Spectroscopy of Tb3+:Sol-Gel Silica Glass

Diplomarbeit

Der Philosophisch-naturwissenschaftlichen Fakultät

der Universität Bern

vorgelegt von

Loredana Di Labio

2005

Leiter der Arbeit: Prof. W. Lüthy und Prof. Th. Feurer

Laserabteilung

Institut für Angewandte Physik

1

Introduction

Rare earth doped crystals and glasses have found wide applications in various laser systems.

Especially Nd3+, Yb3+, Tm3+, Ho3+ and Er3+ are frequently used for lasers in the wavelength

ranges of 1 µm, 2 µm and 3 µm. The spectroscopy of these rare earth ions has thoroughly

been investigated in literature. Other rare earth elements have found less application. An

example is Terbium, a rare earth element that has very limited importance as a laser material.

Laser action has been reported in only few hosts.

A very promising host for all rare earth elements is the core of a glass fiber. In a glass fiber

the intensity of pump light can be concentrated over long interaction lengths thus leading to

very efficient pumping. A novel and very elegant way to produce glass fibers is based on sol-

gel technique. This technique starts from the liquid phase of the future glass and this allows

the addition of dopant materials and network modifiers in a very homogeneous way.

It is therefore very interesting to study the behavior of Tb3+ in a sol-gel silica glass host.

Spectroscopic data of this combination are rather rare. It is the goal of the present work to

investigate the excitation of the Tb3+ 5D4 level and the fluorescence of 5D4 to the lower lying 7F5 level. This includes the investigation of the used detection system, the evaluation of

detection limits as well as the measurement of the line shapes of the transitions involved.

In part II of this work, measurements of the spectrally resolved fluorescence of trivalent

Terbium ions doped in a sol-gel silica glass fiber are reported. The fluorescence of the 5D4 → 7F5 transition is excited with the 488 nm emission line of an Ar-ion-laser. The

detection system consists of a spectrometer in connection with an intensified CCD (ICCD)

camera. With this camera, the signal should exceed 3.5·103 counts per pixel to be clearly

resolved (3 dB above the noise level). The parameters of the detection system are

experimentally verified. An estimation of the detection limit in the given arrangement results

in an optical power of 80 fW. Finally, possibilities to further enhance the sensitivity are

discussed. The conclusions are then realized in part III of this work.

Part III describes experiments with a Tb3+ impurity-doped Al3+:sol-gel silica fiber. The fiber

is actively doped with Al3+ (10 at. % with respect to Si) and it has a natural concentration of

Tb3+ due to the inevitable contamination of the sol-gel-precursors and the Heralux-WG quartz

2

glass. Excitation of the 5D4 state leading to fluorescence in the 5D4 → 7F5 transition is

performed again with the single-line Ar-ion-laser. In this experiment, however, the laser is

tuned to 476.5 nm, 488 nm or 496.5 nm. Different wavelengths were necessary to distinguish

the fluorescence from Raman signals. Although the spectra are dominated by the strong

Raman lines, it is possible to detect the weak signal of the Tb3+ 5D4 → 7F5 transition. This

signal was compared with that of an actively doped sample of Tb3+(100 ppm):Al3+(10 at. %)

sol-gel glass. The comparison shows a natural Tb3+-concentration of 110 ppb. In literature

similar concentrations are reported for natural quartz.

In part IV we report on a suitable way to perform excitation spectroscopy of the 485 nm 7F6 → 5D4 transition of Tb3+:Al3+:sol-gel silica glass. The goal is a measurement of the shape

of the absorption line as well as a determination of the inhomogeneous broadening. To allow

for a continuous tuning of the excitation wavelength in the range of 470 nm to 500 nm, blue-

green light emitting diodes (LEDs) in combination with a monochromator are used. Detecting

the intensity of the Tb3+ 5D4 → 7F5 transition with varying pump wavelength allows

measuring the shape of the 7F6 → 5D4 transition. Three different Tb3+:Al3+(10 at. %):sol-gel

silica glass samples with Tb3+ concentrations of 0.01 at. %, 2 at. %, and 10 at. % have been

measured. The spectral absorptance can be described by a Voigt line with a Lorentz part of

1.6 THz HWHM and a Gauss part with 6 THz HW@1/e. The Gauss part dominates the

narrower Lorentz part thus showing the inhomogeneously broadened transition.

