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  • 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

    [email protected]

    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

    MicroscopeObjective

    Lens

    Computer

    Fiber

    Slit

    SpectrometerComputer

    ICCD-Camera

    OG-515

    Ar-Ion-LaserMirror

    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

    allows excitation with the Ar-ion-laser wavelengths at 476.5 nm, 488 nm and 496.5 nm. A

    possible drawback of Tb3+ is its long lifetime of about 1.7 ms [4] in sol-gel glass that allows

    only about 590 spontaneous emission processes per second.

    A schematic energy level scheme is shown in Fig. 2.

    Excitation is best performed with the 488 nm emission of an Ar-ion-laser. Population of the 5D4 level leads to fluorescence into the lower lying 7F levels. The strongest fluorescence is

    found in the green 5D4 → 7F5 transition at 546 nm.

  • 6

    Fig. 2: Schematic energy level diagram of Tb3+.

    The fiber contains 1 at. % of Tb3+, 2 at. % of Ti3+ and 10 at. % of Al3+ with respect to Si

    (N (Tb3+) = 2.148·1020 cm-3) and was manufactured by the sol-gel technique [4-6]. The

    preform was a quartz-tube with a diameter of 19 mm by 25 mm and a length of 80 cm. The

    inside of the tube was coated with 60 layers of Tb3+-doped sol-gel. After thermal treatment

    and collapsing it was drawn to a fiber with the drawing tower in the Institute of Applied

    Physics at Bern.

    The fiber used for the measurements has a length of 35 cm and diameters of 140 µm

    (cladding) and 6.9 µm (core). The fiber is excited with an Ar-ion-laser (Coherent, Innova 300)

    operated single line at 488 nm with a typical gain bandwidth of 6 to 8 GHz.

    The fluorescence of the fiber is measured at a distance of 233±1 mm from the incoupling end

    (Fig. 3).

    The density of quartz glass (SiO2) is 2.2 g/cm3. The numerical aperture (NA) of the fiber core

    is about 0.1 [7]. The measured extinction length was lext = 5.88 cm @ 488 nm leading to a

  • 7

    cross-section of σext = 7.9·10-22 cm2. With this extinction length the measured part of the fiber

    after a length of 23.3 cm carries only 1.9 % of the power coupled into the core.

    The laser-light at a wavelength of 488 nm from

    the Ar-ion-laser is coupled into the fiber with a

    microscope objective 20x, NA = 0.4. The

    fluorescence from the measured part of the fiber

    emitted nearly perpendicular to the fiber axis is

    imaged with a lens of f2 = 14.5 mm (NA = 0.266)

    as defined by NA = n sin (arctg(R/f)) onto the slit

    of the spectrometer. On the slit the measured part

    of the fiber is magnified by a factor of 1.3. The

    spectrometer (Oriel MS260i) has an f-number of

    f / 3.9 in an asymmetrical in-plane Czerny-Turner

    optical configuration. It has unequal input and

    exit focal lengths of 220 mm and 257 mm

    respectively and is equipped with two gratings,

    one with 150 grooves/mm, blazed for 300 nm

    and one with 1200 g/mm blazed for 500 nm respectively. To collect as much light as possible

    the slit is set to a width of 380 µm, leading to a resolution of about 1.2 nm with the 1200

    g/mm grating.

    Parasitic pump light at 488 nm is blocked by a selective filter (Schott, OG-515). The spectrum

    is detected with a camera (Andor iStar) shown schematically in Fig. 4.

    350

    mm

    233

    mm

    f = 8 mmNA = 0.4

    1

    f = 14.5 mmNA = 0.266

    2

    Slit

    Fig. 3: Fluorescence measurement.

  • 8

    Face Plate

    Photocathode

    Water Cooling

    Fiber OpticCoupler

    CCD Sensor

    FanD-Connector

    Heat SinkImage Intensifier TubePeltier Cooler

    Electronics

    Fig. 4: Side view.

    The camera is an Intensified Charge Coupled Device (ICCD). As shown in Fig. 5, the three

    major components of the image intensifier are the photocathode (standard W-type, modified

    S25) with a peak quantum efficiency of ηc = 13 %, the Microchannel Plate (MCP) and the

    output phosphor screen.

    Lens

    Image WindowPhosphor Coating

    Fiber OpticWindow

    Output Image

    Microchannel PlateVaccum

    Photocathode

    Input Image

    Fiber OpticWindow

    Microchannel Plate

    Photocathode

    Off +50V0V

    On -200V 0V 500 -1000V

    6000V

    Fig. 5: Sectional view of the image intensifier (left), operating voltages (right).

