T. U. Driskell

Measuring Buffer-Gas Pressure in Sealed Glass Cells: An Assessment of the KSK Technique

In alkali rf-discharge lamps used for optical pumping in atomic clocks and magnetometers, a buffer-gas (Kr or Xe) allows electrons to extract energy from an rf-field, and these energized electrons eventually produce alkali resonant light. Contrary to naïve intuition, rf-discharge lamps can lose their noble-gas buffer over time. Recently, we began a long-term experimental program to better understand the mechanism of noble-gas loss in rf-discharge lamps, and needed a nondestructive means of measuring buffer-gas pressure changes in sealed glass cells. For this purpose, we settled on the Kazantsev, Smirnova, and Khutorshchikov (KSK) technique, which is based on inferring buffer-gas pressure from the collision shift of an alkali ground-state hyperfine transition frequency νhfs. Here, we discuss the basic KSK technique and two modifications that we have implemented for its improvement: use of a diode laser for optical pumping, and extrapolation of νhfs to zero magnetic field. Testing our systems long-term performance with a very low pressure reference cell (i.e., 3.3 torr Xe), we find a reproducibility of 0.2% and an absolute accuracy of 5%. Further, our systematic drift is less than 1 mtorr/month.

Measuring buffer-gas pressure in sealed glass cells

In alkali rf-discharge lamps used for optical pumping in atomic clocks and magnetometers, a buffer-gas (Kr or Xe) allows electrons to extract energy from an rf-field, and these energized electrons eventually produce alkali resonant light. Contrary to naïve intuition, rf-discharge lamps can lose their noble-gas buffer over time. Recently, we began a long-term experimental program to better understand the mechanism of noble-gas loss in rf-discharge lamps, and needed a non-destructive means of measuring buffer-gas pressure in sealed glass cells. For this purpose, we employ the Kazantsev, Smirnova, and Khutorshchikov (KSK) technique, which is based on inferring buffer-gas pressure from the collision shift of an alkali ground-state hyperfine transition frequency νhfs. Here, we discuss the basic the KSK technique and two modifications that we have implemented for its improvement: use of a diode laser for optical pumping, and extrapolation of νhfs to zero magnetic field. Testing our systems long-term performance with a very low pressure reference cell (i.e., 3.3 torr Xe), we find a reproducibility of 0.2% and an absolute accuracy of 5%. Further, our systematic drift is less than one mtorr/month.

Carrier quenching in InGaP/GaAs double heterostructures

Photoluminescence measurements on a series of GaAs double heterostructures demonstrate a rapid quenching of carriers in the GaAs layer at irradiance levels below 0.1 W/cm2 in samples with a GaAs-on-InGaP interface. These results indicate the existence of non-radiative defect centers at or near the GaAs-on-InGaP interface, consistent with previous reports showing the intermixing of In and P when free As impinges on the InGaP surface during growth. At low irradiance, these defect centers can lead to sub-ns carrier lifetimes. The defect centers involved in the rapid carrier quenching can be saturated at higher irradiance levels and allow carrier lifetimes to reach hundreds of nanoseconds. To our knowledge, this is the first report of a nearly three orders of magnitude decrease in carrier lifetime at low irradiance in a simple double heterostructure. Carrier quenching occurs at irradiance levels near the integrated Air Mass Zero (AM0) and Air Mass 1.5 (AM1.5) solar irradiance. Additionally, a lower energy pho...

Stabilizing vapor-cell temperature: The isoclinic-point “atomic thermometer”

In this paper, we introduce our isoclinic-point “atomic thermometer.” Briefly, in vapor-cell atomic clocks one of the primary impediments to long-term frequency stability derives from vapor temperature fluctuations. We believe that the isoclinic-point thermometer may be a means of more accurately assessing and stabilizing temperature in these clocks. Here, we review the spectroscopic nature of the isoclinic point, and we outline its use for the thermometry of vapor-phase systems. Our results suggest that it should be possible to stabilize the temperature of these systems at the sub-mK level on a time scale of minutes.

87 Rb isoclinic point thermometry

The concept of atomic thermometry is demonstrated in an experimental system consisting of two lasers frequency stabilized using FM spectroscopy. Utilizing the temperature insensitivity of the 87Rb D1 isoclinic point, a reference laser is beat against a second laser locked to an atomic transition whose frequency is strongly dependent on the vapor temperature. The data presented here indicates that thermal fluctuations down to the mK level could potentially be sensed. This concept may be useful to obtain improved long term performance in Rb vapor cell atomic clocks.

Alkali pressure shifts and their temperature dependence: Measurements with the Rb isoclinic point

Accurately assessing temperature variations of vapor-phase systems is a non-trivial problem, and one that has significant implications for the long-term stability of atomic frequency standards. As an example, consider the straightforward case of attempting to measure and stabilize the vapor temperature of Rb in a gas-cell atomic clock. Routinely, the temperature of the vapor is measured by placing a thermocouple or thermistor at the cold point of the resonance cells exterior (i.e., the region of the liquid Rb pool). As a consequence of vapor-phase/condensed-phase equilibrium, this temperature defines the Rb vapor density and by implication its mean kinetic energy. However, using a temperature probe in this manner has at least two problems: 1) the temperature of the vapor is not measured directly, only the temperature at a point on the exterior of the vapors glass container is measured, and 2) the temperature gradients that likely exist over the vapors volume are ignored. One can, of course, place additional probes on the cells exterior, but this still leaves the question of how cell-exterior temperature gradients map to interior-vapor gradients. Therefore, in order to improve the long-term stability of next-generation atomic clocks, there is a need for more accurate means of assessing actual vapor temperatures.

The influence of laser polarization noise on the short-term stability of CPT atomic clocks

We have completed a series of experiments examining the effect of laser polarization noise on the short-term stability of a CPT atomic clock, when the CPT signal is generated in the “standard” fashion. Specifically, we have examined CPT signal amplitudes, CPT-signal noise, and CPT linewidths for various levels of randomly fluctuating circular polarization. We find that CPT signal amplitudes increase in the presence of polarization noise due to a decreasing efficiency of trapping states. However, we also find that CPT linewidths increase, since polarization fluctuations add to atomic dephasing, and that CPT-signal noise increases. This latter effect is due to bulk population movement between trapping states when the laser polarization changes. Consequently, more sophisticated CPT signal generation schemes that eliminate the trapping of atomic population might be more resilient to laser polarization noise.