IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 7, NO. 6, NOVEMBER 2017

741

Direct Wavefront Measurement of Terahertz Pulses
Using Two-Dimensional Electro-Optic Imaging
Mathilde Brossard, Harsono Cahyadi, Mathias Perrin, Jérôme Degert,
Eric Freysz, Takeshi Yasui, and Emmanuel Abraham

Abstract—We report on the development of a wavefront sensor
for terahertz pulses using a direct two-dimensional electro-optic
imaging system composed of a ZnTe crystal and a CMOS camera.
By measuring the phase variation of the terahertz electric field in
the crystal plane, we are able to reconstruct the terahertz (THz)wavefront in order to determine the optical aberrations of the
optical system. Associated with deformable mirrors or specificallydesigned optics, the sensor will open the route to terahertz adaptive
optics.
Index Terms—Optical aberrations, sensor, terahertz (THz),
wavefront.

I. INTRODUCTIONF

ROM the emergence of photonics technology, it has been
forever essential to characterize and control the wavefrontof optical beams, characterized by the surface of equiphase of
the electromagnetic field, in order to determine the optical aberrations and optimize the performances of the optical systems.
This can be accomplished for instance with the well-known
Shack–Hartmann sensor which can locally measure the wavefront slopes of an incoming optical beam [1]. At present, in thevisible and near-infrared spectral region, commercially available wavefront sensors can measure the optical aberrations and,associated with adaptive optics systems, they are able to correct these defaults in a wide variety of applications including
astronomy, ophthalmology, and microscopy.
In the terahertz (THz) spectral domain, it is still challenging
to measure the spatial profile and wavefront of a THz beam
due to the lack of effective THz two-dimensional (2-D) detectors. However, the wavefront characterization of THz beams isessential to optimize the spatial resolution or illumination uniformity of THz imaging systems, developed for a large variety of
applications such as security and defense, nondestructive test-

Manuscript received April 30, 2017; revised July 22, 2017; accepted September 7, 2017. Date of publication September 26, 2017; date of current version
November 8, 2017. (Corresponding author: Emmanuel Abraham.)
M. Brossard is with the University of Bordeaux, CNRS, LOMA, UMR 5798,
Talence F-33400, France, and also with NeTHIS—New Terahertz Imaging Systems, Mrignac 33700, France (e-mail: mathilde.brossard@nethis-thz.com).
H. Cahyadi and T. Yasui are with the Graduate School of Science and Technology, Tokushima University, Tokushima 770-8506, Japan (e-mail: harsono@
femto.me.tokushima-u.ac.jp; yasui.takeshi@tokushima-u.ac.jp).
M. Perrin, J. Degert, E. Freysz, and E. Abraham are with the Universityof Bordeaux, CNRS, LOMA, UMR 5798, Talence F-33400, France (e-mail:mathias.perrin@u-bordeaux.fr; jerome.degert@u-bordeaux.fr; eric.freysz@
u-bordeaux.fr; emmanuel.abraham@u-bordeaux.fr).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TTHZ.2017.2751249

