ArticlePDF Available

Abstract and Figures

This paper presents experimental results and an analysis of methods of generating terahertz (THz) radiation, using femtosecond laser sources: generation by photoconductive semiconductor antennas, nonlinear-optical generation of the difference frequency or optical rectification, and generation using optical breakdown in gases under the action of femtosecond pulses. An undoped semiconductor crystal of indium arsenide (InAs) located in a magnetic field, an electrooptic ZnTe crystal, an organic DAST crystal, and an optical spark in air were used as generators of THz radiation
Content may be subject to copyright.
LASER PHYSICS AND ENGINEERING
Methods of generating superbroadband terahertz pulses with femtosecond lasers
V. G. Bespalov, A. A. Gorodetski, I. Yu. Denisyuk, S. A. Kozlov, V. N. Krylov,
G. V. Lukomski, N. V. Petrov, and S. É. Putilin
St. Petersburg State University of Information Technologies, Mechanics, and Optics, St. Petersburg
Submitted May 22, 2008
Opticheski Zhurnal 75,3441October 2008
This paper presents experimental results and an analysis of methods of generating terahertz
THz radiation, using femtosecond laser sources: generation by photoconductive semiconductor
antennas, nonlinear-optical generation of the difference frequency or optical rectification, and
generation using optical breakdown in gases under the action of femtosecond pulses. An undoped
semiconductor crystal of indium arsenide InAs located in a magnetic field, an electrooptic ZnTe
crystal, an organic DAST crystal, and an optical spark in air were used as generators of THz
radiation © 2008 Optical Society of America.
INTRODUCTION
The creation of an efficient, powerful, inexpensive, and
compact source of ultrashort a few vibrations of the light
field in width superbroadband terahertz THz pulses that
operates at room temperature is one of the main problems in
contemporary photonics.
1
This is because superbroadband
THz radiation offers enormous potential for a wide range of
technical and scientific applications: the diagnosis of various
materials, including semiconductors, chemical compounds,
biomolecules, and biological tissues; the formation of im-
ages, tomography, and endoscopy for medical purposes and
safety; remote control and monitoring of the environment;
astronomy; etc.
2,3
The Thz range actually covers a wide re-
gion of vibrational, rotational, and translational lines of a
broad class of organic and biological molecules. The unob-
structed penetration through fog and haze, rain, paper, wood,
plastic, ceramic, and other materials because of the smallness
of the Rayleigh scattering of radiation in this region opens up
wide possibilities for endoscopy with resolution all the way
to 100
m and high SNR. The low energy of the THz quanta
and the associated nonionizing character of the action of the
THz radiation opens up wide possibilities for using it in bi-
ology and medicine. At the same time, the energy of THz
quanta corresponds to the vibrational energy of important
biological molecules, including DNA and RNA, and this
makes it possible to accomplish purposeful action on them
both for research and for medical purposes, stimulating or
suppressing the development of viruses, cells, and their com-
ponents. No less promising from a practical viewpoint is the
use of THz radiation in medicine for the visualization, ho-
lography, and tomography of tissues, therapy, and surgery.
In the last fifteen years, along with the development of
femtosecond solid-state lasers especially lasers based on
sapphire crystals doped with titanium ions and microelec-
tronics, a significant shift has been noted in studies of the
THz region. Three methods of obtaining ultrashort THz
pulses have been most actively developed, using femtosec-
ond laser sources: generation by photoconductive antennas,
nonlinear-optical generation of the difference frequency or
optical rectification, and generation with the use of optical
breakdown in gases under the action of femtosecond pulses.
These methods make it possible to obtain THz electromag-
netic radiation with peak electric-field amplitudes up to
100 kV/ cm by using femtosecond laser systems with
amplifiers.
4
THE MAIN METHODS OF GENERATING SUPERBROADBAND
THz PULSES WITH FEMTOSECOND LASERS
Generation by photoconductive antennas
One of the first methods for generating THz radiation
was by implementing a photoconductive antenna by irradia-
tion with femtosecond pulses.
5
The effect by which electro-
magnetic radiation is generated by the surface of a semicon-
ductor that is a photoconductive antenna when it is excited
by supershort femtosecond pulses is explained by the dy-
namics of the formation of photocarriers—electron-hole
pairs—and their superfast motion in a near-surface electric
field. According to Maxwell’s equations, the current Jt that
appears in this case causes an electromagnetic pulse Et
J/
t to be generated, usually in the form of one vibration,
with a spectrum determined by the Fourier transform of its
temporal shape. The surface of the semiconductor thus oper-
ates as a dynamic photoconductive antenna that emits pulses
of broadband electromagnetic radiation with a width of hun-
dreds of femtoseconds. The central frequency of the radia-
tion generated by the photoconductors is usually in the
12-THz region. Semiconductor crystals of GaAs, InP, and
InAs are widely used as generators of THz radiation.
6
To
increase the efficiency of the Thz emission, the crystal
samples are placed in strong electric or magnetic fields.
7
It
should be pointed out that, according to the model of Ref. 8,
the intensity of the THz radiation is proportional to the time
derivative of the concentration of electron-hole pairs and
their speed in an electric or magnetic field, which is deter-
mined by the mobility of the charge carriers. One of the
highest electron mobilities about 310
4
cm
2
/ V sec is pos-
636 636J. Opt. Technol. 75 10, October 2008 1070-9762/2008/100636-07$15.00 © 2008 Optical Society of America
sessed by undoped InAs crystals, and these crystals currently
exhibit the highest conversion efficiency of femtosecond la-
ser radiation into THz pulsed radiation.
