At the heart of a conventional laser is an optical cavity that amplifies light of a certain wavelength to produce a narrow beam of coherent light with high spectral purity. For nanoscale photonics applications, however, it is becoming increasingly difficult to build the tiny cavity structures needed for ‘lasing’. Sang Hyun Lee and colleagues from Tohoku University in Japan1 have now demonstrated that lasing can be induced by the confinement of light in a nanoscale conical taper.

Fig. 1: Simulation showing how a pulse of light propagates along a nanotaper

The researchers studied how pulses of ultraviolet light propagate through the tapered end of zinc oxide nanowires grown on a silicon substrate. As the propagating pulse reaches the end of the ‘nanotaper’, a beacon of light is emitted from the tip, while part of the beam is reflected back into the nanowire (Fig. 1). The incoming and reflected pulses interfere with each other in a way that produces a standing wave strong enough to support lasing. The researchers showed that beyond a certain ‘pump’ power — the intensity of light delivered to the nanotaper — the wavelength of light emitted from the device draws in to a single narrow spectral line, a telltale sign of lasing.

The observation of light emission from a tip much smaller than the wavelength of the light emitted has broader implications for such devices. “Lasing from the nanotaper makes it possible to control light on very small scales and could be used for light modulation in photonic devices such as light-emitting diodes and solar cells,” comments Lee. Although the researchers examined a specific case of ultraviolet light emission from a zinc oxide nanotaper, the fact that the resulting lasing is a geometry effect suggests that other combinations of materials and wavelengths are possible.

The work highlights the importance of geometry in nanophotonic devices, where small structural changes can have a significant impact on performance. Furthermore, the discovery may be relevant not only to light emission but also to light absorption, says Lee. “We are interested in studying the morphological effect of nanostructures on photovoltaic devices and radiation detectors, and how these changes affect performance.”