Imaging quantum confinement with optical and POWER (perturbations observed with enhanced resolution) NMR

Evidence for the light hole in GaAs/AlGaAs quantum wells from optically-pumped NMR and Hanle curve measurements

Optically-pumped (69)Ga NMR (OPNMR) and optically-detected measurements of polarized photoluminescence (Hanle curves) show a characteristic feature at the light hole-to-conduction band transition in a GaAs/AlxGa1-xAs multiple quantum well sample. OPNMR data are often depicted as a "profile" of the OPNMR integrated signal intensity plotted versus optical pumping photon energy. What is notable is the inversion of the sign of the measured (69)Ga OPNMR signals when optically pumping this light hole-to-conduction band energy in OPNMR profiles at multiple external magnetic fields (B0=4.7T and 3T) for both σ(+) and σ(-) irradiation. Measurements of Hanle curves at B0=0.5T of the same sample exhibit similar phase inversion behavior of the Hanle curves at the photon energy for light hole excitation. The zero-field value of the light-hole state in the quantum well can be predicted for the quantum well structure using the positions of each of these signal-inversion features, and the spin splitting term in the equation for the transition energy yields consistent values at 3 magnetic fields for the excitonic g-factor (g(ex)). This study demonstrates the application of OPNMR and optical measurements of the photoluminescence to detect the light hole transition in semiconductors.

High-gradient nanomagnets on cantilevers for sensitive detection of nuclear magnetic resonance

Detection of magnetic resonance as a force between a magnetic tip and nuclear spins has previously been shown to enable sub-10 nm resolution 1H imaging. Maximizing the spin force in such a magnetic resonance force microscopy (MRFM) experiment demands a high field gradient. In order to study a wide range of samples, it is equally desirable to locate the magnetic tip on the force sensor. Here we report the development of attonewton-sensitivity cantilevers with high-gradient cobalt nanomagnet tips. The damage layer thickness and saturation magnetization of the magnetic material were characterized by X-ray photoelectron spectroscopy and superconducting quantum interference device magnetometry. The coercive field and saturation magnetization of an individual tip were quantified in situ using frequency-shift cantilever magnetometry. Measurements of cantilever dissipation versus magnetic field and tip–sample separation were conducted. MRFM signals from protons in a polystyrene film were studied versus rf irradiation frequency and tip–sample separation, and from this data the tip field and tip-field gradient were evaluated. Magnetic tip performance was assessed by numerically modeling the frequency dependence of the magnetic resonance signal. We observed a tip-field gradient ∂B(z)(tip)/∂z estimated to be between 4.4 and 5.4 MT m(–1), which is comparable to the gradient used in recent 4 nm resolution 1H imaging experiments and larger by nearly an order of magnitude than the gradient achieved in prior magnet-on-cantilever MRFM experiments.

Coherent NMR Stark spectroscopy

We demonstrate phase-coherent Stark effects from a radiofrequency E field at twice the NMR frequency (2ω(0)) of (69)Ga in GaAs. The 2ω(0) phase (ϕ(E)) selects component responses from the nuclear quadrupole Hamiltonian (H(Q)). This is possible by synchronizing few-μs 2ω(0) pulses with an NMR line-narrowing sequence, which averages the Stark interaction to dominate spectra on a background with 10(3)× enhanced resolution. Spectra vs ϕ(E) reveal relative sizes of tensorial factors in H(Q). Comparative modeling and numerical simulations evaluate spectral features unexplained by average Hamiltonian theory, and suggest improvements for quantitative calibration of individual response components. Application of this approach to bulk samples is of value to define Stark responses that may later be used to interrogate the internal electrostatics of structured samples.

Using optical markers of nondysplastic rectal epithelial cells to identify patients with ulcerative colitis-associated neoplasia

BACKGROUND

Current surveillance guidelines for patients with long-standing ulcerative colitis (UC) recommend repeated colonoscopy with random biopsies, which is time-consuming, discomforting, and expensive. A less invasive strategy is to identify neoplasia by analyzing biomarkers from the more accessible rectum to predict the need for a full colonoscopy. The goal of this pilot study was to evaluate whether optical markers of rectal mucosa derived from a novel optical technique, partial-wave spectroscopic microscopy (PWS), could identify UC patients with high-grade dysplasia (HGD) or cancer (CA) present anywhere in their colon.

METHODS

Banked frozen nondysplastic mucosal rectal biopsies were used from 28 UC patients (15 without dysplasia and 13 with concurrent HGD or CA). The specimen slides were made using a touch prep method and underwent PWS analysis. We divided the patients into two groups: 13 as a training set and an independent 15 as a validation set.

