Absorption sensor based on graphene plasmon quantum amplifier

Plasmon spectroscopy methods are highly sensitive to the small volumes of material due to subwavelength localization of light increasing light-matter interaction. Recent research has shown a high potential of plasmon quantum generator (spaser) or amplifier (sped) for sensing in the infrared optical region. Trinitrotoluene (TNT) molecules fingerprints are considered as an example. Basing on Lindblad equations, we implement full quantum mechanical theory of graphene plasmon generator to investigate how a small amount of absorbing atoms influences the spectrum of a graphene spaser. We analyze the optimal type of an active medium, the number of active molecules, and the pump level to achieve the highest sensitivity and show that optimized structure is sensitive to dozens of atoms. Our research is useful for the development of near- and mid-IR spectroscopy based on plasmon quantum amplifier.

Figure 1

A sketch of graphene quantum amplifier based TNT sensor. It consists of a graphene flake with the Fermi level being controlled by a gate electrode. Graphene plasmons are generated by quantum dots. The generation intensity and spectrum are affected by TNT molecules, which have absorption peak near the generation frequency.

Figure 2

The number of photons as a function of the number of gain particles and the pump rate. The wavelength is $\lambda =3.2\mu \text{m},d_{12}=1.4\mathrm{D}.$

Figure 3

Relative intensity change in the presence of 100 absorbing molecules at the wavelength of the first line ($\lambda =3.2\mu \mathrm{m}$) (a) and of the second line ($\lambda =6.5\mu \mathrm{m}$) (b).

Figure 4

Optimized dependence of the relative intensity change (sensitivity) on the gain medium transition dipole moment in the presence of 100 absorbing molecules for $\lambda =3.2\mu \mathrm{m}$ (solid curve) and for $\lambda =6.5\mu \text{m}$ (dashed curve).

Figure 5

Spectrum change with the presence of 15 absorbing molecules for the first line ($\lambda =3.2\mu \text{m}$) (a) and for the second line ($\lambda =6.5\mu \text{m}$) (b). The frequency is shown with respect to the lasing frequency. This number of molecules is smaller than that in Figs. 3 and 4 because of the higher spectral sensitivity.