Volume 8 Issue 6, June 2014

It has been 20 years since near-infrared spectroscopy was first used to investigate human brain function. The technique has subsequently been extended to offer high-resolution imaging of the cortex and has now become a viable alternative to functional magnetic resonance imaging.

The dot-to-dot variation of the optical transition frequency makes it impractical to use single-photon sources based on semiconducting quantum dots in quantum computing, which requires indistinguishable photons. This can now be overcome by using coherently scattered single photons from a dot and tuning them using a microcavity.

Laser systems designed for fusion research are able to produce a high density of X-ray photons in a metal cavity. Scientists have now proposed that this environment could be used to create matter from light and test a fundamental prediction of quantum electrodynamics.

A high energy conversion efficiency and a low fabrication cost are required to make the widespread implementation of solar cells attractive. Researchers are striving to enhance cell performance by developing heterojunction techniques, introducing photonic-crystal structures and proposing new device designs.

A new ‘photon–photon collider’, which may enable elusive Breit–Wheeler pair production in an optics laboratory setting, is predicted. Using this concept, it is potentially possible to produce 10⁵ Breit–Wheeler electron–positron pairs by firing a gamma-ray beam into a high-temperature radiation field of a laser-heated hohlraum cavity.

Little attention has been devoted to development and characterization of below-threshold harmonic sources, which are critical for extending time-resolved photoemission spectroscopy to megahertz repetition rates and for developing high-average-power vacuum-ultraviolet sources. Now, a new regime of below-threshold harmonic generation accompanied by the bright, coherent emission of vacuum-ultraviolet lines is reported.

Cavity-stimulated Raman spin-flip emission is demonstrated by coupling a negatively charged InAs/GaAs quantum dot to a photonic crystal defect cavity. The emission is spectrally narrow and tunable over a range of about 125 GHz. The process can be made spin selective by tuning the scattered photons to be in resonance with the cavity.

High-resolution diffuse optical tomography employing a large array of light sources and detectors arranged around the head can perform functional brain imaging. It provides an alternative to magnetic resonance imaging for monitoring activity in different areas of the brain.

An investigation of the use of nonlinear upconversion effects like second-harmonic generation and four-wave mixing within biological tissue indicates that it should be possible to perform photodynamic therapy with near-infrared laser light at greater depths than previously.

Frequency combs based on terahertz quantum cascade lasers, which combine the high power of lasers with the broadband capabilities of pulsed sources, are demonstrated. The frequency combs generate 5 mW of terahertz power covering a frequency range of almost 500 GHz and produce more than 70 lines at 3.5 THz.

Theoretical analysis reveals that spasers do not differ fundamentally from conventional semiconductor lasers; differences are mainly technical and result from loss in the metal. Spasers are shown to have significantly inferior threshold currents and linewidths to those of vertical-cavity surface-emitting lasers, but their speed can be slightly greater.

Large-scale densely integrated optical memory on a single photonic crystal chip is demonstrated. The wavelength-division-multiplexing (WDM) capabilities of nanophotonic memories are exploited for optical addressing. This work may enable optical random-access memories and a large-scale WDM photonic network-on-chip.

The integration of germanium quantum-well devices and low-loss waveguides with silicon substrates shows promise for realizing low-loss, on-chip photonic interconnects.

Perovskite solar cells containing tin rather than lead, which is usually employed, are reported. These cells have a power conversion efficiency of 5.7% and retain 80% of their performance over a period of 12 hours.

Oliver Pike explains to Nature Photonics that the so far elusive electron–positron pair production from light may now be possible using existing technology.