RESULTS

We identified six optical markers, ranked by measuring the information gain with respect to the outcome of cancer. The most effective markers were selected by maximizing the cross-validated training accuracy of a Naive Bayes classifier. The optimal classifier was applied to the validation data yielding 100% sensitivity and 75% specificity.

CONCLUSIONS

Our results indicate that the PWS-derived optical markers can accurately predict UC patients with HGD/CA through assessment of rectal epithelial cells. By aiming for high sensitivity, our approach could potentially simplify the surveillance of UC patients and improve overall resource utilization by identifying patients with HGD/CA who should proceed with colonoscopy.

Quantitative calibration of radiofrequency NMR Stark effects

Nuclear magnetic resonance (NMR) Stark responses can occur in quadrupolar nuclei for an electric field oscillating at twice the usual NMR frequency (2ω(0)). Calibration of responses to an applied E field is needed to establish nuclear spins as probes of native E fields within material and molecular systems. We present an improved approach and apparatus for accurate measurement of quadrupolar Stark effects. Updated values of C(14) (the response parameter in cubic crystals) were obtained for both (69)Ga and (75)As in GaAs. Keys to improvement include a modified implementation of voltage dividers to assess the 2ω(0) amplitude, |E|, and the stabilization of divider response by reduction of stray couplings in 2ω(0) circuitry. Finally, accuracy was enhanced by filtering sets of |E| through a linear response function that we established for the radiofrequency amplifier. Our approach is verified by two types of spectral results. Steady-state 2ω(0) excitation to presaturate NMR spectra yielded C(14) = (2.59 ± 0.06) × 10(12) m(-1) for (69)Ga at room-temperature and 14.1 T. For (75)As, we obtained (3.1 ± 0.1) × 10(12) m(-1). Both values reconcile with earlier results from 77 K and below 1 T, whereas current experiments are at room temperature and 14.1 T. Finally, we present results where few-microsecond pulses of the 2ω(0) field induced small (tens of Hz) changes in high-resolution NMR line shapes. There too, spectra collected vs |E| agree with the model for response, further establishing the validity of our protocols to specify |E|.

Quantitative Carré differential interference contrast microscopy to assess phase and amplitude

We present a method of using an unmodified differential interference contrast microscope to acquire quantitative information on scatter and absorption of thin tissue samples. A simple calibration process is discussed that uses a standard optical wedge. Subsequently, we present a phase-stepping procedure for acquiring phase gradient information exclusive of absorption effects. The procedure results in two-dimensional maps of the local angular (polar and azimuthal) ray deviation. We demonstrate the calibration process, discuss details of the phase-stepping algorithm, and present representative results for a porcine skin sample.

Enhancing time-suspension sequences for the measurement of weak perturbations

We detail key features for implementation of time-suspension multiple-pulse line-narrowing sequences. This sequence class is designed to null the average Hamiltonian (H¯(⁰)) over the period of the multiple-pulse cycle, typically to provide for high-resolution isolation of evolution from a switched interaction, such as field gradients for imaging or small sample perturbations. Sequence designs to further ensure null contributions from correction terms (H¯((¹)) and H¯(²)) of the Magnus expansion are also well known, as are a variety of approaches to second averaging, the process by which diagonal content is incorporated in H¯(⁰) to truncate unwanted terms. In spite of such designs, we observed spin evolution not explicable by H¯(⁰) using 16-, 24- and 48-pulse time-suspension sequences. We found three approaches to effectively remove artifacts that included splitting of the lineshape into unexpected multiplets as well as chirped evolution. The noted approaches are simultaneously compatible for combination of their benefits. The first ensures constant power deposition from RF excitation as the evolution period is incremented. This removes chirping and allows more effective 2nd averaging. Two schemes for the latter are evaluated: the noted introduction of a diagonal term in H¯(⁰), and phase-stepping the line-narrowing sequence on successive instances during the evolution period. Either of these was sufficient to remove artifactual splittings and to further enhance resolution, while in combination enhancements were maintained. Finally, numerical simulations provide evidence that our experimental line-narrowing results with ⁷⁵As in crystalline GaAs approach performance limits of idealized sequences (e.g., with ideal square pulses, etc.). The three noted experimental techniques should likewise benefit ultimate implementation with switched interactions and corresponding new error contributions, which place further demand on sequence performance.

