Research
Research on frequency-agile ultra-widely tunable
THz-wave sources | Research on highly sensitive
THz-wave detection
Research on THz-wave applications using frequency-agile
THz-wave source | THz spectroscopic database
Research on frequency-agile ultra-widely tunable THz-wave sources
BNA crystal growth towards BNA-DFG THz-wave source
We
have been developing
THz-wave source
with ultra-widely frequency tunability based on the difference
frequency
generation (DFG) scheme by utilizing large nonlinearity of organic DAST
crystals. Generated THz-waves in the organic crystals are attenuated
owing to
the interaction with several phonon modes existing at wide THz
frequency
regions. Therefore, DAST-DFG source never fail to have intensity dips
in the
THz spectrum although it has ultra- widely frequency tunability. In
order to
overcome this problem, we are developing a new DFG source using BNA
crystal
which has different phonon modes compared to DAST crystal. By combining
DAST-
and BNA-DFG sources, a sophisticated ultra-widely tunable THz-wave
source
without intensity dips can be realized. In particular, it is important
to
establish a robust method to grow high quality and large BNA crystal
because
the BNA is an immature organic crystal. In general, it is difficult to
grow
high quality and large single organic crystals since the molecular
weights of
them are huge and their structure are quite complex. We have tried to
grow high
quality and large single BNA crystal from solution phase. Recently, as
shown in
Fig.1 (a), we have succeeded in growing single BNA crystal with high
quality
and practical size by optimizing various parameters such as kind of
solutions,
range of solution temperature, cooling rate and so on. From the crystal
quality
evaluation using X-ray diffraction, it is found that full width at half
maximum
(FWHM) of rocking curve was reduced down to one-fifth compared to that
for
conventional BNA crystal grown by Bridgeman method (Fig.1 (b)). This
FWHM value
is comparable to that for commercial silicon wafers, therefore, high
quality
BNA single crystal with extremely high molecular orientation can be
obtained.
In addition, enhancement of damage threshold level to pump lights was
confirmed
and generation of higher THz-wave output power can be possible now.
Tunable terahertz-wave generation from DAST crystal pumped by dual-phase matched (DPM) single-KTP-based OPO
An
ultra-widely tunable OPO based on
dual KTP crystals has been demonstrated in our group to generate
THz-wave
through difference-frequency generation (DFG) process in organic DAST
crystals.
However, the two-crystal based OPO cavity is costly and complicated. We
here
demonstrate tunable two-color OPOs based on a single KTP crystal. Both
collinear and non-collinear phase matched parametric processes are
established
in our double-pass system with a slightly tilted rear reflecting mirror
(Fig.
2(a)). The frequency difference between
two wavelengths (~1.3 µm) can be adjustable
continuously with the rear mirror tilting angle δ and the KTP crystal angle β. As an
immediate application depicted in Fig. 2(b), we perform a DFG process
with the
DAST crystal pumped by this newly developed OPO. Monochromatic tunable
0.5- 3
THz-wave has been successfully radiated. The OPO wavelength can be
easily
tailored by proper crystal cutting angle and cavity mirror coating,
this
two-color double-pass singly resonant oscillator (DPSRO) promising for
various
applications.
Compact continuously tunable terahertz CW source based on
monolithic dual-wavelength fiber laser
The
monochromatic THz-wave generation
utilizes difference-frequency generation (DFG) process induced by the
two color
near infrared (NIR) laser fields in bulk nonlinear optical (NLO)
crystal. In
order to achieve simultaneous dual-wavelength lasing with improved
stability
and robustness, it is advantageous to exploit compact single cavity
fiber laser
made of optical single mode fibers (SMFs). So far, the reported
wavelength
spacing of two color fiber laser was lack of tunability, always
constrained in
the microwave/millimeter wave region. In this work we demonstrated
THz-wave
generation infrastructure pumped by a monolithic fiber laser via DFG
process.
The single cavity two color tunable fiber ring laser, depicted in Fig.
