Application Notes
Broadband terahertz generation and detection
Widely tunable and narrowband THz sources
Applications of THz waves

The interest in terahertz electromagnetic radiation stems from the unique interactions ofthese rays with matter, which can be exploited in various applications. Terahertz waves
excite molecular vibrations and lattice vibrations in materials, which make THz radiation
very interesting for spectroscopy and material identification.
Terahertz radiation is non-ionizing, very sensitive to water and hydration state, andtransparent to non-polar substances such as nonconductive polymers, paper, packing
material, etc. Therefore material irregularities, defects, enclosures, not seen with othertechniques, can be visualized with terahertz radiation. THz radiation has therefore ahigh potential for the non-destructive materials testing.
There are several techniques to produce terahertz radiation. We have developed novel
organic THz generators, with optimized properties that make them ideal materials forthe efficient generation and detection of terahertz radiation. Based on these materials
we developed compact systems for THz time-domain spectroscopy, TeraSys® and
TeraKit®, as well as for THz imaging, TeraIMAGE®. We have also developed a unique
tunable single-frequency THz source, TeraTune®, with a very wide tunability range 1–
20 THz and a narrow linewidth < 100 GHz.
Terahertz radiation
The terahertz range of the electromagnetic spectrum is situated between highfrequency electronics (microwaves) and long wavelength photonics (infrared light).
Terahertz radiation is easily accessible by means of blackbody radiation, but it is achallenge
toseparatethesignalsfromthenaturalbackground.
Several techniques have been used to generate electromagnetic waves below andabove this so-called "THz frequency gap". Figure 1 shows the rough power levels ofsome of them. Electronic techniques can be used to generate waves with frequencies(mainly by electronic frequency multiplication of lower frequency sources) up to about
0.5 THz. From 0.3 to 3 THz Auston switches are very popular sources. Nonlinear opticaltechniques (optical rectification and difference-frequency generation) can be used tocover the frequency range between 0.3 to 50 THz and quantum-cascade lasers
between roughly 20 THz and 100 THz.

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Figure 1: Approximate output
power (either CW or peak
power) as a function of
frequency (wavelength) ofelectromagnetic radiation in
the THz range and aroundwhen using different
generation techniques.
Adapted and updated from
M. Tonouchi, Nat.
Photonics 1, 97 (2007).

Broadband THz sources
Most broadband THz sources are based on the excitation of different materials withultrashort laser pulses in the femtosecond range. Photoconduction and opticalrectification are two of the most common approaches for generating broadband THz
pulses. Optical methods are used for the generation of broadband terahertz sources,and due to the increasing progress in laser technology these methods have been themost developed in the last 20 years.
In the photoconductive approach a femtosecond laser generates an ultrafast
photocurrent in a photoconductive switch or semiconductor using electric-field carrier
acceleration. The achievable bandwidth is limited to a few THz due intrinsic limits of thecarrier lifetime in semiconductors.
Optical Rectification is an alternative mechanism for pulsed THz generation. A
femtosecond laser is used as well, and the energy of the terahertz radiation comes
directly from the exciting laser pulse. In this case the conversion efficiency depends,
beside the parameters of the pump laser, mainly on the electro-optic coefficient and theproximity of the phase-matching conditions of the material.
In optical rectification, a high-intensity ultrashort laser pulse passes through atransparent crystal material that emits a terahertz pulse without any applied voltages.
Figure 2 shows a schematic of optical rectification with a pulsed femtosecond laser withorganic crystal generators DAST or DSTMS. In this nonlinear-optical process, anonlinear material is quickly electrically polarized at high optical intensities. Thischanging electrical polarization emits terahertz radiation. It is called rectification
because the rapid oscillations of the electric field of the laser pulse are “rectified” andonly the envelope of the oscillations remains. Since the medium absorption is low, thepolarization instantaneously follows the pulse envelope implying that there is practicallyno limit on the speed at which the polarization can be switched on and off, i.e. there isno intrinsic limit on the bandwidth as for the photoconductive antennas.
The bandwidth of the THz pulse generated by the optical rectification process is limitedby the length of the pump laser and the velocity matching conditions in the generator
and detector material. THz bandwidth achievable for pump laser sources with various
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pulse lengths is illustrated in Figure 3. The materials for THz generation by opticalrectification are discussed in the following section.
Figure 2: Scheme of the setup for
generation and detection of THz pulses inorganic THz generators DAST or DSTMS.
The generation is based on opticalrectification of femtosecond pulses, while
the detection is based on electro-opticallyinduced changes of the refractive index in a
THz detector, induced by the THz electricfield and probed by an optical probe beam.
For inorganic semiconductors most often
the electro-optic sampling is used,1 while we
use THz induced lensing in case of organic
crystals.2

Figure 3: Terahertz frequency range forfemtosecond laser sources with various
pulse lengths (FWHM from 25 fs to 100 fs)
in case of perfectly phase-matched THz
generation materials.