Measurements of fluorescence light in the transition of Tb3+ 5D4 → 7F5 at 542 nm show a

slight shift of less than 2 nm of the fluorescence peak as a function of excitation wavelength

and concentration.

Outside the frame of this diploma work contributions have been made to two additional

reports and one poster presentation written by R. Renner-Erny et al. For completeness, these

reports and the poster are added in the appendix (part V).

3

Spectral Measurements of Tb3+:Sol-Gel

Glass Fluorescence with fW Power

Levels

L. Di Labio, R. Renner-Erny and W. Lüthy

Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

loredana.dilabio@iap.unibe.ch

Abstract

The spectrally resolved fluorescence of Tb3+ excited with the 488 nm line of an Ar-ion-laser

on the 5D4 → 7F5 transition is measured. The fluorescence is detected out of a volume of

4.4·10-4 mm3 and a photon flux of 2.1·105 photons per sec leads to a clearly resolved signal of

about 2.5·103 counts per pixel. An estimation of the detection limit of this presently non

optimized setup shows that an optical power of about 80 fW can be measured in the given

arrangement. Possibilities to considerably lower the detection threshold are discussed.

4

Introduction

Detection of single ion or atom fluorescence offers the possibility to measure Doppler free

spectra and to monitor the particle in its quantum state. The observation of single particles is

possible with laser cooling and trapping. Several techniques for cooling particles have been

published in the last twenty years [1,2]. The basic idea is to slow particles down by

momentum transfer from a counter propagating resonant laser beam. Due to the change in

their velocity ergo in their frequency, however, the particles run out of resonance. This

Doppler shift problem can either be solved by chirping the laser frequency to keep it resonant

with the Doppler-shifted decelerating particle or by changing the particles energy level. The

resonance frequency of the particle can be Zeeman tuned in an inhomogeneous magnetic

field. Then, the slow particle can be caught in a trap and further cooled down to few µK or

even nK. To trap ions after cooling, magnetostatic, electrostatic and hybrid magnetostatic-

radiative traps can be used. Trapping of neutral atoms is more challenging. A suitable

technique for trapping and further cooling makes use of Doppler cooling with six laser beams

propagating and counter propagating in the three spatial directions. The laser radiation is

tuned slightly below resonance with the atomic transition. Atoms moving towards the laser

are Doppler shifted into resonance and will suffer a momentum transfer pushing them back

into the trap. With a single trapped ion or atom ultrahigh-resolution Doppler free spectroscopy

is possible. Further information can be obtained on the quantum state of the system (quantum

jumps) [3].

Besides monitoring a single ion in vacuum, however, it is also interesting to study a single ion

in a glassy host. The inhomogeneous broadening due to the possible sites is omitted and an

individual site can spectrally be investigated. The single ion in the glass is further

permanently available. A highly diluted rare-earth-doped fiber can be used to host these

distinguished single ions.

In this paper we report on the setup of a system for spectrally resolved detection of

fluorescence in trivalent Terbium ions doped in a sol-gel silica glass fiber. The detection

system consists of a spectrometer in connection with an intensified CCD camera (ICCD). The

parameters of the detection system are experimentally investigated and a first estimation of its

sensitivity is made. Possibilities to further enhance the sensitivity are discussed.

5

Experimental

The experimental arrangement is shown in Fig. 1.

Ar-Ion-Laser

Microscope Objective

Lens

Computer

Fiber

Slit

SpectrometerComputer

ICCD- Camera

OG-515

Ar-Ion-Laser Mirror

Mirror

Fig. 1: Experimental arrangement.

For the active ion Terbium was chosen for its high fluorescence efficiency, suitable emission

wavelengths in the visible spectrum with good detection efficiency in the standard W-type

(modified S25) photocathode that is sensitive in the range from 180 nm to 850 nm. Further it

a