Deep ultraviolet (UVC) laser for sterilisation
and fluorescence applications
E.A. Boardman, L.S.-W. Huang, J.J. Robson-Hemmings, T.M. Smeeton, S.E. Hooper, J. Heffernan
Sharp Laboratories of Europe, Ltd.
Deep ultraviolet (UV) light can be used to sterilise bacteria and viruses, decontaminate drinking water
and in fluorescence sensors to detect chemicals. We are developing a new deep UV laser technology
targeting these applications. The UV laser components emit UV light in the wavelength range between
205 nm and 230 nm and we have demonstrated an output power of more than 1 mW. The laser devices
offer at least × 200 increase in output power compared with LEDs emitting at the same wavelengths.
We have demonstrated the use of the laser to sterilise bacteria and as the light source for fluorescence
measurements.
be characteristic of the fluorescing
is currently a significant R&D effort
compound. This means that UVC
to develop UVC LEDs and these are
New light-emitting components
light sources are very effective in
now becoming commercially available.
which generate deep ultraviolet (UV)
fluorescence sensors to detect chemicals
The performance and lifetime of the
light will enable new markets in areas
or bacteria in air or in water.
devices is improving but performance
1
Introduction
such as drinking water purification, bio-
Exploiting these applications for
at very short UVC wavelengths – less
sensing and medical devices. These
UVC light requires suitable UVC-
than 240 nm – remains poor. There are
applications are possible because of
emitting components. Mercury-based
no low-cost and compact UVC lasers
some useful properties of ultraviolet
UV lamps are the most common sources
on the market – in particular there
light with wavelength between 200 nm
of UVC light. Low-pressure mercury
are no laser diodes which emit in this
and 280 nm – the so-called UVC
lamps emit a narrow wavelength band
spectral range. If a UVC laser source
band. Firstly, UVC light sterilises
centred near 254 nm and are widely
is required for an application then the
bacteria, viruses and fungi so it can be
used in water, air and surface sterilising
only options are expensive and bulky
used in appliances for chemical-free
systems. Although these UV lamps can
industrial or laboratory sources which
disinfection of surfaces, air or water
generate high optical output powers
are impractical for many applications.
such as point-of-use drinking water
of at least several watts, they do have
At Sharp Laboratories of Europe we
sterilisers, air-conditioning systems and
significant disadvantages: they contain
are developing a novel UVC laser light
medical systems. Secondly, UVC light
the toxic element mercury which is an
source which has the potential to be a
can initiate photocatalytic reactions
environmental concern; are slow to
mass-producible, compact and low-cost
which break down organic pollutants,
warm up; cannot be modulated rapidly;
component and could be used for the
such as pesticide residues, which
show performance degradation at low
applications described above. In this
are in water. This decontamination
temperatures; are fragile; require high
paper we give a review of the device
function could be used in point-of-
voltages; and have lifetimes which are
and show demonstrations of some of its
use drinking water purifiers. A third
typically no more than 10,000 hours
unique properties.
useful property of UVC light is that it
and are reduced by cycling.
The diode-pumped UVC
laser
2
is strongly absorbed by most biological
These disadvantages of UV lamps
and chemical compounds and it induces
will be overcome by development
fluorescence (re-emission of light
of solid-state UVC emitting devices
Our novel UVC laser light source is
with a longer wavelength) which can
such as LED and laser sources. There
based on blue/violet laser diode
シャープ技報 第104号・2012年9月
31
technology. The device exploits a non-
is limited because the majority of
a waveguide increases the intensity
linear optical property possessed by
common non-linear optical materials
and increases the frequency-doubling
certain materials known as ‘second
absorb deep UV light. Furthermore, to
efficiency. We have developed a novel
harmonic generation’(SHG) or ‘frequency
meet the critical condition for efficient
method to create thin films of BBO
doubling’. A schematic of our device is
SHG (known as phasematching) at the
single crystal with thickness in the
shown in Fig. 1. The blue light output
wavelengths we desire we also need a
range of 5-20 μm which act as planar
by the laser diode has a wavelength, λ,
highly birefringent material. The most
waveguides and significantly increase
in the range 410 nm-460 nm. This blue
suitable non-linear optical material to
the frequency-doubling efficiency. We
light ‘pumps’ a frequency-doubling
use is β-BaB 2 O4 which is commonly
also modify these thin films to form
component and is converted to UVC
known as BBO.
two-dimensional waveguides which
light with wavelength λ/2 which is in
BBO does not naturally offer efficient
provide further confinement of the
the range 205 nm-230 nm. The
frequency-doubling of light from blue
propagating light into even smaller
frequency-doubling process, which is
laser diodes because it has a relatively
areas. A challenge for these methods is
the same as ‘wavelength-halving’
, can be
small second order non-linear optical
that BBO is hygroscopic – it absorbs
thought of as pairs of blue photons
coefficient. We are developing several
moisture – and water absorption leads
combining to form single UVC
methods to increase the efficiency of
to BBO becoming an absorber of deep
photons. The emitted light has the
the frequency-doubling in BBO and
UV radiation which extinguishes all
features characteristic of a UVC laser.