8
Optical rectification
The large peak value of the electric field of the radiation
of a femtosecond pulse in the visible or near-IR regions
makes it possible to use the second-order nonlinear suscep-
tibility
2
of electrooptic crystals to generate THz radiation.
The nonlinear interaction between any two frequency com-
ponents within the spectrum of a femtosecond pulse causes
polarization P
THz
of the medium, as a result of which the
electromagnetic waves are emitted at the beat frequency,
with the polarization of the medium being proportional to the
incident pulse intensity; i.e, it is possible to write
P
THz
兲⬃
2
E
1
E
2
兲⬃
2
E
0
2
1
in the frequency region, where E
1
and E
2
are the Fou-
rier components of the spectrum of the femtosecond pulse,
while
THz
=
1
2
. In the dipole approximation and in the
far zone of diffraction, the amplitude of the THz wave is
proportional to the second derivative with respect to time of
the optically induced polarization, E
THz
2
P/
t
2
. Since the
width of the spectrum the pulse width of femtosecond ra-
diation is usually 10 THz 100 fs, the upper limit of the
spectral width and the lower limit of the pulse width of THz
radiation must be about the same.
Optical rectification has been used for the generation of
THz radiation in many electrooptic crystals, such as ZnSe,
GaSe, and ZnTe,
9
as well as in the organic ionic salt N-4-
dimethylamino-4-N-methylstilbazolium tosylate DAST.
10
Besides the value of the second-order susceptibility, the con-
version efficiency into THz radiation depends on the rela-
tionship of the phases of the interacting waves; i.e., the fol-
lowing phase-synchronization condition should be satisfied:
k = k
1
k
2
k
THz
=0, 2
where k is the wave detuning between the wave vectors k
1
and k
2
of the pump waves and the wave vector k
THz
of the
THz pulse. In many nonlinear optical materials, such as
LiNbO
3
, phase matching between the THz wave and the
pump wave cannot be achieved, because the refractive index
of the given materials at THz frequencies is significantly
greater than that in the visible and near-IR regions. It has
been shown
11
that the length of coherent interaction the co-
herence length l
coh
depends largely on the mismatch of the
group velocity of the femtosecond pump pulse and the phase
velocity of the THz pulse and is determined by
L
coh
=
c
THz
n
gr
n
THz
, 3
where n
gr
=n
n/
is the refractive index of the
crystal for the group velocities of the femtosecond pulse,
n
THz
is the refractive index of the medium at the THz fre-
quency, and c is the speed of light. Phase matching is ob-
served in such nonlinear materials as ZnTe, GaSe, and
DAST, in which l
coh
is 0.11 mm. It should be pointed out
that the organic crystal DAST has the greatest nonlinear sus-
ceptibility d
111
=1010 pm/ V among these nonlinear media at
a wavelength of 1318 nm.
11
Many groups of researchers use
ZnTe crystals, which have a nonlinear susceptibility of d
14
=4 pm/ V at the titanium-sapphire-laser wavelength of
=800 nm, with the coherence length making it possible to
generate electromagnetic vibrations in the range from
0 to 2 THz.
GENERATION USING OPTICAL BREAKDOWN
The generation of THz radiation in which the fundamen-
tal and the second harmonic of a femtosecond laser are fo-
cused in air is one of the newest methods of generating THz
radiation and does not require the presence of any special
medium. There are several explanations of the generation
mechanism. Thus, Cook and Hochstrasser
12
connect the ap-
pearance of radiation of the difference frequency with four-
wave mixing of the radiation of the first and second harmon-
ics of a femtosecond pump laser at third-order plasma
nonlinearity
3
. The process is described as follows: Polar-
ization P
THz
at a THz frequency arises when three waves
interact—two pump waves at the fundamental frequency,
E
1
and E
2
, and a wave of the second harmonic,
E2
; i.e.,
P
THz
兲⬃
i,j,k ,l
3
E2
E
1
E
2
. 4
It should be pointed out that, for THz radiation to appear,
the presence of a plasma optical breakdown of the gas—the
appearance of free electrons—is necessary in this case. An-
other explanation of the effect is based on the transverse-
plasma-current model, resulting from the liberation of elec-
trons from the gas molecules as a consequence of tunnel
ionization. The resulting electrons are accelerated in the
asymmetric laser field formed by adding the vibrations of the
first and second harmonics,
13
and this results in the appear-
ance of a nonzero projection of the velocity in the transverse
direction—transverse current. Since the process is strongly
nonsteady-state and occurs at the instant that the laser pulse
acts
50 fs, current Jt causes the generation of an elec-
tromagnetic pulse Et兲⬃
J/
t, thus generating an electro-
magnetic pulse at THz frequencies.
As a consequence of the given method, THz pulses were
obtained with an energy of several microjoules, a lasing band
width of 70 THz, and an electric field around 100 kV/ cm at
a frequency of 2 THz.
14
EXPERIMENTAL APPARATUS
Experiments on the generation of THz radiation were
carried out using two laser femtosecond systems: a laser sys-
tem with an active medium based on sapphire crystals with
titanium according to the master-generator—stretcher—8-
pass-amplifier-compressor layout in what follows, an FLS
Fig. 1 and a femtosecond fiber system FFS, based on
erbium-doped fibers, EFOA-SH.