ing of industrial materials, food and pharmaceutical industry,
communications, or cultural heritage [2]. Wavefront analysis
of THz pulses is also mandatory for the control of far-field
intensity distribution of time-domain spectrometers or to increase the peak power of intense terahertz sources and optimize
the beam focalization of scanning THz time-domain imaging
systems.
Previous studies were oriented toward the measurement ofTHz wavefronts for both continuous wave and pulsed sources.
In 2008, Bitzer et al. determined the beam profiles of THz
pulses after passing through a hyper-hemispherical silicon lens[3]. They observed an asymmetric spatiotemporal electric field
dynamic at the focus of the lens attributed to a distortion of theincident THz wavefront. However, they did not measure the THz
wavefront, but only the THz beam profile. Moreover, their indirect and time-consuming method used a xy-scanning system inorder to determine point-by-point the beam profile. In 2012, Cuiet al. proposed the first THz wavefront measurement associatedwith a Hartmann sensor and indirect xy-scanning method, detecting the radiation with a pyroelectric sensor [4], [5]. The same
year, Richter et al. presented a similar arrangement with a 2-D
detection consisting in a microbolometer camera [6]. However,their method cannot be applied with short THz pulses producedby femtosecond laser pulses owing to the low sensitivity of thethermal incoherent detector. Another aperture scanning device
was proposed in 2013 to characterize the THz beam propagation in a time-domain spectrometer [7]. Associated with a
Hartmann mask, it has been used to produce 2-D image of theTHz wavefront. More recently, we proposed the wavefront characterization of THz pulses using a Hartmann sensor associatedwith a 2-D electro-optic (EO) imaging system [8]. We quantitatively determined the deformations of planar and converging
spherical wavefronts using the modal Zernike reconstruction
least-squares method. Nevertheless, our indirect method uses amask which limits the beam transmission and requires an initial
calibration or reference measurement.
In this paper, we propose a new direct method for the
wavefront measurement of THz pulses. The principle of ourmeasurement is as follows. It is well known that spatiotemporal profiles of THz transient electric fields can be directly
determined through EO sampling in a nonlinear crystal [9].
By varying the time delay between a short optical laser probe
pulse and the THz pulse, EO sampling can record the spatial
dependence of the full (amplitude and phase) THz electric field,
ETHz , in the crystal plane. Then, from the phase variation of2156-342X © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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742IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 7, NO. 6, NOVEMBER 2017

Fig. 1. Experimental setup of the direct THz wavefront sensor using 2-D EO
imaging.

Fig. 2. Analysis of a convergent THz beam. (a) E THz (x, y) for a t = 0 pstime delay between the THz and laser probe pulses, from [8]. (b) Amplitude andphase of the complex electric field E THz (Ω), calculated for the central pixel ofthe image presented in (a).

ETHz in this plane, we will demonstrate that a simple proceduremakes it possible to determine the equiphase surface that is the
wavefront.
II. EXPERIMENTAL DETAILSFor the demonstration of the method, broadband THz pulses
were generated by optical rectification of amplified femtosecond laser pulses (800 nm, 1 mJ, 150 fs, 1 kHz repetition rate) in
a 1 mm-thick ZnTe crystal with a clear aperture of 25 mm. The
collimated THz beam is sent into another 1 mm-thick ZnTe crystal for collinear EO detection with a time-delayed femtosecond
laser probe pulse, reflected by the front left face of the crystal (see Fig. 1). There, owing to the Pockels effect, the spatial
distribution of the broadband THz electric field ETHz (x, y) modifies the polarization distribution of the probe beam [9], whose
intensity is finally captured by a 232 × 232 pixels CMOS camera (1000 fps) after passing through an analyzer and a lensthat images the crystal plane. For a given time delay t between the THz and the probe pulses, the system is able toprovide a 2-D (x-horizontal and y-vertical directions) image
at 800 nm corresponding to the distribution of the THz electricfield ETHz (x, y, t) at the crystal position. This image can beobtained with a 500 Hz acquisition rate, thanks to the synchronization with the optical chopper, the laser repetition rate, and a
dynamic subtraction method [10]. However, signal-to-noise ratio higher than 100 usually requires an averaging time of 0.5 s.
By changing the time delay between the THz and probe pulses,
it is possible to record the temporal evolution of the THz electricfield ETHz (x, y, t). Thanks to this THz imaging system, we will
demonstrate that we are able to fully analyze the spatial distribution of the THz electric field at the crystal position and finallydetermine the THz wavefront and extract the optical aberrations.
III. RESULTS
A. Analysis of a Convergent THz beamFirst, to implement our method, we introduced a planoconvex THz lens (f = 100 mm focal length) at e = 65 mm
before the ZnTe crystal. This configuration provides a converging spherical THz beam at the crystal position with a theoretical R = (f ′ − e) + (πw02 )2 /(Λ2 (f ′ − e)) = 38 mm radiusof curvature at 1 THz assuming an incident Gaussian beam

