# Graphene Terahertz Absorption

## TL;DR

This paper investigates how bias voltage affects terahertz absorption in graphene, revealing that electron coherence length enables intraband transitions responsible for absorption changes, aligning with sum-rule constraints.

## Contribution

It provides the first direct calculation linking bias-induced absorption changes in graphene to electron coherence, challenging phonon-assisted models.

## Key findings

- Enhanced far-infrared absorption with bias voltage
- Diminished infrared absorption with bias voltage
- Results agree with experimental data

## Abstract

The unique terahertz properties of graphene has been identified for novel optoelectronic applications. In a graphene sample with bias voltage added, there is an enhanced absorption in the far infrared region and a diminished absorption in the infrared region. The strength of enhancement(diminishment) increases with the gate voltage, and the enhancement compensates the diminishment. We find that it is the coherence length of electrons in graphene that allows pure electronic transitions between states differing by small momentums and makes intraband transition possible, is responsible for the far infrared enhancement. Phonon assisted processes are not necessary and would not in any case contribute to a sum rule. This naturally leads to results obeying the general sum-rule in optical absorptions. Our prediction of the strength of enhancement(diminishment) in terms of the bias agrees with experiments. This is the first direct calculation we are aware of, since the prior phonon assisted model for indirect transition should not obey a sum rule.