Two-beam coupling in the production of quantum correlated images by four-wave mixing.

We investigate the effect of 2-beam coupling in different imaging geometries in generating intensity-difference squeezing from four-wave mixing (4WM) in Rb atomic vapors. A recently-introduced dual-seeding technique can cancel out the classical noise in a seeded four-wave mixing process. This dual-seeding technique, however, can introduce new complications that involve 2-beam coupling between different seeded spatial modes in the atomic vapor and can ruin squeezing at frequencies on the order of the atomic linewidth and below. This complicates some forms of quantum imaging using these systems. Here we show that seeding the 4WM process with skew rays can eliminate the excess noise caused by 2-beam coupling. To avoid 2-beam coupling in bright, seeded images, it is important to re-image the object in the gain medium, instead of focussing through it.

Signal advance and delay due to an optical phase-sensitive amplifier.

Fast and slow light media exploit a steep frequency dependence in their index of refraction in order to advance or delay a modulated signal. Here we observe a qualitatively similar advance and delay from an optical phase-sensitive amplifier (PSA). Unlike in the case of slow and fast light, this effect is due to a redistribution of power between imbalanced signal sidebands, and the advance or delay is dependent on the optical phase of the input. The PSA adds energy and also changes the frequency spectrum of the input. We show that the advances and delays observed in a PSA implemented using four-wave mixing in a warm rubidium vapor are consistent with the expected behavior of an ideal PSA.

Twin-beam intensity-difference squeezing below 10 Hz.

We report the generation of strong, bright-beam intensity-difference squeezing down to measurement frequencies below 10 Hz. We generate two-mode squeezing in a four-wave mixing (4WM) process in Rb vapor, where the single-pass-gain nonlinear process does not require cavity locking and only relies on passive stability. We use diode laser technology and several techniques, including dual seeding, to remove the noise introduced by seeding the 4WM process as well as the background noise. Twin-beam intensity-difference squeezing down to frequencies limited only by the mechanical and atmospheric stability of the lab is achieved. These results should enable important low-frequency applications such as direct intensity-difference imaging with bright beams on integrating detectors.

Assessing Medical Student's Ability to Interpret Traumatic Injuries on Computed Tomography Before and After the Third Year Clerkships.

INTRODUCTION: Exposure to radiologic images during clinical rotations may improve students' skill levels. This study aimed to quantify the improvement in radiographic interpretation of life-threatening traumatic injuries gained during third year clinical clerkships (MS-3). METHODS: We used a paired-sample prospective study design to compare students' accuracy in reading computed tomography (CT) images at the beginning of their third year clerkships (Phase I) and again after completion of all of their third year clerkships (Phase II). Students were shown life-threatening injuries that included head, chest, abdomen, and pelvic injuries. Overall scores for Phase II were compared with Phase I, as well as sub-scores for each anatomical region: head, chest, abdomen, and pelvis. RESULTS: Only scores from students participating in both Phase I and Phase II (N = 57) were used in the analysis. After completing their MS3 clerkship, students scored significantly better overall and in every anatomical region. Phase I and Phase II overall mean scores were 1.2 ± 1.1 vs. 4.6 ± 1.8 (p < 0.001). Students improved the most with respect to injuries of the head and chest and the area of least improvement was in interpreting CT scans of the abdomen. Although improvements in reading radiographic images were noted after the clerkship year, students accurately diagnosed only 46% of life-threatening images on CT scan in the trauma setting. CONCLUSIONS: These results indicated that enhanced education is needed for medical students to interpret CT scans.

Comparisons of Medical Student Knowledge Regarding Life-Threatening CT Images Before and After Clinical Experience.

