Graphene-based optical materials & devices

While graphene – single-atomic-layer graphite – has existed a long time as a theoretical construct (or more recently also here), it was only recently that graphene was actually discovered in the laboratory. It garnered an enormous amount of interest – so much interest, in fact, that some of the pioneering researchers on graphene won the Nobel Prize in Physics, 2010. This interest stems from graphene’s many extraordinary properties. Graphene is a fantastic conductor, allowing for sheet resistance as low as 125 Ohms/square. As shown in Fig. 2 it is a zero-bandgap semiconductor with linear band structure near the K-point. This unique band structure gives graphene a universal 2.3% linear optical absorption that makes it transparent, despite its high conductivity. Now there is active research in using graphene as a transparent electrode for flexible touch screens, as a material in high-speed transistors, and in many more applications. Furthermore, graphene is nature’s closest approximation to a true two-dimensional material, since it is only a single atom thick – which lets us use it to experimentally probe fundamental physics.

Our interest in graphene focuses on the optical and opto-electronic applications of this novel material. We are currently working on graphene-based broadband saturable absorbers for femtosecond pulse generation (Fig. 2–4), on Quantum Interference Control (QIC) devices (Fig. 5–6), and on graphene-based electro-optic modulators (Fig. 7–8).

A single atomic layer of graphene is visible to the naked eye, as it shows a wavelength-independent absorption of a whopping 2.3%.

Graphene synthesis

We produce our graphene by low pressure chemical vapor deposition (LPCVD) on copper, as previously reported here and here. At high temperatures up to about 1300 K and low pressures (below 10 mTorr), we flow methane or acetylene gas over copper foil. The foil catalyzes the breakdown of the gas into carbon and hydrogen. The carbon deposits itself on the exposed copper foil as a honeycomb lattice, while the hydrogen diffuses away. Because exposed copper is required to catalyze the reaction, growth self-terminates when a single atomic layer of carbon covers the whole copper surface. We then etch the underlying copper away and transfer the graphene onto an arbitrary substrate, where it is held in place by Van der Waals forces.

Ultrafast optical properties

Optically, one of the most interesting features of graphene is its universal 2.3% absorption over every wavelength of light. Furthermore, as a semiconductor, the optical absorption of graphene saturates at high light intensities. The wavelength-independent saturation of optical absorption is shown in Fig. 2: at low intensities graphene absorbs 2.3% of the light passing through it. At high optical intensities the photo-generated carriers block further absorption.

This effect can be used to shorten optical pulses: as an optical pulse travels through the graphene, the low-intensity pulse “wings” are attenuated by the graphene’s absorption more than the high-intensity pulse center (see Fig. 4). This enables novel ultra-broadband mode-locking devices for solid-state as well as fiber lasers to produce pulses mere femtoseconds wide and of any color of light. Mode-locking with graphene is particularly interesting for new types of long-wavelength light sources, such as chip-scale lasers, and also for novel mid-IR laser sources based on thulium-doped glasses or chromium-doped II-VI semiconductors. Such mid-IR sources would open up new avenues for ultra-sensitive molecular detection.

Graphene electro-optic modulators

While we can change the optical absorption of a graphene sample by saturating it with laser light, we can also use chemical or electrostatic doping. As shown in Fig. 7, a change in the Fermi level can be used to block the optical absorption. Instead of chemical doping one can also use electrostatic doping similar to a field-effect transistor (FET). Electrostatic doping enables graphene-based electro-optic modulation of light. Unlike most modulators, however, a graphene-based device is ultra-broadband and produces little to no signal distortion or dispersion as an optical pulse travels through it. We have successfully used such modulators to stabilize optical frequency combs with record-low residual phase noise.