Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Strong nonlinear optical response of graphene flakes measured by
  four-wave mixing   We present the first experimental investigation of nonlinear optical
properties of graphene flakes. We find that at near infrared frequencies a
graphene monolayer exhibits a remarkably high third-order optical nonlinearity
which is practically independent of the wavelengths of incident light. The
nonlinear optical response can be utilized for imaging purposes, with image
contrasts of graphene which are orders of magnitude higher than those obtained
using linear microscopy.

19
The categories of this paper are: eess
13
The categories of this paper are: quant-ph
9
The categories of this paper are: nlin
5
The categories of this paper are: gr-qc
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
36
The categories of this paper are: cmp-lg
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
29
The categories of this paper are: funct-an
18
The categories of this paper are: econ
15
The categories of this paper are: nucl-ex
37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th