Cover story

Fabricating photovoltaics from an array of miniature silicon spheres rather than a continuous wafer potentially promises to slash material costs and usher in an era of cost-effective and flexible solar cells. However, difficulties in manufacturing such spherical cells have prevented their commercialization so far. Now, a small company in Japan claims to have solved these problems and has commenced mass production at its plant in Kyoto. The resulting product is a solar module 5 × 15 cm2 in size with an efficiency of 10%. Each module contains 1,794 1-mm-diameter silicon antireflection-coated spheres, individually connected to electrodes and positioned in metallic reflectors on a foil substrate. [Profile p558 ]

Counting photons

Being able to directly measure the number of photons in a pulse of light is a highly attractive capability for researchers involved in quantum communications and computing. Eric Gansen and co-workers from the National Institute of Standards and Technology in Colorado, USA, have now achieved this feat by exploiting a quantum-dot, optically gated field-effect transistor (QDOGFET). When the GaAs/AlGaAs device is illuminated, photo-generated carriers cause a change in electrical current that is proportional to the number of photons. The team says that the approach is well-suited to counting the number of photons in weak laser pulses and has conducted experiments using 20-μs duration pulses with a photon energy of 1.54 eV. It says that future work will try and enable measurements at telecommunication wavelengths and higher speeds. [Letter p585 ]

Nanolasers

The successful development of miniature optical circuits analogous to today's microelectronics will require nanoscale electrically pumped lasers. However, so far it has been widely thought that lasers based on metallic cavities would not be up to the task, as they would suffer from too large an optical loss. Now Martin Hill and colleagues from Technische Universiteit Eindhoven in the Netherlands and Korea Advanced Institute of Science and Technology in Korea have designed and fabricated metallic-coated nanocavity lasers that they claim are the smallest electrically pumped lasers reported so far. Formed from a semiconductor heterostructure encapsulated in a thin gold film and cooled to cryogenic temperatures, the nanocavity is able to lase at a very low threshold current when electrically pumped. Thanks to its combination of a small modal volume, moderate quality factor, high spontaneous-emission enhancement and high confinement factor, the researchers foresee that such nanocavity lasers could ultimately become useful for a variety of low-power applications. [Article p589; News & Views p563 ]

Phase-change promise

Phase changes in nanoparticles could be the answer to achieving nanoscale optical circuits.

When it comes to shrinking optical circuits down to the nanoscale how to modulate and encode nanobeams of light with information has been a longstanding puzzle. It now appears that the answer may lie with phase changes of nanoparticles, according to Nikolay Zheludev, the author of this month's commentary. The concept is already prevalent in photonics albeit on a much larger scale in for example rewritable optical storage disks (DVD-RWs). On the nanoscale it appears that such changes are potentially far easier to induce, requiring much less energy, and in principle just a single or a handful of photons are required to change the phase state of a nanoparticle. If we can learn to harness and optimize such effects the result could be a generation of power-efficient optical memory devices and switches that are ideal for optical circuits of the future. [Commentary p551 ]

Infrared metrology

Semiconductor photodetectors do not operate at in the mid- to far-infrared, making precise frequency measurements in this range difficult. Now, the task might get easier thanks to a non-synchronized electro–optic sampling technique demonstrated by scientists from Max-Born-Institut, Germany, and the University of Washington, USA. Peter Gaal and co-workers have developed a time-domain method and demonstrate the feasibility of the approach by measuring the electric field of a continuous-wave CO2 laser operating at 10 μm using femtosecond light pulses from a Ti:sapphire laser as a probe. The highest measurable optical frequency is determined by the pulse length of the probe laser, and with 12-fs probe pulses, the scientists report frequency measurements of up to 40 THz. [Letter p577 ; News & Views p564 ; Interview p602 ]

Organic observation

Imaging the dynamic behaviour of charge carriers in organic materials is important for optimizing the performance of polymer electronics and optoelectronics, in particular devices such as thin-film transistors and photoconductors. However, it's not easy to visualize and image such carrier motion given that it takes place deep inside a device. Now Takaaki Manaka and colleagues from Tokyo Institute of Technology in Japan have demonstrated an elegant non-destructive solution based on time-resolved microscopic optical second-harmonic generation. By probing the local electric field generated by a two-photon process the researchers can not only directly visualize carrier motion in pentacene field-effect transistors, but also determine the carrier velocity. Applications of the imaging technique could also spread beyond polymers to inorganic and biomaterials. [Letter p581 ; News & Views p570 ]