Materials for THz generation
Due to the larger nonlinear optical susceptibilities and velocity matching between THz
and optical pump waves of organic materials compared to the inorganic ones, muchlarger power levels, limited by the damage thresholds of the materials, can be obtainedby using organic materials as THz generators.
Table 1 shows most relevant material parameters for THz wave generation for bestorganic crystals compared to inorganic crystals as well as for an electro-optic polymer
LAPC. As it can be seen in this table, the organic crystals OH1, DSTMS and OH1 show
the largest figure of merit FMTHz. Phase matching is possible when the THz refractive
index nTHz is close to the group index ng at the mean optical pump wavelength.3 Thiskind of phase matching is usually referred to as group velocity matching and iscommonly used for broadband THz-wave generation by optical rectification of fs pumplasers. 3
1

G. Gallot and D. Grischkowsky, "Electro-optic detection of terahertz radiation," J. Opt. Soc. Am. B 16, 1204
(1999)
2
A. Schneider, I. Biaggio, P. Günter, “Terahertz-induced lensing and its use for the detection of terahertz pulses in abirefringent crystal,” Appl. Phys. Lett. 84, 2229 (2004)
3
Schneider, A.; Neis, M.; Stillhart, M.; Ruiz, B.; Khan, R. U. A. & Gunter, P., “Generation of terahertz pulses
through optical rectification in organic DAST crystals: theory and experiment,” J. Opt. Soc. Am. B, 2006, 23, 18221835
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no

ng

nTHz

r
(pm/V)FMTHz(pm/V)2

DAST
DSTMS
OH1
LAPC
GaAs
ZnTe
InP
GaP
ZnS
CdTe
LiNbO3

2.13
2.08
2.16
1.6
3.37
2.83
3.2
3.12
2.3
2.82
2.2

2.26
2.19
2.33
1.8
3.61
2.18
3.16

2.29
2.20
2.28
1.7
3.63
3.16
3.54
3.34
2.88
3.24
4.96

47
49
52
52
1.6
4
1.45
1
1.5
6.8
28

5600
6100
7400
1700
66
160
40
17
7
470
1100

FM THz =n o6 r 2
16n THz

2.18

Table 1: Relevant parameters oforganic and inorganic nonlinear
optical materials that have been
investigated for optical-to-THz
frequency conversion. Refractive
index no and the group index ng at thepump optical wavelength; refractive
index nTHz in the THz range; electrooptic coefficient r; figure of merit
FMTHz for THz generation by opticalrectification.
Organic crystals:
DAST, DSTMS, OH1
LAPC: guest-host polymer[X.M. Zheng et al, J. Nanoelectron.
Optoelectron. 2, 58 (2007)]
Inorganic crystals: GaAs, ZnTe, InP,
GaP, ZnS, CdTe, LiNbO3Best inorganic electro-optic materials, such as LiNbO3 are far from optimal phasematching conditions and can only be used in special configurations. Therefore, althoughhaving lower electro-optic coefficients and figures of merit, most commonly
semiconducting electro-optic materials, such as ZnTe are used because they canoperate close to phase matching. Organic materials combine both high figures of meritand possibility for velocity matching, therefore we chose these materials for our
instruments. Organic crystals DAST, DSTMS and OH1 show best phase matching for fs
laser sources in the telecom wavelength range 1200–1600 nm, which makes them very
attractive for compact table-top THz instruments. They can also operate close tovelocity matching in a broad range of THz frequencies, making possible broadband THz
generation with low-power fs laser sources. Figure 4 shows an example of a broadband
THz field generated in organic crystal DSTMS using TeraKit® in comparison with the
filed generated by a semiconducting antenna using a compact fs fiber laser source.

Figure 4: THz field
amplitude as a function of
frequency for a THz pulse
generated in an organic
crystal DSTMS (inTeraKit®) using
femtosecond pump lasers
and THz time-domain
spectroscopy, andtypical range reachedwith PC antennas.

