a piece of paper and looking at it in a security camera. The jumps over threshold are obvious.

L ASER C ONTROLLING A PPARATUS

E XPERIMENTAL S ETUP - L ASER F REQUENCY C ONTROLS Laser frequency was monitored by a wave meter. The coarse adjustment on our Laser was controlled by adjusting the temperature of the lasing medium. Medium range adjustment was done by adjusting the “x” adjustment of the diffraction grating. Fine adjustment was done by current control.

E XPERIMENTAL S ETUP - M ODE H OPPING Our laser was not of high enough quality for the frequency emitted to stay fixed. After sometime of use, our laser would begin hopping back and forth between two different frequencies. This was dependent on what frequency we tuned the laser to. To alleviate this problem, we readjusted the temperature and current combinations in order to move the frequencies where mode hopping occurred away from our base frequency of GHz.

H ALF WAVE PLATE Half wave plates are made of special crystal of certain thickness such that light of one polarization orientation travels slower than the light polarized perpendicular to it. This is used to rotate the Polarization of a light beam.

P OLARIZED B EAM S PLITTER Polarizing beam splitters are made of 2 wedged shaped gratings that transmit light of one polarization and reflects light of the other polarization by 90 degrees.

R UBIDIUM C ELL

P HOTO D IODE D ETECTOR Our Photodiode detector converts the light of the laser into a voltage that was fed to an oscilloscope for observation.

R ESULTS This is our first gathering of usable information. We notice the one big peak, which corresponds to the energy level of a photon. We concluded that our probe beam was of too high intensity to resolve more individual hyperfine energy levels.

R ESULTS Probe beam at lowest intensity 5 Hz

R ESULTS

C ONCLUSION We were able to observe the three hyperfine allowed energy transitions in Rubidium 87.

D ISCUSSION – H YPERFINE I MPROVEMENTS Why were we only able to resolve three hyperfine energy transitions? One suggestion to obtain would be a more sensitive and accurate oscilloscope. Additionally it would be helpful to direct our data into a computer so more quantitative data analysis could be performed.