Fig. 3. Convergent THz beam. Amplitude (a) and relative phase (b) of
E THz (x, y, Ω = 1 THz) in the ZnTe crystal plane. (c) Evolution of the relative phase along the horizontal black line indicated in (b).

and a w0 = 1 mm beam-waist at the wavelength Λ = 300 µm.
Fig. 2(a) displays the distribution of the converging sphericalTHz electric field, ETHz (x, y), for a t = 0 ps time delay between the THz and laser probe pulses. This result is taken from[8]. However, in this paper, the THz wavefront reconstruction
was made with the insertion of a Hartmann mask, whereas in thepresent work, we will show that a direct analysis of the 2-D EO
imaging will provide the THz wavefront without any prior calibration. In Fig. 2(a), the central blue circle indicates a negativevalue of ETHz , whereas the red ring is for a positive one, indicating the focusing of the THz beam at the crystal position. Then,by changing the time delay t between the THz and probe pulses,
we can record a set of 2-D EO transversal images representing
the time-dependent amplitude ETHz (x, y, t), which is in fact a
3-D matrix whose analysis will provide us all the information
we need for the wavefront reconstruction. The recording time of
the whole 3-D matrix composed of 256 images is about 2 min.
It is clear that, for every pixels of the camera, i.e., for every x
and y positions, it is possible to plot the corresponding temporal
waveform ETHz (t). Then, through a Fourier transformation of
the temporal data, it is possible to calculate the amplitude andthe phase of the complex electric field ETHz (Ω), where Ω is theTHz frequency [see Fig. 2(b)]. The THz spectrum extends from0 to 3 THz with an expected linear dependence of the phase.
From the data presented in Fig. 2, it is straightforward toplot the amplitude and the phase of ETHz (x, y, Ω) for every
pixels of the camera, as shown for Ω = 1 THz in Fig. 3(a) and(b), respectively. For the sake of simplicity, the phase map at
1 THz plotted in Fig. 3(b) does not represent the absolute phase

BROSSARD et al.: DIRECT WAVEFRONT MEASUREMENT OF TERAHERTZ PULSES USING TWO-DIMENSIONAL ELECTRO-OPTIC IMAGING

743

also by other optical aberrations since the surface does notexhibit a perfect circular symmetry. To quantify these aberrations, the standard method consists in developing the wavefront surface W (x, y) onto a linear combination of the Zernike
polynomials [11]!
ai Zi (x, y)(1)
W (x, y) =i

Fig. 4. Analysis of a convergent THz beam at 1 THz. (a) Reconstructed
wavefront. (b) Amplitude of the Zernike coefficients. The dashed vertical blacklines indicate the noise level or measurement accuracy. a 1 : y-tilt , a 2 : x-tilt,
a 3 : astigmatism 45, a 4 : defocus, a 5 : astigmatism 0/90, a 6 : vertical trefoil, a 7 :y-coma, a 8 : x-coma, a 9 : oblique trefoil, a 1 0 : oblique quadrafoil, a 1 1 : oblique
2nd astigmatism , a 1 2 : spherical aberration, a 1 3 : vertical 2nd astigmatism, a 1 4 :vertical quadrafoil.

