Practitioners in many fields, including environmental science, homeland security, and health care, are keenly interested in identifying exquisitely tiny amounts of certain gases in, for example, the atmosphere or one's breath. Laser-based techniques are useful because they can selectively probe spectral lines with great sensitivity. In the mid-IR, where many molecules have a multitude of lines, the quantum cascade laser is fast becoming the instrument of choice. (For more on QCLs, see the article in Physics Today, May 2002, page 34.) Typically, an optical cavity is loaded with the gas at millibar pressure and the laser light is finely tuned to resonate with both a single cavity mode and a spectral line of interest. In a new twist, Gottipaty Rao and Andreas Karpf (Adelphi University, New York) use gases at atmospheric pressure, combining so-called off-axis cavity enhanced spectroscopy with multiple-line integrated absorption spectroscopy, which they developed. The off-axis alignment of the laser excites a near-continuum of cavity modes in addition to reflecting many times for a long path length through the gas. After tuning the QCL to a dense spectral region, the spectroscopists enhance the sensitivity by integrating over the resulting absorption spectrum, which includes more than 100 transitions. Their proof-of-principle detection of nitrogen dioxide has a sensitivity of 28 parts per trillion, which compares favorably with other laser-based systems. Unlike other systems, this one works well at high pressure, is less sensitive to instrumental vibrations, does not require fast electronics, and is effective for molecules with dense, poorly resolved spectra. (G. N. Rao, A. Karpf, Appl. Opt. 50, 1915, 2011.)—Stephen G. Benka
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Fiber lasers are commonly run with their frequency modes locked in phase by a saturable absorber, an optical element opaque to light below a threshold intensity but increasingly transparent above it. The mode locker enforces pulsed operation at a repetition rate inversely proportional to the cavity length. Fiber lasers typically run at tens of megahertz because of the long fiber lengths—on the scale of meters—needed to accumulate gain. But for portable metrology and data-transmission devices, researchers are striving to push that pulse rate higher. The University of Tokyo’s Amos Martinez and Shinji Yamashita now report the latest milestone in that effort: the development of an erbium-doped fiber laser that delivers 20-GHz pulses from a cavity just 5 mm long, as shown here. Key to the achievement are co-doping the fiber with ytterbium and incorporating carbon nanotubes into the laser cavity. Ytterbium’s absorption cross section is two orders of magnitude greater than that of Er3+ and thus it generates high gain over short lengths. Thanks to the nanotubes’ subpicosecond charge-carrier dynamics, low losses, and essentially negligible space requirement, a thin film of them functions as a nearly ideal saturable absorber when sprayed onto one of two Fabry–Pérot mirrors that form the cavity. As a demonstration of the laser’s applicability, Martinez and Yamashita use its ultrashort pulses to generate a broadband spectrum of frequencies that may be used as a precise frequency comb. (A. Martinez, S. Yamashita, Opt. Express 19, 6155, 2011.)—R. Mark Wilson
Are there any unexpected differences between matter and antimatter? The international ALPHA collaboration has taken an important step toward answering that question by constructing an apparatus at CERN that can confine freshly made atoms of antihydrogen, the bound state of an antiproton and a positron, for nearly 0.2 seconds—long enough for the antimatter to be examined spectroscopically. A hot plasma of roughly 104 antiprotons—produced by slamming 26-GeV protons into a metal target—is cooled and introduced into one end of the apparatus, while about 106 low-energy positrons from the decay of radioactive sodium are introduced into the other. Electric fields gently nudge the charged species together in the heart of the device, pictured here, where they mix at cryogenic temperatures and form antihydrogen. If their kinetic energies are low enough—in temperature units, less than 0.5 K—the antihydrogen atoms are held in the grip of a superconducting octupole magnet and solenoidal “mirror” coils that together interact with the atoms’ magnetic moments. When the magnetic fields are abruptly turned off, the atoms are released and their spatial distribution captured by a three-layer silicon detector, which locates the atoms’ annihilations and distinguishes them from events triggered by lone antiprotons and stray cosmic rays. In 335 trial runs, the researchers confirmed that 38 antihydrogen atoms had survived in the trap for at least 172 ms. Although the trapping rate per atom produced is low—about 10−5—the achievement sets the stage for precision spectroscopy and antihydrogen tests of fundamental symmetries and gravitation. (G. B. Andresen et al., Nature 468, 673, 2010.)—R. Mark Wilson
