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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
共Submitted May 22, 2008兲
Opticheski Zhurnal 75,34–41共October 2008兲
This paper presents experimental results and an analysis of methods of generating terahertz
共THz兲 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 共InAs兲 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 共a few vibrations of the light
field in width兲 superbroadband terahertz 共THz兲 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 共especially lasers based on
sapphire crystals doped with titanium ions兲 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 J共t兲 that
appears in this case causes an electromagnetic pulse E共t兲
⬃
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
1–2-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 共about 3⫻10
4
cm
2
/ V sec兲 is pos-
636 636J. Opt. Technol. 75 共10兲, 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
共2兲
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共
THz
兲 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共
THz
兲⬃
共2兲
E共
1
兲E共
2
兲⬃
共2兲
E
0
2
共1兲
in the frequency region, where E共
1
兲 and E共
2
兲 are the Fou-
rier components of the spectrum of the femtosecond pulse,
while
THz
=兩
1
−
2
兩. 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 共the pulse width兲 of femtosecond ra-
diation is usually 10 THz 共100 fs兲, 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 共DAST兲.
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, 共2兲
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 共the co-
herence length兲 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
共n
gr
− n
THz
兲
, 共3兲
where n
gr
共兲=n共兲−共
n/
兲
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 0.1–1 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
共3兲
. The process is described as follows: Polar-
ization P共
THz
兲 at a THz frequency arises when three waves
interact—two pump waves at the fundamental frequency,
E共
1
兲 and E共
2
兲, and a wave of the second harmonic,
E共2
兲; i.e.,
P共
THz
兲⬃
i,j,k ,l
共3兲
E共2
兲E共
1
兲E共
2
兲. 共4兲
It should be pointed out that, for THz radiation to appear,
the presence of a plasma 共optical breakdown of the gas兲—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 共
⬍50 fs兲, current J共t 兲 causes the generation of an elec-
tromagnetic pulse E共t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 共in what follows, an FLS兲
共Fig. 1兲 and a femtosecond fiber system 共FFS兲, 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 共MO兲 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 共10兲, 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 共10兲, 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 共the operating frequency
of the pump laser of the amplifier兲.
The discriminated pulse train enters a multipass tele-
scopic amplifier, where it passes through the active medium
共a Ti: sapphire crystal兲 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
600–700-
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 30–40 fs,
• width of the emission spectrum 共at half the maximum in-
tensity兲 less than 30 nm,
• energy in a single laser pulse no less than 1 mJ,
• beam diameter at the output from the compressor 共at half
the maximum intensity兲 5 mm,
• radiation divergence no worse than 10
−3
rad rad,
• pulse-repetition rate 50 Hz.
A nonlinear beta-barium borate 共

-BBO兲 crystal
200
m thick was used to convert the radiation of the FLS to
the second harmonic 共=400 nm兲, 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 D共L兲=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 共1 THz兲 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. 共a兲 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. 共b兲 Magnetic sys-
tem in which the sample is placed; 共c兲 N-4-dimethylamino-4-N-
methylstilbazolium tosylate 共DAST兲 crystal.
639 639J. Opt. Technol. 75 共10兲, October 2008 Bespalov et al.
filter made from black Teflon eliminated the pump radiation
incident on the optoacoustic detector 共OAD兲. 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 50–600
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 共Fig. 2b兲. 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 共Fig. 2c兲,
and an optical spark in air.
EXPERIMENTAL RESULTS
Photoconductive antenna
An undoped InAs crystal, cut on the 关100兴 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 3⫻10
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 共Fig. 3兲 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 共Fig. 4兲. 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 共10兲, 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 共 =400 nm兲, 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兲
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 共the mean
power is 5 mW兲. 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 共Fig. 5兲. 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 共T=1.33 fs兲 and a quarter of the
vibrational period at a wavelength of =800 nm 共T
=2.67 fs兲.
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-
ofiគts 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 共10兲, October 2008 Bespalov et al.
1兲
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 共http://www.kayelaby.npl.co.uk/generalគphysics/
2គ5/2គ5គ7.html兲.
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