We used a Femos-2 femtosecond laser based on a Ti:sap-
phire crystal as the master oscillator MO of the FLS, with
the following parameters: half-width of the lasing spectrum
40 nm, single-pulse width about 20 fs, pulse-repetition rate
637 637J. Opt. Technol. 75 10, October 2008 Bespalov et al.
FIG. 1. Optical block diagram of titanium-sapphire femtosecond laser system. M1–M6—flat mirrors, C1 and C2—wedge substrates, R1—mirror polarization
rotator, L1–L2—lens, SP—ASP100 spectrometers, FD1 and FD2—fast photodiodes, GS—stretcher grating, SM—spherical mirror of the stretcher, MS1–
MS5—flat mirrors of the stretcher, MA1–MA3 and MA6—flat mirrors of the amplifier, FI—Faraday isolator, PC—Pockels cell, —polarizer, —mirror
periscope, MA4 and MA5—spherical mirrors of multipass amplifier, TiSa—titanium-sapphire crystal, MA7 and MA8—reflective telescope, S1—polarization
rotator, S2—polarizer, MP1 and MP2—flat mirrors, L4 and L5—lens telescope, L6—lens that focuses the pump on the crystal, GC1 and GC2—compressor
gratings, MC1 and MC2—flat mirrors, K corner reflector, ME1—flat mirror, AC—ASF-20 autocorrelator.
638 638J. Opt. Technol. 75 10, October 2008 Bespalov et al.
80 MHz, single-pulse energy 1.25 nJ, and mean radiation
power 100 mW. The MO radiation was also used in experi-
ments on the generation of THz radiation.
The vertical polarization of the radiation at the Femos-2
output laser was rotated by 90° by means of mirror polariza-
tion rotator F
mir
, and a femtosecond pulse from the MO was
then incident on a standard two-pass stretcher, using only
reflective optics, where it was stretched in time to about
60 ps. The stretcher consists of a 600-line/ mm diffraction
grating, a spherical mirror with 750-mm focus, and two flat
mirrors.
To prevent the amplified luminescence from the multi-
pass amplifier from affecting the operation of the MO after
the stretcher, an optical-decoupling unit—a broadband Fara-
day isolator—is placed in the layout. Behind the Faraday
isolator, from a sequence of 60-ps pulses with a repetition
rate of 80 MHz, a Pockels cell discriminates a train of opti-
cal pulses with repetition rate 50 Hz the operating frequency
of the pump laser of the amplifier.
The discriminated pulse train enters a multipass tele-
scopic amplifier, where it passes through the active medium
a Ti: sapphire crystal eight times. A pulse of second-
harmonic radiation from a Nd: YAG lamp laser with 12-ns
pulse width and energy up to 14 mJ is simultaneously inci-
dent on the crystal. An attenuator consisting of quartz polar-
ization rotator S
1
and polarizer S
2
is used to smoothly vary
the energy of the pump pulse. In order to amplify a single
60-ps pulse by a factor of 10
6
, the pumping provides an
energy density in the amplifier crystal of about 4 J/ cm
2
in a
600700-
m-diameter beam, and this corresponds to ampli-
fying the field by about a factor of 8 on the first pass in the
amplifier.
After the amplifier, the radiation arrives at a compressor
consisting of two diffraction gratings and a vertical corner
reflector to provide a double pass through the gratings. The
beam coming out of the compressor propagates parallel to
the input beam.
After it comes out of the compressor, the radiation was
analyzed by means of an ASP100 spectrometer and a single-
pulse femtosecond autocorrelator; measurements of the mean
power were made using calibrated calorimeters or semicon-
ductor photodetectors.
At the output of the FLS, the radiation pulses had the
following parameters:
width 3040 fs,
width of the emission spectrum at half the maximum in-
tensity less than 30 nm,
energy in a single laser pulse no less than 1 mJ,
beam diameter at the output from the compressor at half
the maximum intensity 5 mm,
radiation divergence no worse than 10
−3
rad rad,
pulse-repetition rate 50 Hz.
A nonlinear beta-barium borate
-BBO crystal
200
m thick was used to convert the radiation of the FLS to
the second harmonic =400 nm, and the conversion effi-
ciency in this case reached 25%.
The EFOA-SH femtosecond fiber system uses a fiber
doped with Er
3+
ions as an active medium and includes a
master ring fiber laser with passive mode locking, a fiber
amplifier with pumping by two laser diodes, a prism com-
pressor for temporal compression of the pulses after ampli-
fication, and an optical frequency-doubling unit. At the out-
put of the system, the pulses have the following parameters:
wavelengths 1560 and 780 nm,
single-pulse width less than 120 fs,
spectral width about 7.5 nm,
repetition rate 50 MHz,
repetition-rate stability 0.0001%,
mean output power more than 120 mW for =1560 nm
and more than 40 mW for =780 nm,
single-pulse energy 2.4 nJ for =1560 nm and 0.8 nJ for
=780 nm,
beam diameter at the output of the system 5 mm.
The generalized layout of the apparatus for generating
THz radiation using various methods is shown in Fig. 2. The
beam diameter of the THz radiation with central wavelength
at distance L from the radiating surface of the generator
can be estimated from DL=L sin
, where
=1.22 / 2r
0
is
the diffraction divergence of a beam of radius r
0
, which is
determined by the size of the excited region. Computations
show that, for =300
m 1 THz and 2r
0
=500
mata
distance of 120 mm from the THz-radiation generator, the
beam has a diameter of 80 mm. Therefore, to carry out ex-
periments with this radiation, we used parabolic mirrors with
a principal focus of 120 mm and an aperture of 90 mm. A
FIG. 2. a Optical layout for measuring the mean power of THz radiation.
