Biophotonics: Glucose Detection

research highlights

MATERIALS to the electrons through a second-harmonic the processes of absorption and emission Topological electrodes interaction, which can have an azimuthal into a single coherent event, the researchers Nature Chem. 4, 281–286 (2012) dependence on the laser mode profile. The decreased the excitation power so that researchers passed a short 3 ps electron the Rabi frequency was less than a fifth bunch (enery of around 12 MeV) from of the spontaneous emission rate. Strong the Neptune Inverse Free Electron Laser antibunching in the intensity-correlation through a 1.9-cm-period helical undulator measurement suggests that the generated together with a 10.6 μm, 100 ps pulse from photons were non-classical in nature. The

a CO2 laser. They measured the coherent researchers measured a coherence time of transition radiation emitted from the electron 22 ns — 30 times longer than the lifetime of beam by focusing the output into a liquid- the quantum dot transition — through field- nitrogen-cooled mercury cadmium telluride correlation measurements using a Michelson detector. The data confirmed the existence interferometer. The single photons exhibited of microbunching and gave a measure of the a linewidth of 7 MHz, which is a metric process efficiency. DP inherited from the excitation laser. NH © 2012 NPG PLASMONICS OPTOELECTRONICS Topological insulators, materials that have a Laser-written conductors Double-layer graphene boost conductive surface but an insulative interior, Adv. Func. Mater. http://dx.doi.org/10.1002/ Nano Lett. 12, 1482–1485 (2012) have attracted much attention in recent years adfm.201102665 (2012) owing to their intriguing properties. Scientists have now shown that topological insulators Shining green laser light onto nanostructured could be used as transparent flexible electrodes gold or silver films coated with amorphous for next-generation optoelectronics operating carbon can induce a phase transformation in the near-infrared regime. Hailin Peng that makes the carbon highly conductive, and co-workers from Peking University, according to researchers from the the University of Oxford and Stanford Georgia Institute of Technology in the USA.

University fabricated thin, multilayer Bi2Se3 The nanostructures exhibit a plasmonic © 2012 ACS nanostructures onto mica substrates by van resonance that strongly absorbs 514 nm laser der Waals epitaxy. The resulting nanosheets light, causing intense localized heating of The strength of graphene’s optical absorption functioned as transparent conductors with the carbon and a resulting phase change into can be tuned by controlling its charge carrier sheet resistances as low as 330 Ω ☐–1 and a nanocrystalline or disordered graphitic density, which makes it promising for use as transparencies exceeding 70% for wavelengths phase, as confirmed by Raman spectroscopy a miniature broadband optical modulator. of 1–3 μm. Tests indicated a high degree of and atomic force microscopy. Conductive Ming Liu and co-workers from the University mechanical robustness, with the nanosheet force microscopy suggests that the changed of California in Berkeley and Lawrence electrodes proving durable to 1,000 bending state offers an electrical resistivity that is at Berkeley National Lab in the USA have now

cycles. The researchers say that the Bi2Se3 least seven orders of magnitude lower than presented a device design that comprises electrodes could prove useful for applications the initial insulating surface or unexposed a 400-nm-wide, 40-μm-long silicon wire

such as infrared imaging and sensing, near- regions. In contrast, tests with a reference waveguide covered by a graphene–Al2O3– infrared solar cells and optoelectronic devices silicon substrate showed no changes in graphene sandwich, which functions as for optical communication. OG morphology or conductivity, which indicates a p–oxide–n junction. Grating couplers the importance of the metal nanostructures with a period of 780 nm are used to couple X-RAYS for the phase change. The researchers 1,537 nm light in and out of the device. The Orbital angular momentum suggest that the technique might be useful researchers report a modulation depth of up Appl. Phys. Lett. 100, 091110 (2012) for fabricating complex patterns of metal– to 6.5 dB and a peak transmission of −25 dB, carbon electrical interconnects. OG with most of the loss being due to the grating Erik Hemsing and colleagues from the couplers. Their measurements also indicate University of California Los Angeles in the QUANTUM DOTS that the modulator has a 3 dB bandwidth of USA have presented evidence of the helical Superior single photons 1 GHz and a modulation depth of around microbunching of charge within an electron Phys. Rev. Lett. 108, 093602 (2012) 0.16 dB μm–1 per unit length at a drive voltage beam. Previous theory suggested that such of approximately 5 V. The researchers say beams could be realized in principle, but an Clemens Matthiesen and co-workers from the that using multiple pairs of double-layer experimental demonstration was lacking until University of Cambridge and the University graphene could further reduce the footprint now. The ability to induce microbunching of Rochester have reported the emission and energy consumption of the device. RW may ultimately lead to the development of highly coherent (sub-natural linewidth) of high-gain, high-mode-generation free- single photons from InAs quantum dots. The BIOPHOTONICS electron lasers capable of emitting light with researchers used a confocal microscope to Glucose detection orbital angular momentum. For this to be collect optically excited resonant fluorescent Biomed. Opt. Express 3, 667–680 (2012) possible, the electrons must be arranged spectra at a temperature of 4 K. The InAs with a spiral period equal to the wavelength quantum dots were embedded in a Schottky Diabetes is a widespread disease that affects of light emitted from the free-electron laser. diode heterostructure, which allowed 346 million people around the world and Theory suggested this could be achieved by their excitonic energy levels to be tuned causes an estimated 3.4 million deaths tuning the laser such that it imparts energy through the d.c. Stark effect. To combine every year. Although glucose levels can be

