PHY 451, Spring 2019| Experiments

Diode Laser Spectroscopy (Doppler Free Spectroscopy)

Laser is one of the most widely employed technologies in science and industry. In this experiment, you will be introduced to the concept of semiconductor diode laser, and its applications in spectroscopy and atomic physics. You will learn the basic technique of aligning a laser and observing phenomena, such as absorption and fluorescence. You will also learn about how to counter Doppler effect using saturated absorption to perform precision measurement in atomic physics. The basic technologies are relevant to the achieving of Bose Einstein Condensation using atomic gasses and the readout technique in the development of optical storage medium.

Pulsed Nuclear Magnetic Resonance (NMR)

In 1945 Felix Bloch (Stanford) and Edward Purcell (Harvard) discovered nuclear magnetic resonance in ordinary matter, for which they were awarded the Nobel Prize in 1952. The phenomenon has found many applications in science and technology, including magnetic resonance imaging (MRI) used in medical practice. In an NMR experiment, nuclear dipoles (the samples) are placed in a static magnetic field of about 4000 Gauss and in a time-varying radio-frequency magnetic field perpendicular to the static field. The static field causes Zeeman-effect splitting between sub-states, and the radio frequency field is tuned to the Larmor frequency so that it induces transitions between the sub-states. The resonance condition can be observed using the Bloch two-coil induction technique. You can observe proton and fluorine nuclei with proper lock-in detection and signal averaging.

In your experiment, we use a sequence of pulsed radiofrequency (~ 10MHz) fields ("Pulsed NMR") rather than a continuous-wave (CW) field. Transient signals ("spin echoes") are detected shortly after the pulsed excitation stops. The observable effects are comparable to the free vibration or ringing of a resonant cavity on an atomic scale. This is the basis of Magnetic Resonance Imaging in the medical field today. You will measure the spin-lattice and spin-spin relaxation time-constants of various substances containing protons (water, glycerin, etc.). You will observe the effects of temperature on glycerin and of paramagnetic ions on the relaxation time-constants in water.

Nuclear Physics (Gamma Ray Spectroscopy & Muon Lifetime)

This is a set of two experiments in which you will learn some experimental techniques in Nuclear Physics to study (1) the energy spectrum of the nuclear decay process and the absorption of the emitted gamma radiation and (2) the lifetime of muons that stop in your detector after being created near the top of our atmosphere.

Nuclear Physics 1 (Gamma-Ray Spectroscopy):

Learn the properties of different detectors and the electronics to operate those detectors.

Analyze the energy spectrum of several different radioactive decay processes.

Study the interaction of emitted radiation with matter

Nuclear Physics 2 (Muon Lifetime):

Learn how muons are detected in the apparatus.

Measure the lifetime of muons that come to rest in your detector and then decay 2.2 ms later on-average.

Check the time-calibration of the instrument and prove that muon detection obeys Poisson statistics.

Optical Pumping of Rb

This experiment is quantum mechanics in action. Optical pumping has been widely used to exploring atomic energy states, atomic transitions, and atomic collisions. Several Nobel Prizes have been awarded for work in this area (eg. Alfred Kastler 1966). When a sample of gaseous atoms is placed in a static magnetic field, the electronic states undergo Zeeman energy level splittings in addition to fine-structure and hyperfine-structure splittings. By applying polarized light at the proper frequency, we can induce transitions from ground state levels to excited state energy levels. The atoms then decay to higher ground state levels until we have "pumped" all of them into the same (highest) ground state energy level. At this point we can see an increase in light passing through the sample because no more photons can be absorbed by the gas. However, when we apply a radiofrequency signal of just the right energy to stimulate transitions to a lower level, we see a sudden decrease in the light signal as photon absorption reoccurs. By determining the frequency of the RF signal we gain information about the atomic energy levels.

Our apparatus uses electromagnetic waves at optical- and radio-frequencies in the presence of a uniform, constant magnetic field. The atom you will be studying is rubidium because of its simple hydrogen-like qualities - a "one-electron atom" with core electrons forming closed shells. You will measure the energy splittings of the two isotopes of rubidium, the nuclear moments of these isotopes, and the strength of the earth's magnetic field in the laboratory. You will also observe such interesting phenomena as the recovery time for pumping and "Rabi oscillations".

Sonoluminescence

As the name implies this experiment connects sound with light emissions. Ultrasonic waves are applied to partially-degassed water in a small transparent tank under resonance conditions. A current-pulse-heated filament generates vapor bubbles in the water that become trapped at the anti-nodes of the sound wave. At the antinodes, when the sound pressure rises toward a maximum, the bubble can collapse violently emitting a pulse of light in the process! There are all sorts of interesting experiments for you to try. For example, using a photomultiplier tube, you can determine the time duration of the pulse of light, at what time during the period of the sound wave the flash occurs and how the light intensity varies with temperature. You can also scatter laser light off the trapped bubble to estimate its size as a function of time. Finally, you can add a noble gas to the trapped air bubble and see what happens.

Superconductivity and Superconducting Tunneling Juction

Superconducting phenomena are among the most fascinating features in physics, and the Josephson effect is the most interesting of all. In 1962, Brian Josephson predicted that electron pairs could tunnel without resistance through an insulating barrier between two superconductors. A DC current can flow through the junction with no potential difference; but when a DC voltage is applied together with a small alternating voltage, the current-voltage curve shows a characteristic step structure. From this step structure the value of 2e/h can be calculated easily and accurately, an exciting consequence of the properties of the junction. The ratio e/h appears throughout atomic and condensed matter physics. In this experiment you will learn some vacuum-evaporation and low-temperature techniques which are used in many research laboratories. You will fabricate your own SIN (superconductor-insulator-normal metal) and SIS (superconductor-insulator-superconductor) tunnel junctions in a vacuum evaporator, dip them into liquid helium and measure their voltage-current characteristics. The SIS junctions exhibit the famous Josephson Effect, whereby an electrical current flows across the junction without any voltage drop. When you make a circuit with 2 such junctions, you get a "SQUID", i.e. a Superconducting Quantum Interference Device.

Superfluidity of Liquid Helium-4

Helium gas liquefies at 4.2 K at atmospheric pressure. When it's cooled to lower temperatures, there is a phase transition at 2.17K to a new liquid phase which exhibits the property of superfluidity. The superfluid has the properties of a Bose-Einstein condensate: extraordinarily low viscosity and nearly infinite thermal conductivity. Heat propagates as a wave called second sound. Using an acoustic resonator, you will measure the speed of sound in gases and in liquid helium. In the superfluid phase, you will measure the speed of second sound as a function of temperature. You can show that the speed of second sound becomes quite slow at the superfluid-normal fluid transition (the lambda point).. This experiment uses a phase-sensitive lock-in detector to measure standing wave resonances in a cylindrical acoustic cavity. You will also learn how to handle cryogenic fluids such as liquid nitrogen and liquid helium.