January 15, 2010 / Vol. 35, No. 2 / OPTICS LETTERS
103
Mid-IR to near-IR image conversion by thermally
induced optical switching in vanadium dioxide
S. Bonora,1,2,* U. Bortolozzo,3 S. Residori,3 R. Balu,4 and P. V. Ashrit4
1
CNR-INFM, Laboratory for Ultraviolet and X-ray Optical Research INFM-CNR, via Gradenigo 6/b,
35131 Padova, Italy
2
National Laboratory for Ultrafast and Ultraintense Optical Science—INFM-CNR, Piazza Leonardo da Vinci 32,
20133 Milano, Italy
3
INLN, Université de Nice Sophia Antipolis, CNRS; 1361 route des Lucioles, 06560 Valbonne, France
4
Thin Films and Photonics Research Group (GCMP), Department of Physics and Asrtonomy, Université de Moncton,
Moncton, NB, Canada E1A 3E9
*Corresponding author: [email protected]
Received September 1, 2009; revised November 17, 2009; accepted November 19, 2009;
posted December 10, 2009 (Doc. ID 116517); published January 11, 2010
We demonstrate the image conversion from mid-IR to near-IR (NIR) exploiting high-contrast optical switching in vanadium oxide thin-film layers. The intensity distribution of a mid-IR beam is converted to NIR
wavelengths exploiting the strong reflectivity changes induced by optical pumping in the mid-IR. We show
an experimental setup in which the radiation of a Tm-doped fiber laser at 1940 !m and a carbon dioxide at
10.6 !m has been converted to both 850 nm and 1064 nm. The resolution was 35 !m and was reached by
using an inexpensive CCD camera. The sensitivity of the device increases linearly with sample temperature.
We measured a threshold of 144 mW/ cm2, with a sample temperature of 62° C. © 2010 Optical Society of
America
OCIS codes: 040.6808, 160.6840, 110.3080, 040.3060.
The conversion of IR radiation to spectral ranges
where silicon array detectors can operate allows one
to detect IR light. Interest in the detection of mid-IR
radiation is increasing with the availability of lasers
emitting in the spectral range from 2 !m to 10 !m.
At the state of the art, the detection of shortwavelength mid-IR !2 – 5 !m" radiation can be carried out with cooled InSb focal-plane-array detectors,
which are usually cooled down to 77 K in order to reduce thermal noise [1]. The usual detector for the
characterization of a high-power laser for wavelength
longer than 1.8 !m is a pyroelectric slit-scanning
profilometer based on lithium tantalate !LiTaO3".
These devices can operate from 200 nm to 100 !m.
Other IR detectors such as an IR viewer can convert
IR light to visible by phosphor electronic bombardment and can see IR light up to 1.9 !m. Recently, microbolometers have become available in the form of
an uncooled array for the detection of light between
8 !m and 12 !m [2].
The spectral region between 1.7 !m and 8 !m still
does not offer any possibility of image detection without cooling the detector.
In this Letter, we demonstrate that using a VO2
layer makes it possible to convert mid-IR wavelength
to NIR wavelength detectable with silicon cameras
such as CCD and CMOS. We used a mid-IR beam
(called writing beam or pump beam) to locally change
the reflectivity in the VO2 layer. This reflectivity
change depends on the local temperature, which is
related to the intensity of the writing beam, and it is
extended from the NIR to the mid-IR [3,4]. Thus a
second beam in the NIR compatible with CCD and
CMOS technology (called reading beam or probe
beam) is used for the detection of the reflectivity pattern imposed by the writing beam. Figure 1(a) shows
the experimental setup. Here we characterize the de0146-9592/10/020103-3/$15.00
tector with a thulium-doped fiber laser !1940 nm"
and with a CO2 laser !10.6 !m". Both writing and
reading beam can be broadband or monochromatic,
coherent or noncoherent.
Several vanadium oxides change their properties
due to a crystallographic phase change between
monoclinic (insulator) and tetragonal (metal) [3].
Among them VO2 is the most studied, because the
transition happens at 68°, very close to room temperature. For the effect of the phase transition, both
electrical and optical properties undergo a strong
change. The optical characteristics, such as dynamic
range, spectral transmission, reflectivity, and absorption are strongly dependent on sample preparation.