where ai represents the amplitudes of the polynomial Zi , eachpolynomial corresponding to one specific optical aberration.
Fig. 4(b) represents the amplitude of the 14 first Zernike polynomials. The main observation is obviously the strong defocus
a4 = 350 µm (1.17Λ at 1 THz). To compare this value to thetheoretical R = 38 mm radius of curvature of the THz beam at
the sample position, we can assume a perfect spherical wavefront (i.e., ai = 0, ∀i ̸= 4) and writes
W (r) =of the beam, but the relative one with a zero value arbitrary
fixed at the center. Moreover, in the following of the paper, we
will restrict our analysis to 1 THz but this latter can be easily
generalized to every other frequency within 0–3 THz spectral
range, emphasizing the ability of our sensor to characterizebroadband THz pulses. Next, the final reconstruction of the THz
wavefront at the crystal position will simply use the values of therelative phase of ETHz (x, y, Ω), represented by the phase map
image in Fig. 3(b), as follows. First, we select a pixel (x0 , y0 ),e.g., the center of the phase map image, and we note the phasevalue φ0 (x0 , y0 ) at this position. Then, except if the wavefront
is perfectly planar without any optical aberrations, all the otherpixels in the phase map image must have a different phase value.
However, for every pixel position (x, y), it is simple to calculate
the distance ∆z(x, y) along the propagation axis for which thephase shift ∆φ = 2π∆z/Λ will provide a new phase value being
exactly φ0 . The distribution of the resulting ∆z(x, y) values will
design a new specific 3-D map, which will exactly correspondto the equiphase surface, demonstrating that our method is able
to reconstruct the 3-D THz wavefront surface for every pixelsof the camera.
To evaluate the accuracy of the method, we have to determine the minimal phase shift we can measure. Fig. 3(c) showsthe evolution of the relative phase along the horizontal blackline indicated in Fig. 3(b). The amplitude of the point-to-point
fluctuations of the phase provides the minimum phase shift ∆φwe can measure. For Ω = 1 THz, we found ∆φ = 100 mrad,
which corresponds to a minimum wavefront deformation of(∆φΛ)/(2π) = 5 µm, i.e., Λ/60. This wavefront measurement
accuracy is comparable to standard commercial wavefront sensors designed for visible and infrared radiation.
First, the method is tested for the convergent THz beam,
previously described in Figs. 2 and 3. Fig. 4(a) shows the reconstructed wavefront in the crystal plane with a quasi-sphericaldeformation from the center to the edge of the THz beam. The
colored scale indicates the local wavefront deformation in µm.
However, the precise observation of this surface indicates thatthe wavefront is not only governed by a single defocus but√
r2= 2 3a4 ρ2
2R(2)

where r = (x2 + y 2 )1/2 and ρ = r/α. The parameter α is a normalization factor since the Zernike polynomials are defined fora maximum circular aperture with ρ = 1. In our measurement,
we have α = 8.5 corresponding to a 17 mm beam diameter.
From (2), we calculate R = 30 mm which is about the theoretical value. The 20% deviation can be attributed to the imperfectspherical wavefront. Fig. 4(b) can actually quantify these additional aberrations with a x-tilt a2 = 38 µm and a sphericalaberration a12 = −15 µm. All the other aberrations are of theorder of the measurement accuracy (5 µm), indicated by thedashed vertical black lines in the figure, and cannot be strictly
considered as relevant in this experiment.
B. Analysis of an x-Tilted THz Beam
After this first demonstration of validity of the method witha spherical wavefront, we removed the focusing lens before thedetection crystal and intentionally horizontally tilted the THz
beam with respect to the laser probe beam so that these two
beams interact noncollinearly in the crystal. The angle between
the two beams has been precisely measured to be θ = 3.9 mrad.
We introduced this precise tilt because it constitutes anotheroptical aberration that we can finely control, providing anothertest to verify the validity of our method. According to the relation
W (r) = r tan θ = 2a2 ρ(3)the x-tilt parameter we introduced in the crystal plane is thena2 = 90 µm. Fig. 5 shows the reconstructed wavefront and theamplitudes of the corresponding Zernike coefficients. As expected, the major optical aberration is the horizontal x-tilt witha calculated value a2 = 86 µm very close to the actual tilt we intentionally introduced. This additional test validates once again
our wavefront reconstruction method using 2-D EO imaging.
C. Analysis of a Planar THz Beam
Finally, a third experiment has been performed in order todetermine the optical aberrations of the collimated THz beam

744IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 7, NO. 6, NOVEMBER 2017

Fig. 5. Analysis of a x-tilted THz beam at 1 THz. (a) Reconstructed wavefront. (b) Amplitude of the Zernike coefficients. The dashed vertical black lines
indicate the noise level or measurement accuracy.

