Optical detection of liquid-state NMR.

Nuclear magnetic resonance (NMR) in liquids and solids is primarily detected by recording the net dipolar magnetic field outside the spin-polarized sample. But the recorded bulk magnetic field itself provides only limited spatial or structural information about the sample. Most NMR applications rely therefore on more elaborate techniques such as magnetic field gradient encoding or spin correlation spectroscopy, which enable spatially resolved imaging and molecular structure analysis, respectively. Here we demonstrate a fundamentally different and intrinsically information-richer modality of detecting NMR, based on the rotation of the polarization of a laser beam by the nuclear spins in a liquid sample. Optical NMR detection has in fact a long history in atomic vapours with narrow resonance lines, but has so far only been applied to highly specialized condensed matter systems such as quantum dots. It has been predicted that laser illumination can shift NMR frequencies and thus aid detection, but the effect is very small and has never been observed. In contrast, our measurements on water and liquid 129Xe show that the complementary effect-the rotation of light polarization by nuclear spins-is readily measurable, and that it is enhanced dramatically in samples containing heavy nuclei. This approach to optical NMR detection should allow correlated optical and NMR spectroscopy on complex molecules, and continuous two-dimensional imaging of nuclear magnetization with spatial resolution limited only by light diffraction.

Zero-field remote detection of NMR with a microfabricated atomic magnetometer.

We demonstrate remote detection of nuclear magnetic resonance (NMR) with a microchip sensor consisting of a microfluidic channel and a microfabricated vapor cell (the heart of an atomic magnetometer). Detection occurs at zero magnetic field, which allows operation of the magnetometer in the spin-exchange relaxation-free (SERF) regime and increases the proximity of sensor and sample by eliminating the need for a solenoid to create a leading field. We achieve pulsed NMR linewidths of 26 Hz, limited, we believe, by the residence time and flow dispersion in the encoding region. In a fully optimized system, we estimate that for 1 s of integration, 7 x 10(13) protons in a volume of 1 mm(3), prepolarized in a 10-kG field, can be detected with a signal-to-noise ratio of approximately 3. This level of sensitivity is competitive with that demonstrated by microcoils in 100-kG magnetic fields, without requiring superconducting magnets.

Detection of NMR signals with a radio-frequency atomic magnetometer.

We demonstrate detection of proton NMR signals with a radio-frequency (rf) atomic magnetometer tuned to the NMR frequency of 62 kHz. High-frequency operation of the atomic magnetometer makes it relatively insensitive to ambient magnetic field noise. We obtain magnetic field sensitivity of 7 fT/Hz1/2 using only a thin aluminum shield. We also derive an expression for the fundamental sensitivity limit of a surface inductive pick-up coil as a function of frequency and find that an atomic rf magnetometer is intrinsically more sensitive than a coil of comparable size for frequencies below about 50 MHz.

Method for accurate measurements of nuclear-spin optical rotation for applications in correlated optical-NMR spectroscopy.

The nuclear-spin optical rotation (NSOR) effect recently attracted much attention due to potential applications in combined optical-NMR spectroscopy and imaging. Currently, the main problem with applications of NSOR is low SNR and accuracy of measurements. In this work we demonstrate a new method for data acquisition and analysis based on a low-power laser and an emphasis on software based processing. This method significantly reduces cost and is suitable for application in most NMR spectroscopy laboratories for exploration of the NSOR effect. Despite the use of low laser power, SNR can be substantially improved with fairly simple strategies including the use of short wavelength and a multi-pass optical cell with in-flow pre-polarization in a 7 T magnet. Under these conditions, we observed that NSOR signal can be detected in less than 1 min and discuss strategies for further improvement of signal. With higher SNR than previously reported, NSOR constants can be extracted with improved accuracy. On the example of water, we obtained measurements at a level of accuracy of 5%. We include a detailed theoretical analysis of the geometrical factors of the experiment, which is required for accurate quantification of NSOR. This discussion is particularly important for relatively short detection cells, which will be necessary to use in spectroscopy or imaging applications that impose geometrical constraints.