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Organic THz generators and detectors
Terahertz generators and detectors are produced and optically prepared at the facilities
of Rainbow Photonics in Switzerland. Rainbow Photonics AG is the worldwide only
commercial producer of organic single crystalline THz generators.
Figure 5: THz generators/detectors optically
prepared and mounted for applications. Standard
aperture sizes range from 2 to 5 mm; larger sizes
are available upon request.
The generators exhibit a high damage threshold ofover 150 GW/cm2 at 150 fs pulse length and 1500
nm.
High THz electric fields of over MV/cm have been
generated by our generators.4,5

Figure 6 shows best application ranges of our THz generators: pump lasers in the

wavelength range 1200–1600 nm are most suitable. Shorter the pulse, larger will be thegenerated THz bandwidth when using optical rectification, as shown in Figure 3.
The list of publications using our organic crystals DAST, DSTMS and OH1 can be foundhere: http://www.rainbowphotonics.com/pdf/Publications_THz_generators.pdf

Figure 6: Best application ranges of organic single-crystalline THz generators DAST, OH1 andDSTMS. The shaded ranges present areas of coherence length exceeding 0.5 mm. Pulsed
lasers with wavelengths between 1200 and 1600 nm are best suited as pump lasers for our THz
generators.

Detection: In THz time domain spectroscopy, one can use common techniques for thedetection of THz signals generated in organic crystals. To achieve best detection
efficiency in a broad THz range, same organic materials can be also used for electrooptic detection using a different principle as for standard electro-optic sampling,1 which
is limited to optically isotropic materials. For organic crystals (or other birefringent
materials) THz-induced lensing can be used with a similar sensitivity as the electro-optic
sampling.2

4

Hauri, C. P.; Ruchert, C.; Vicario, C. & Ardana, F., "Strong-field single-cycle THz pulses generated in an organic
crystal," Appl. Phys. Lett. 99, 161116 (2011)
5
Ruchert, C.; Vicario, C.; Hauri, C. P., "Scaling submillimeter single-cycle transients toward megavolts per
centimeter field strength via optical rectification in the organic crystal OH1," Opt. Letters 37, 899 (2012)
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Terahertz time-domain spectroscopy and imaging with TeraKit® and TeraIMAGE®
The TeraKit® provides a flexible solution for laboratory terahertz spectroscopy. It isbased on organic crystals, to allow access to terahertz frequencies not yet available
with conventional antennas. The TeraKit® includes all optical, mechanical andelectronic components for the generation and detection of THz waves such as delayline, terahertz generator, terahertz detector, optics, electronics, dedicated software andlaptop. It can be used with any femtosecond laser source at telecom wavelengths.

Figure 8: TeraIMAGE® optical board (scanning
range 50 mm x 50 mm).

Figure 7: TeraKit® optical board (38 cm x 38
cm).

The THz detection in TeraKit® is optimized using special optical and electronic
components (the details are confidential) so that a high SNR (signal to noise ratio) canbe achieved already using relatively low-power femtosecond fiber lasers. An example ofthe time-domain THz signal and its spectrum is shown in Figure 9.

Figure 9: THz time-domain signal amplitude and amplitude spectrum in TeraKit®, using 0.45 mm
thick DSTMS crystals for THz generation and detection, and a pump laser with 65 fs pulse
length, average power of 180 mW and 1.8 nJ energy/pulse.

The THz time-domain spectrometer with imaging option, TeraIMAGE®, includes, besidethe spectroscopic part, which is the same as in TeraKit®, the imaging part with allnecessary mechanics control and data acquisition software for scanning objects up to
50 mm x 50 mm (larger available upon request). An example of THz imaging with
TeraIMAGE® is shown in Figure 10.

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Figure 10: Opticalimage (made by a usualphotocamera) andterahertz image (madeby TeraIMAGE®) of apiece of plastics with
optically invisible
defects.

Narrowband widely tunable THz source: TeraTune®
Many materials exhibit specific absorption features (fingerprints) not only in the THz
range up to about 3 THz, mostly investigated because this range that can be reachedby photoconductive antennas, but also higher above, see Table 2. Also, the attenuationin air due to water-vapor absorption, which limits the range of application possibilities
below about 10 THz becomes much smaller (up to four orders of magnitude at 18 THz!)above 10 THz (see Figure 11), which makes it interesting to extent the THz range up to
20 THz.
Table 2: THz fingerprints of explosives and related compounds [from Liu et al., Proc. IEEE].6

6

H.B. Liu, H. Zhong, N. Karpowicz, Y. Chen, X.C. Zhang, "Terahertz Spectroscopy and Imaging for Defense andSecurity Applications," Proc. IEEE 95, 1514 (2007).

