Laser technology

Over the rainbow

Many laser diodes provide light in only a limited range of the visible spectrum. A hybrid laser made out of plastic, driven by a high-power light-emitting diode, looks to offer a more flexible approach.

In the early days of semiconductor lasers, the choice of wavelengths was reminiscent of a famous Monty Python skit: it was a case of spam, spam or spam. The spectrum of available colours has since expanded impressively, but large gaps still exist, particularly at yellow wavelengths. Writing in Applied Physics Letters, Yang, Turnbull and Samuel1 join up the dots, describing an ingenious laser that uses an inorganic light-emitting diode (LED) to activate an organic lasing material. This cheap and compact device promises an unbroken rainbow of lasing wavelengths for optical communications and analytical spectroscopy.

In its classic form, laser emission is brought about by pumping a medium with energy, either as light or as electric current. The aim is to heave — or 'pump' — so many atoms or molecules within the medium up from their ground state into an excited state that a population inversion is established, with more atoms in the higher-energy state than in the lower. Each excitation boosts an atomic electron into a higher energy level, leaving behind a positively charged hole where the electron used to be. Electron and hole recombine after a short while, and stimulate others to follow suit. The result is the emission of amplified, coherent light of a single wavelength.

Plastics have long seemed to hold promise as lasing materials2,3,4, largely because of their structure — or rather, their comparative lack of it. Inorganic semiconductors such as gallium arsenide, which are traditionally used as lasing media, have rigid atomic lattices with long-range order. Charge carriers can therefore wander through them relatively unimpeded, making pumping them using electric current easy. The downside is that the wavelengths of optical transitions in these materials are equally rigidly fixed. The rather disordered structure of plastic semiconductors, on the other hand, can be synthesized with widely varying optical and electronic properties.

During the past two decades, four main applications of plastic semiconductors have been identified: organic light-emitting diodes (OLEDs); solar cells; field-effect transistors (the bedrock of integrated circuits); and lasers. Of these, only lasers have so far resisted serious commercial exploration. Naively, one might assume that all one has to do to induce laser action in a plastic is to pump an OLED with sufficient power. So what's been holding us back?

Broadly speaking, three things. First, plastics have comparatively poor charge-transport characteristics, and so large numbers of charge carriers — and very high currents — are needed to generate a population inversion through electrical pumping3,4. To add insult to injury, the presence of this horde of charge carriers would impede the electron–hole recombination by which laser light is generated.

Second, laser action requires the use of mirrors at the boundaries of the laser medium to reflect light to and fro, and thus to build up sufficient intensity gain. Because OLEDs are extremely thin, the device's metal electrodes interfere with this 'optical feedback'.

But the third, and most daunting, obstacle to lasing OLEDs is that much of the energy they generate is funnelled into particular electron-spin states known as triplet excitations5. Triplets are 'dark states', the nemesis of molecular photophysics. An electron can fall back into a hole and emit a photon only if the electron and hole spins match up; in triplet states, this isn't the case, and radiative recombination is forbidden. Long-lived triplets cause photobleaching — the chemical destruction of the surrounding emitting structure — and quench laser action in conventional lasers (if that weren't enough indication of villainy, they have also been implicated as a cause of skin cancer6). Triplets arise through strong quantum-mechanical interactions on the small length scales characteristic of OLED materials; in larger systems such as semiconductor crystals, their effects are negligible.

Hence the impetus behind Yang and colleagues' development1 of a hybrid device, the two separate components of which play to the fortes of both inorganic semiconductors (ease of light generation) and organic semiconductors (flexibility in the wavelength generated). First, a high-power inorganic LED — unconventionally operated in a pulsed mode with its focusing lens removed — generates incoherent, spectrally broad light. That light is then converted into coherent radiation in an organic, plastic lasing medium situated immediately beneath the LED (Fig. 1a). For this medium, the authors chose a conjugated polymer derived from polyfluorene, with a backbone consisting of paired phenylene rings (Fig. 1b). The characteristic alternation of single and double covalent (shared-electron) bonds in this hydrocarbon chain means electrons can move along it efficiently, such that its response to the optical pumping from the LED is strong.

Figure 1: Hybrid technology.
figure1

a, In Yang and colleagues' hybrid laser system1, a bright, pulsed inorganic (indium gallium nitride) light-emitting diode (LED) pumps light into an organic laser structure. A thin layer bridges the refractive-index gap between the LED and the organic semiconductor (a conjugated polymer) beneath, minimizing refractive losses. Light generated in the polymer bounces back and forth in the plane of the film owing to reflections from a periodically corrugated silica substrate. This provides optical feedback, and thus the gain necessary for laser action. A dichroic mirror reflects pump light back into the laser medium, while allowing laser light of the specific wavelength produced by the medium to leave. This wavelength can, in principle, be tuned continuously by varying the polymer material and the corrugation period of the grating9. b, The alternation of single and double carbon bonds in the paired phenylene rings of the lasing polymer allows electrons to move easily along the backbone, and thus produces efficient lasing. c, The disordered nature of the polymer plastic means that individual molecules have slightly different absorption spectra. Broadband LED light (yellow) can excite more molecules than narrow-band pumping by an external laser (orange), reducing the threshold for lasing, and promising cost and efficiency savings.

The new device is more compact and much cheaper than plastic lasers pumped with inorganic laser diodes7,8. Whereas such diodes emitting blue or ultraviolet light come with price tags of hundreds of dollars, high-power LEDs (which are also increasingly edging out traditional incandescent bulbs for lighting applications) are available for just cents. But that's not the best of it: because plastics are inherently disordered, made up of polymer chains jumbled up like a plate of spaghetti, different units on a chain emit light of slightly different colours. The absorption spectrum of the whole ensemble is made up of a superposition of narrower transitions corresponding to these units (Fig. 1c). Whereas a narrow-band pump laser will excite only a small subset of the molecules available, an LED with a broad emission spectrum can shovel more optically active units into the excited state, potentially lowering the threshold power needed to stimulate lasing.

By changing the laser medium and varying the corrugation of the silica substrate on which the device rests, it will be easy to tune such a laser system across the visible spectrum9. Plastics are not good conductors of heat, and so plastic lasers are unlikely to provide high power output, but in many applications — biomedical diagnostics and optical communications10, to name but two areas — precise wavelength control trumps brute force. The lasing future of plastics might not be as bright as that of other materials; but it certainly promises to be more colourful.

References

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Lupton, J. Over the rainbow. Nature 453, 459–460 (2008). https://doi.org/10.1038/453459a

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