Polymer composites are one of the most attractive near‐term means to exploit the unique properties of carbon nanotubes and graphene. This is particularly true for composites aimed at electronics and photonics, where a number of promising applications have already been demonstrated. One such example is nanotube‐based saturable absorbers. These can be used as all‐optical switches, optical amplifier noise suppressors, or mode‐lockers to generate ultrashort laser pulses. Here, we review various aspects of fabrication, characterization, device implementation and operation of nanotube‐polymer composites to be used in photonic applications. We also summarize recent results on graphene‐based saturable absorbers for ultrafast lasers.

Ultrashort-pulse lasers with spectral tuning capability have widespread applications in fields such as spectroscopy, biomedical research and telecommunications [1][2][3] . Mode-locked fibre lasers are convenient and powerful sources of ultrashort pulses 4 , and the inclusion of a broadband saturable absorber as a passive optical switch inside the laser cavity may offer tuneability over a range of wavelengths 5 . Semiconductor saturable absorber mirrors are widely used in fibre lasers [4][5][6] , but their operating range is typically limited to a few tens of nanometres 7,8 , and their fabrication can be challenging in the 1.3 -1.5 mm wavelength region used for optical communications 9,10 . Single-walled carbon nanotubes are excellent saturable absorbers because of their subpicosecond recovery time, low saturation intensity, polarization insensitivity, and mechanical and environmental robustness [11][12][13][14][15][16] . Here, we engineer a nanotube -polycarbonate film with a wide bandwidth (>300 nm) around 1.55 mm, and then use it to demonstrate a 2.4 ps Er 31 -doped fibre laser that is tuneable from 1,518 to 1,558 nm. In principle, different diameters and chiralities of nanotubes could be combined to enable compact, mode-locked fibre lasers that are tuneable over a much broader range of wavelengths than other systems.The development of compact, diode-pumped, ultrafast fibre lasers as alternatives for bulk solid-state lasers is fast progressing. To date, short pulse generation has been particularly effective using passive mode-locking techniques 4 . At present, the dominant technology in passively mode-locked fibre lasers is based on semiconductor saturable absorber mirrors (SESAMs) 6 . Conventional SESAMs use III -V semiconductor multiple quantum wells grown on distributed Bragg reflectors (DBRs) 3 . Their fabrication involves molecular beam epitaxy (MBE) 6 . To reduce the relaxation time to sub-picosecond levels, either postgrowth ion-implantation or low-temperature growth is normally required 6,17 . Furthermore, SESAMs are based on a resonant nonlinearity, which tends to limit wavelength tuneability 18,19 for the shortest pulse lasers 20 . Their operating bandwidth is further limited by the bottom DBR section, which has a finite bandwidth for high reflectivity 19 . For example, the bandwidth of conventional Al x Ga 12x As/AlAs SESAMs is limited to about 60 nm by the bottom Bragg mirrors 21 . Wider bandwidth, &200 nm, was achieved using novel material pairs with larger refractive index difference (for example, AlGaAs/CaF 2 ) 21 , or by replacing the DBRs with metallic mirrors 22 . However, so far, no widely tuneable mode-locked laser has been reported using these novel structures. Trade-offs between design parameters have to be made in order to obtain targeted device characteristics 10 . A tuning range over 100 nm was achieved by SESAMs in solidstate and fibre lasers 18,23 . The widest was 125 nm, for a Yb-doped fibre laser operating at 1 mm. However, two SESAMs with complementary spectral properties had to b...

Photoluminescence is commonly used to identify the electronic structure of individual nanotubes. But, nanotubes naturally occur in bundles. Thus, we investigate photoluminescence of nanotube bundles. We show that their complex spectra are simply explained by exciton energy transfer between adjacent tubes, whereby excitation of large gap tubes induces emission from smaller gap ones via Förster interaction between excitons. The consequent relaxation rate is faster than nonradiative recombination, leading to enhanced photoluminescence of acceptor tubes. This fingerprints bundles with different compositions and opens opportunities to optimize them for opto-electronics.

We extract the distribution of both center-of-mass and angular fluctuations from three-dimensional tracking of optically trapped nanotubes. We measure the optical force and torque constants from autocorrelation and cross-correlation of the tracking signals. This allows us to isolate the angular Brownian motion. We demonstrate that nanotubes enable nanometer spatial and femtonewton force resolution in photonic force microscopy, the smallest to date. This has wide implications in nanotechnology, biotechnology, nanofluidics, and material science.