Fluorescence of laser-created electron-hole plasma in graphene

We present an experimental observation of nonlinear up- and down-converted optical luminescence of graphene and thin graphite subject to picosecond infrared laser pulses. We show that the excitation yields to a high-density electron-hole plasma in graphene. It is further shown that the excited charge carriers can efficiently exchange energy due to scattering in momentum space. The recombination of the resulting nonequilibrium electron-hole pairs yields to the observed white-light luminescence. Due to the scattering mechanism, the power dependence of the luminescence is quadratic until it saturates for higher laser power. Studying the luminescence intensity as a function of layer thickness gives further insight into its nature and provides a new tool for substrate independent thickness determination of multilayer flakes.

Figure 1

(Color online) Emission spectra of a single graphene layer under picosecond excitation of 820 nm wavelength. The periodic modulation on top of the spectrum is due to multiple reflections inside the optical elements of the setup and can therefore be considered an artifact. The spectrum has been corrected for the sensitivity of the spectrometer according to the specifications of the manufacturer. The gap from 800 to 900 nm is due to optical filters blocking the excitation light. The inset shows the dependence on laser power.

Figure 2

(Color online) Schematic representation of the 2D dispersion relation of graphene. The gray arrow shows the optical excitation of initially monoenergetic electron-hole pairs. Collisions lead to a broadening of the energy distribution as shown by the green and red curves on the right. Recombination of shifted electron-hole pairs leads a broad fluorescence centered around the excitation energy.

Figure 3

(Color online) Flake consisting of several parts of different thickness examined by (a) AFM and by (b) imaging of the blueshifted fluorescence signal. In (c) the profiles along the white lines are shown with the number of layers indicated at the bottom. The blueshifted fluorescence was recorded in the spectral range from 650 to 750 nm. The substrate was $\text{Si}/{\text{Si}}_3{\text{N}}_4$.

Figure 4

(Color online) Calculated dependence of the luminescence intensity on the number of graphene layers together with the experimental data. The inset shows a sketch of the underlying model. The red arrow symbolized the laser excitation while the black arrows show the luminescence contributions from a single graphene sheet taking into account multiple reflections at the different interfaces. We describe here the case of graphene on glass substrate however the model can be modified for any dielectric substrate.

Figure 5

(Color online) Left: optical wide-field microscopy image of a typical flake. Right: imaging the same flake by mapping the fluorescence between 650 and 750 nm (excitation with 890 nm). The flake was prepared on a $150\text{−}\mu \text{m}$-thick glass substrate.