Since its advent 20 years ago, room-temperature optical detection of single molecules has found application in biology, materials science, and other fields. Typically, the molecules are detected by their fluorescence, but not all molecules fluoresce. Now, three groups have independently detected single molecules by their optical absorption. Each group used a different technique. Vahid Sandoghdar and his colleagues at ETH Zürich in Switzerland measured the absorption directly, by detecting the minute intensity change of a laser beam passing through the sample. Michel Orrit and his colleagues at Leiden University in the Netherlands used a two-laser technique based on the photothermal effect: The molecule to be detected absorbed photons from the first laser and converted their energy to heat, thereby changing the refractive index in the surrounding material. The researchers detected the scattering of a second laser beam off that refractive-index inhomogeneity. And Sunney Xie and his colleagues at Harvard University used a different two-laser technique called ground-state depletion microscopy. One laser, whose amplitude was modulated at 1.75 MHz, repeatedly pumped the molecule out of its ground state, which caused the absorption of a second laser to be modulated at the same frequency. By isolating the high-frequency fluctuations of the second laser, the researchers separated the absorption signal from the laser’s inevitable intensity noise. (P. Kukura, M. Celebrano, A. Renn, V. Sandoghdar, J. Phys. Chem. Lett 1, 3323, 2010; A. Gaiduk, M. Yorulmaz, P. V. Ruijgrok, M. Orrit, Science 330, 353, 2010; S. Chong, W. Min, X. S. Xie, J. Phys. Chem. Lett. 1, 3316, 2010.)—Johanna Miller
Like their optical-tweezer cousins, magnetic tweezers have become a standard tool for stretching individual biological molecules to gain insight into their physical properties and behavior. Magnetic tweezers (and optical ones, too) can also apply torques to rotate or twist specimens. Last year, Sean Sun of the Johns Hopkins University and colleagues developed a technique to control and measure the applied torque with magnetic tweezers. In their approach a biomolecule, such as DNA, was connected to the middle of a 2-µm-long nanorod that was magnetically latched at one end to a superparamagnetic bead. In a dipole magnetic field, the bead held the nanorod and the top of the molecule in place while the experimenters introduced a controlled amount of twist by rotating the substrate bound to the molecule's other end. Nynke Dekker and her coworkers at the Delft University of Technology have now presented a new take on magnetic torque tweezers, this time using standard spherical beads. A nonmagnetic bead 1 µm in diameter served as a landmark on the 2.8-µm-diameter superparamagnetic bead to which it was attached (a third, nonmagnetic reference bead corrected for mechanical drift). The larger bead was tethered to a substrate by a single DNA molecule, and the beads were placed in a slightly asymmetric dipole magnetic field. Rotating the magnet twisted the DNA, and by monitoring the beads' orientation the team could extract the DNA's torsional stiffness and the torque. With their tweezer setup, Dekker and colleagues could document torque-induced twisting, buckling, and denaturing of DNA under a wide range of torques and stretching forces and could study protein–DNA interactions. (J. Lipfert et al., Nat. Meth., in press, doi:10.1038/nmeth.1520.)—Richard J. Fitzgerald
There’s no reason to think that the lengthening of time predicted by the theory of relativity does not hold even for everyday speeds, but the effect is so minuscule that it has taken two of the world’s most accurate optical clocks to measure it. Trapped at the heart of each clock is an aluminum ion: Displacing the ion in one clock just slightly from the trap’s center induces a relative average speed difference between the two clocks’ ions of 10 m/s (22 mile/hr). The resulting fractional frequency difference is on the order of 10−16. That small shift was measured recently by James Chin-Wen Chou and his colleagues at NIST in Boulder, Colorado, using NIST’s newest optical clock, whose accuracy is 8.6 x 10−18, and a slightly less accurate older clock. The group also measured the frequency difference between the clocks’ ions when one clock was raised by 33 cm relative to the other. The measurements not only demonstrate the high performance reached by optical clocks but also show that they may play an important role in geodesy, the measurement of Earth’s gravitational potential. The time keepers might be sensitive to elevation changes as small as 1 cm if they can attain the current goal of 10−18 accuracy. Such measurements would complement those of satellite-borne instruments, which also have 1-cm sensitivity but average over large areas of Earth’s surface. (C. W. Chou et al., Science 329, 1630, 2010.)—Barbara Goss Levi