BACKGROUND: Currently, no national standard exists for educating medical students regarding radiography or formal research indicating the level of improvement regarding computed tomography (CT) interpretation of medical students during clinical rotations. METHODS: Students were evaluated based on their response to twenty-two open-ended questions regarding diagnosis and treatment of eleven de-identified CT images of life-threatening injuries. The number of incorrect answers was compared with correct or partially correct answers between students starting third-year clinical rotations and those starting their fourth year. RESULTS: Survey results were collected from 65 of 65 (100%) beginning third-year students and 9 of 60 (15%) beginning fourth-year students. Students in their fourth-year had less incorrect answers compared to third-year students, with five questions reflecting a statistically significant reduction in incorrect responses. The image with the least incorrect for both groups was epidural hemorrhage, 33.9% and 18.5% incorrect for third-year students for diagnosis and treatment, respectively, and 11.1% and 0% incorrect for fourth-year students. Outside of this image, the range of incorrect answers for third-year students was 75.4% to 100% and 44.4% to 100% for fourth-year students. CONCLUSION: Baseline CT knowledge of medical students, regardless of clinical experience, indicated a strong deficit, as more students were incorrect than correct for the majority of CT images.

Photooxidation of self-assembled monolayers by exposure to light of wavelength 254 nm: a static SIMS study.

Self-assembled monolayers (SAMs) of alkanethiols have been photooxidized by exposure to light from a lamp emitting light with a wavelength of 254 nm. The data confirm that SAM oxidation on exposure to UV light sources occurs in the absence of ozone, but also suggest that the mechanism is different from that observed in previous studies using broad-spectrum arc lamps. In particular, for monolayers on both gold and silver, carboxylic acid-terminated SAMs oxidize significantly faster than methyl-terminated SAMs, in contrast to earlier observations for monolayers exposed to light from a mercury arc lamp. The difference in rates of photooxidation for the two classes of monolayer is significantly greater on silver than on gold. These data support our recent suggestion that while methyl-terminated SAMs are able to pack much more closely on silver than on gold, carboxylic acid-terminated thiols are not able to adopt the same close-packed structures, and their rates of photooxidation on silver are similar to, or slightly greater than, those measured for the same adsorbates on gold. Surface potential measurements were made for carboxylic acid- and methyl-terminated SAMs using a Kelvin probe apparatus. It was found that the work functions of carboxylic acid-terminated SAMs are significantly greater than those of methyl-terminated monolayers. It is concluded that these data are consistent with the oxidation reaction being initiated by "hot" electrons generated following the interaction of photons with the metallic substrate.

Friction force microscopy: towards quantitative analysis of molecular organisation with nanometre spatial resolution.

Friction force microscopy (FFM) is a technique based upon scanning force microscopy that provides information on the properties of molecular materials. Continuum mechanics provides models that may be used to conduct quantitative analyses of data. While there are some important unresolved issues associated with the contact mechanics of the tip-sample interaction, there is a growing body of data that demonstrates the sensitivity of FFM to changes in molecular organisation and surface composition. Importantly, FFM provides these data with nm spatial resolution, making it in many respects a unique tool for exploring the structures of organic materials on small length scales. Some of the capabilities of FFM are illustrated by drawing on both the literature and work performed in the authors' laboratory on self-assembled monolayers. For example, the compositions of mixed monolayer systems may be determined, with control of tip chemistry providing an additional element of chemical specificity; the alkyl chain organisation may be investigated; and the rates of surface chemical reactions may be measured. FFM is a powerful tool for the quantitative investigation of nm scale chemistry.

Chemical force microscopy of mixed self-assembled monolayers of alkanethiols on gold: evidence for phase separation.

Mixed self-assembled monolayers formed by the coadsorption of hydroxyl- and methyl-terminated alkanethiols with similar chain lengths have been characterized by friction force microscopy. Friction coefficients have been determined by assuming a fit to Amonton's law. The friction coefficients vary linearly with the fraction of polar-terminated adsorbates in the self-assembled monolayer (SAM). With carboxylic acid-terminated tips, the coefficient of friction increases with the fraction of hydroxyl-terminated thiols, while with methyl-terminated tips it decreases. Similar trends are observed for pull-off forces, which increase and decrease as a function of the fraction of polar-terminated adsorbates for carboxylic acid- and methyl-terminated adsorbates, respectively. Analysis of histograms of adhesion forces has yielded insights into the phase structure of mixed SAMs. Single-component monolayers yield histograms that may be fitted to symmetric Gaussian distributions, irrespective of the nature of the terminal group on either the tip or the SAM. However, mixed monolayers yield broad, asymmetric distributions that could not be fitted with a Gaussian distribution. The best explanation for these data is that mixed SAMs of hydroxyl- and methyl-terminated alkanethiols of similar chain length form phase-separated structures.