    The image intensifier can rapidly be switched, allowing it to be used as a shutter with few ns

    resolution. The gain of the MCP can be set between 0 and 255 [arbitrary units]. The effective

    gain of the ICCD is variable in the range from about G = 0.6 to G = 274 counts per

    photoelectron. The CCD is a silicon-based semiconductor chip consisting of 256 rows and

  • 9

    1024 columns of photo-sensors (pixels), whereas one pixel has an area of 26 µm2. The

    sensitivity of the camera is 10 electrons per count (ηccd = 0.1). This is in best case about 36

    counts in the CCD chip per photon. The signal of the CCD chip is recorded and analyzed with

    a computer.

    Results and Discussion

    The number of ions in the field of view is given by

    3 2( ) 2obs core coreHN N Tb r for w r MM

    π+= ≥ (1)

    with

    Nobs = Number of ions in the observed volume

    N(Tb3+) = 2.148·1020 cm-3, the density of ions in the fiber core

    H = 15 mm, the height of the spectrometer slit

    w > 10 µm, the width of the spectrometer slit

    M = 1.3, the magnification of lens f2

    rcore = 3.45 µm, the radius of the fiber core

    With the given values the number of ions in the field of view is

    Nobs = 9.37·1013 (2)

    The effective aperture of the lens f2 with R = 4 mm imaging the fiber from a distance of

    21 mm is 0.187. This corresponds to the emission half-angle of 10.78° in the plane

    perpendicular to the fiber in the direction of the lens axis. In the plane defined by the lens axis

    parallel to the fiber, refraction has to be considered, thus reducing the angle to 7.36° inside the

    fiber. These two angles define a solid angle of Ω = 0.076 steradians corresponding to 0.6 % of

    emission that can reach the slit of the spectrometer. This resulting solid angle is obtained from

    a numerical calculation (Maple 9.5).

    Excitation of Tb3+

    The pump power in the measured volume element is given by

  • 10

    Laser Filters Mirrors Obj Spot Apt Fr FibP P T T T T Tη η= (3)

    with

    PLaser = Laser power at 488 nm = 0.57 W

    TFilters = Transmittance of the NG-4 and NG-11 filter positioned in the feeding path = 14.43 %

    TMirrors = Transmittance of the feeding path with 5 mirrors = 66 %

    TObj = Transmittance of the 20x microscope objective = 86 %

    ηSpot = Coupling efficiency into the fiber with respect to the spot size.

    Assuming a Gaussian beam with 1.6 mm diameter focused with f = 8 mm leads to a

    waist of 1.55 µm radius, clearly smaller than the core radius of 3.45 µm. Therefore

    ηSpot = 100 %

    ηΑpt = Coupling efficiency into the fiber with respect to the core aperture.

    Assuming a Gaussian beam with 1.6 mm diameter focused with f = 8 mm leads to an

    aperture angle of sin(α) = 0.1. With a NA of the fiber core of about 0.1 this leads to

    ηApt = 100 %

    TFr = Transmittance considering Fresnel reflection at the fiber end = 96.5 %

    TFib = Transmittance of the first L = 23.3 cm of the fiber = 1.9 %

    With (3) the value of

    P = 0.86 mW results. (4)

    The effective number of excited Tb3+ ions in the 5D4 level within the measured volume can

    then be calculated by:

    ( )3 21 extN Tb Lext coreP HN e r

    hcV Mσλ τ π

    +−⎛ ⎞= −⎜ ⎟⎝ ⎠

    (5)

    whereas

    P = Laser power in the test sample

    λ = 488 nm, the laser wavelength in vacuum

    h = 6.626·10-34 Js, the Planck constant

    c = 3·108 m/s, the vacuum speed of light

    H/M = 1.17 cm, the length of the test sample

  • 11

    τ = 1.7 ms, the lifetime of Tb3+

    σext = 7.9·10-22 cm2, the absorption cross-section

    V = unit volume = 1 cm3

    L = unit length = 1 cm

    With the pump power of 0.57 W and with (3,5) an emission rate of

    Next/τ = 1.17·108 [s-1] results. (6)

    With this emission rate the number of counts measured in the CCD camera is:

    24ext

    CCD green f OG Spec cNN T T T Gη ητ π

    Ω= (7)

    with

    NCCD = Number of counts measured at the output of the camera

    ηgreen = 71 %, Percentage emitted in the green line

    Tf2 = 70 %, the transmittance of lens f2

    TOG = 89 % @ 542 nm, 90.3 % @ 588 nm and 90.5 % @ 625 nm, the transmittance of the

    selective filter OG-515

    TSpec = 80 %, the transmittance of the spectrometer, as estimated from manufacturers

    information [8]

    With the emission rate of 1.17·108 and the gain G, a rate of counts

    NCCD = 3.3·104 G [s-1] (8)

    is expected.

    Settings of the Camera

    To increase the detection sensitivity of the ICCD camera, several settings can be changed and

    modified. The sensitivity depends on the gain (0-255), the number of accumulations, the pulse

    width and the temperature of the CCD chip. Fig. 6 shows an overview of the terbium emission

    spectrum.

