Upconversion Spectroscopy of
3+
Tm
doped sol-gel glasses
Carlos Ortiz, Dan Boye
Physics Department, Davidson College
Abstract
Detection of Water Quenching
A series of samples with different Tm3+ molar concentrations and with 1%
Al3+ as a co-dopant were made. All samples were heated to 900°C and
subsequently held at 500°C. During the laser experiments, samples were
placed in a vacuum chamber to avoid atmospheric water effects. Enough
fluorescence was obtained from the 0.5% sample to photograph it using a
blue filter (see top-right corner of poster).
The excitation spectrum of the 456nm fluorescence of 1%Tm was
measured every 20 minutes with a Fluoromax-3 spectrometer. Time is
measured with respect to the time the sample left the oven at 500°C.
1
3
Excitation spectrum of D2 → F4 transition
2.E+05
Intensity (counts/sec)
Upconversion refers to processes that cause materials to emit light of shorter
wavelength (higher energy) than the light incident upon them. Because of the
numerous potential applications of materials with high upconversion
efficiency, we study upconversion fluorescence of Tm3+ in sol-gel glass hosts.
The time-evolution of the upconversion fluorescence was detected following
pulsed excitation. The fluorescence’s power dependence indicates a two-step
upconversion mechanism via real energy levels. Maximum upconversion
efficiency occurred for 0.5%Tm3+ and 1%Al3+. The fluorescence quenching
effects of ion-clustering and non-radiative relaxation from water diffusion
into the sample were individually observed.
Experimental Setup
Sample Synthesis
1.E+05
0 hr
1.E+05
1 hr
1.E+05
4 hr
6 hr
8.E+04
12 hr
6.E+04
18 hr
4.E+04
22 hr
2.E+04
0.E+00
200
250
300
350
400
wavelength (nm)
Intensity of 456nm fluorescence excited at 352nm
₪ Upon mixing TMOS (tetramethoxyorthosilicate) with water,
Tm nitrate, and catalytic amounts of nitric acid, the gelling begins
by sequentially undergoing the above reactions.
Intensity (a.u.)
1
Power Dependence of
Upconversion Intensity
0.8
0.6
0.4
0.2
Log(Fluorescence)
4
3.5
3
2.5
2
1.5
1
0.5
0
-1.32
500
1000
1500
2000
2500
3000
3500
Time in atmosphere (min)
-1.12
-0.92
-0.72
-0.52
-0.32
₪ When exposed to the atmosphere, water molecules diffuse into the
sample.
₪ Hydrogen bonded water complexes around Tm ions increase the rate
of nonradiative relaxation producing a quenching in the fluorescence.
-0.12
₪ Power dependence obtained by passing the beam through neutral
density filters .
₪ The quadratic relationship between power and intensity implies a
two-photon absorption upconversion mechanism.
1000
0.3
delta m [mg]
0.25
0.10%
1.2
1
0.1
0
0
20
40
60
80
100
800
600
400
time [min]
200
500
New sample
0.25%
0.15
0.05
0
1.4
0.2
100
90
80
70
60
50
40
30
20
10
0
120
Temperature [C]
0
-2
-4
-6
-8
-10
-12
1000
time [min]
1500
0
2000
Temperature [ºC]
Concentration Dependence of
Upconversion Intensity
Reheat
0.50%
Reheat #3 in N2
1%
0.8
Temperature
2% (x100)
0.6
0.4
0.2
Upconversion processes in Tm3+
Detection of Rehydration by
Thermogravimetric Analysis
Dmass
₪ All samples are co-doped
with the same amount of Al3+
to break up clusters.
₪ Before using a sample in
an experiment, they are heated
to 900°C at 1°C/min for
further densification and water
extraction.
₪ Following annealing, the
sol-gel glass displays the lack
of large-scale order
characteristic of amorphous
solids.
0
Log(Laser pow er)
Intensity (V)
₪ After a “wet” gel is formed by
hydrolysis and condensation reactions
in an aqueous environment, proper
heating removes a large volume of
water from the sample, forming a dry
sol-gel.
₪ The drying procedure on the left
follows the heating schedule from the
figure to the side.
0
slope = 2.2 +/- 0.2
0
0.00E+00
2.00E-06
4.00E-06
6.00E-06
8.00E-06
1.00E-05
₪ ~25% mass loss on initial heating
₪ ~3% gain in mass after annealing due to
rehydration
₪ No mass gain when cooled in N2 atmosphere
time (sec)
Conclusions & Future Work
Tm Concentration Dependence of Upconversion Intensity
1.4
Intensity of 457nm
Emission
₪ The energy levels shown below are from the Dieke diagram,
which identified the levels in a LaCl3 host.
₪ Because the optically active 4f electrons are shielded by the filled
5s and 5p shells, the energy level positions of Tm are relatively
insensitive to their host environment.
₪ Two possible upconversion pathways are observed with cw
pumping at 652nm.
1.2
1
0.8
0.6
0.4
0.2
0
0.00%
0.25%
0.50%
0.75%
1.00%
1.25%
1.50%
1.75%
2.00%
Concentration (molar %)
₪ Initially, increasing the concentration increases fluorescence
probability and thus the fluorescence intensity.
₪ As concentration increases further, clustering effects are
observed in decay profile and overall intensity.
₪ Only one of the two possible upconversion pathways fluoresced
significantly.
₪ Upconverted intensity has a quadratic dependence on laser power.
This helps to identify one of the well-known two-step upconversion
processes in Tm.
₪ Ion clustering negatively affects upconversion fluorescence. The
optimal concentration of Tb ions is 0.5mol%.
₪ Exposing sol-gels to the atmosphere dramatically quenches
fluorescence due to water diffusion.
₪ We will investigate the effect of Al3+ co-doping on the upconverted
fluorescence intensity of 0.5%Tm.
₪ Measuring the upconversion intensity over time exposed in air would
allow us to quantify the effects of phonon relaxation. These effects
could be advantageous or detrimental.
₪ The ultimate goal of this project would be to maximize the
upconverted fluorescence intensity and measure the quantum yield of
the overall process.
Acknowledgements
Support for this project has come from NSF MRI grant DMR-0421023.
Acknowledgement is also made to the donors of the American Chemical
Society Petroleum Research Fund for partial support of this research.