VO2 layer thickness, deposition technique, and annealing have to be controlled in order to prepare
sample with characteristics suitable for the application for which they are going to be employed. Recently, high contrast with near-zero transmission in
the switched state has been demonstrated. An experimental demonstration in [4] reports that exploiting
the absorption of the VO2 film in the visible makes it
possible to induce a thermal heating of the film beyond 68° C. Switching time in the order of 2 ms has
been measured for different film thicknesses for
pumping wavelength of 532 nm (frequency-doubled
Nd:YAG 5 W) with a switching contrast of 35% at
1.6 !m, which increases for longer wavelengths [5].
The main property we exploit for our devices is
that the reflectivity change induced in the VO2 layer
in any wavelength range extends with a contrast
larger than 10% from 700 nm to the mid-IR. This
wavelength range intersects the band of the silicon
cameras sensitivities defining a window of functioning of the device that goes from 700 nm to 1.1 !m. A
very interesting similar setup is used in Hughes liquid crystal (LC) light valves to convert NIR images to
© 2010 Optical Society of America
104
OPTICS LETTERS / Vol. 35, No. 2 / January 15, 2010
Fig. 1. (Color online) (a) Outline of the experimental
setup. The mid-IR laser (Tm fiber laser at 1940 nm or
CO2at 10.6 !m) induces local transmission change in the
VO2 layer, which modulates the NIR reading beam
(850 nm diode laser or 1064 nm Nd:YAG laser), which copropagates with the mid-IR after the beam splitter. The inset shows the switching transmission of the VO2 film for
sample temperatures below and over 68°. (b) Beam profile
of a thulium-doped fiber laser (10 Hz, 8 ms, 10 W / cm2)
probed with a reading light at 850 nm, 1 mW. The FWHM
of the beam is 2.241 mm" 2.209 mm.
visible [6] using a photoconductor associated with a
LC layer.
In this experiment vanadium films were initially
deposited by rf sputtering, and then they were oxidized in a vacuum furnace under carefully controlled
conditions of oxygen flow and temperature [5]. Then
the VO2 light valve was built by attaching a 100 nm
thin VO2 film deposited on a glass substrate with a
Peltier heater, which allows one to control the sample
temperature from room temperature to 100°. Figure
1(a) shows the experimental setup used to demonstrate the detection of the beam profile of a Tm fiber
laser at 1940 nm by means of the thermal pumping
technique. In this demonstration the writing beam is
a pulsed thulium-doped fiber laser emitting at
1940 nm (IPG Photonics 20 W at 1940 nm) and the
reading beam is an expanded 1 mW, 850 nm diode laser. The beam profile is recorded by a CCD camera
that detects the probe transmission due to the optically induced thermal heating of the vanadium film.
Figure 1(b) shows the 3D profile of the Tm fiber laser
under test. We can notice a very good imaging of the
beam profile under an input writing intensity of
10 W / cm2, with a pulse length of 8 ms at 10 Hz. The
system in a single shot gives a detailed characterization of the beam profile. The response time of the VO2
valve is 15 ms, so dynamical following can be
achieved under variation of the writing beam in the
range of 60 Hz. The same compact setup can be extended to other laser sources, in the wavelength
range from the visible [4] up to mid-IR, thus allowing
for laser-beam profiling. This characterization is very
useful, for example for extracting the M2 value, for
beam waist measurements and for the determination
of laser coherence length. Therefore the VO2 valve
constitutes a detector that can find applications in
beam characterization for lasers currently used in
different disciplines, such as surgery lasers emitting
at 2 !m and 3 !m (Tm fiber laser and holmium and
erbium lasers), vertical-cavity surface-emitting lasers for gas detection spectroscopy at 2 !m, highpower CO2 lasers, and tunable sources such as ultrashort tunable mid-IR optical parametric
amplifiers [7–9].
Having verified the functionality of the device as a
detector for beam profiles, we now investigate the device sensitivity at room temperature. Spectroscopic
measurements of the sample under normal conditions show a transmission of !25% in the visible
spectra, and this increases as the wavelength increases, finally showing a ! 50% transmission at
2.5 !m. The same sample in the switched-state conditions, i.e., above 68° C, shows an unaltered transmission in the visible state, which starts to decrease,
however, as the wavelength increases and finally
shows a near-zero transmission beyond 1.5 !m. The
experimental setup for the determination of the sensitivity is shown in Fig. 1(a), where we used a silicon
photodiode as detector and a Nd:YAG laser as reading beam. Figure 2 shows the transmittance change
upon illumination of the sample with intensities up
to 109 W / cm2 with a pulse length of 5 ms. At room
temperature the threshold for the transition is of
about 54 W / cm2. Using the Peltier temperature control, we can increase the device sensitivity by heating
the sample, which results in reducing the temperature gap to reach the switched state and also to reduce the amount of heat flux to be supplied by the la-
Fig. 2. Transmission measurements under Tm laser
square pulses. VO2 layer temperature was 22° C. Pump laser repetition rate, 100 Hz; Ton= 5 ms.