3(a),
consists of 1) chirped fiber Bragg gratings (CFBGs) for wavelength
selection,
and 2) a semiconductor optical amplifier (SOA) for lasing. The local
thermo-optical phase change along the linearly chirped FBG contributes
to the
seamless wavelength tuning. As a result, the CW dual-wavelength fiber
laser
emission around 1060 nm has been achieved with single longitudinal mode
operation, preserving excellent beam quality and spatial coherence,
then
amplified by a polarization maintaining Ytterbium fiber amplifier
(PM-YDFA) to
pump the DAST crystal. Collinear phase matched DFG in DAST crystal has
led to
the generation of 0.5- 2 THz CW radiations successfully. This
continuous wide
tunability is only limited by the CFBG spectrum and gain bandwidth of
the fiber
laser. Suitable active fiber plus the well-established FBG fabrication
technique can make our system tailored easily to match the other NLO
crystals
such as organic BNA and inorganic GaSe pumped by NIR laser at 800-1000
nm and
1530-1560 nm, respectively. Our THz-wave source based on a narrow
linewidth
fiber laser is expected to be promising for THz spectroscopic and
imaging
compact apparatus with high resoution.
High efficient dual-wavelength Nd:YAG oscillator for THz-wave generation by utilizing DAST-DFG
A
DAST organic crystal is one of the
promising materials for high power THz-wave generation because of its
very
large nonlinear optical coefficient. Our group has developed the
monochromatic
and ultra-widely tunable THz-wave source based on KTP-OPO pump source
for
DAST-DFG method pumped by two near infrared (NIR) light near 1.3 m. In order to achieve high efficient conversion to NIR
light,
instead of the conventional KTP-OPO system, we newly developed the
dual-wavelength
Nd:YAG oscillator for pumping the DAST crystal (Fig. 4(a)). Nd:YAG
crystal has
the similar cross sections, 9.5x10-20 and 10x10-20 cm2, for stimulated
emission
at 1319 and 1338 nm respectively, which allows us to generate two NIR
laser
simultaneously under the suppression of Nd:YAG fundamental (1064 nm),
as
displayed in Fig. 4(b). The plots in the bottom are NIR output energy
as a
function of the laser diodes (LDs) excitation energy in the Nd:YAG
oscillator
and the slope efficiency reached 3% which value is over ten times
higher than
the conventional KTP-OPO system. Also, output power from this
oscillator can be
easily scaling up by increasing of the LD excitation energy. This kind
of pump
source is an attractive candidate for high power and high efficiency
monochromatic THz-wave source. This joint project is promoted with
Prof. Asai
in Tohoku institute of technology.
Research on highly sensitive THz-wave detection
The
detection of
monochromatic terahertz
(THz) waves using nonlinear optical crystals has attracted considerable
interest, due to the lack of a practical THz detector with high
sensitivity and
fast response time at room temperature. In this report, we demonstrate
THz-wave
detection with high sensitivity, rapid response, and room-temperature
operation
using an organic DAST crystal.
Figure
5 shows the
experimental setup,
including the THz-wave source and detection system. The THz-wave source
is the
DAST-difference frequency generation we reported, and has ultra-wide
tunability
(from 1.5 to 37 THz). In the part of THz-wave detection, the radiation
of
approximately 1.3 m from another
KTP-OPO with a single crystal was used for the pump beam. A KTP crystal
(10 mm × 5 mm × 27 mm) with cut
angles of 60° was mounted on a
Galvano optical scanner so that the wavelength could be tuned quickly
and randomly.
In
order to investigate the
condition of
THz-wave detection, 19.2-THz radiation was used because of the higher
THz
emission from the DAST-DFG source in the vicinity of this frequency.
When a THz
wave and a pump beam are simultaneously irradiated into a DAST, a
near-infrared
signal is generated by wavelength up-conversion, using DAST
nonlinearity as a
THz-wave detection signal with Type 0 phase matching. The signal beam
was
separated from the intense 1.3-µm pump beam with two
long-wavelength-pass filters. An InGaAs (818-BB-30; Newport Corp.)
photodetector was used to capture the signal beam.
Figure
6 shows the detected
signal
output as a function of THz-wave input power. A minimum peak power of
about 270
W was achieved for THz-wave detection. From
the minimum detection power determined in our experimental results, the
noise
equivalent power (NEP) was estimated to be about 6 nW/Hz1/2. Thus
THz-wave
detection was also carried out using a DLATGS pyroelectric detector,
resulting
in a minimum THz-wave detection power of ~ 6 mW, which is larger by a
factor of
200 than the power obtained for DAST detection. Taking the time
response of a
few milliseconds into account, the NEP of the pyroelectric detector was
estimated to be 4 orders of magnitude larger than the NEP of DAST
detection.