The public-health and biosecurity communities need biosensors that are sensitive, operate in real time, and can be easily deployed. Many approaches are being pursued, including one by a group from Lawrence Livermore National Laboratory that uses a photonic crystal (PC) slab connected to two waveguides. The idea is for viruses or other tiny pathogens to randomly infiltrate the pores of a silicon PC, whose optical properties—specifically, the in-plane transmission spectrum's band edge—change accordingly. First, the researchers used simulations to determine a PC geometry suitable for a specific virus, vaccinia, and for their laser's wavelength. They then fabricated an appropriate 17 × 17 array of 280-nm pores and exposed it to a flux of polystyrene beads with two different sizes; those with 260-nm diameter entered the pores (see the figure, with empty and filled pores enlarged) while 320-nm ones did not. The measured band-edge redshifts were then used to calibrate the simulations and predict detection limits. Theoretically, as few as 10 vaccinia viruses could be detected with the PC, comparable to other biodetection methods. Advantages accrue, however, from the small sensor size, the ability to tune the geometry for different particles, and the ease of integration into lab-on-a-chip setups. The authors say that their random-binding scheme is more practical than methods that rely on binding organisms to single PC defects. (S. E. Baker et al., Appl. Phys. Lett. 97, 113701, 2010.)—Stephen G. Benka
X-ray diffraction has long been an important tool for finding crystal structures by mapping their electron densities. In recent decades, time-resolved x-ray diffraction has probed ever-faster structural changes in single crystals, including atomic motions on the femtosecond time scale. But many materials of interest, such as the transition metal complexes used in organic photovoltaic cells, can’t easily be made into crystals of sufficient size and quality. Now Michael Woerner, Thomas Elsaesser, and colleagues at the Max Born Institute in Berlin have demonstrated femtosecond x-ray powder diffraction, in which the sample is an ensemble of randomly oriented microcrystals of ammonium sulfate, (NH4)2SO4, and the diffraction pattern is composed of concentric rings rather than discrete peaks. The innovation was in engineering the x-ray source—a laser-driven plasma source that produced an ultrafast x-ray pulse from an equally brief optical pulse—to operate stably and at high repetition rate for long enough to reveal small changes in the diffraction ring intensities. From those changes, the researchers calculated the change in the sample’s electron density. As shown in the figure, which depicts the equilibrium electron density and the resulting changes over one slice through the crystal, electrons briefly pool (red blobs) where no nucleus exists in the equilibrium structure—so a nucleus, specifically a proton, must have migrated there. Ultrafast IR spectroscopy confirmed that NH4+ ions were reversibly breaking apart; surprisingly, the observed structural change bears no resemblance to either of ammonium sulfate’s known phase transitions. (M. Woerner et al., J. Chem. Phys., in press.)—Johanna Miller
Just as accelerating charge produces electromagnetic radiation, accelerating mass is predicted to produce gravitational radiation. The effect of a gravitational wave’s alternating distortions of space could be detected by a Michelson interferometer, but gravity’s weakness means that extraordinary sensitivity is needed to observe even a relatively intense wave. The Laser Interferometer Space Antenna (LISA) is a proposed mission to achieve that sensitivity with an interferometer 5 million km on a side, its vertices located on three spacecraft orbiting the Sun. But the inevitable fluctuations in laser frequencies introduce noise a billion times more intense than the signal from the gravitational waves researchers hope to see. The solution, a technique called time-delay interferometry (TDI), is to reduce noise not through better stabilization of the physical components but by signal processing. In essence, the phase gained by light traversing each arm is subtracted from the phase from the same arm offset by the round-trip time for the other arm; subtracting those two differences then yields a quantity unmarred by laser frequency fluctuations. In two more steps, noise due to clock error can likewise be eliminated. Now, researchers at NASA’s Jet Propulsion Laboratory have demonstrated TDI in a laboratory experiment designed to mimic LISA’s noise environment. They’ve shown that the technique can indeed reduce laser frequency noise and clock noise by the necessary nine orders of magnitude. (G. de Vine et al., Phys. Rev. Lett. 104, 211103, 2010.)—Johanna Miller