  • 12

    In all the following experiments background noise is subtracted. All spectra are taken with the

    1200 g/mm grating. The pulse width of the detection is set to 10 ms and Peltier cooling is not

    operated.

    To be sure that all the light imaged onto the slit of the spectrometer is collected, in a first

    experiment the width of the slit has been varied. On the axis of lens f2 ideal imaging would

    lead to a width of about 10 µm. The 15 mm height of the slit, however, will lead to blurring of

    the image.

    The result is shown in Fig. 7:

    Fluorescence Spectrum of the 1 at. % Tb3+ Fiber excited at 488 nm

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    1.2

    1.4

    1.6

    480 530 580 630 680 730 780 830

    Wavelength [nm]

    Cou

    nts [

    108 ]

    Fig. 6:

    The three fluorescence lines

    at 546 nm, 589 nm and 622

    nm as measured with the

    150 g/mm grating. The gain

    is set at 255, the slit width is

    380 µm and 5000

    accumulations were made.

  • 13

    0.0

    0.5

    1.0

    1.5

    2.0

    520 530 540 550 560 570 580 590 600Wavelength [nm]

    Cou

    nts [

    107 ]

    25 mu 30 mu 40 mu 60 mu 80 mu 100 mu 120 mu 140 mu 160 mu180 mu 200 mu 220 mu 240 mu 260 mu 280 mu 300 mu 320 mu 340 mu360 mu 380 mu 400 mu 420 mu 440 mu 460 mu 480 mu 500 mu

    40 µm

    300 µm

    Fig. 7: The fluorescence lines in the green at 542 nm to 550 nm and in the orange at 589 nm

    wavelength range recorded with different slit width. The gain is set at 200 and 5000

    accumulations are made. The slit width is varied in the range from 25 µm to 500 µm

    according to the table below the figure.

    From Fig. 7 it can be seen that the signal is not further enhanced if the slit width exceeds 360

    µm. All the following experiments are therefore made with a slit width of 380 µm.

    The effective gain of the camera is determined in a second experiment (Fig. 8). The

    sensitivity rises exponentially.

  • 14

    Gain @ 542 nm

    0

    1

    2

    3

    4

    5

    6

    7

    0 50 100 150 200 250

    Gain [a.u.]

    Cou

    nts [

    107 ] 0.0232e0.022x

    Fig. 8:

    Measured gain for

    settings in the range

    from 0 to 255. The

    solid line is an

    exponential fit.

    The ICCD camera is capable to accumulate a large number of counts. The third experiment

    shows the effect of accumulations in the range of 1000 up to 100'000 (Fig. 9):

    0

    2

    4

    6

    8

    10

    12

    520 530 540 550 560 570 580 590 600

    Wavelength [nm]

    Cou

    nts [

    108 ]

    A = 1000

    A = 5000

    A = 10'000

    A = 20'000

    A = 40'000

    A = 60'000

    A = 80'000

    A = 100'000

    Fig. 9: Numbers of accumulation A (G=255).

    The signal shows a linear increase with the number of accumulations.

  • 15

    To estimate the detection limit, the power of the 488 nm laser was reduced with neutral

    density filters. The different filter sets and the corresponding transmittances are listed in

    Tab.1.

    Filter combination Transmittance [%]

    Glass + NG-4 + NG-5 + NG-12 1.90

    NG-3 0.86

    NG-3 + NG-11 0.45

    NG-9 0.18

    NG-10 0.02

    Tab 1: Filter combinations and corresponding

    transmittances. All filters are neutral density filters

    from Schott.

    The resulting spectra are shown in Fig. 10.

    Variation of the Laser Power

    -5

    0

    5

    10

    15

    520 530 540 550 560 570 580 590 600

    Wavelength [nm]

    Cou

    nts [

    105 ]

    T = 0.02 %

    T = 0.18 %

    T = 0.45 %

    T = 0.86 %

    T = 1.90 %

    Fig. 10: Spectra with reduced laser power (5000 accumulations).

  • 16

    The curve with T = 0.18 % can be estimated to be the detection limit (3 dB above noise).

    Estimation of the Sensitivity

    In (6) it has been shown that with 570 mW of laser power an emission rate of 1.17·108

    photons per s occurs. Further it is shown that the detection limit with 5000 accumulations and

    a gain of 255 is reached at 0.18 % of the excitation power. With these present boundary

    conditions a photon rate of 2.1·105 s-1 can be detected. With (8) this corresponds to NCCD =

    0.0018· 3.3·104 G [counts/s]. With the maximum gain of G = 274 this leads to a number of

    1.6·104 counts. With 5000 fold accumulations this leads to 8·107 counts distributed in

    (1/8)·1024 by 256 = 33'000 pixels. About 1/8 of the pixels are contained in the green emission

    region. A signal of 2.5·103 counts per pixel results. The present sensitivity of the spectrometer

    with respect to the total spectral emission of the Tb3+ 5D4 → 7FJ transitions is limited to about

    80 fW of optical power. The error in this value is mainly due to i) neglecting scattering in the

    fiber by equating extinction with absorption, ii) an error in the lifetime of excited Tb3+ that

    depends on the preparation of the sol-gel glass and iii) the error in the coupling efficiency of

    pump light into the fiber. These errors are difficult to determine. Therefore the 80 fW limit

    just gives a rough estimation.