thereby increase the efficiency and
U V emis s ion. Therefore, w e have
In particular, the output can be modulated
output power of the dpUVC-laser.
developed all of our material handling
at high speed and the emitted UVC
One very effective method is to
processes so that they can be carried out
light is monochromatic, coherent,
fabricate an optical waveguide which
without the use of water or any water-
linearly polarised and can be collimated
confines the blue and UVC light into
containing chemicals. In the device
into a beam or focused to a small spot.
a small cross-sectional area as they
we cover the material in a protective
We call it the diode-pumped UVC laser
pass through the BBO. The waveguide
coating and/or encapsulate it in a dry
(dpUVC-laser).
is effective because the frequency-
atmosphere and this provides stable
One of the biggest challenges
doubling efficiency depends on the
operation over long lifetimes.
in developing the dpUVC-laser is
intensity of the light within the BBO
There are several key challenges
t o a c h i e v e g o o d e ff i c i e n c y i n t h e
(intensity = power ÷ cross-sectional
relating to the integration of the
frequency-doubling component. The
area). Decreasing the cross-sectional
laser diode with the frequency-
choice of frequency-doubling material
area of the light by confining it in
doubling component. One important
design consideration is to ensure that
the light emitted by the laser diode
has a sufficiently narrow spectral
linewidth and stable wavelength to
remain matched with the wavelength
acceptance range of the frequencydoubling component. A second
important feature is to increase the
overall UVC output power by filtering
and re-using the blue light which
passes through a frequency-doubling
component without being converted to
UVC.
Our frequency-doubling components
can be adapted to generate UVC with
Fig. 1　A schematic diagram of the diode-pumped UVC laser.
32
wavelength from 205 nm to more
than 280 nm (i.e. laser diode emission
LEDs (blue triangles) [1-7] and those
spots with power densities of at least
wavelengths between 410 nm and
of our dpUVC-lasers (red square is
400 mW·cm -2 (222 nm wavelength)
at least 560 nm). At the moment the
continuous operation(cw); red circle is
and we expect values of up to several
highest performance laser diodes emit
pulsed operation; red lines are projected
W·cm -2 are achievable. High power
wavelengths between 410 nm and
power for devices with laser diodes
densities are attractive for fluorescence
460 nm so we are focusing on dpUVC-
emitting shorter wavelengths). The
excitation applications or very rapid
laser devices with UVC wavelengths
plot shows that for wavelengths of
sterilisation of small volumes or surfaces.
between 205 nm and 230 nm.
between 205 nm and 230 nm the UV
If an application requires UVC light
3
laser power is between ×200 higher and
over a large area then the light emitted
×2000 higher than the best reported
from the UVC laser can be spread into
We have fabricated dpUVC-laser
R&D LEDs. We are not aware of any
a broad beam using simple optics.
devices emitting at several wavelengths
commercially available LEDs emitting
in the range from 207 nm to 223 nm.
in this 205 nm-230 nm wavelength
The UVC spectra for four of our
range. The laser-like beam properties
devices are shown in Fig. 2. The
of the dpUVC-laser mean that the UVC
We h a v e c a r r i e d o u t s u r f a c e -
output from each device is essentially
beam can be collimated or focused into
sterilisation tests to demonstrate the
monochromatic with a wavelength full-
a much smaller area than is possible
sterilising effect of the dpUVC-laser.
width half-maximum (FWHM) of less
with large-area light sources such as
Bacteria were seeded onto the surface
than 1 nm. Our highest power device
mercury lamps or LEDs. For example,
of an agar plate and then the UVC laser
has a wavelength of 222 nm and output
we have focused the full power of a
beam was rastered over the infected
power of 1.1 mW (pulsed operation).
dpUVC-laser into an area of ~10 μm
surface to sterilise the bacteria in
Device properties
Sterilisation using the
UVC laser
4
Tw o a d v a n t a g e s o f t h e d p U V C -
× 10 μm using a single lens. The ready
predetermined regions. The dpUVC-
laser over the emerging technology of
focusability means that the UV light can
laser beam with wavelength 222 nm
UVC LEDs are the very short operating
be efficiently coupled into optical fibres
was loosely focused with a lens to form
wavelength of less than 230 nm and
so that the action of the UV light may
a spot size of approximately 0.5 mm
the laser-like nature of the emitted
be spatially remote from the UV source.
× 0.5 mm and an energy density of
light. The plot in Fig. 3 shows a
Also, the UV light can be provided
1 0 m J · c m-2 w a s d e l i v e r e d t o e a c h
comparison of the wavelength and
with very high power densities: we
“pixel” (this would take 0.025 seconds
output powers reported for R&D UVC
have already demonstrated focused
with a 1 mW beam). This dose of UVC
Fig. 2　O utput spectra from four different dpUVC-laser
devices.
Fig. 3　A comparison of the output wavelength and power of R&D
UVC LEDs reported in the literature (blue triangles) and
the SLE dpUVC-laser (red square, circle and lines).