Fig. 6. Analysis of a planar THz beam at 1 THz. (a) Reconstructed wavefront. (b) Amplitude of the Zernike coefficients. The dashed vertical black lines
indicate the noise level or measurement accuracy.

generated by optical rectification in the first ZnTe crystal. If theincident pump laser beam is supposed to have a planar wavefront, as illustrated in Fig. 1, a planar THz wavefront is also
expected. Using our reconstruction procedure, Fig. 6 shows the
wavefront and the amplitudes of the corresponding Zernike coefficients. Clearly, the THz wavefront is not exactly planar even
if all optical aberrations are smaller than 20 µm (Λ/15). Besides
the x- and y-tilts, which are dependent on the angle between
the probe and THz beams, the main optical aberration is the
0/90 astigmatism (a5 = −9 µm). This measurement is important since it indicates that the THz beam is not perfectly planar.
Further investigations may concern the wavefront measurementof the incoming pump laser beam onto the ZnTe crystal using acommercially available wavefront sensor in order to determine
if the THz wavefront deformation arises from the laser beamitself or from the THz generation process in the crystal.
D. Comparison With the Hartmann Method
As explained in the introduction, we also recently demonstrated the wavefront characterization of THz pulses using a
Hartmann sensor associated with the similar a 2-D EO imaging
system [8]. For this paper, the experimental setup was exactly thesame as here, except that a Hartmann mask was inserted 10 mm
before the detection crystal. Consequently, the THz wavefront
was determined in the plane of the mask instead of the planeof the crystal, as it is the case for the present work. Therefore,
if we want to compare the measurement presented in this pa-

Fig. 7. Analysis of a convergent THz beam at 1 THz. (a) Zernike coefficientswith the Hartmann method. (b) Zernike coefficients with the direct method.

per to that of [8], we need to propagate the THz electric field
10 mm before the crystal in order to reconstruct the THz wavefront at the same position. To this end, we have implemented abeam propagation method [12] to compute the field distribution
at the position z2 , from the knowledge of the amplitude andphase of the field distribution at the position z1 = z2 + 10 mm.
The relationship between both scalar fields is computed using
the Helmholtz equation in free space to obtain"#$%kx2 + ky2
−1
E(kx , ky , z1 ) exp(z1 − z2 )E(x, y, z2 ) = F
2ik0(4)
where E(kx , ky , z1 ) is the Fourier transform of E(x, y, z1 ),with ki the wave vectors and the symbol F −1 denotes the inverse Fourier transform. Note that the knowledge of both theamplitude and phase of the electric field at the position z1 isnecessary to back-propagate the field and compute E(x, y, z2 ).
Based on the precedent equation, a freely available MATLAB
program has been computed in order to propagate the THz waves[13]. This procedure makes it possible to compare both methods. However, we want to stress again that, the direct method we
propose in the present paper only relies on the phase distributionof the THz electric field.
Let us recall that the Hartmann mask used in [8] consists ofa 1-mm thick metallic plate with 9 by 9 circular holes (1 mmdiameter with 2 mm periodicity in the horizontal and vertical directions). The accuracy of the Hartmann method depends on thedesign of the mask (distance between two holes and size of theholes). In our case, we estimated that the accuracy was equal to(Λ/35) which is less precise than the direct measurement method
proposed in this paper. In the case of the spherical convergent
THz beam, we can see in Fig. 7 that the Zernike coefficients
we obtain for the two methods are in good agreement. First, theamplitude of the defocus term is similar with a4 = 270 µm with
the Hartmann mask and a4 = 310 µm with the direct method
associated with the back propagation of 10 mm. Note that the decrease of a4 , i.e., the increase of the radius of curvature, from theposition z1 to the position z2 , is consistent with the convergent
spherical THz wave. Both methods also indicate similar amplitudes for x-tilt and spherical aberration. Only the direct method
is able to measure the 45 astigmatism with a3 = −9 µm, since
the value is lower than the accuracy of the Hartmann method,
indicated by the dashed vertical black lines in the figure. It is