    For the detection of single ion emission, this sensitivity has to be considerably enhanced.

    Possibilities involve:

    i) Waiving the spectrometer.

    ii) Optimized integration by either selection of an optimized measuring time or

    “Integration on Chip”, a function that allows enhancing the accumulated charge

    per readout of the CCD chip. Thereby the noise of the readout can be reduced.

    iii) Cooling of the camera. Water cooling of the Peltier cooler will further reduce the

    temperature and thereby reduce thermal noise.

    Conclusion

    In summary the spectrally resolved fluorescence of Tb3+ excited with the 488 nm line of an

    Ar-ion-laser on the 5D4 → 7F5 transition has been measured. The fluorescence out of a volume

    of 4.4·10-4 mm3 has been detected. 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 the

  • 17

    non optimized setup shows that it is possible to detect an optical power of about 80 fW in the

    given arrangement. This is still about a factor of 105 above the sensitivity required for single

    ion detection.

  • 18

    Acknowledgments

    We would like to thank Th. Feurer for helpful discussions, V. Romano, U. Pedrazza, M.

    Locher, D. Weber and H.J. Weder we are grateful for their help with fiber drawing. This work

    was supported in part by the Swiss National Centers of Competence in Research (NCCR)

    program “Quantum Photonics” subprogram 8: “High Power Fiber Lasers”.

    References

    [1] W.D. Phillips, J.V. Prodan, H.J. Metcalf

    “Laser Cooling and Electromagnetic Trapping of Neutral Atoms”

    J. Opt. Soc. Am. B 2 (11), 1751-1767 (1985)

    [2] W.D. Phillips

    “Laser-Cooling and Trapping Neutral Atoms”

    Ann. Phys. Fr. 10, 717-732 (1985)

    [3] W. Nagourney, J. Sandberg, H. Dehmelt

    “Shelved Optical Electron Amplifier: Observation of Quantum Jumps”

    Physical Review Letters 56 (26), 2797-2799 (1986)

    [4] H. Marthaler

    “Design of Solid State Lasers at 3µm and Fibre Preforms with Sol-Gel Technology”

    PhD thesis, University of Bern, November 2003

    [5] M. Locher, V. Romano, H.P. Weber

    “Rare-earth Doped Sol-Gel Materials for Optical Waveguides” Optics and Lasers in Engineering 43, 341-347 (2005)

    [6] C.J. Brinker, G.W. Scherer

    “Sol-Gel Science – The Physics and Chemistry of Sol-Gel Processing”

    Academic Press, San Diego (1990)

  • 19

    [7] R. Renner-Erny, W. Lüthy

    “Measuring the Numerical Aperture of Uncoated Lossy Fibres and Preforms: Traps

    and Tips”

    IAP scientific report 2004/05 SL-5

    [8] N.N.

    Catalogue: “New Oriel Photonics Tools”

    Thermo Electron Photonics p. 51 (2002)

  • 20

    Tb3+ Impurities in Sol-Gel Glass

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

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

    [email protected]

    Abstract

    Pure quartz as well as sol-gel precursors contain a natural concentration of terbium ions. The

    spectrally resolved Tb3+ fluorescence of an Al3+ (10 at. % with respect to Si) doped silica fiber

    is investigated. The fluorescence is measured in the Tb3+ 5D4 → 7F5 transition excited with the

    488 nm emission of an Ar-ion-laser. The detected fluorescence shows a Tb3+-concentration of

    110 ppb resulting from impurities in the sol-gel chemicals.

  • 21

    Introduction

    Single photons on demand are very important for quantum information technology, such as

    quantum computers and quantum cryptology, with single photons operating as carriers of

    quantum information. Physical realization of sources are quite varied, including quantum dots

    in pillar micro-cavities, parametric down-conversion, trapped atoms in optical cavities, single

    defects in diamond nano-crystals and individual molecules in a solid [1].

    Another possibility is the observation of single ions in a dielectric host. A first attempt led to

    spectroscopy of terbium in a glass fiber with an emission as low as 80 fW [2] using a probe

    volume with 9.37·1013 ions.

    It is, however, desirable to use a probe volume with much less - ideally only one - active ions.

    This leads to additional problems with impurity contamination of the sol-gel precursors and

    the quartz tubes.

    In the present paper we report on experiments with a Tb3+ impurity-doped Al3+:sol-gel silica

    fiber. As in [2] the fluorescence is excited with the 488 nm emission of an Ar-ion-laser.