シャープ技報 第104号・2012年9月
33
light is sufficient to achieve between
area of the spot size and the surface
excitation source must be strongly
99.9% to 99.99% deactivation of
immediately adjacent was unaffected,
absorbed by the target molecule and
bacteria on a surface. For this edition of
even though the UVC laser device was
must have photon energy higher than
the SHARP Technical Journal, which
at least 300 mm away from the agar
the fluorescent photon energy. The very
falls in SHARP Corporation’s centenary
plate (we stress that the pattern in Fig. 4
deep ultraviolet light generated by the
year, we used the UVC light to sterilise
is obtained without any masking of
dpUVC-laser (wavelength less than
a “SHARP 100” pattern on the infected
the surface, and simply by rastering
230 nm) is excellent in both of these
surface.
of the UVC beam). This function may
respects: it is very strongly absorbed
A photograph of the agar plate
enable new applications using UVC-
by many organic chemicals, including
after incubation for 24 hours at 37°C
based sterilisation. As well as this, the
some amino acids in proteins, and has
is shown in Fig. 4. Where the agar
very short wavelength of our device
very high energy which enables many
surface was irradiated by the UVC laser
(less than 230 nm) may show good
fluorescence processes. It is often
the seeded bacteria were irreparably
potential for sterilisation of bacteria and
advantageous for the excitation light
damaged and were unable to reproduce.
viruses which are resistant to longer-
used in a fluorescence measurement to
The agar surface remains a dark brown
wavelength UVC light.
be monochromatic and to be focused
colour which indicates no bacteria
reproduction and demonstrates the
effective sterilisation of the bacteria.
Fluorescence using
the UVC laser
5
into a small spot. This means that the
laser-like beam emitted by our device is
very well suited to this application.
Elsewhere the seeded bacteria could
We h a v e a l s o d e m o n s t r a t e d t h e
The plots in Fig. 5 show the
reproduce as normal and they multiplied
potential of the dpUVC-laser for use
fluorescence spectra measured when
during the incubation period to form
as a light source for fluorescence bio-
the dpUVC-laser was used to pump an
colonies which are visible as pale spots
or chemical-sensors which could be
aqueous solution of naphthalene (top)
in the photograph.
used to detect chemical or biological
and a surface coating of the bovine
The clear definition of the “Sharp
molecules or bacteria. In fluorescence
serum albumin (BSA) protein (bottom).
100” logo in Fig. 4 demonstrates the
measurements, photons of the excitation
We use naphthalene to illustrate the
sterilising effect of the UVC laser.
light are absorbed by a specimen and
potential for chemical-sensors and BSA
Importantly, it also demonstrates that
fluorescent photons with lower energy
to illustrate the potential for use in bio-
the UVC laser output beam can be very
(longer wavelength) are emitted.
sensors. In each case the UVC laser is
easily and effectively focused into a
Analysis of the fluorescent light
visible as a sharp peak at a wavelength
small spot size and delivered from a
can be used in a sensor to detect the
of approximately 220 nm and the
long distance away. The sterilisation
presence and/or quantity of a particular
fluorescence is a broader peak in the
occurred only within the intended
chemical or biological molecule. An
280 nm-400 nm range (note that the
Fig. 4　A demonstration of sterilisation of bacteria using the dpUVC-laser. The dpUVC-laser beam was rastered to
sterilise the surface in the pattern of a“SHARP 100”logo. The pale spots are bacterial growth; the dark
brown area of the logo was sterilised. The diagram on the right shows the sterilisation setup.
34
Fig. 5　Plots of fluorescence spectra from naphthalene (top) and BSA protein (bottom) after excitation
with the dpUVC-laser. The diagram on the right shows a typical fluorescence setup.
spectral width of the laser emission is
enough for the device to be used in
as the monochromatic and short UVC
overestimated due to the low-resolution
some applications, in particular for
emission wavelength and the ability to
of the spectrometer used for this
fluorescence excitation and for some
focus the UV light into small spots with
measurement). These results show the
sterilisation applications. We are now
very high power density.
strong potential of the dpUVC-laser as
working to reduce the size of the mW-
a fluorescence excitation source.
class device into a compact component
Conclusion and future
plans
6
and one which can operate in continuous
wave (cw) mode. In addition, we are
carrying out several developments to
We have demonstrated a UVC laser
achieve large increases in the efficiency
device with output power of more than
of the frequency-doubling component
1 mW. Our device is all-solid-state
so that the UVC output power can be
and can generate light in the important
increased to 10 mW and above. This
wavelength range between 205 nm and
increased output power will open up
at least 230 nm. The output power is
new applications for the device.
more than ×200 higher than the output
from LEDs at the same wavelengths.
The mW-class output power is high
We will also continue to investigate
new applications which benefit from
References
1）Taniyasu et al., Nature 441 p325 (2006)
2）Noguchi et al., physica status solidi (c) 6
S459 (2009)
3）Hirayama et al., Applied Physics Express
1 051101 (2008)
4）Hirayama et al., physica status solidi (a)
206 1176 (2009)
5）Shatalov et al., Applied Physics Express
3 062101 (2010)
6）G r a n d u s k y e t a l . , A p p l i e d P h y s i c s
Express 4 082101 (2011)
7）We have calculated the average output
power where published data were only
provided for pulsed testing conditions.
the unique properties of the device such
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