Professor

Faculty

Department of Physics

** Research: **Atomic, molecular and optical physics, surface interactions of spin polarized atoms, magnetometry.

We are interested in the surface interactions of spin polarized alkali metal atoms and inert gas atoms. Currently we are studying the surface interactions of spin polarized Rb atoms, a good understanding of which is important to the development of miniature atomic devices such as atomic magnetometers and atomic clocks, and will also help develop new methods that are intrinsically sensitive to surface properties. The method of our study is based on the use of evanescent waves. The highlights of our study are the following.

(1) Due to surface interactions the polarization of optically pumped alkali vapor near the cell surfaces can be quite different from that in the bulk. This fact can be important in miniature atomic devices. Using evanescent waves, we have developed a method for measuring the spatial variation of the polarization with sub-micron spatial resolution in the normal direction to the cell surface, and this allows us to deduce the normal gradient coefficient of the polarization. We also measured the spatial variation of the polarization parallel to the cell surface, and this provides an interesting 2D imaging method to map the depolarization property of the surface, allowing one to "see" for the first time the "hot spots" on the coated-surface, where spin polarized Rb atoms relax faster than on other parts of the coating.

(2) Edge enhancement, the increased magnetic resonance signal due to the restriction of diffusion near a boundary, is well-known in nuclear magnetic resonance. We made a first observation of edge enhancement in electron paramagnetic resonance in optically pumped alkali (Rb) vapor. A detailed experimental and theoretical study is made of the line shape of the edge enhancement, which represents a first experimental realization of the transition from a real to a complex spectrum for a non-hermitian, but PT-invariant Hamiltonian.

(3) Based on edge enhancement, a new method has been developed for measuring the surface interaction parameters of spin polarized alkali atoms using edge enhancement and light shift. The methods should be helpful in the development of better anti-relaxation coatings.

(4) Using evanescent waves we have demonstrated a method for measuring the dwell time of spin polarized alkali atoms (Rb) on cell surfaces. The method does not depend on the microscopic details of surface interactions. The information of dwell times can help us understand anti-relaxation properties of various coatings.

(5) We demonstrated the operation of a new type of atomic magnetometer, the evanescent wave magnetometer, which will be useful in the development of sub-millimeter atomic magnetometers. Evanescent wave magnetometers have some unique features. For example, they can achieve a spatial resolution of a few tens of microns without using submillimeter glass cells. This spatial resolution is comparable to that of the sensors based on Bose-Einstein condensates. When operated in the edge enhanced mode, the line width of an evanescent wave magnetometer is independent of the cell length, and is given by zo= (σ2*D*)1/3/2, where zo=1.018 is the negative root of the first derivative of the Air function, σ is the field gradient in Hz/cm, and *D* the diffusion constant (cm2 sec-1). The two-thirds power dependence on the field gradient σ has an interesting consequence: the spatial resolution of an evanescent wave magnetometer is δz = zo(*D*/σ)1/3/2 which improves with increasing field gradient and buffer gas pressure.

**Figure 1**

**Figure 1.** Edge enhancement in an OTS-coated cell (No. 128) with adjustable thickness *L*. As *L* increases, the localized modes, which manifest themselves as enhanced magnetic resonance signals near the front and back cell surfaces, start to appear. The Larmor frequency gradient along the cell axis is 400 kHz/cm. The cell is filled with 5 Torr N2 gas and the Rb density is 4.3 x 1012 cm-3. Symbols denote experimental data points, and solid lines are calculated from theory.