    Although the spectra are dominated by strong Raman lines, it is possible to detect the weak

    signal of Tb3+ 5D4 → 7F5 fluorescence.

  • 22

    Experimental

    The experimental arrangement is shown in Fig. 1.

    Ar-Ion-Laser

    Pinhole

    Pinhole

    Prism

    Mirror

    MicroscopeObjective

    MicroscopeObjective

    Lens

    Lens

    Computer

    CCD-CameraFiber

    Slit

    Spectrometer

    ComputerICCD-Camera

    OG-515

    Ar-Ion-Laser

    Fig. 1: Experimental arrangement.

    The fiber was manufactured by sol-gel technology. A quartz tube made of Heralux-WG with

    a diameter of 19 mm by 25 mm and 80 cm length was coated on its inner side with 50 layers

    of sol-gel glass. Between subsequent layers the tube was dried in nitrogen for 30 minutes. The

    sol-gel glass is actively doped with 10 % Al3+.

    The most important source of impurities is found in the chemicals for sol-gel preparation. As

    an example the AlN3O9 x 9H2O (Fluka puriss. p.a.) contains declared metal impurities ranging

    from

  • 23

    filtered with a prism between two pinholes to avoid emission from the discharge to reach the

    fiber. The pump beam is focused with a microscope objective (10x, illuminated NA = 0.05)

    onto the fiber core (radius 6 µm, NA = 0.09). With these parameters, theoretically, the

    coupling efficiency is η = 0.965, only reduced by Fresnel reflection at the fiber end. Coupling

    is optimized with a microscope (magnification 25x) in connection with a CCD camera at the

    rear end of the fiber.

    A length of 13.2 mm of the fiber is imaged onto the 15 mm high slit of a spectrometer with a

    lens of 14.5 mm focal length. This lens collects a solid angle of 0.079 steradian of the

    fluorescence and transforms it to the numerical aperture of about 0.13 of the spectrometer.

    Pump-light is blocked with a selective filter (Schott, OG-515).

    The spectrometer (Oriel MS 260i, 220/257 mm focal length, with a 1200 grooves/mm grating

    blazed for 500 nm) is equipped with an intensified CCD Camera (Andor iStar) cooled down

    to -20° C with a water-cooled Peltier system. A detailed description of the system is found in

    [2].

    Results and Discussion

    A typical spectrum is shown in Fig. 2:

    0.0

    0.4

    0.8

    1.2

    1.6

    500 520 540 560 580

    Wavelength [nm]

    Cou

    nts [

    109 ]

    Fig. 2:

    Fluorescence

    spectrum of the

    Al3+:quartz glass

    fiber excited with

    1.1 W launched

    power @ 488 nm.

    Exposure time is

    50 ms, with 30'000

    accumulations. The

    arrows indicate the

    position of expected

    fluorescence peaks

    at 542 nm and 549

    nm.

  • 24

    Fig. 2 also shows additional peaks below 540 nm. Different sources can be responsible for

    these peaks:

    i) Lines from the argon-ion-laser discharge are firstly assumed because part of the peak

    wavelengths 529 nm and 514 nm well correspond with argon emission lines. Filtering the

    pump beam with a prism and two pinholes (cf. Fig. 1), however, had no influence.

    Therefore this source can be excluded.

    ii) The peaks are shifted with a change of pump wavelength. Fig. 3 shows the lines for

    different pump wavelengths. This behavior excludes fluorescence from microscope

    objectives or the OG-515 selective filter as well as narrow band parasitic light not counted

    in the background correction.

    0

    1

    2

    3

    4

    5

    6

    7

    8

    500 520 540 560 580

    Wavelength [nm]

    Cou

    nts [

    108 ]

    476.5 nm

    488 nm

    496.5 nm

    Fig. 3: Shifted peaks when excited with 476.5 nm, 488 nm and 496.5 nm. The fluorescence

    peaks are only visible for 488 nm excitation. When excited with 476.5 nm and 496.5

    nm the fluorescence is too weak to be visible.

    Two potential sources are still possible: ghost lines from the ruled spectrometer grating or

    Raman lines of quartz. The comparison between measurements with quartz, sapphire and

    YAG allowed excluding ghost lines. The different spectra are shown in Fig. 4.

  • 25

    Raman Spectra @ 496.5 nm

    0

    1

    2

    3

    4

    5

    6

    200 700 1200 1700 2200 2700

    Wavenumber [cm-1]

    Cou

    nts [

    108 ]

    Tb-Fiber

    Nd:YAG-Rod

    Sapphire

    Fig. 4:

    Raman

    spectra of

    quartz glass,

    YAG and

    sapphire

    recorded

    with a pump

    wavelength

    of 496.5 nm.

    For quartz glass the measured peaks are situated at wave numbers of 474.1 cm-1, 590.2 cm-1,

    789.7 cm-1, 1042.9 cm-1, 1178.8 cm-1 and 1591.1 cm-1.

