To map molecules in cells and tissue, researchers prefer biomedical imaging techniques that rely solely on the intrinsic responses of chemical bonds to optical stimulation. Although fluorescence microscopy and other chemical tagging methods yield high-resolution images, they also introduce foreign species or synthetic derivatives that can alter the dynamics of intracellular processes. Spontaneous Raman scattering, which uses a single laser beam to excite the vibrational and rotational modes in chemical bonds, requires no chemical labels but generates a weak signal that gets muddled by Rayleigh scattering. A more sensitive technique known as coherent anti-Stokes Raman scattering uses multiple laser beams to generate coherent optical signals that enhance resonant frequencies in the sample; that method, however, also produces nonresonant background noise. Recently a team led by Harvard University chemist Sunney Xie demonstrated a new technique based on stimulated Raman scattering that tunes the difference between the frequencies of two laser beams to match a desired molecule's resonant frequency, thus amplifying the Raman signal. The measurable intensities of the transmitted beams change only when a match occurs; nonresonant signals are not picked up. The images show the top view (a) and the depth profile (b) of an acne medication (blue) that penetrated a mouse's skin, thus demonstrating the potential of the new technique to monitor drug delivery. (C. W. Freudiger et al., Science 322, 1857, 2008.)
— Jermey N. A. Matthews

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Supercontinuum emission extends from the IR through the visible to the UV. As Robert Alfano and Stanley Shapiro discovered 40 years ago, one can generate supercontinuum pulses by sending bright, narrowband pulses through an optical fiber or other highly nonlinear material. Sometimes, depending on the noise, the process of a generating supercontinuum pulse also begets rare, intense pulses known as rogue waves. The artist's impression depicts the process. Ordinarily, rogue waves are sporadic and unpredictable, but if they could be produced to order, researchers would have access to bright, amplified pulses of supercontinuum light. UCLA's Daniel Solli, Claus Ropers, and Bahram Jalali have done just that, at least over a broad range in the IR. Their technique relies on sending in a second seed pulse right after the main pump pulse. The seed pulse is 10000 times weaker than the pump pulse and, with a central wavelength of 1630 nm, redshifted from the pump pulse by about 100 nm. Adjusting the relative timing of the two pulses has a dramatic effect. When timed optimally, the two pulses always generate a supercontinuum pulse of rogue-sized magnitude. What's more, the supercontinuum pulses are uniform in both intensity and spectrum. How does the technique work? Supercontinua begin from a nonlinear process called modulation instability, which produces lobes at either side of the pump pulse spectrum. In the presence of noise, the supercontinuum pulses can vary erratically from pulse to pulse, occasionally yielding a rogue wave. According to the UCLA team's numerical analysis, seed pulses tame the modulation instability and prompt the controllable formation of rogue waves. Data switching and routing in optical networks are among the applications the UCLA team envisions. (D. R. Solli, C. Ropers, B. Jalali, Phys. Rev. Lett. 101, 233902, 2008). — Charles Day

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Soft biological tissue is often subjected to forces that affect the tissue’s geometry, and finite elasticity provides a robust theoretical framework for analyzing the mechanical behavior of those tissues. Although the theory can accommodate anisotropic, nonlinear, and inhomogeneous processes subjected to large stresses and strains, its complexity makes many problems intractable. For growing tissue, though, the slow addition of cells generates shape- or size-changing stresses that are small enough to model successfully (see PHYSICS TODAY, April 2007, page 20). So, too, are simple geometries for tissues in equilibrium, even after those tissues are subjected to large stresses. Two recent papers have looked at applying the theory to those cases in thin elastic disks. In one recent study, Julien Dervaux and Martine Ben Amar (both of École Normale Supérieure, Paris) looked at anisotropic growth rates: If growth was faster in the radial than in the circumferential direction, the disk became conelike, while a reversal of rates generated saddle shapes. A separate study by Jemal Guven (National Autonomous University of Mexico) along with Martin Müller (ENS) and Ben Amar looked at excessively large circumferences for a given radius. Using the fully nonlinear theory, the researchers found an infinity of quantized equilibrium states for an ever-increasing perimeter at fixed radius. The ripples around the edge grew in size and number—not unlike the flower petals shown here—eventually crowding together enough to touch, like the ruffled collar in a portrait by Rembrandt. For more on the elasticity of thin sheets, see the article in PHYSICS TODAY, February 2007, page 33. (J. Dervaux, M. Ben Amar, Phys. Rev. Lett. 101, 068101, 2008; M. M. Mueller, M. Ben Amar, J. Guven, Phys. Rev. Lett., in press.) — Stephen G. Benka

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The recently elucidated crystal structure of a promising class of inorganic polymer salts reveals why these materials generate strong second-harmonic generation (SHG) responses to optical stimulation. In general, asymmetric inorganic polymer thin films with highly polarizable bonds exhibit strong nonlinear optical behavior, and are used in some tunable, coherent IR lasers to probe the electronic or structural properties of molecules or surfaces. A team from Northwestern University and Argonne National Laboratory used Argonne’s Advanced Photon Source to study the quaternary salts formed from the zirconium selenophosphate (ZrPSe₆^-) polyanion and its complementary metal cation (K⁺, Rb⁺, or Cs⁺)—this class of salt tends to crystallize as microneedles (see figure 1). The crystal structure (see schematic in figure 2) revealed a distortion in the molecular backbone from its ideal geometry, which contributes to the salt's high polarity. The second harmonic—a beam generated in the crystal and emitted at half the wavelength and twice the frequency of incident light—for the sample with the largest cation, Cs⁺, had an intensity 15 times that produced by a typical commercial nonlinear optical material. Even the smallest cation, K⁺, mixed with Cs⁺, produced about double the SHG response of the commercial benchmark material. The new salts exhibit strong photoluminescence in solution; they are also optically transparent from the mid- to the near-IR region, which gives them potential for use in a range of applications, from broadband communication to medical devices. (S. Banerjee et al., J. Amer. Chem. Soc. 130, 12270, 2008.) — Jermey N. A. Matthews

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A ferrofluid is a colloidal suspension of nanometer-sized magnetic particles in a nonmagnetic carrier fluid. As you might expect, it can be easily manipulated with external magnetic fields and often exhibits different patterns and instabilities. For example, when a sufficiently strong magnetic field is applied perpendicular to the flat surface of a ferrofluid, the Rosensweig instability produces a stationary array of peaks protruding above the surface. When a similar field is applied to a ferrofluid droplet immersed in a confined immiscible liquid, the labyrinthine instability produces horizontal fingering as the two fluids interpenetrate. A new experiment reveals a hybrid situation in which those two normally distinct instabilities occur simultaneously. Scientists from Taiwan and Brazil immersed a ferrofluid droplet in a thin layer of a miscible nonmagnetic fluid. The images of the experiment, with a side view in the upper panels and a top view in the lower ones, show what the researchers found after switching on the field. The Rosensweig instability grows rapidly to its greatest amplitude in 0.43 s (left panels), at which time diffusion is already affecting the base of the droplet, decreasing the magnetic body force that sustains the peak against gravity and surface tension. At 1.2 s (middle panels), the peak is clearly decaying as the fingering progresses and after 5 s (right panels) the surface is again flat and radial diffusion dominates. (C.-Y. Chen, W.-K. Tsai, J. A. Miranda, Phys. Rev. E 77, 056306, 2008 [SPIN].)    — Stephen G. Benka

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