1—FLS, 2—modulator, 3—lens, 4
-BBO crystal, 5—generator of THz
radiation, 6 and 8—parabolic mirrors, 7—filter, 9—OAD. b Magnetic sys-
tem in which the sample is placed; c N-4-dimethylamino-4-N-
methylstilbazolium tosylate DAST crystal.
639 639J. Opt. Technol. 75 10, October 2008 Bespalov et al.
filter made from black Teflon eliminated the pump radiation
incident on the optoacoustic detector OAD. The mean
power of the generated THz radiation was measured by a
nonselective OAD with internal filters that transmitted elec-
tromagnetic radiation in the range 50600
m. The OAD
was a sealed chamber filled with xenon, in which a spectrally
nonselective radiation absorber and an optical microphone
were placed. The electric signal from the optical microphone
arrived at an amplifier with a gain of up to 10
4
and then at a
synchronous detector coupled with the modulator of the in-
put optical radiation. The minimum power that could be re-
corded by the detector system described above was about
1nW.
To create a magnetic field parallel to the surface of the
semiconductor crystal, which is most effective for generating
THz radiation, the sample was placed in a specially devel-
oped magnetic system based on a Nd:B:Fe composite with a
magnetic field of 1.8 kOe at the point of excitation of THz
radiation Fig. 2b. The magnetic system is a vertical cylin-
der 100 mm in diameter and 140 mm high, with two hori-
zontal wedge-shaped recesses that communicate at the center
of the magnetic system, 10.5 mm high. The semiconductor
crystal was placed at the center of the cylinder on its axis, so
that the pump radiation was incident on it through one aper-
ture, while the reflected and THz radiation escaped through
another.
As generators of THz radiation we used an undoped
InAs semiconductor crystal located in the magnetic field, a
ZnTe electrooptic crystal, a DAST organic crystal Fig. 2c,
and an optical spark in air.
EXPERIMENTAL RESULTS
Photoconductive antenna
An undoped InAs crystal, cut on the 100 plane and
consisting of a 5 5-mm plate 300
m thick, was used as a
THz generator. The concentration of majority carriers in the
crystal was about 3 10
16
cm
−3
and the electron mobility
was 310
4
cm
2
/ V sec.
To increase the energy density, the radiation of a femto-
second laser having a lens with f =100 cm was focused on
the surface of the crystal in a spot 500
m in diameter, with
the plane of the InAs crystal at an angle of 45° to the incident
beam, since, as a consequence of the high refractive index
for the far-IR region, THz radiation experiences total internal
reflection, and the reflected radiation can be totally directed
to a parabolic mirror.
When pulses of the MO of the FLS with a single-pulse
energy up to 1 nJ and a mean power of 50 mW were focused
on the InAs surface with no magnetic field, THz radiation
with mean power 2 nJ was recorded. Placing the InAs in a
1.8-kOe magnetic field in a direction parallel to the crystal
surface increased the power of the generated radiation to
150 nW. The dependence of the mean power of the THz
radiation on the mean pump power was quadratic, and this
agrees with the experimental data of Ref. 8 and the theory of
Ref. 15 for optical-excitation energy densities below the
saturation density. The maximum conversion efficiency
,
defined as the ratio of the emitted mean power W
THz
of the
THz radiation to the mean power W
0
of the optical radiation
incident on the crystal, was obtained at a value of W
0
=100 mW and equalled
10
−6
.
When pulses of the FLS with a single-pulse energy of up
to 1 mJ and a mean power of up to 50 mW were focused on
the surface of the InAs in the magnetic system, the maximum
conversion efficiency increased to
10
−5
. When the de-
pendence of the mean power of the THz radiation on the
mean power was studied, saturation was clearly observed
when the power density of the radiation was 10
−4
J/ cm
2
or
with a radiation flux of 10
14
photon/ cm
2
, approximately cor-
responding to the number of majority carriers in the near-
surface layer of the crystal.
Optical rectification
When pulses of the FLS with single-pulse energy up to
1 mJ and mean power up to 50 mW were focused and after
they passed through a ZnTe crystal 4 mm thick or a DAST
crystal 100
m thick, a spectral supercontinuum was
generated;
16
therefore, a starting laser beam 5 mm in diam-
eter was used in the experiments. Under these conditions, a
mean THz-radiation power reaching 100 nW was obtained in
the ZnTe crystal. When the DAST crystal was used in the
same geometry and with the same pump parameters, the
mean THz-radiation power increased and reached 800 nW.
As is well known,
10
DAST is a uniaxial nonlinear-optical
crystal; the generation efficiency of the THz radiation ac-
cordingly depends on the relative alignment of the DAST
crystal’s crystallographic axes and the polarization of the ra-
diation. Figure 3 shows how the THz-radiation intensity de-
pends on the crystal’s angle of rotation. The pronounced
maxima show that the generation efficiency depends on the
relative alignment of the crystal’s axes and the polarization
of the exciting radiation. The 0°–180° direction Fig. 3 cor-
responds to the vertical axis of the DAST crystal in Fig. 2c,
with the polarization of the pump radiation being horizontal.
The high conversion efficiency in the DAST crystal
made it possible to measure the beam’s spatial profile by the
Foucault knife-edge method Fig. 4. Knowing the beam pro-
file between the parabolas and the size of the excited region
FIG. 3. Dependence of the generation intensity I of THz radiation on the
rotation angle of the DAST crystal.