    The peak at 474.1 cm-1 may be strongly deformed by the steep edge of the OG-515 filter that

    completely cuts off the lower Raman lines.

    The spectrum of the Al:(10 at. %) silica glass corresponds to the Raman spectra presented for

    Herasil type II SiO2 glass with a strong, diffuse band at 440 cm-1 and weaker features near

    800 cm-1, 1060 cm-1 and 1190 cm-1 [4,5]. It is also similar to phosphate glass [6] and titanium-

    doped SiO2 [7].

    The two peaks assigned to Tb3+ 5D4 → 7F5 indicated in Fig. 2 are determined in a long-term

    measurement with the following parameters: launched laser power: 1.1 W, slit width: 0.5 mm,

    exposure time: 40 ms, accumulation: 350'000, with maximum gain and cooling. This was the

    maximum exposure possible within the dynamics of 2·109 counts of the camera. With the

    determination of the background the duration of the measurement was 8 hours. The result is

    shown in Fig. 5.

  • 26

    0.0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    536 541 546 551 556 561 566

    Wavelength [nm]

    Cou

    nts

    [10

    9 ]

    Fig. 5: Fluorescence around 550 nm. The measured curve (inset) was corrected assuming a

    baseline with constant slope. The two arrows indicate the position of the peaks at 542 nm

    and at 549 nm as expected from earlier work [2].

    The result given in Fig. 5 shows that the two peaks can be assigned to the Tb3+ 5D4 → 7F5

    transition. The additional peak at 546 nm is not identified. It could possibly be a higher order

    Raman line or an additional contamination of the fiber. In [2] it has been shown that the peak

    at 542 nm was about 50 % higher than the peak at 549 nm. In Fig. 5 the peak at 549 nm is

    higher by about 17 % than the peak at 542 nm. In view of the extreme dilution of the Tb3+, the

    10 % Al dopant level and the good correspondence of the wavelengths we nonetheless do not

    doubt the assignment.

    0.0

    0.5

    1.0

    1.5

    2.0

    2.5

    3.0

    536 541 546 551 556 561 566

    Wavelength [nm]

    Cou

    nts

    [10

    9 ]

  • 27

    Fluorescence Spectum of the Undoped Fiber

    0

    1

    2

    3

    4

    535 540 545 550 555 560

    Wavelength [nm]

    Cou

    nts [

    107 ]

    4.23·107 Counts

    Raman Lines of the Undoped Fiber

    0.0

    0.4

    0.8

    1.2

    505 507 509 511 513 515 517 519 521 523

    Wavelength [nm]

    Cou

    nts [

    109 ]

    1.47·109 Counts

    Fluorescence Spectrum of the 0.01 at. % Tb3+-doped Fiber

    0

    1

    2

    3

    535 540 545 550 555 560

    Wavelength [nm]

    Cou

    nts [

    108 ]

    2.94·108 Counts

    Raman Lines of the 0.01 at. % Tb3+-doped Fiber

    0.0

    0.4

    0.8

    1.2

    505 507 509 511 513 515 517 519 521 523

    Wavelength [nm]

    Cou

    nts [

    107 ]

    1.11·107 Counts

    Fig. 6: The Raman lines and fluorescence spectra of the undoped and low doped fiber. Comparison

    of the Raman and fluorescence lines of the terbium fiber with a dopant level of 100 ppm and

    the undoped fiber shows that the undoped fiber has a concentration of Tb3+ of 110 ppb.

    The Raman lines are very useful to calibrate the fluorescence signals of the doped and the

    undoped fibers. The fluorescence-to-Raman-counts ratio amounts to 0.029 for the undoped

    fiber and to 26.49 for the 0.01 at. % Tb3+ doped fiber. The signal in the undoped fiber is thus

    913 times weaker than in the 100 ppm fiber. This leads to a Tb3+ concentration level of 110

    ppb. In [8] values are given for the terbium concentration in natural quartz. Concentrations

    vary between 20 ppb and 80 ppb, depending on the origin of the quartz. This corresponds very

    well with the concentration measured in our sol-gel silica fiber core.

    These results show that the approach of single ion detection with a low-doped fiber of

    standard geometry is not possible. The contamination of the fiber material is by far too high.

    Even a 1 mm length of a fiber with 1 µm core diameter would still contain 1.9 million Tb3+

    ions.

  • 28

    Conclusion

    The spectrally resolved 5D4 → 7F5 fluorescence of an Al3+(10 at. %)-doped silica fiber with a

    natural concentration of Tb3+ has been measured. Excitation was performed with a single-line

    Ar-ion-laser tuned to 476.5 nm, 488 nm or 496.5 nm. A Tb3+-concentration of 110 ppb was

    found. This result is due to impurities of the sol-gel chemicals and the Heralux-WG quartz

    glass. With the given chemicals minimum Tb3+ dopant concentration should clearly exceed 1

    ppm to obtain a well defined dopant concentration. It is therefore not possible to achieve

    single ion detection with a low-doped fiber of standard geometry.