Research on THz-wave applications using frequency-agile THz-wave source
Multi-parameter carrier-density mapping of semiconductor wafers using frequency-agile, widely tunable THz-wave reflectometry system
A
non-destructive detective with surface
property mapping is highly desirable for online quality control of
semiconductor industry and it has been widely recognized that
contactless
optical methods are efficient to characterize the electrical parameters
of
semiconductor materials. Ohno et al. and Hamano et al. applied our tunable DAST-DFG THz-wave source to obtain
high spatial resolution reflection mappings of GaN wafers with
different
carrier density (Fig. 7(a)&(d)), uncovering the relative
differences within
free-carrier concentration at the single THz-wave frequency. However,
the
information from this single frequency probing was insufficient to real
material inspections. In this work, by fully exploiting the capability
of our
frequency agile DAST-DFG THz-wave source tunable ultra-widely, we
proposed a
general methodology to study the spatial distribution of multiple
free-carrier
parameters by examining the reflection mapping at a set of discrete
THz-wave
frequencies. Probing at appropriate five THz-wave frequencies sensitive
to the
carrier property variation in terms of THz-wave reflectivity (within
20- 24 THz
for GaN) enables fast and accurate numerical fitting based on
Drude-Lorentz
equation in which the dielectric function of semiconductors is
connected to its
unique carrier dynamics (Fig. 7 (e)). Without tedious frequency
scanning along
the whole THz-wave spectrum, for any given point on the surface we
could
extract simultaneously and accurately the information of GaN wafer
properties
such as carrier concentration, transverse optical phonon mode,
longitudinal
optical phonon mode, lattice phonon damping constant and carrier
mobility (Fig.
7 (f)). The surface reflectivity mapping/ imaging of the whole GaN
wafer was
further obtained at each probing THz-wave frequency. The combination of
reflectivity mapping data at five THz frequencies leads to a
two-dimensional
(2D) fitting, which reveals the spatial distributions of multiple free
carrier
parameters.
2D mapping of water content ratio in thin fresh tissues using the monochromatic THz-wave
The variation of water content ratio in
biological system accompanying the pathological/ histological alteration is
expected to be a good indicator for tracing when addressing many unsolved
biological and medical diagnosis problems. We propose to utilize THz- wave,
which has large absorption for water but no damage to biological tissues, as a
viable sensitive tool for the sake. A novel sample preparation is performed to
preserve freshness of thin tissues at room temperature, then the monochromatic
THz-wave from our LN- DFG system irradiating to the small area on the tissues.
Repeated measurement of THz-wave transmittance at 1.45 THz on the 1D pixel line
(Fig. 8(b) & (c)) reveals that volume water content ratio of animal tissues
is invariant to time up to 70 min. with low <3%, whereas the previous preparation dried the tissues quickly
within 2 min., reducing water content significantly. As exemplified in Fig.
8(b) & (d), 2D mapping of volume water content ratio is also demonstrated
with sufficient stability and resolution (240 m/ pixel), suggesting our measurement method can apply medical
diagnosis potentially.
THz spectroscopic database
Construction of spectroscopic database
for THz-wave region is extremely important to activate basic research and
commercial applications of THz-wave. In cooperation with NICT we have developed
THz-wave spectroscopic database, disclosing it online since 2009
(http://www.thzdb.org/). The database covers much variety materials such as
living organ tissues, polymer macro molecules, sugar groups, agricultural
chemicals, various organic and inorganic materials as shown in Fig. 9(b). Total
number of posting data has exceeded 1200. Our database is accessed 3500 times
in one year and running as capital THz database because of superior quantity
and quality to other databases at overseas site. In addition, our database was
adopted as priority subject of Grant-in-Aid for Publication of Scientific
Research Result (KAKENHI). Now the database is under reconstruction to be more
user-friendly. We also have plan in the near future to expand the database by
updating new data offered from outside research institutions.