640 640J. Opt. Technol. 75 10, October 2008 Bespalov et al.
of the crystal, it is easy to calculate the frequencies that
contribute the most to the emission spectrum. Starting from
the diffraction calculations, it was shown that the maximum
of the THz emission spectrum comes at about a frequency of
1.2 THz.
Experiments were also carried out on the excitation of
THz radiation in the DAST crystal, using the pulses of the
FFS with a single-pulse energy of 0.8 nJ, a mean power of
40 mW, and a wavelength of 780 nm. In the geometry of
Fig. 2, focusing the pump radiation with a lens having f
=10 cm into a spot 50
m in diameter, a mean THz-
radiation power was obtained of up to 80 nW, and this
opened up prospects of using compact sources of femtosec-
ond radiation for THz applications. It should be pointed out
that, in a layout with the same geometry and the same pa-
rameters, the mean THz power was only 8 nW after the
pump beam passed through a ZnTe crystal 4 mm thick.
Generation using optical breakdown
The experiments were carried out using the layout
shown in Fig. 2, with a
-BBO crystal that generates second-
harmonic radiation =400 nm, placed between the lens
and its focus in order to make the pulses of the first and
second harmonic coincide in time. The crystal was attached
to a linear translator and was displaced along the beam axis
to provide accurate phase locking between the waves of the
first and second harmonics.
1
An FLS was used in the experiments, and the optical-
breakdown threshold in air was at a total single-pulse energy
of the first and second harmonic of about 100
J the mean
power is 5 mW. Radiation of the THz range and a spectral
supercontinuum appeared at the same time as the breakdown.
As the BBO crystal moved along the beam axis in the direc-
tion of the focus of a lens having f =15 cm, a sinusoidal
variation of the mean THz-radiation power with contrast
above 50% was observed Fig. 5. With the maximum total
single-pulse energy of the first and second harmonic of W
1 mJ and a mean power of 50 mW, a mean THz power of
up to 20 nW was obtained, with the dependence of the mean
THz power on the mean pump power being close to expo-
nential.
The sinusoidal variation of the mean THz-radiation
power when the BBO crystal moves along the beam axis can
be explained by the addition of the electric fields of the
waves of the first and second harmonics of the radiation of
the FLS and by the motion of the plasma electrons in the
given biharmonic field. This is shown by the fact that a pe-
riod of variation equal to 30 mm corresponds to a delay time
of 0.77 fs between pulses of the first and second harmonics
of the FLS radiation because of the dispersion of air and
approximately corresponds to half the vibrational period at a
wavelength of =400 nm T=1.33 fs and a quarter of the
vibrational period at a wavelength of =800 nm T
=2.67 fs.
CONCLUSION
This paper has presented experimental results and an
analysis of the generation of THz radiation using femtosec-
ond laser sources: generation by photoconductive semicon-
ductor antennas, nonlinear-optical generation of the differ-
ence frequency or optical rectification, and generation using
optical breakdown in gases by means of femtosecond pulses.
The following were used as generators of THz radiation: an
undoped InAs semiconductor crystal in a magnetic field as a
photoconductive antenna, electrooptic crystals of ZnTe and
DAST, and an optical spark in air with two-frequency exci-
tation. It has been determined that the most promising gen-
erators of THz waves are DAST crystals, which efficiently
generate a radiation pulse with central frequency 1.2 THz
even when they are excited by relatively low-power laser
sources. This opens up prospects of using compact
femtosecond-radiation sources for applications.
A sinusoidal character has been detected for the depen-
dence of the mean power of the THz radiation on the dis-
placement of a BBO crystal along the beam axis with gen-
eration using optical breakdown. This is explained by the
influence of the addition of electric fields of the waves of the
first and second harmonics and by the formation of a plasma.
This work was carried out with the support of Grants
Nos. 06-02-08317-ofi, 06-02-17303-a, and 07-02-13562-
ofits of the Russian Foundation for Basic Research. The
authors express gratitude to Yu. É. Burunkova for fabricating
and providing the samples of DAST crystals.
FIG. 4. Profile of THz beam accompanying excitation in the DAST crystal.
FIG. 5. Mean THz-radiation power vs shift of the BBO crystal along the
axis of the pump beam.
641 641J. Opt. Technol. 75 10, October 2008 Bespalov et al.
1
Calculations show that, when two-color radiation passes through a
lens 10 mm thick, as a consequence of the dispersion of the glass and,
accordingly, the different group velocities of the two pulses, the pulse of
the first harmonic outruns the pulse of the second by 50 fs. When it propa-
gates in air, the dispersion between the pulses of the first and second
harmonics is 0.257 fs/ cm http://www.kayelaby.npl.co.uk/generalphysics/
25/257.html.
1
K. Reimann, “Table-top sources of ultrashort THz pulses,” Rep. Prog.
Phys. 70, 1597 2007.
2
E. Pickwell and V. P. Wallace, “Biomedical applications of terahertz tech-
nology,” J. Phys. D: Appl. Phys. 39, 301 2006.
3
D. A. Newnham and P. F. Taday, “Pulsed terahertz attenuated total reflec-
tion spectroscopy,” Appl. Spectrosc. 62, 394 2008.
4
M. Kress, T. Löffler, S. Eden, M. Thomson, and H. G. Roskos, “Terahertz-
pulse generation by photoionization of air with laser pulses composed of
both fundamental and second-harmonic waves,” Opt. Lett. 29, 1120
2004.