measured through invasive procedures atomic force microscope tip as both a local MICROFLUIDICS such as polarimetry, Raman spectroscopy thermal source and a scattering probe, and High-speed cell sorter and photoacoustic spectroscopy, scientists employed a Michelson-type interferometer Lab Chip 12, 1378–1383 (2012) have so far been unsuccessful in realizing a to perform spectral analysis in the mid- reliable non-invasive alternative monitoring infrared. They observed an enhanced spectral technique. Markus Sigrist and colleagues energy density in the near-field associated in Switzerland have now developed a mid- with phonon, phonon–polariton and infrared photoacoustic technique that can vibrational resonances. The researchers told be used to track glucose by analysing deep Nature Photonics that the findings may have epidermal layers in the skin. Their set-up wide implications, ranging from chemical comprises an external-cavity quantum cascade vibrational nano-imaging without the laser (tuning range of 1,010–1,095 cm–1) need for an external light source, to a better and a 78 mm3 photoacoustic cell for understanding of nanoscale heat transport detection. They controlled the glucose levels and the origin of the Casimir force. RW of epidermal skin samples housed in the photoacoustic cell by introducing aqueous QUANTUM OPTICS glucose solutions of different concentrations Multimode frequency combs in the range of 1–100 g l–1. The detection limit Phys. Rev. Lett. 108, 083601 (2012) of this technique is around 1 g l–1, which, although within the physiological range Optical frequency combs, perfect tools of 0.3–5 g l–1, is still too high for the in vivo for studies in the field of high-precision glucose monitoring of diabetes patients. metrology, could now benefit tasks such as The researchers say that a lower detection quantum computation. Oliver Pinel and limit could be achieved by placing the sensor co-workers from France, China and Germany directly on human skin, which would provide now claim to have realized a multimode a rigid seal for the photoacoustic cell. JB non-classical frequency comb with a singly resonant synchronously pumped optical SPECTROSCOPY parametric oscillator (SPOPO). They sent

Near-field effects 120 fs pulses from a Ti:sapphire mode-locked © 2012 RCS Nano Lett. 12, 1475–1481 (2012) laser and its second harmonic (wavelength of 397 nm) into a cavity, whose length was locked One of the major applications of flow Experimentally investigating the using the Pound–Drever–Hall technique. cytometry is separating cells according characteristics of thermal electromagnetic They confirmed the non-classicality of the to subtype for further biological studies. radiation in the near-field is challenging generated field by measuring a noise intensity Current cell-sorting techniques are limited because of its short-range evanescent below the standard quantum limit, and in their purity and throughput, and the character. Now, Andrew Jones and revealed the multimode nature of the comb by ability to sort mixed populations of live cells Markus Raschke from JILA and the studying the distribution of quantum intensity has yet to be demonstrated. Pei-Yu Chiou University of Colorado Boulder in the USA fluctuations in the frequency comb over its and co-workers in the USA have now have demonstrated that this task can be optical spectrum. In particular, they conclude developed a high-speed, high-purity laser- achieved by combining scattering scanning that two of the modes of the SPOPO are in a triggered fluorescence-activated cell sorter. near-field optical microscopy with Fourier- squeezed (quantum-enhanced) state with a Their device is capable of sorting up to transform spectroscopy. They used a heated normalized intensity noise of <1. NH 20,000 mammalian cells per second with a purity of 37%, or 1,500 cells per second INTERFEROMETRY with a purity of more than 90%. The device achieves fast switching (30 μs) in a small Overcoming Coriolis Phys. Rev. Lett. 108, 090402 (2012) volume (around 90 pl) by using 532 nm

focused pulses from a Q‑switched Nd:YVO4 It is well-known that the performance of large or very sensitive interferometers can be laser to produce vapour bubbles in a affected by the rotation of the Earth. Indeed, this dependence is put to good use by laser microfluidic circuit made from PDMS. The ring gyroscopes, which can detect and measure the Earth’s rotation through the Sagnac laser-induced bubbles create a liquid jet that effect. Now, Shau-Yu Lan and colleagues from the University of California, Berkeley, temporarily switches the flow of cells from Lawrence Berkeley National Laboratory and the University of Vienna have significantly the main channel to a second (collection) improved the contrast of their large space–time atom interferometer by compensating channel. The volume and location of the for the Coriolis effect associated with the Earth’s motion. In their experiment, laser fluid flow can be precisely controlled by the beams are generated from a 6 W Ti:sapphire laser using acousto-optic modulators for laser pulse energy and focus position, thus frequency tuning. The Doppler effect is used to select a single pair of counter-propagating allowing multiple cell types to be sorted frequencies that are resonant with atoms launched ballistically from a 1.5-m-tall fountain in any biological liquid. The researchers of caesium atoms. A retroflection mirror mounted flexibly on top of the vacuum chamber say that shorter switching times should be is calibrated by a tilt sensor and moved by piezoelectric actuators to compensate for possible by utilizing smaller bubbles and a the Earth’s rotation. This Coriolis-corrected interferometer could be used to measure modified channel design. JB gravity at improved accuracies; the authors predict that the uncertainty in gravitational measurements could be reduced from 6 × 10–8g to around 1 × 10–9g. DP Written by James Baxter, Oliver Graydon, Noriaki Horiuchi, David Pile and Rachel Won.