Spectroscopy of Tb3+:Sol-Gel Silica Spectroscopy of Tb3+:Sol-Gel Silica Glass Diplomarbeit Der...

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Transcript of Spectroscopy of Tb3+:Sol-Gel Silica Spectroscopy of Tb3+:Sol-Gel Silica Glass Diplomarbeit Der...

  • 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