    Acknowledgments

    We would like to thank Th. Feurer for helpful discussions, V. Romano, D. Weber and H.J.

    Weder for their help with fiber drawing. This work was supported in part by the Swiss NCCR

    program „Quantum Photonics” subprogram 8: „High Power Fiber Lasers”.

    References

    [1] B. Sanders, J. Vuckovic, P. Grangier

    ”Single Photons on Demand“

    Europhysics News, 36 /2, March/April (2005)

    [2] L. Di Labio, R. Renner-Erny, W. Lüthy

    ”Spectral Measurements of Tb3+:Sol-Gel Glass Fluorescence with fW Power

    Levels“

    IAP Scientific report [2005-01-LA] (2005)

    [3] N.N.

    ”Heraeus Quartz: Fused Quartz and Fused Silica for Optics“

    Heraeus Publication 40-1015-079 printed in USA

    [4] R.J. Hemley, H.K. Mao, P.M. Bell, B.O. Mysen

    ”Raman Spectrocsopy of SiO2 Glass at High Pressure”

    Physical Review Letters, 57 (6) 747-750 (1986)

  • 29

    [5] F.L. Galeener, A.E. Geissberger

    ”Vibrational Dynamics in 30Si-substituted Vitreous SiO2“

    Physical Review B, 27 (10) 6199-6204 (1983)

    [6] Y. Li, B. Ashton, S.D. Jackson

    ”Raman Oscillation on a New Vibrational Mode Setup in Phosphosilicate Binary

    Glass Systems“

    Opt. Express, 13 (4) 1172-1177 (2005)

    [7] V.F. Lebedev, V.V. Koltashev, E.B. Kryukova, V.G. Plotnichenko, A.O.

    Rybaltovsky

    ”Luminescence, Raman and ESR Spectroscopy of Ti Doped SiO2 Hosts”

    Proc. XIX Int. Congr. Glass, Edinburgh, 1–6 July 2001 Phys. Chem. Glasses 43C

    141–144 (2002)

    [8] Qi-Cong Ling, Cong-Qiang Liu

    ”Behavior of the REEs and Other Trace Elements During Fluid-rock Interaction

    Related to Ore-forming Processes of the Yinshan Transitional Deposit in China”

    Geochemical Journal, 36 443-463 (2002)

  • 30

    Excitation of the Tb3+ 5D4 → 7F5

    Transition in Sol-Gel Glass

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

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

    [email protected]

    Abstract

    Three different Tb3+:Al3+(10 at. %):sol-gel silica glass samples are excited with blue-green

    light emitting diodes (LEDs) filtered with a monochromator. The absorption in the Tb3+ 7F6

    → 5D4 transition is measured in the range of 470 nm to 500 nm. The peak absorption is at 485

    nm. The spectral absorptance can be described by a Voigt line with a Lorentz part of 1.6 THz

    HWHM and a Gauss part with 5 THz HWHM. The Gauss part dominates the narrower

    Lorentz part thus showing the inhomogeneous broadening of the transition. Measurements of

    fluorescence light in the transition of Tb3+ 5D4 → 7F5 show a slight shift of less than 2 nm of

    the fluorescence peak at 542 nm with excitation wavelength and concentration.

  • 31

    Introduction

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

    chelates [1,2], Tb(10 at. %):Gd(10 at. %):YLF [3,4] and sodium terbium borate [5] no other

    host has been found that allows laser emission at 544.5 nm. Strong fluorescence is found also

    in Tb3+:Al3+(10 at. %):sol-gel silica glass fibers [6] but as yet no laser activity has been

    measured. A possible reason could be the strongly inhomogeneous broadening of rare earth

    ions in sol-gel silica glass that has already been described in Nd3+:Al3+:sol-gel silica glass [7].

    It has been shown that inhomogeneous broadening is not only a problem in the laser transition

    but also in the pump transition [7].

    In the case of Nd3+ or Yb3+ the properties of the pump transition can easily be measured with

    a Ti:sapphire-laser. In the case of Tb3+, however, with a pump transition at 485 nm a suitable

    tunable laser source is more difficult to find. Therefore, with the exception of [6], the aspect

    of inhomogeneous broadening in the pump transition has not yet been treated for the 485 nm 7F6 → 5D4 transition of Tb3+:Al3+:sol-gel silica glass.

    In this paper we report on a suitable way to perform excitation spectroscopy of the 485 nm 7F6

    → 5D4 transition of Tb3+:Al3+:sol-gel silica glass by the use of strong blue-green LEDs.

    Detection of the 542 nm 5D4 → 7F5 transition allows determination of the 7F6 → 5D4 line

    shape.

  • 32

    Experimental

    The experimental arrangement is shown in Fig. 1.