BROSSARD et al.: DIRECT WAVEFRONT MEASUREMENT OF TERAHERTZ PULSES USING TWO-DIMENSIONAL ELECTRO-OPTIC IMAGING

also important to notice that the Hartmann method measures the
local slopes of the wavefront, which is finally calculated after
integration of the data and application of a least-squares method[11]. Here, on the contrary, we directly measure the equiphasesurface of the THz beam, which constitutes the wavefront.

745

Mathilde Brossard was born in St. Etienne, France, in 1992. She received
the Engineering degree from the Institut d’Optique Graduate School, Palaiseau,
France, in 2015, and is currently working toward the Ph.D. degree on THz wavefront measurement at Bordeaux University, Bordeaux, France, in collaboration
with the NeTHIS Company, Mérignac, France.

IV. CONCLUSION
In conclusion, by means of 2-D EO imaging of THz waveforms and subsequent calculation of the phase variation of theTHz electric field, we were able to determine the wavefront witha precision of Λ/60, without any prior reference measurement.
We believe that our THz wavefront analyzer could provide aneffective tool to control the generation of pulsed THz sources.
Another interesting application concerns the association of thesensor with deformable mirrors in order to open the route to
THz adaptive optics.
ACKNOWLEDGMENT
The authors would like to thank the referee of [8] whose
useful comments and suggestions triggered this work.

Harsono Cahyadi was born in Indonesia, in 1980. He received the Ph.D. degreefrom the Graduate School of Engineering Science, Osaka University, Japan, in
2013.
He continued as a Postdoctoral Researcher with the same laboratory until April 2014. He then joined the Institute of Technology and Science, The
University of Tokushima, Tokushima, Japan. During his time with The University of Tokushima, he joined a research exchange program with Bordeaux
University for the development of THz measurement systems. In May 2015,
he became an Assistant Professor with the Department of Pathology and Cell
Regulation, Kyoto Prefectural University of Medicine. He is currently a Senior
Optics Engineer with a private company in Singapore where he is involved withthe development of medical instrumentation based on optical spectroscopy. His
research interests include various optical spectroscopy/microscopy techniques
and biomedical optics.
Dr. Cahyadi is a member of the Optical Society of America, the International
Society of Optics and Photonics, and the Spectroscopical Society of Japan.

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[13] [Online]. Available: https://www.loma.cnrs.fr/mathias-perrin/

Mathias Perrin is a CNRS Researcher involved in the field of nano-optics,
plasmonics, guided (fiber) and nonlinear optics from a theoretical and numericalpoint of view. He participated in the development of quasi-normal mode theory todescribe resonances in electromagnetism (from visible to THz) and is currently
involved with the modeling of phase masks for THz imaging.

Jérôme Degert received the Ph.D. degree in atomic and molecular physics fromthe University of Toulouse, Toulouse, France, in 2002.
In 2003, he was a Postdoctoral Research Fellow with the Max Planck Institutefor Quantum Optics, Garching, Germany. During these years, he was involved in
the field of optical pulse shaping and its application to quantum coherent controlof ultrafast phenomena in simple atomic and molecular systems. From 2003 to
2016, he was an Assistant Professor with the University of Bordeaux, working
with the Laboratoire Ondes et Matière d′ Aquitaine. He is currently a Professor.
His research interests range from the development of intense THz sources andtheir applications to nonlinear optics, THz pulse shaping and coherent control,
to ultrafast time-resolved spectroscopy in the visible and THz ranges applied tothe study of the optical properties of systems like spin crossover complexes andorganic conducting polymers.