5
D. H. Auston, “Picosecond optoelectronic switching and gating in sili-
con,” Appl. Phys. Lett. 26,1011975.
6
D. M. Mittleman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd,
and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. 68,
1085 1999.
7
V. G. Bespalov, V. N. Krylov, S. É. Putilin, and D. I. Stasel’ko, “Gener-
ating radiation in the far-IR region with femtosecond optical excitation of
the semiconductor InAs in a magnetic field,” Opt. Spektrosk. 93, 158
2002兲关Opt. Spectrosc. 93, 148 2002兲兴.
8
N. Sarukura, H. Ohtake, S. Izumida, and Z. Liu, “High average-power
THz radiation from femtosecond laser-irradiated InAs in a magnetic field
and its elliptical polarization characteristics,” J. Appl. Phys. 84,654
1998.
9
J. Hebling, K.-L. Yeh, M. C. Hoffmann, B. Bartal, and K. A. Nelson,
“Generation of high-power terahertz pulses by tilted-pulse-front excitation
and their application possibilities,” J. Opt. Soc. Am. B 25,62008.
10
Y. Mori, Y. Takahashi, T. Iwai, M. Yoshimura, Y. Khin Yap, and T. Sasaki,
“Slope nucleation method for the growth of high-quality
4-dimethylamino-methyl-4-stilbazolium-tosylate DAST crystals,” Jpn. J.
Appl. Phys. 39, L1006 2000.
11
A. Schneider, M. Neis, M. Stillhart, B. Ruiz, R. U. A. Khan, and P. Gunter,
“Generation of terahertz pulses through optical rectification in organic
DAST crystals: theory and experiment,” J. Opt. Soc. Am. B 23, 1822
2006.
12
D. J. Cook and R. M. Hochstrasser, “Intense terahertz pulses by four-wave
rectification in air,” Opt. Lett. 25, 1210 2000.
13
X. Xie, J. Xu, J. Dai, and X. C. Zhang, “Enhancement of terahertz wave
generation from laser-induced plasma,” Appl. Phys. Lett. 90, 141104
2007.
14
K.-Y. Kim, B. Yellampalle, J. H. Glownia, A. Taylor, and G. Rodriguez,
“Intense coherent terahertz radiation from two-color photocurrent mixing
in atmospheric air,” Optical Terahertz Science and Technology. OSA Tech-
nical Digest Series CD兲共Optical Society of America, 2007, paper TuD7.
15
P. K. Benicewicz, J. P. Roberts, and A. J. Taylor, “Scaling of terahertz
radiation from large-aperture biased photoconductors,” J. Opt. Soc. Am. B
11, 2533 1994.
16
A. M. Zheltikov, “Let there be white light: supercontinuum generation by
ultrashort laser pulses,” Usp. Fiz. Nauk 176,6232006兲关Phys. Usp. 49,
605 2006兲兴.
642 642J. Opt. Technol. 75 10, October 2008 Bespalov et al.
... To study the optical and electrical properties of graphene thin films in the THz range, we used an upgraded spectroscopy system [11,12]. This system is based on the THz radiation generation using a InAs crystal in a static magnetic field [13], and the detection using a CdTe crystal with the electro-optic sampling. Using a system of high-precision passive THz polarizers, we have added the ability to perform polarimetric measurements [14][15][16]. ...
Article
Full-text available
Efficient devices for control temporal and spatial properties of electromagnetic waves are essential for the development of terahertz (THz) technologies. But despite the great progress achieved in a study of graphene, the influence of the number of graphene layers on its optical and electrical properties in the THz frequency range has not yet been sufficiently studied. In this work, we experimentally studied optical and electrical properties of multilayer graphene (MLG) thin films in the frequency range 0.2–0.8 THz (corresponding to a wavelength range ∼1.5–0.37 mm), at a controlled room temperature of 291 K, and a relative humidity of 40 %. Using our custom-made THz time-domain spectroscopic polarimetry system, we obtained spectra of the complex relative permittivity and the electrical conductance of the chemical vapor deposition graphene with ∼14, ∼40, and ∼76 layers of graphene on borosilicate glass substrates. It is shown that the conductance increases nonlinearly with an increase in the graphene layer number and reaches, for ∼76 layers, 0.06 S for the real, and 0.03 S for the imaginary part, respectively. Our results show that by using MLG it is possible to create tunable devices that can be used in the advanced areas of THz photonics.
... Bulk InAs crystal in 2.4 T magnet field used as THz generator. 19 Lens 3 position regulates the pump power density at InAs crystal. Filter cuts off the reflected pump beam. ...
Conference Paper
In this contribution, we discuss the features of design and polarimetric inspection of terahertz achromatic waveplates. The design of the crystalline quartz half-wave plate was performed taking into account the ellipticity and the introduced phase difference between the orthogonal components of the output radiation polarization vector. The designed waveplate are relatively thin, work equally efficiently in the frequency range from 0.4 to 1.4 THz, and, most importantly, are cheap to manufacture. The modification of the experimental terahertz time-domain spectroscopy polarimetric setup involving direct waveform detection is proposed. The proposed polarimetry THz time-domain spectrometer with electro-optic detection shows parasitical signals absence and easier measurement procedure.
