Zeeman relaxation of cold iron and nickel in collisions with ³He

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 143-148).This thesis describes a measurement of the ratio of elastic to Zeeman-projection changing collision cross sections ([gamma]) in the Fe-3He and Ni-3He systems. This ratio is a probe of the anisotropy of the interaction between the colliding species. Theory and experiment confirm that Zeeman-projection collisions are suppressed in transition metals due to the presence of a spherically symmetric, full 4s shell, making them good candidates for loading a magnetic trap with the buffer gas cooling method. Nickel and iron atoms are introduced via laser ablation into an experimental cell containing a background 3He buffer gas. Elastic collisions with the buffer gas thermalize the atoms to less than 1K. The highest energy mJ = J Zeeman state decays via diffusion through the buffer gas and collisional Zeeman relaxation. Therefore the mJ = J lifetime depends on the buffer gas density of the cell. By measuring the mJ = J lifetime as a function of buffer gas density we determine [gamma]. We find [gamma] for Ni [3F4, mJ = 4] is between 2 x 103 and 1:1 x 104 at 0.75 K in a 0.8 T magnetic field. Zeeman relaxation in Fe [5D4, mJ = 4] occurs on time scales too rapid for us to measure accurately, and we are only able to set an upper bound of [gamma] < 3 x 103. The nickel result confirms that Zeeman relaxation is highly suppressed in submerged shell transition metal atoms.by Cort Nolan Johnson.Ph.D

Directly pumped 10 GHz microcomb modules from low-power diode lasers

Soliton microcombs offer the prospect of advanced optical metrology and timing systems in compact form factors. In these applications, the pumping of microcombs directly from a semiconductor laser without amplification or triggering components is desirable to reduce system power and to simplify system design. At the same time, low-repetition-rate microcombs are required in many comb applications as an interface to detectors and electronics, but their increased mode volume makes them challenging to pump at low power. Here 10 GHz repetition rate soliton microcombs are directly pumped by low-power (<20  mW) diode lasers. High-Q silica microresonators are used for this low-power operation and are packaged into fiber-connectorized modules that feature temperature control for improved long-term frequency stability

Atomic magnetometer for human magnetoencephalograpy.

We have developed a high sensitivity (&lt;5 fTesla/{radical}Hz), fiber-optically coupled magnetometer to detect magnetic fields produced by the human brain. This is the first demonstration of a noncryogenic sensor that could replace cryogenic superconducting quantum interference device (SQUID) magnetometers in magnetoencephalography (MEG) and is an important advance in realizing cost-effective MEG. Within the sensor, a rubidium vapor is optically pumped with 795 laser light while field-induced optical rotations are measured with 780 nm laser light. Both beams share a single optical axis to maximize simplicity and compactness. In collaboration with neuroscientists at The Mind Research Network in Albuquerque, NM, the evoked responses resulting from median nerve and auditory stimulation were recorded with the atomic magnetometer and a commercial SQUID-based MEG system with signals comparing favorably. Multi-sensor operation has been demonstrated with two AMs placed on opposite sides of the head. Straightforward miniaturization would enable high-density sensor arrays for whole-head magnetoencephalography

Zeeman Relaxation of Cold Atomic Iron and Nickel in Collisions with 3He

We have measured the ratio of the diffusion cross-section to the angular momentum reorientation cross-section in the colliding Fe-3He and Ni-3He systems. Nickel (Ni) and iron (Fe) atoms are introduced via laser ablation into a cryogenically cooled experimental cell containing cold (< 1 K) 3He buffer gas. Elastic collisions rapidly cool the translational temperature of the ablated atoms to the helium temperature. The cross-section ratio is extracted by measuring the decays of the atomic Zeeman sublevels. For our experimental conditions, thermal energy is comparable to the Zeeman splitting. As a result, thermal excitations between Zeeman sublevels significantly impact the observed decay. To determine the cross-section ratio accurately, we introduce a model of Zeeman state dynamics that includes thermal excitations. We find the cross-section ratio for Ni-3He = 5 x 10^3 and Fe-3He <= 3 x 10^3 at 0.75 K in a 0.8 T magnetic field. These measurements are interpreted in the context of submerged shell suppression of spin relaxation as studied previously in transition metals and rare earth atoms.Comment: 10 pages, 5 figures; submitted to Phys. Rev.

Late pregnancy thyroid-binding globulin predicts perinatal depression

Previously we found that late pregnancy total and free thyroxine (TT4, FT4) concentrations were negatively related to greater pre and/or postpartum depressive symptoms. In a much larger cohort, the current study examined whether these thyroid indices measured earlier in the third trimester (31-33 weeks) predict subsequent perinatal depression and anxiety ratings as well as syndromal depression. Thyroid-binding globulin (TBG) concentrations increase markedly during pregnancy and may be an index of sensitivity to elevated estrogen levels. TBG was examined in this study because prior findings suggest that postpartum depression is related to sensitivity to mood destabilization by elevated sex hormone concentrations during pregnancy. Our cohort was 199 euthyroid women recruited from a public health obstetrics clinic (63.8% Hispanic, 21.6% Black). After screening and blood draws for hormone measures at pregnancy weeks 31-33, subjects were evaluated during home visits at pregnancy weeks 35-36 as well as postpartum weeks 6 and 12. Evaluations included psychiatric interviews for current and life-time DSM-IV psychiatric history (M.I.N.I.-Plus), subject self-ratings and interviewer ratings for depression and anxiety (Edinburgh Postnatal Depression Scale, Montgomery-Ǻsberg Depression Rating Scale; Spielberger State-Trait Anxiety Inventory, Hamilton Anxiety Inventory), as well as a standardized interview to obtain life-time trauma history. Numerous covariates were included in all regression analyses. Trauma and major depression history were robustly significant predictors of depression and anxiety ratings over the study period when these variables were analyzed individually or in a combined model including FT4 or TBG (p<.001). When analyzed alone, FT4 levels were a less strong but still significant predictor of all depression and anxiety ratings (p<.05) while TBG levels was a significant or nearly significant predictor of most ratings. FT4, TBG and trauma history, but not major depression history, were significant individual predictors of syndromal depression during the study period (p<.05) in single predictor models. In models combining each with trauma and major depression history, FT4 and TBG generally were not significantly predictive of depression or anxiety ratings, and FT4 was also not a significant predictor of syndromal depression: however, in the combined model TBG was a particularly strong predictor of perinatal syndromal depression (p=.005) and trauma history was also significant (p=.016). Further study of the interactions among TBG, FT4, sex hormones, trauma history and perinatal depression may provide insights into the pathophysiological basis of individual variance in vulnerability to mood destabilization by the hormone conditions of pregnancy

Directly pumped 10 GHz microcomb modules from low-power diode lasers

Soliton microcombs offer the prospect of advanced optical metrology and timing systems in compact form factors. In these applications, the pumping of microcombs directly from a semiconductor laser without amplification or triggering components is desirable to reduce system power and to simplify system design. At the same time, low-repetition-rate microcombs are required in many comb applications as an interface to detectors and electronics, but their increased mode volume makes them challenging to pump at low power. Here 10 GHz repetition rate soliton microcombs are directly pumped by low-power (<20  mW) diode lasers. High-Q silica microresonators are used for this low-power operation and are packaged into fiber-connectorized modules that feature temperature control for improved long-term frequency stability

Architecture for the photonic integration of an optical atomic clock

Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities to explore fundamental physics and enable new measurements. However, their size and use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 10^(13). These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks

The specific surface area and chemical composition of diamond dust near Barrow, Alaska

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95687/1/jgrd17349.pd