Phys 726: JQI Rotation 1

Michael Jarret & Jiehang Zhang

Table of Contents

  1. Introduction
    1. Goals
    2. Theory
  2. Experimental Setup
    1. The Basic Setup
    2. Notes, Details, and Preliminary Findings
  3. Results
    1. Discussion of Results
  4. Moving Forward
  5. References
  6. Acknowledgements

Introduction

    First discovered by Penning in 1928, the Ogtogalvanic Effect (OGE) is a useful laboratory technique for obtaining spectroscopic data without the use of absorption or emission spectroscopy. Despite an early discovery, the technique did not become popular until 1976, when the tunable dye laser made the technique practical. Since then, optogalvanic spectroscopy (OGS) has seen many applications, including laser calibration and stabilizaton.[1,2]

Goals

    In the long term, this project will create a laser locking apparatus for a 399nm laser. Because OGS allows us to access the spectrum of our discharge directly through an electronic signal, we can use it as a feedback mechanism to stabilize our laser. If implemented properly, this technique can lock the laser to a known frequency to within a high degree of accuracy.
    The first task in the development of this apparatus is to effectively measure the optogalvanic spectrum of our neutral Ytterbium hollow-cathode lamp. We constructed an apparatus to observe the OGE.

Theory

    The OGE is a cumulative effect of one or more different ionizing reactions. [1] (For a list of the contributing ionizing mechanisms, see [2].) When a laser at or near a discharge's transition wavelength strikes the discharge, the ionization balance of the discharge changes. Because electrons are excited to higher energy states, there is a net effect on the electrical properties of the discharge. Although it can be observed in multiple ways, we observe it as a change in impedence across the discharge. [1] The change in electrical effect is sufficiently small that we can consider our observed effect as "directly proportional to the numer of photons absorbed." [2]  
    In the case of Ytterbium, we can stimulate a transition between (6s 6p)1P1 and (6s2)1S0 by using 399nm light. [3] A diagram of the energy levels of neutral Ytterbium (taken from [3]) is shown below:


Experimental Setup

The Basic Setup

    We adopt the standard experimental setup for an OGE measurement. Light from a 399nm laser is collimated and sent through a chopper wheel set to around 1.1kHz. This light is then focused on the Ytterbium lamp. A reference frequency is provided to a Lock-in Amplifier [5] by a Sharp GP1A52HRJ00F photointerrupter.
    A high voltage supply (HVS) set to ~300V is connected across a Hollow Cathode Ytterbium Lamp in series with a 30kΩ Ballast Resistor. The lock-in measures the potential across the resistor, with a 100nF capacitor acting as a high-pass filter. (See Figures 1 and 2)
Figure 1. A power supply is connected to a hollow cathode Ytterbium lamp in series with a Ballast Resistor. The OGE is measured using a Lock-in Amplifier.
 Figure 2. Actual realization of Figure 1.

Notes, Details, and Preliminary Findings

   The HVS could be limited by either current or potential. We found that when current limited, the signal was highly unstable, so that we typically voltage limited the supply. Also, we observed a more discernable signal for lower currents, and thus we chose to operate at around 300V with an approximate current of 6mA. (Note: the lamp requires about 210V to fire and about 130V to maintain operation.)
    The chopper wheel has a slight wobble, which causes some complications when attempting to lock the signal. Using the photointerrupter improved the stability of the reference frequency and, when operated at around 1kHz, allowed us to lock to the chopper frequency. (The layout of the pins for the photointerrupter are diagrammed elsewhere. See References.)
    Additionally, there might be a warm-up time associated with the lamp (discussed in detail below). After attempting to observe the OGE for a brief period, we observed the signal make a 180 Degree phase change. Only after this phase change did the effect become apparent.
    Last, we discovered that to observe the OGE the laser light must land directly in the center of the cathode. We are still unsure of the reason, but when the light lands elsewhere an effect (not believed to be the optogalvanic effect) is produced. This effect may disappear if the orientation of the lamp is altered. (See Below.)

Results

    Our first observation of the OGE was incomplete. (Figure 3) The spectrum produced by the impedance change is approximately what one would expect for the doppler-broadened spectrum of Neutral Ytterbium. We were unable to complete this data set because of an observed shift in impedance while at constant frequency. (Figure 4)
Figure 3. The first observation of OGS. The data is incomplete between 751526 and 751527 GHz.
 Figure 4. Impedance shift as a function of time at constant frequency. The observed discontinuity around t=300s is because of an intentional retuning of the laser frequency.

    After seeing this time dependent nature of the impedence, we altered our experimental procedure. In between consecutive data points, we returned the laser to a reference frequency. Then, our data were plotted with respect to the observed impedence at this reference. (Figures 5 and 6)
Figure 5. Second run, fitted with a Lorentzian.
 Figure 6. Second run, fitted with a gaussian

Discussion of Results

    We believe that we have observed the OGE. Our measurements were, however, not precise enough to form a practically useful spectrum. From Figure 5 we can see that the Lorentzian profile does seem to fit the data, with an observed width of around 1.5GHz. Figure 6 shows that a Gaussian seems to more accurately describe the data of the right-hand tail.
    This time-dependency is quite strong and, as one can readily observe from Figure 4, fairly consistent. (Although, this signal was observed to both increase and decrease at different occassions.) Because we observed an anomalous 180 Degree phase shift and a continual change in impedance, we are led to speculate that there may be some continual, potentially predictable, dephasing between the lock-in and the OG signal. Alternatively, there may be some thermal effects or signal contributions not specifically observed.

Moving Forward

    Before proceeding to use this technique as a laser-locking method, we need to work out either precisely what causes the observed time-dependent effects or a means of counteracting them. A first step would be to attempt a longer term observation of the impedance change as a function of time with the laser held at constant frequency. Additionally, we might change the orientation of the lamp.
    Because we are unsure of the precise construction of the lamp, we are not convinced that the light is landing on the Ytterbium alone. It is possible that, in addition to the OGE we are also seeing the photoelectric effect, or some other interaction between the laser and the electronics. Changing the direction that the laser enters the lamp may enable the beam to land primarily upon the Ytterbium while avoiding other electronic components.
    Once the time-dependency issue is worked out, one might proceed by attempting to sharpen the spectrum peak. One means of accomplishing this is to use saturation spectroscopy. [4]
    (As a final note, it might be worthwhile to attempt to find a means of mounting the chopper wheel in a more stable fashion.)

References

[1] Goldsmith, J. E. M., and Lawler, J. E., 1981, Contemp. Phys., 22, 235.
[2] Barbieri, B., and Beverini, N., 1990, Rev. Mod. Phys., 62,  603.
[3] Andrés David Cimmarusti's Rotation Page (http://www.terpconnect.umd.edu/~candres/rotation1.html).
[4] Ohmukai, R. et al., 1994, Jpn. J. Appl. Phys., 33, 311.
[5] Dunlap, R. A., 1988, Experimental Physics, 102-106

(photointerrupter pin layout)
(photointerrupter spec sheet)

Acknowledgements

Thanks to Prof. Chris Monroe for providing us with a project and workspace for this rotation.
Thanks to Dr. Wes Campbell for his imeasurable amount of help and guidance.
Thanks to Dr. Qudsia Quraishi for her help and advice.
Thanks to Dr. Kihwan Kim for his help in using the 399nm laser.
Thanks to Prof. Luis Orozco for organizing the course.