124
S. Banerjee et al.
THz = (2ω + THz − ω − ω)
(2)
where THz, ω, and 2ω, represent the angular frequencies of the THz, laser fundamental, and the second harmonic photons, respectively. The difference between a
photon of energy (2ω + THz ) and the sum of energies of two photons (ω) yields
THz radiation. The THz field amplitude is proportional to the intensity of the fundamental (ω) and the square root of the second harmonic (2ω) beams. The optimal THz
output is obtained when the fundamental, the second harmonic, and the THz wave
have parallel polarization.
According to the asymmetric transient current (ATC) model [42, 48], an intense
laser field causes suppression of the Coulomb barrier leading to tunnel ionization
of electrons from the atoms or the molecules of gases present in the air. Thus, THz
radiation is emitted from a non-diminishing transverse photocurrent via Maxwell’s
equations. In the presence of fundamental and second harmonic beam where the
laser field symmetry is perturbed, asymmetric current results in THz emission. It
was demonstrated that both four-wave mixing and the plasma current contribute to
the generation process [49].
Broadband detection of THz radiation was first realized in 2006 [22]. As shown
in Fig. 5, the THz field induces a second harmonic (TFISH) of the gate beam through
a third-order non-linear process. We can explain the second harmonic generation as
E
THz
2ω ∝ χ
(3) E ω E ω E THz
(3)
where χ
(3) is the third-order non-linear susceptibility, E
T Hz
2ω , E ω and E T Hz are the
electric field amplitude of the 2ω, ω, and THz waves, respectively. Here, E
T Hz
2ω
∝
E T Hz , and the intensity of the second harmonic is proportional to the intensity of the
THz field (I
T Hz
2ω
∝ I T Hz ). Hence, the phase information, in this case, is not recovered,
making the measurement incoherent. The introduction of a local oscillator (LO) at
the plasma (overlap) point turns the THz detection to be heterodyned and phasesensitive [50]. An AC external bias applied across a pair of electrodes kept at the
plasma point can act as the LO. This heterodyned technique has been termed as the
‘air biased coherent detection’ (ABCD) [51].
Including the local oscillator, the second harmonic electric field is given by the
equation
E 2ω ∝ χ
(3) E ω E ω
E
THz
2ω + E
LO
2ω
(4)
and the second harmonic intensity has the form,
I 2ω ∝ (E 2ω )
2
∝
χ
(3)
2 I
2
ω
E
THz
2ω
2 + 2E
THz
2ω E
LO
2ω cosφ +
E
LO
2ω
2
(5)
where E LO is the electric field supplied by the electrodes and ϕ is the phase
difference between the E
T Hz
2ω and E
L O
2ω . The above equation can be written as
S. Banerjee et al.
THz = (2ω + THz − ω − ω)
(2)
where THz, ω, and 2ω, represent the angular frequencies of the THz, laser fundamental, and the second harmonic photons, respectively. The difference between a
photon of energy (2ω + THz ) and the sum of energies of two photons (ω) yields
THz radiation. The THz field amplitude is proportional to the intensity of the fundamental (ω) and the square root of the second harmonic (2ω) beams. The optimal THz
output is obtained when the fundamental, the second harmonic, and the THz wave
have parallel polarization.
According to the asymmetric transient current (ATC) model [42, 48], an intense
laser field causes suppression of the Coulomb barrier leading to tunnel ionization
of electrons from the atoms or the molecules of gases present in the air. Thus, THz
radiation is emitted from a non-diminishing transverse photocurrent via Maxwell’s
equations. In the presence of fundamental and second harmonic beam where the
laser field symmetry is perturbed, asymmetric current results in THz emission. It
was demonstrated that both four-wave mixing and the plasma current contribute to
the generation process [49].
Broadband detection of THz radiation was first realized in 2006 [22]. As shown
in Fig. 5, the THz field induces a second harmonic (TFISH) of the gate beam through
a third-order non-linear process. We can explain the second harmonic generation as
E
THz
2ω ∝ χ
(3) E ω E ω E THz
(3)
where χ
(3) is the third-order non-linear susceptibility, E
T Hz
2ω , E ω and E T Hz are the
electric field amplitude of the 2ω, ω, and THz waves, respectively. Here, E
T Hz
2ω
∝
E T Hz , and the intensity of the second harmonic is proportional to the intensity of the
THz field (I
T Hz
2ω
∝ I T Hz ). Hence, the phase information, in this case, is not recovered,
making the measurement incoherent. The introduction of a local oscillator (LO) at
the plasma (overlap) point turns the THz detection to be heterodyned and phasesensitive [50]. An AC external bias applied across a pair of electrodes kept at the
plasma point can act as the LO. This heterodyned technique has been termed as the
‘air biased coherent detection’ (ABCD) [51].
Including the local oscillator, the second harmonic electric field is given by the
equation
E 2ω ∝ χ
(3) E ω E ω
E
THz
2ω + E
LO
2ω
(4)
and the second harmonic intensity has the form,
I 2ω ∝ (E 2ω )
2
∝
χ
(3)
2 I
2
ω
E
THz
2ω
2 + 2E
THz
2ω E
LO
2ω cosφ +
E
LO
2ω
2
(5)
where E LO is the electric field supplied by the electrodes and ϕ is the phase
difference between the E
T Hz
2ω and E
L O
2ω . The above equation can be written as