Radiofrequency quadrupolar NMR stark spectroscopy: steady state response calibration and tensorial mapping

Radiofrequency electric (E) fields oscillating at twice the usual NMR frequency (2ω(0)) can induce double-quantum transitions in quadrupolar nuclei, an NMR Stark effect. Characterization of such is of interest to aid understanding of electrostatic effects in NMR spectra. Calibration of Stark responses to an applied electric field may also be used to assess native fields within molecules and materials. We present high-field (14.1 T), room-temperature NMR experiments to calibrate the 2ω(0) Stark response in crystalline GaAs. This system presents an important test of current techniques and conditions, as historical studies at low field (500-900 mT) and low temperature (77 K) provide a basis for comparison. Our measurements of steady state response reveal the quadrupolar Stark tuning rate for (69)Ga in this material. The value, β(Q) = (11.5 ± 0.1) × 10(12) m(-1), is 3.6 times larger than the most-reliable prior result. In the process, we also uncovered a previously unobserved double-quantum steady state coherence. It appears as a completely separable dispersive signal component in quadrature-detected presaturation spectra versus offset from 2ω(0). The new component may eventually afford an independent route to calibrating β(Q). Finally, we demonstrated exceptional agreement with theory of the orientation-dependent Stark response for rotation of the sample relative to B(0) over a range of 90° and for E-field amplitudes from 30-180 V/cm.

Study of back-scattering microspectrum for stomach cells at single-cell scale

A fiber confocal backscattering (FCBS) spectrometer is developed based on fiber confocal microscopy and light scattering theory. The FCBS spectrometer can provide imaging and spectral information simultaneously at the cellular scale. Normal stomach epithelial cell line GES-1 and cancerous cell line NCI-N87 are measured and their spectral results show that backscattering intensity from NCI-N87 cells is stronger than that from GES-1 cells in 500 to 800 nm, and the GES-1 cells scattering spectra show regular intensity changes, while the NCI-N87 cells do not. The experiments prove that the FCBS spectrometer is able to distinguish cancerous cells from normal stomach cells at the cellular level. The spectrometer could be further developed into a noninvasive optical technology for early cancer detection.

Prospects for sub-micron solid state nuclear magnetic resonance imaging with low-temperature dynamic nuclear polarization

We evaluate the feasibility of (1)H nuclear magnetic resonance (NMR) imaging with sub-micron voxel dimensions using a combination of low temperatures and dynamic nuclear polarization (DNP). Experiments are performed on nitroxide-doped glycerol-water at 9.4 T and temperatures below 40 K, using a 30 mW tunable microwave source for DNP. With DNP at 7 K, a 0.5 microL sample yields a (1)H NMR signal-to-noise ratio of 770 in two scans with pulsed spin-lock detection and after 80 db signal attenuation. With reasonable extrapolations, we infer that (1)H NMR signals from 1 microm(3) voxel volumes should be readily detectable, and voxels as small as 0.03 microm(3) may eventually be detectable. Through homonuclear decoupling with a frequency-switched Lee-Goldburg spin echo technique, we obtain 830 Hz (1)H NMR linewidths at low temperatures, implying that pulsed field gradients equal to 0.4 G/d or less would be required during spatial encoding dimensions of an imaging sequence, where d is the resolution in each dimension.

Quasi one-dimensional light beam generated by a graded-index microsphere

An optically illuminated micron-scale dielectric sphere can generate a photonic nanojet - a nonresonant propagating beam phenomenon of high amplitude, narrow waist, and substantial sensitivity to the presence of nanometer-scale particles and geometric features located within the beam. Via three-dimensional finite-difference time-domain computational electrodynamics modeling of illuminated graded-index microspheres, we have found that the useful length of a photonic nanojet can be increased by an order-of-magnitude to approximately 20 wavelengths. This is effectively a quasi one-dimensional light beam which may be useful for optical detection of natural or artificially introduced nanostructures deeply embedded within biological cells. Of particular interest in this regard is a potential application to visible-light detection of nanometer-scale anomalies within biological cells indicative of early-stage cancer.

Partial-wave microscopic spectroscopy detects subwavelength refractive index fluctuations: an application to cancer diagnosis

Existing optical imaging techniques offer us powerful tools to directly visualize the cellular structure at the microscale; however, their capability of nanoscale sensitivity is restricted by the diffraction-limited resolution. We show that the mesoscopic light transport theory analysis of the spectra of partial waves propagating within a weakly disordered medium, such as biological cells [i.e., partial wave spectroscopy (PWS)] quantifies refractive index fluctuations at subdiffractional length scales. We validate this nanoscale sensitivity of PWS using experiments with nanostructured models. We also demonstrate the potential of this technique to detect nanoscale alterations in cells from patients with pancreatic cancer who are otherwise classified as normal by conventional microscopic histopathology.