Optical Materials: Variety pays off for nanotubes.

T he global communications infrastructure relies on optical fibre lasers and other photonics components. As traffic on these networks continues to increase by a stunning 40% per year, hardware companies are under pressure to reduce costs. There is thus a need for new, low-cost optical technologies that deliver high performance and can be manufactured in an environmentally friendly manner. On page 738, Andrea Ferrari of Cambridge University and co-workers1 demonstrate that carbon nanotubes, incorporated into a transparent polymer matrix, can be used as a practical and efficient photonic material for the generation of ultrashort pulses at communications wavelengths by fibre lasers. Pulsed laser sources are deployed in a wide variety of applications ranging from basic scientific research and metrology to eye surgery and materials processing. Regardless of wavelength, the majority of laser systems producing pulses with durations of picoseconds and shorter use a mode-locker — a nonlinear optical element called a saturable absorber — that turns a continuous wave output into a train of ultrashort pulses. The main requirements for the nonlinear materials used in mode-lockers are a fast response time, strong nonlinearity, broad wavelength range, low optical loss, high power handling, low power consumption, low cost and ease of integration into an optical system. Currently, multiple-quantum-well semiconductor heterostructures are regularly used as saturable absorbers. However, the techniques used to fabricate and package these structures are expensive and environmentally unfriendly (because they involve toxic materials such as arsenic), even though they are well-established. Moreover, each design tends to be application-specific, covering a relatively narrow wavelength range (about 100 nm). Nanotube composites have the potential to overcome all of these shortcomings. First, they function well as nonlinear optical elements. Semiconducting nanotubes can absorb photons with energies that are close to the value of their bandgap (the difference in energy between their conduction and valence bands), creating electron–hole pairs in the process. However, this causes the conduction and valence bands in the nanotube to fill up with electrons and holes, respectively, causing the absorption to saturate. Under saturation, or ‘bleaching’, an increase in the laser power will not result in increased absorption by the sample. It is this nonlinear behaviour that makes nanotubes suitable for ultrashort pulse generation. Furthermore, the window of interest for optical communications is between 1,300 and 1,600 nm, which Carbon nanotubes are usually produced in samples that contain a mixture of different diameters and electronic properties; this is a problem for applications in nanoelectronics but is advantageous when generating ultrashort laser pulses. OPTICAL MATERIALS