Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: All-optical generation and detection of sub-picosecond ac spin current
  pulses in GaAs   Sub-picosecond ac spin current pulses are generated optically in GaAs bulk
and quantum wells at room temperature and 90K through quantum interference
between one-photon and two-photon absorptions driven by two phase-locked
ultrafast laser pulses that are both circularly polarized. The dynamics of the
current pulses are detected optically by monitoring in real time and real space
nanoscale motion of electrons with high-resolution pump-probe techniques.

18
The categories of this paper are: econ
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
9
The categories of this paper are: nlin
20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
10
The categories of this paper are: q-bio
21
The categories of this paper are: adap-org
22
The categories of this paper are: chao-dyn
3
The categories of this paper are: physics
16
The categories of this paper are: stat
25
The categories of this paper are: patt-sol