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Excitation wavelength dependences of
terahertz emission from surfaces of InSb
and InAs
A. Bicˇiu
¯nas, Y.V. Malevich and A. Krotkus
The terahertz (THz) power radiated by the femtosecond laser excited
semiconductor surfaces was measured by the Golay cell. Intrinsic
InSb crystals as well as n- and p-type InAs were investigated by
using three different wavelength, 780, 1030, 1550 nm, femtosecond
lasers. It has been shown that p-type InAs crystal is the most efficient
THz emitter for all three laser wavelengths with a nearly constant
optical-to-THz power conversion efficiency of approximately 10
26
.
Introduction: Unbiased semiconductor surfaces excited by femtose-
cond laser pulses are an attractive alternative to photoconductive anten-
nae as the radiation sources for terahertz time-domain spectroscopy
(THz-TDS) systems [1] because of their simplicity and the power
scaling possibilities. Most efficiently the THz pulses are generated at
the illuminated surfaces of the narrow-gap semiconductors in which
the interaction with the laser radiation is the strongest. When using the
femtosecond Ti:sapphire laser pulses with a central wavelength at
around 780 nm for the sample excitation, the largest THz pulse ampli-
tudes are emitted by p-type InAs crystals [2]. From the application
point of view, it is important to compare the performance characteristics
of different THz emitters at various light wavelengths, especially at 1
and 1.55 mm, for which efficient, cheaper and more compact than the
Ti:sapphire laser femtosecond fibre lasers are available. Previous
attempts of such comparison [3– 5] were successful only in part. THz
emission spectra for InAs and InSb [3] as well as for InAs and InN
[4] were investigated by using tunable wavelength femtosecond pulses
from an optical parametric amplifier at very high photoexcitation fluen-
cies not achievable by the fibre lasers. On the other hand, the sensitivity
of the THz detector made from low-temperature grown GaAs that was
changing with the wavelength did not allow direct comparison with
the optical-to-THz conversion efficiencies for various lasers used in [5].
In this Letter, absolute values of the optical-to-THz energy conver-
sion for InAs and InSb illuminated by the pulsed radiation of three
different wavelength femtosecond lasers are presented. It is shown that
p-type InAs crystal is the best THz emitter over the whole wavelength
range from 0.78 to 1.55 mm with a constant conversion efficiency
approximately equal to10
26
. It is also demonstrated that clear azimuthal
angle dependences of the emitted THz power are manifested on all
measurements, evidencing that in the narrow-gap semiconductors this
effect cannot be explained by a simple photo-Dember effect.
Samples and setup: Three different (111) cut crystals, InSb as well as
n- and p-type InAs, all having a free carrier density of 2×10
16
cm
23
were investigated. The surface of the crystals was illuminated at
an angle of 458to their surface. The lasers used were: Ti:Sapphire
laser (MIRA Coherent) with the wavelength of 780 nm and pulse dur-
ation of 150 fs, Yb:KGW laser (Light Conversion) with the wavelength
of 1030 nm and pulse duration of 70 fs, and Er:fibre laser (Toptica) with
the wavelength of 1550 nm and pulse duration of 80 fs. The pulse
repetition rates of all three lasers were close to each other – 75 to
80 MHz; the experiments were performed at the same optical excitation
levels (the average optical power up to 300 mW) with the beams focused
at the semiconductor surfaces to 1.9 mm diameter spot. The emitted
average THz power was measured in the quasi-reflection direction by
a Golay cell (Tydex).
Experimental results: p-type InAs crystal was the most efficient THz
emitter for all three laser wavelengths. Fig. 1 shows the dependences
of the average THz power emitted by this crystal on the optical excitation
level. The optical-to-THz energy conversion efficiency is the largest
(2×10
26
) for the 780 nm wavelength; it is only about two times
smaller at the 1550 nm wavelength – much weaker reduction than the
one that has been observed when measuring the spectral dependences
of the THz emission on n-type InAs [3]. This fact can be considered
as additional evidence of the electric-field-induced optical rectification
(EFIOR) effect in the surface inversion layer of p-InAs as the main
cause of the THz emission from this crystal [6].