... To study the polarization properties of the experimental samples using the THz-TDSP method [11], a system based on a THz time-domain spectrometer [12], three wire grid polarizers, a 980 nm laser for creating the external OP of 0.2 Wcm −2 , 0.6 Wcm −2 , and 1.0 Wcm −2 , and an axially magnetized NdFeB magnet for creating an external static MF of ∼0.3 T were used. ...
Conference Paper
Full-text available
Terahertz time-domain spectroscopic polarimetry (THz-TDSP) method was used to experimental study polarization properties of unaligned single-wall carbon nanotube thin films with different geometric parameters on transparent float glass substrates in a frequency range from 0.2 THz to 0.8 THz (corresponding to a wavelength range from ~1.50 mm to ~0.37 mm) at a controlled room temperature of 291–293 K, and a relative humidity of 40–45%. Frequency dependences of azimuth and ellipticity angles of a polarization ellipse (PE) of electromagnetic waves transmitted through the samples, and PEs at the frequencies of 0.2 THz, 0.5 THz, and 0.8 THz were obtained for values of 0.2 W cm−2, 0.6 W cm−2, and 1.0 W cm−2 of an external 980 nm near infrared optical pumping, with an external static magnetic field of ~0.3 T. Polarization properties were calculated from temporal waveforms of signals transmitted through the samples at the parallel and the crossed by 45° positions to a transmission direction of the polarizers. A change of 15° in the azimuth angle, and of 10° in the ellipticity angle was achieved. The results show that by using carbon nanomaterials-based structures it is possible to devise efficient and affordable magneto-optically tunable polarization modulators that can be used in the advanced areas of terahertz nanoscience and nanotechnologies.
Article
Full-text available
A method for creating of a three-dimensional nonlinear optical grating based on the alternation of layers with different nonlinear optical properties has been developed. The spatial structure of the lattice is formed by an active layer from a polymethylmethacrylate (PMMA) matrix with dimethyl amino -4-n-methylstilbazolium-tosylate (DAST) nanocrystals and a photopolymer used as an inactive layer. The absorption and refraction terahertz spectral dependencies of the photopolymerizable composition and the DAST - PMMA nanocomposite have been studied. Obtained results allow us to consider this material as a good candidate for terahertz photonics. The processes of terahertz generation in the DAST - PMMA nanocomposite by the optical rectification of femtosecond laser pulses have been investigated and high generation efficiency have been demonstrated. The nonlinear optical grating based on the indicated components was created and its structure was investigated.
Conference Paper
Efficient devices for control properties of electromagnetic waves are essential for the development of terahertz (THz) technologies. But despite the great progress achieved in a study of graphene, the influence of the number of graphene layers on its properties in the THz frequency range has not yet been sufficiently studied. In this work, we experimentally studied properties of multilayer graphene (MLG) films in the frequency range 0.2–0.8 THz, at a room temperature, and a relative humidity of 40%. Using our custom-made THz time-domain spectroscopic polarimetry system, we obtained spectra of the complex relative permittivity and the electrical conductance of the chemical vapor deposition graphene with ~14, ~40, and ~76 layers of graphene on glass substrates. It is shown that the conductance increases nonlinearly with an increase in the graphene layer number and reaches, for ~76 layers, 0.06 S for the real, and 0.03 S for the imaginary part, respectively.
Presentation
Full-text available
Magneto-optic Faraday effect in unaligned single-wall carbon nanotube thin films with different geometric parameters on transparent float glass substrates was experimental studied in a frequency range 0.2–0.8 THz (corresponding to a range from ~1.50 mm to ~0.37 mm) at a controlled room temperature of 291–293 K, and a relative humidity of 40–45%. A change of 15° in an azimuth angle, and of 10° in an ellipticity angle was achieved. The results show that by using carbon nanomaterials-based structures it is possible to devise efficient tunable polarizers that can be used in the advanced areas of terahertz nanophotonics.
Conference Paper
Magneto-optic Faraday effect in unaligned single-wall carbon nanotube thin films with different geometric parameters on transparent float glass substrates was experimental studied in a frequency range 0.2–0.8 THz (corresponding to a range from ~1.50 mm to ~0.37 mm) at a controlled room temperature of 291–293 K, and a relative humidity of 40–45%. A change of 15° in an azimuth angle, and of 10° in an ellipticity angle was achieved. The results show that by using carbon nanomaterials-based structures it is possible to devise efficient tunable polarizers that can be used in the advanced areas of terahertz nanophotonics.
Article
Full-text available
We proposed a simple and cost-effective method to manipulate the temporal and spectral properties of pulsed terahertz waves. A deep modulation of a pulse spectrum was both numerically and experimentally verified using Fresnel apertures with a radius ranging from several to several tens of the central wavelength of the broadband terahertz radiation. N-fold frequency minima were formed in the spectrum at a specific axial position behind the filter. Non-paraxial properties of this filter were also analyzed. A significant value (35%) of the ratio of the longitudinal to the transverse field component at the filter frequency was obtained. The measured results agree well with the simulation and theoretical predictions. The property of such a diffractive Fresnel notch filter can benefit the generation of longitudinal terahertz fields and relevant applications.
Article
Full-text available
The THz-radiation power from bulk InAs irradiated with femtosecond optical pulses is significantly enhanced and reaches 650 μW in a 1.7-T magnetic field with 1.5-W excitation power. The THz-radiation power is related almost quadratically both to the magnetic field and excitation laser power. We have also found that the power of the THz-radiation from an InAs sample in a magnetic field is over one order of magnitude higher than that from GaAs. Additionally, a dramatic change of ellipticity is observed, and the spectra of the horizontal and vertical polarization components are found to differ. © 1998 American Institute of Physics.