**Figure 2**

**Figure 2.** Mapping silicone-coated surfaces using regionally specific 85Rb hyperfine polarization. The Rb density 2.9 x 1013 cm-3. The penetration depth of the evanescent wave is 1.4 µm. (a) Images of an area (8 mm x 10 mm) of the front and back surfaces of a cell coated according to the standard procedure. The probe beam size is 2 mm x 2 mm. The average hyperfine polarization, while having vastly different values, is quite uniform across the mapped areas on both the front and back surfaces. (b) Image of an area (7 mm x 13 mm) on a good coated surface that is intentionally damaged by high voltage discharge (a few tens kV at 500 kHz), using a Tesla coil, which has a tip in the shape of a knife edge more or less parallel to the x-direction. The probe beam size is 1.2 mm x 1.2 mm. The hyperfine polarization measured at the same penetration depth on the coated surface, before it was damaged, was 1.20 ± 0.03 across the mapped area. Thus, one sees that the lower right part of the coating in the mapped area is completely damaged whereas the top part of the coating is only slightly damaged.

## Courses Taught

General Physics I, II

Introduction to Modern Physics

Electricity and Magnetism

Quantum Mechanics

## Education

*M.A., M.Phil, Ph. D., Columbia University*## Publications

*Publications Related to the Current Research:*(1) E. Ulanski and Z. Wu,

*Measurement of dwell times of spin polarized rubidium atoms on octadecyltrichlorosilane- and paraffin-coated surfaces*. Appl. Phys. Lett.**98**, 201115 (2011).(2) K. F. Zhao, M. Schaden, and Z. Wu,

*Enhanced magnetic resonance signal of spin-polarized Rb atoms near surfaces of coated cells*, Phys. Rev. A**81**, 042903 (2010).(3) K. Zhao, M. Schaden, and Z. Wu,

*Method for measuring the dwell time of spin polarized Rb atoms on coated Pyrex glass surfaces using light shift*, Phys. Rev. Lett.**103**, 073201 (2009).(4) K. Zhao, M. Schaden, and Z. Wu,

*Method for measuring surface interaction parameters of spin polarized Rb atoms on coated Pyrex glass surfaces using edge enhancement*, Phys. Rev. A**78**, 034901 (2008).(5) K. F. Zhao, M. Schaden, and Z. Wu,

*Nonperturbative broadening of paramagnetic resonance lines by transverse magnetic field gradients*, Phys. Rev. A**78**, 013418 (2008.(6) K. F. Zhao and Z. Wu,

*Evanescent wave magnetometers with ultrathin (~100 µm)**cells*, Appl. Phys. Lett.**93**, 101101 (2008).(7) M. Schaden, K. Zhao and Z. Wu,

*Effects of surface interactions and diffusion on the lineshape of electron paramagnetic resonances in the presence of a magnetic field gradient*, Phys. Rev. A**76**, 062502 (2007);**77**, 049903(E) (2008).(8) K. Zhao and Z. Wu,

*Evanescent wave magnetometer*, Appl. Phys. Lett.**89**, 261113 (2006).(9) K. Zhao and Z. Wu,

*Regionally Specific Hyperfine Polarization of Rb in the Vicinity (~10⁻⁵ cm) of Surfaces*, Phys. Rev. A**71**, 012902 (2005).(10) K. Zhao and Z. Wu,

*Mapping Surfaces Using Regionally Specific Hyperfine Polarization*, Phys. Rev. A**70**, 010901(R) (2004).(11) K. Zhao and Z. Wu,

*Hyperfine Polarization and its Normal Gradient Coefficient of ⁸⁷Rb Atoms in the Vicinity (~10⁻⁵ cm) of Coated and Uncoated Pyrex Glass Surfaces*, Phys. Rev. Lett.**91**, 113003 (2003).(12) Z. Wu, W. Happer, J. Daniels, and M. Kitano,

*Experimental study of the coherent surface interactions of spin polarized ¹³¹Xe nuclei*, Phys. Rev. A**42**, 2774 (1990).(13) Z. Wu, S. Schaefer, G. Cates, and W. Happer,

*Coherent interactions of the polarized nuclear spins of gaseous atoms with the container walls*, Phys. Rev. A**37**, 1161 (1988).(14) Z. Wu, W. Happer, and J. Daniels,

*Coherent Nuclear-Spin Interactions of Adsorbed ¹³¹Xe Gas with Surfaces*, Phys. Rev. Lett.**59**, 1480 (1987).