Terahertz Emission Mechanisms in III–V Semiconductors …
177
In this section, we shall discuss the terahertz emission emerging out of nonlinear
phenomena and transient currents in GaAsBi. One must keep in mind that both these
emission phenomena are present in the GaAs system as well.
9.1 Optical Rectification
The terahertz emission from GaAsBi due to 800 nm excitation with moderate fluences shows a very clear indication of OR. An essential characteristic of the NL
processes is the strong dependence of THz emission on the relative orientation of
the excitation laser polarization and the crystal axes. For instance, in a zinc-blende
structure, following relations dictate the dependence of p-polarized THz emission
on the azimuthal angle (φ) with p-polarized laser excitation, due to bulk OR [43].
E
100
p− p ≈ cos 2φ
E
110
p− p ≈ sin φ + sin 3φ
E
111
p− p ≈ a + b cos 3φ
(2)
The surface field-assisted OR has a different set of rules for azimuthal dependence. The studies so far have looked at the epilayers grown on the <100> and
<311>B substrates. The GaAsBi epilayers grown on (311)B face has a rather interesting azimuthal dependence [44]. The bare <311>B GaAs substrate and the GaAsBi
epilayers show entirely different trends in terahertz emission with azimuthal rotation.
With the careful analysis of the second- and third-order susceptibility tensor in zincblende crystals, one could, in principle, arrive at the azimuthal dependence for any
crystal plane. In doing so, the azimuthal dependence from (311)B GaAs substrate
agrees well with that of bulk OR. While the trends in epilayer differed significantly,
this could well be explained by considering a combination of bulk as well as surface
field-assisted OR [44].
The surface field-assisted OR phenomenon arises due to the presence of a static
electric field in the zinc-blende structure. Such a static electric field can be present on
the surface of semiconductors, as discussed earlier. In such cases,
say, E
surf
z
= 0
the third-order nonlinearity
χ
{3}
i jkz
can contribute to an effective second-order
nonlinearity, as expressed below.
χ
{2}eff
i jk
= χ
{2}bulk
i jk
+ 3χ
{3}
i jkz E
surf
z
(3)
While the (311)B-grown GaAsBi layers show terahertz emission via bulk and
surface OR, studies have shown no evidence of surface OR contribution in the case
of (100) grown GaAsBi epilayers. Also, in (100) orientation, zinc-blende structures
show no azimuthal dependence in the case of surface OR (Fig. 4).
177
In this section, we shall discuss the terahertz emission emerging out of nonlinear
phenomena and transient currents in GaAsBi. One must keep in mind that both these
emission phenomena are present in the GaAs system as well.
9.1 Optical Rectification
The terahertz emission from GaAsBi due to 800 nm excitation with moderate fluences shows a very clear indication of OR. An essential characteristic of the NL
processes is the strong dependence of THz emission on the relative orientation of
the excitation laser polarization and the crystal axes. For instance, in a zinc-blende
structure, following relations dictate the dependence of p-polarized THz emission
on the azimuthal angle (φ) with p-polarized laser excitation, due to bulk OR [43].
E
100
p− p ≈ cos 2φ
E
110
p− p ≈ sin φ + sin 3φ
E
111
p− p ≈ a + b cos 3φ
(2)
The surface field-assisted OR has a different set of rules for azimuthal dependence. The studies so far have looked at the epilayers grown on the <100> and
<311>B substrates. The GaAsBi epilayers grown on (311)B face has a rather interesting azimuthal dependence [44]. The bare <311>B GaAs substrate and the GaAsBi
epilayers show entirely different trends in terahertz emission with azimuthal rotation.
With the careful analysis of the second- and third-order susceptibility tensor in zincblende crystals, one could, in principle, arrive at the azimuthal dependence for any
crystal plane. In doing so, the azimuthal dependence from (311)B GaAs substrate
agrees well with that of bulk OR. While the trends in epilayer differed significantly,
this could well be explained by considering a combination of bulk as well as surface
field-assisted OR [44].
The surface field-assisted OR phenomenon arises due to the presence of a static
electric field in the zinc-blende structure. Such a static electric field can be present on
the surface of semiconductors, as discussed earlier. In such cases,
say, E
surf
z
= 0
the third-order nonlinearity
χ
{3}
i jkz
can contribute to an effective second-order
nonlinearity, as expressed below.
χ
{2}eff
i jk
= χ
{2}bulk
i jk
+ 3χ
{3}
i jkz E
surf
z
(3)
While the (311)B-grown GaAsBi layers show terahertz emission via bulk and
surface OR, studies have shown no evidence of surface OR contribution in the case
of (100) grown GaAsBi epilayers. Also, in (100) orientation, zinc-blende structures
show no azimuthal dependence in the case of surface OR (Fig. 4).