Eric Freysz received the Ph.D. degree in physics from Bordeaux University,
Bordeaux, France, in 1983.
He is a Senior Researcher (DR1) with the CNRS and Deputy Director with
the Department of Science and Technology, University of Bordeaux, Bordeaux,France. He joined the CNRS in 1984 after he earned the Ph.D. degree. He has
authored or co-authored more than 130 publications in international journals,
holds 14 patents, and has presented more than 220 conference proceedings.
From 1991 to 1993, he was a Visiting Scientist with the Department of Physics,
University of Berkeley, where he worked with Prof. Y. R. Shen. In 1994, he
was a Invited Professor with the University of Pittsburgh, Pittsburgh, PA, USA.
Since 1995, he has been the Director of different institutes and laboratory ofphysics in Bordeaux. He has a well-established expertise in lasers, nonlinear
optics, linear and nonlinear THz spectroscopy, time resolved spectroscopy, andlaser-induced phase transitions.

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IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 7, NO. 6, NOVEMBER 2017

Takeshi Yasui received the Ph.D. degree in mechanical engineering from
Tokushima University, Tokushima, Japan, in 1997.
From 1997 to 1999, he was a Postdoctoral Research Fellow with the National
Research Laboratory of Metrology, Japan. He was with the Graduate Schoolof Engineering Science, Osaka University, Osaka, Japan, from 1999 to 2010,
and was briefly with the University of Bordeaux I in 2007, and the Universityof Littoral Côte d′ Opale in 2010, as an Invited Professor. He is currently aProfessor with the Graduate School of Science and Technology, Tokushima
University, and an Invited Professor with the Graduate School of Engineering
Science, Osaka University. His research interests include THz instrumentationand metrology, nonlinear optical microscopy, and optical combs.
Prof. Yasui is a member of the Optical Society of America, the Japan Societyof Applied Physics, the Optical Society of Japan, the Laser Society of Japan,
the Japan Society of Medical Electronics and Biological Engineering, and theJapan Society of Mechanical Engineers. He was the recipient of the award for
the Most Promising Young Scientist of the Optical Society of Japan in 1998, theSakamoto Award of the Japan Society of Medical Electronics and BiologicalEngineering in 2006, the Optics Paper Award of the Japan Society of Applied
Physics, and the Funai Award of the Japan Society of Mechanical Engineers in2009.

Emmanuel Abraham was born in Reims, France, in 1970. He received the
Ph.D. degree in physics from Bordeaux University, Bordeaux, France, in 1997.
From 1997 to 1998, he was a Postdoctoral Research Fellow with the National
Research Laboratory of Metrology, Tsukuba, Japan, and the FemtoSecond Technology project to study industrial applications of femtosecond lasers, including
ultrafast optical Kerr gates. From 1998 to 2013, he was an Assistant Professor with Bordeaux University, working with the Laboratoire Ondes et Matièred′ Aquitaine (LOMA). He joined Osaka University as an Invited Researcher
in 2007 and the University of Tokushima as a Short-Term Invited Fellow Researcher of Japan Society for the Promotion of Science in 2010. Since 2013, hehas been a Professor with LOMA, Bordeaux University. He developed intense
terahertz sources and studied their applications for nonlinear spectroscopy. Dueto the international collaboration with Tokushima University, he also developed
innovative instrumentation for THz science and technology. He has authored or
co-authored more than 50 peer-reviewed papers, 4 patents, 4 book chapters, andmore than 100 conference proceedings. His research interests include femtosecond time-resolved spectroscopy and nonlinear optics. He also has investigatedthe potential of terahertz radiation for the analysis of objects related to cultural
heritage such as paintings, sculptures, ceramics, etc., by using pulsed and continuous millimeter-wave sources for 2-D imaging and 3-D terahertz computed
tomography.