MRI with an atomic magnetometer suitable for practical imaging applications.

Conventionally implemented MRI is performed in a strong magnetic field, typically >1T. The high fields, however, can lead to many limitations. To overcome these limitations, ultra-low field (ULF) [or microtesla] MRI systems have been proposed and implemented. To-date such systems rely on low-Tc Superconducting Quantum Interference Devices (SQUIDs) leading to the requirement of cryogens. In this letter, we report ULF-MRI obtained with a non-cryogenic atomic magnetometer. This demonstration creates opportunities for developing inexpensive and widely applicable MRI scanners.

Simple method for obtaining electron scattering phase shifts from energies of an atom in a cavity.

We present a simple method for obtaining elastic scattering phase shifts and cross sections from precise ab initio many-body perturbation theory energies of atoms in variable cavities. This method does not require calculations of wave functions of continuum states, can be generalized to many atoms and ions, and is extremely convenient because existing codes developed for energy calculations can be used without modification. The high precision of the method and close agreement with experiment are illustrated on examples of e-Ar and e-Kr scattering. Correlations as well as relativistic corrections are systematically considered.

NMR detection with an atomic magnetometer.

We demonstrate detection of NMR signals using a noncryogenic atomic magnetometer and describe several novel applications of this technique. A nuclear spin-precession signal from water is detected using a spin-exchange-relaxation-free potassium magnetometer. We also demonstrate detection of less than 10(13) 129Xe atoms whose NMR signal is enhanced by a factor of 540 due to Fermi-contact interaction with K atoms. The possibility of using a multichannel atomic magnetometer for fast 3D magnetic resonance imaging is also discussed.

Nonlinear amplification of small spin precession using long-range dipolar interactions.

In measurements of small signals using spin precession the precession angle usually grows linearly in time. We show that a dynamic instability caused by spin interactions can lead to an exponentially growing spin-precession angle, amplifying small signals and raising them above the noise level of a detection system. We demonstrate amplification by a factor of greater than 8 of a spin-precession signal due to a small magnetic field gradient in a spherical cell filled with hyperpolarized liquid 129Xe. This technique can improve the sensitivity in many measurements that are limited by the noise of the detection system, rather than the fundamental spin-projection noise.

Tunable atomic magnetometer for detection of radio-frequency magnetic fields.

We describe an alkali-metal magnetometer for detection of weak magnetic fields in the radio-frequency (rf) range. High sensitivity is achieved by tuning the Zeeman resonance of alkali atoms to the rf frequency and partially suppressing spin-exchange collisions in the alkali-metal vapor. We demonstrate magnetic field sensitivity of 2 fT/Hz(1/2) at a frequency of 99 kHz with a resonance width of 400 Hz. We also derive a simple analytic expression for the fundamental limit on the sensitivity of the rf magnetometer and show that a sensitivity of about 0.01 fT/Hz(1/2) can be achieved in a practical system with a measurement volume of 200 cm3.

Numerical and experimental studies of long-range magnetic dipolar interactions.

We describe several numerical methods developed to analyze the behavior of spin polarized liquids in the presence of long-range magnetic dipolar interactions and external field gradients. Two of the methods use a discrete lattice of spins. In the first we calculate the magnetic field from the lattice of spins directly, either in the rotating frame, or in the lab frame. In the second method we include the dipolar fields from linear magnetization gradients analytically and calculate the dipolar fields from higher order gradients in Fourier space, where they are a local function of the magnetization. In the third method the magnetization is expanded in a Taylor series and the dipolar fields are calculated analytically for each term. The results of these calculations are compared to experimental data, in which we use two superconducting quantum interference device magnetometers adjacent to a spherical sample of hyperpolarized liquid 129Xe to detect the evolution of magnetization gradients. In particular, we observe an increase by a factor of 100 of the spin dephasing time in a longitudinal magnetic field gradient due to dipolar interactions of the spins. While each of the numerical techniques has certain limitations, they are generally in agreement with each other and with experimental data.