January 15, 2010 / Vol. 35, No. 2 / OPTICS LETTERS
ser. We tested the dependence of the device
sensitivity on the VO2 temperature, and we found a
linear
dependence
with
a
coefficient
of
−0.7 W / ! ° C cm2". The last stable point was detected
at 62° C with a threshold of 144 mW/ cm2. It is shown
in [3] that VO2 switches between the insulating state
and the conducting state around an hysteretic cycle
with the heat-cycle transition at 68° C and the cooling cycle transition at around 64° C, which is in good
agreement with the behavior of our device. Figure 3
shows the characterization of the device at 62° C. The
minimum detectable signal was about 144 mW/ cm2.
Longer pulses lead to saturation of the device because of heat accumulation in the device substrate.
Finally, we demonstrate that we can use the device
for acquisition of high-resolution images in the midIR, converting them to NIR images. To demonstrate
the resolution limit, we put a target mask in front of
the writing beam heating the sample at 53° C. The
reading beam is an Nd:YAG 1 mW laser. Figure 4(a)
shows an image of the target mask acquired. Details
of about 35 !m have been detected. The resolution
was limited by the CCD imaging system !1 pixel
= 12 !m" and by the speckle noise of the reading
beam, which is clearly visible in the image. The image contrast and resolution depends on pulse length
and laser intensity, which were 18 ms at 10 Hz and
3 W / cm2. We then demonstrate high-resolution image conversion by the combination of different lasers
for both writing and reading process. Figures
4(b)–4(d) show image acquisitions using both a Tm fiber laser and a carbon dioxide laser at 10.6 !m. The
reading beams were both Nd:YAG laser at 1064 nm
and a diode laser at 850 nm. The images were acquired by subtracting from the signal beam the background given by the reading light. The pulse length
and repetition rate were acquired in two different
ways. Figures 4(a), 4(c), and 4(d) were obtained using
a software video trigger for the acquisition of pulsed
images. The writing-beam pulse length and repetition rate were chosen in order to have a good contrast
105
Fig. 4. (a) Detection of a USAF x1 target mask image illuminated with Tm laser (18 ms at 8 Hz, 3 W / cm2, TVO2
53°). (b) Detection of Tm fiber laser (8 ms at 10 Hz,
3 W / cm2, TVO2 60°) by an 850 nm diode laser. The image is
the interference pattern generated by an opaque cross
printed on a glass support. (c) and (d) Image of a cross and
a circular aperture printed in an aluminum frame with a
CO2 (4 ms at 10 Hz, 88 W / cm2, TVO2 40° C) laser and an
850 nm as a reading beam.
and avoid sample saturation due to heat accumulation. Acquisition of Fig. 4(b) was carried out by a fine
tuning of the pulse length and repetition rate of the
writing beam in order to allow for the sample thermalization and the formation of a stationary image.
In conclusion VO2 thin films have been found to be
very effective for image detection of mid-IR laser radiation by exploiting the strong reflectivity change in
the NIR induced by optically pumping the film in the
mid-IR. We demonstrate the effectiveness of the device for the detection of thulium-doped fiber lasers at
1940 nm and for carbon dioxide lasers 10.6 !m using
lasers as reading beams at both 1064 nm and
850 nm. The spatial resolution was about 35 !m and
is limited essentially by the speckle noise of the reading beam. We have shown as well that the sensitivity
increases linearly when heating the sample, with
a minimum value for the Tm laser of
144 mW/ cm2 at 62° C. The advantages of this technique are the use of a nonpixellated device, the compactness of the setup, the absence of mechanical
parts, and the low cost when compared with other
techniques currently employed (see, e.g., slit profilometers).
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Fig. 3. Transmission change in 100-nm-thick VO2 layer at
a temperature of 62° C. Pump laser repetition rate, 100 Hz;
Ton= 5 ms.
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