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Figure 11: Attenuation ofTHz radiation in air [fromAppleby et al, IEEE 2007]7.
Above 10 THz, theattenuation in air
dramatically decreases
compared to the attenuation
below 10 THz. This opens
up a wide range of new
application possibilities, suchas remote imaging andsensing.

For some applications a high THz beam power in a narrow band can be preferable thana broadband pulse. The total THz power generated with broadband generation
techniques is distributed over the spectral content of the pulses; therefore the power
density at any particular frequency is inherently low. To obtain reasonable conversion
efficiency for a certain THz frequency, a narrower band pulsed output with high beam
peak power is preferred.
TeraTune® is a tunable narrowband THz source with a narrow linewidth of less than
100 GHz and a large tuning range 1–20 THz. Rainbow Photonics introduced this unique
THz source to the market in 2012. It is based on difference frequency generation ofnanosecond pulses in organic THz generators DSTMS and OH1. The suitable infrared
pump pulses are generated in a specially designed dual-wavelength OPO (opticalparametric oscillator), which is tunable in the range of 1330–1480 nm and produces two
narrowband nanosecond pulses with their frequency difference in the THz range. The
wavelengths can be tuned by angular tuning of OPO crystals, which is motorized andcontrolled by the accompanying software. The generator crystals can be externallyswitched for achieving the best efficiency at different THz frequencies. High THz peak
power of more than 30 W can be reached at 1.25 THz using OH1 generator crystals.
The tunability spectrum using 1-mm thick DSTMS crystals is shown in Figure 13.

Figure 12: TeraTune®:Tunable (1–20 THz) narrow
linewidth (<100 GHz) THz
source.

7

R. Appleby, H. B. Wallace, “Standoff detection of weapons and contraband in the 100 GHz to 1 THz region”,
IEEE Trans. Antennas Propag. 55, 2944 (2007)
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Figure 13: Tuning curve of
TeraTune® using a 1-mm DSTMS
THz generator. In some regions
the achievable THz peak power islow, which is either due to the THz
absorption of the generationmaterial itself or due to non-perfectphase matching. The generation
crystal is relatively thick (1 mm), sothat high efficiency is achieved
where the generation is phasematched.

Applications of THz waves
Some of the applications of THz waves are related to the unique properties of thesewaves to excite molecular vibrations and lattice vibrations in the "Reststrahlen" range.
In addition the THz waves show low absorption and are transmitted through most nonconductive homogeneous plastics, paper, cartoon, most clothes, etc, and can therefore
detect hidden hazardous substances. Therefore besides THz spectroscopy of materials
these waves are potentially useful for security applications, but also for the identificationof defects in non-conductive materials. For conductive and partially conductive
materials, THz spectroscopy can give useful insights into the mechanisms of chargetransport in these materials. Here we give some examples of THz spectra and materials
testing demonstrated using organic nonlinear optical materials for generation anddetection of THz waves.
Figure 14(a) shows the THz spectra of several explosives as measured using THz time-

domain spectroscopy.
Figure 14(b) shows a Semtex explosive sample hidden behind two teflon plates as seenby optical waves (left) and THz waves (right; Semtex: green; the yellow area on theupper right corresponds to the red paper sticker shown in the left picture).
Figure 14(c) shows a picture and THz image of a bacillus cereus spores (anthrax)
sample hidden in an envelope. Examples of materials testing are shown in
Figure 14(d) and (e).
Figure 14(d) shows the optical and THz pictures of a pile of overhead transparencies,
where the label "ETH" has been cut out in one of the transparencies (not seen by visible
light), and its THz transmission image giving a full contrast image due to the phase shiftof the THz wave in the cut-out area (the "defect"). The second picture shows how
metallic defects or inclusions (a metal wire with the symbol "NLO") embedded in aplastic can be made visible by THz waves.
The last picture shows the identification of a void in a piece of plastics andFigure 14(e) the impressed credit card number from a credit card hidden in an envelope.
Figure 14(f) shows examples of THz reflection images of polyethylene samples with andwithout defects. The voids can be made visible in 3D with a resolution of less than
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10 Qm. This lower than the wavelength (longitudinal) resolution is due to the fact thatthe phase shift and temporal resolution of the reflected wave can be determined very
precisely.

Figure 14: Examples of applications demonstrated by using organic nonlinear optical materials
for THz generation and detection.

For more information please contact:
Rainbow Photonics AG, Farbhofstrasse 21, CH-8048 Zurich, Switzerland
Tel: +41 44 419 0505, Fax +41 44 419 0506
info@rainbowphotonics.com
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