Ongoing Research
Projects
Frontier Carbon Materials • Nanowires,
Nanobelts, and Novel Nanostructures • Molecular
Electronics • Chemical and Biological
Sensors • Wide Band Gap Crystals
Wide Band Gap Crystals
Nonlinear Optical Crystals • Nitride
Single Crystals
Nonlinear Optical Crystals
Ultraviolet (UV) radiation is a clean energy source for
materials synthesis and processing. Rare-gas halides (Excimer: ArF,
KrF...) lasers are the useful source of intense UV radiation. Operation
of these lasers involved high-voltage discharge of corrosive gas. A compact,
maintenance-free, all-solid-state alternative is in high demand. An effective
technique for producing UV light is cascaded sum-frequency generation of
nonlinear optical (NLO) crystals as pumped by solid-state IR lasers (λ ~
1 mm). Until 1975, most NLO crystals
were based on the P-O, I-O and Nb-O bonds like those in KH2PO4,
LiIO3 and LiNbO3. Because of high resistance against
laser-induced damages and good transparency of UV light, borate NLO crystals
are identified as the choice for generation of high-power UV light. The
first borate crystal that described for this purpose was KB5O8.4H2O
(KB5). Intense research on borates was initiated after the development
of b-BaB2O2 (BBO)
and LiB3O7 (LBO).
The performance of solid-state UV light source has been
greatly improved after the discovery of CsLiB6O10 (CLBO)
crystals (discovered in Osaka University, 1995). Average UV (@266nm)
output power reported so far was enhanced by one order of magnitude
within the first six years after its discovery. It is noteworthy
that CLBO was demonstrated to produce deep-UV light (@193nm) by
eighth harmonic generation (ω +7 ω)
of the Er3+-doped fiber amplifier. The conversion efficiency
from the fundamental wave was 7 %. These progresses are due to the
significant nonlinearity, relatively small walk-off angle and relatively
large angular, spectral and temperature acceptance bandwidths of CLBO.
As evidenced, the discovery of materials with desired properties can significantly
enhance the progress of solid-state UV light source. We have continued
interests on the investigation of prospective NLO crystals and the
search of new crystals for high-power, coherent, all-solid-state
UV radiation.
Click images to enlarge.
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CsLiB6O10 (CLBO) crystals
(Courtesy of Osaka University)
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GdxY1-xCa4O(BO3)3 (GdYCOB) crystals
(Courtesy of Osaka University) |
Progress on output power of coherent UV light from NLO crystals |
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Nitride Single Crystals
Nitride crystals like AlN, BN and GaN are prospective wide band-gap semiconductors
for high-power devices and light-emitting diodes (LEDs). The Wurtzite-AlN
and cubic-BN are attractive for their direct energy band-gap of about
6 eV, which can serve as miniature UV light and field electron emitters
for future industrial, medical and environmental applications. On the other
hand, Wurtzite-GaN is a promising material for blue LEDs and diode
laser, which are important for full-color display and high-density data
storage. At present, the blue LEDs and diode lasers are fabricated by depositing
the GaN-based active layers on sapphire substrates. However, there
are still significant problems arising from the large lattice mismatch
(13.8%) and the different in thermal expansion coefficients (25.5%) between
the film and the substrate. These have prevented the fabrication of reliable
GaN-based devices with long operation lifetime. These obstacles can
be avoided by using GaN as the substrates. However, high-quality GaN substrates
from single-crystals are still not accessible because the melt growth
of GaN single-crystal still prohibited due to the extremely high decomposition
pressure (~45000 atm) involved at the melting point (~2500 °C) . At
present, GaN single-crystals can be grown by the solution method
at 1300 to 1600 °C and N2 gas pressure of 10000 to 17000 atm . Because
of the need of high temperatures and gas pressures, the solution growth
method is not suitable for mass-production of GaN single-crystals.
We have been growing high-quality GaN single-crystals by a Na flux method
at 800 °C and <50
atm. This technique is promising for mass production of GaN single-crystals
at significantly low temperatures and pressures. Similar approach
is also applicable for growing other nitride crystals like w-AlN and h-BN.
We are interested to understand the growth mechanism and thus its applications
for other prospective nitride materials.
Click images to enlarge.
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| Hexagonal boron nitride crystals |
Gallium nitride crystals |
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