Much of the light emitted from stars and other astrophysical objects is absorbed by dust and reemitted at far-IR or submillimeter wavelengths—radiation that is notoriously difficult to detect. Last year researchers from the Jet Propulsion Laboratory proposed a new type of detector for that regime, with an eye toward future, more sensitive space missions. The team has now built a prototype microdevice (see figure), called a quantum capacitance detector (QCD), which would be one pixel in an eventual array. The detection chain goes like this: Photons are received at an antenna and fed into a superconducting absorber where they break Cooper pairs and generate quasiparticles. A superconducting island, called a single Cooper-pair box (SCB), is connected to the absorber in such a way that, at most, one quasiparticle at a time can tunnel onto it; that changes the island’s capacitance, which is so small that the charging energy of a single electron has a large effect. With a resonant circuit, the physicists monitor the frequency of capacitance changes from which they can determine the density of quasiparticles in the absorber and thus the photon flux at the antenna. The device’s performance is already comparable to that of other superconducting detectors. The advantage of the QCD, say the researchers, is the ease with which it can be read out from an array of detectors. For example, each pixel detector could be fabricated with a different resonance and simultaneous readout could be done with a frequency comb. (J. Bueno et al., Appl. Phys. Lett. 96, 103503, 2010.) —Stephen G. Benka
One of the hallmarks of lasing is a dramatic narrowing of the light's frequency spread. In 1958 Arthur Schawlow and Charles Townes deduced that the laser linewidth is fundamentally limited by unavoidable spontaneous emission. (Thanks to other sources of noise, a real laser's linewidth is usually considerably broader.) Semiconductor diode lasers required a revision of the intrinsic linewidth formula to account for additional inherent broadening, but quantum cascade lasers (described in Physics Today, May 2002, page 34) had been thought to obey the original limit. Now Saverio Bartalini and colleagues at Italy's National Institute of Optics–CNR, the European Laboratory for Non-linear Spectroscopy, and the Second University of Naples have confirmed a recent theory predicting that QCLs can in fact beat the Schawlow–Townes limit and yield significantly improved spectral purity. Key to the 2008 theory by Masamichi Yamanishi and coworkers at Hamamatsu Photonics was the recognition that nonradiative transitions in QCLs strongly suppress spontaneous emission. To test the prediction, the Italian researchers tuned their IR QCL to be halfway down a carbon dioxide absorption peak at 4.33 µm (69.3 THz). Thanks to the steep slope of the absorption curve there, frequency fluctuations were converted into detectable intensity variations. That technique enabled the team to measure the noise spectrum over seven decades of frequency and to extract the intrinsic QCL linewidths for various pump currents. The obtained widths, in the range of 500 Hz, agreed well with the new theory and were three orders of magnitude smaller than predicted by the venerable Schawlow–Townes formula. (S. Bartalini et al., Phys. Rev. Lett. 104, 083904, 2010.) —Richard J. Fitzgerald
Colossal magnetoresistance is aptly named. By subjecting a piece of appropriately doped manganite to a strong magnetic field and a moderately low temperature, one can raise its electrical conductivity by 10 000%. Despite its prodigious magnitude, CMR has not led to any commercial devices since its discovery 15 years ago. Still, it continues to fascinate physicists. At its most basic level, CMR arises as a paramagnetic insulating phase yields to a ferromagnetic conducting phase. But evidence suggests that a third, charge-ordered antiferromagnetic phase could play a role too. To elucidate the issue, Jing Tao of Brookhaven National Laboratory and her collaborators developed a new experimental technique. Called scanning electron nanoscale diffraction (SEND), their technique combines electron diffraction's ability to reveal the presence of ordered structures with scanning microscopy's ability to reveal those structures' real-space distribution. The patches in the figure correspond to charge-ordered regions. As the temperature approaches the 253-K CMR transition, the volume occupied by the charge-ordered phase increases. Simulations by Elbio Dagotto of Oak Ridge National Laboratory and his collaborators suggest an explanation: The charge-ordered phase vies with the ferromagnetic phase to become the predominant phase below the transition temperature. Although it loses the battle, the charge-ordered phase nevertheless delays and thereby intensifies the onset of CMR. (J. Tao et al., Phys. Rev. Lett. 103, 097202, 2009.)—Charles Day