Article
Full-text available
We review the development of terahertz (THz) technology and describe a typical system used in biomedical applications. By considering where the THz regime lies in the electromagnetic spectrum, we see that THz radiation predominantly excites vibrational modes that are present in water. Thus, water absorption dominates spectroscopy and imaging of soft tissues. However, there are advantages of THz methods that make it attractive for pharmaceutical and clinical applications. In this review, we consider applications ranging from THz spectroscopy of crystalline drugs to THz imaging of skin cancer.
Article
Full-text available
We present a combined theoretical and experimental investigation of the generation of few-cycle terahertz (THz) pulses via the nonlinear effect of optical rectification and of their coherent detection via electro-optic sampling. The effects of dispersive velocity matching, absorption of the optical and the THz waves, crystal thickness, pulse diameter, pump pulse duration, and two-photon absorption are discussed. The theoretical cal-culations are compared with the measured spectra of THz pulses that have been generated and detected in crystals of the highly nonlinear organic salt 4-N , N-dimethylamino-4-N-methyl stilbazolium tosylate (DAST). The results are found to be in agreement with the theory. By the selection of the optical pump wavelength between 700 and 1600 nm, we achieved several maxima of the overall generation and detection efficiency in the spectral range between 0.4 and 6.7 THz, with an optimum at 2 THz generated with 1500 nm laser pulses.
Article
Full-text available
We review recent progress in the field of terahertz “T-ray” imaging. This relatively new imaging technique, based on terahertz time-domain spectroscopy, has the potential to be the first portable far-infrared imaging spectrometer. We give several examples which illustrate the possible applications of this technology, using both the amplitude and phase information contained in the THz waveforms. We describe the latest results in tomographic imaging, in which waveforms reflected from an object can be used to form a three-dimensional representation. Advanced signal processing tools are exploited for the purposes of extracting tomographic results, including spectroscopic information about each reflecting layer of a sample. We also describe the application of optical near-field techniques to the THz imaging system. Substantial improvements in the spatial resolution are demonstrated.
Article
The principles and most-recent results of high-power THz generation through optical rectification using a tilted optical pulse front are described. Single-cycle THz pulses of multimicrojoule energies are generated at kHz repetition rates, and average THz power levels exceeding 1 mW can be generated at kHz–MHz repetition rates. Applications in nonlinear THz spectroscopy and THz coherent control are discussed.
Article
We have developed a new technique called the slope nucleation method (SNM) for the growth of high-quality 4-dimethylamino-methyl-4-stilbazolium-tosylate (DAST) crystals. This technique combines the spontaneous nucleation and subsequent growth of a single crystal into one process. The SNM features the ability to control the nucleation position and the growth orientation of DAST crystals. Many single crystals can be grown simultaneously in one process. X-ray diffraction (XRD) indicates that the SNM is effective for growing higher-quality DAST crystals as compared to conventional spontaneous nucleation and the top-seeded solution growth technique. DAST crystals with an XRD rocking curve as narrow as 20.2 arcsec full-width at half maximum (FWHM) were obtained.
Conference Paper
A transient photocurrent model is developed to explain terahertz emission from ultrafast ionization of air irradiated by femtosecond two-color laser fields. THz power scalability was also examined resulting in generation of 150 kV/cm field amplitudes.
Article
Three centuries after Newton's experiments on the decomposition of white light into its spectral components and the synthesis of white light from various colors, nonlinear-optical transformations of ultrashort laser pulses have made it possible to produce an artificial white light with unique spectral properties, controlled time duration, and a high spectral brightness. Owing to its broad and continuous spectrum, such radiation is called supercontinuum. The laser generation of white light is an interesting physical phenomenon and the relevant technology is gaining in practical implications — it offers novel solutions for optical communications and control of ultrashort laser pulses, helps to achieve an unprecedented precision in optical metrology, serves to probe the atmosphere of the Earth, and suggests new strategies for the creation of compact multiplex light sources for nonlinear spectroscopy, microscopy, and laser biomedicine. Here, we provide a review of physical mechanisms behind the laser generation of white light, examine its applications, and discuss the methods of generation of broadband radiation with controlled spectral, temporal, and phase parameters.
Article
In this paper techniques for the generation and measurement of ultrashort pulses in the frequency range from about 0.1 to 10 THz are reviewed. The methods for generation are restricted to table-top systems based on short-pulse lasers in the visible or in the near-infrared. Three techniques are dealt with in detail: photoconductive switches, difference frequency generation and plasma sources. Definitions and methods to measure the pulse width are given, among them cross-correlation and measurements of the electric field of these pulses as a function of time by photoconductive switches and electro-optic sampling.
Article
Generation of a coherent electromagnetic radiation in the far IR (THz) spectral range upon excitation of a semiconductor InAs crystal by 70-fs Ti: sapphire laser pulses is studied. The effect of a magnetic field of different orientation on generation in the submillimeter-wavelength range is analyzed. Placing the crystal into the magnetic field of an optimized permanent magnet with a strength of 5 kOe aligned along the surface of the semiconductor increased the power of generated radiation by a factor of six compared with that in the absence of the field. For the average pump-laser output power of 150 mW and repetition rate of 80 MHz, the average power of the THz radiation reached 100 nW. For detection of ultrashort pulses of the THz radiation, we used, for the first time, a highly sensitive uncooled optoacoustic detector, which detected signals with a power lower than 1 nW.