0 50 100 150 200 250 300
0
0.1
0.2
0.3
0.4
0.5
0.6
1550 nm
1030 nm
780 nm
THz power, mW
laser power, mW
p-InAs
Fig. 1 Dependences of average THz power emitted by p-InAs crystal excited
by femtosecond pulses of three different wavelength lasers on optical ex-
citation level
Azimuthal angle dependences measured at three femtosecond laser
wavelengths and presented in Fig. 2 support this conclusion. All three
azimuthal angle dependences show a characteristic for the EFIOR
effect on (111) crystallographic plane cos(3
w
)type dependence;
however, the angle dependent component is the strongest for the
1030 nm wavelength. The most probable cause of the enhanced THz
radiation from p-type InAs illuminated by femtosecond laser pulses is
the nonlinear optical interaction in the strong built-in electric field that
is present on its surface due to the natural formation of the inversion
layer [7]. The thickness of this layer depends on the hole density in
the crystal and is around 130 nm for p ¼2×10
16
cm
23
, whereas the
optical absorption length in InAs varies from 110 nm at
l
¼800 nm
to 700 nm at
l
¼1550 nm. The wavelength of 1030 nm is, therefore,
close to the spectral range where the absorption region overlaps with
the depletion layer thus providing the optimum conditions for the non-
linear light interaction with the material.
0 60 120 180 240 300 360
0
0.1
0.2
0.3
0.4
0.5
0.6 780nm
1030nm
1550nm
THz power, mW
azimuthal angle, deg
Fig. 2 Azimuthal angle dependences of THz emission from p-InAs photoex-
cited at three different wavelengths
0 60 120 180 240 300 360
0
0.1
0.2
0.3
p-InAs
n-InAs
InSb (111)
InSb (112)
power, mW
azimuthal angle, deg
Fig. 3 Azimuthal angle dependences of THz power emitted from InAs (111)
and InSb (111) and (211) surfaces photoexcited by femtosecond 1550 nm
wavelength laser with average power equal to 280mW
Well pronounced azimuthal angle dependences of the emitted THz
power were observed on all investigated semiconductor surfaces photo-
excited by femtosecond 1550 nm wavelength pulses (Fig. 3). In this
case, n-type InAs crystal radiates the weakest THz signals, whereas
the THz emission from the (111) InSb crystals is nearly as efficient as
that from p-InAs. The excess energy of the electrons excited by this
laser in the G-valley of the conduction band of InSb is smaller than
the energy separation to the higher lying, large effective mass L
valleys, therefore the physical mechanism of the THz emission in this
material becomes similar to that of InAs – EFIOR effect in the electric
field induced by the photoexcited electron and hole spatial separation
ELECTRONICS LETTERS 13th October 2011 Vol. 47 No. 21
[6]. An additional enhancement of the radiated THz power can be
obtained by illuminating the crystalline planes with a lower symmetry
[8]. As shown in Fig. 3, the THz power radiated from (112) cut InSb
is more that 10% stronger than from the (111) plane.
Conclusion: The THz power radiated by the femtosecond laser excited
semiconductor surfaces was measured by the Golay cell. Intrinsic InSb
crystals as well as n- and p-type InAs were investigated by using three
different wavelength, 780, 1030, 1550 nm, femtosecond lasers. In all
those spectral ranges most efficiently the THz radiation is emitted by
p-InAs, and its optical-to-THz conversion efficiency is of the order of
10
26
.
Acknowledgment: Y.V. Malevich thanks the Marie Curie Initial
Training Network (MITEPHO) for financial support.
#The Institution of Engineering and Technology 2011
21 June 2011
doi: 10.1049/el.2011.1925
A. Bicˇiu
¯nas, Y.V. Malevich and A. Krotkus (Department of
Optoelectronics, Center for Physical Sciences and Technology, 01108,
A. Gostauto 11, Vilnius, Lithuania)
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ELECTRONICS LETTERS 13th October 2011 Vol. 47 No. 21