    Ar-Ion-Laser

    Mirror

    MicroscopeObjective

    MicroscopeObjective

    Lens

    Lens

    Slit

    SpectrometerComputer

    ICCD-Camera

    Preform Sample

    OG-515

    Mirror

    Beam-ExpanderMonochromator

    Fig. 1: Experimental arrangement.

    Three different preforms are investigated. They are manufactured with Tb3+:Al3+(10 at.

    %):sol-gel silica glass. The Tb3+ concentrations are 0.01 at. %, 2 at. % and 10 at. % with

    respect to Si. Each preform is made from a Heralux-WG quartz tube with diameters of 19 mm

    by 25 mm and a length of 80 cm. The inner side is coated with 15 layers of sol-gel glass (16

    layers for the 2 at. % sample). The detailed treatment is found in [8]. After collapsing, the

    diameter of the preform is about 15 mm. The core has a diameter of about 1 mm. 15 cm long

    samples of the preforms are optically polished at both end faces.

    Depending on the required wavelength the excitation of Tb3+ is provided by one of two

    different (Fig. 2) LEDs: Kingbright L7113PBCH (blue, peak wavelength at 468 nm, about 1.0

    cd @ 20 mA) or Sloan L5BG1N (bluish-green, peak wavelength at 500 nm, about 3.6 cd @

    20 mA).

  • 33

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    400 450 500 550 600

    Wavelength [nm]

    Inte

    nsity

    [a.u

    .]Fig. 2:

    Relative spectral

    intensity of the two

    LEDs used for

    excitation. The

    spectra are

    measured with a

    spectrometer (Ocean

    Optics, AVS-USB

    2000) with a

    600 grooves/mm

    grating blazed for

    400 nm.

    The desired excitation wavelength λexc is then filtered out of the LED-spectra using a

    monochromator (Acton Research Corporation, SpectraPro-500) with a focal length of 500

    mm, a 1200 grooves/mm grating and an f-number of f / 6.5. With slit widths of 3 mm the

    spectral width of the transmitted light is about 5 nm. This large spectral width is a

    compromise to provide a sufficiently strong fluorescence signal. A 4x-microscope objective is

    focusing the pump beam onto the core of the preform.

    Fluorescence at 542 nm is imaged with a lens of 14.5 mm focal length onto the 15 x 2 mm slit

    of the spectrometer (Oriel MS 260i, 220/257 mm focal length, with a 1200 grooves/mm

    grating blazed for 500 nm). Again, the slit width of 1.5 mm is required for sufficient

    sensitivity. It leads to a spectral resolution of 5 nm. To block pump light, a selective filter

    (Schott, OG-515) is placed directly in front of the spectrometer slit. The signal is detected

    with an intensified CCD Camera (Andor iStar), Peltier cooled to -15° C. A detailed

    description is found in [8].

    To adjust the different samples optimally in front of the spectrometer, fluorescence can also

    be excited with the 488 nm emission of an argon-ion-laser (Coherent, Innova 300). The laser

    beam is expanded by a factor of 12.5 to obtain a sufficiently large range for adjustment.

    This arrangement, without beam expander, is also used for measurements which require a

    stronger excitation source to allow for a better resolution.

  • 34

    Results and Discussion

    The wavelength dependence of the 7F6 → 5D4 transition of Tb3+:Al3+:sol-gel silica glass is

    determined in the range from 470 nm to 500 nm. For sufficient sensitivity the ICCD camera is

    operated at 44’000 accumulations with an exposure time of 40 ms each at maximum gain. The

    result is shown in Fig. 3.

    0

    0.2

    0.4

    0.6

    0.8

    1

    470 475 480 485 490 495 500

    Excitation Wavelength [nm]

    Inte

    nsity

    [a.u

    .]

    Fig. 3:

    The fluorescence

    efficiency as a

    function of

    excitation

    wavelength. The

    broad curve (black)

    is measured with a

    spectral resolution

    of 5 nm. The narrow

    curve (red) is

    calculated assuming

    a triangularly

    shaped intensity

    distribution of the

    pump light with 5

    nm FWHM.

    The broad curve in Fig. 3 gives the spectral fluorescence intensity as a function of excitation

    wavelength. For sufficient sensitivity it is measured with low spectral resolution of 5 nm. Its

    maximum value is at 485 nm. To correct the low spectral resolution a deconvolution has to be

    performed. For practical reasons the opposite way has been chosen: a “real” curve with a

    maximum at 485 nm is assumed and then broadened with the triangular intensity distribution

    of 5 nm FWHM provided by the monochromator. Fitting the broadened curve to the

    experimental values leads to the corrected “real” curve. This is the narrower curve in Fig. 3.

    The same curve is given in Fig. 4 as a function of frequency (black dots). The curve is

    compared with a Lorentz line with HWHM of 5.75 THz and with a Gauss line with 7.5 THz

    at HW@1/e. Both curves do not fit. The best fit is a Voigt curve with a Lorentz part of 1.6

  • 35

    THz HWHM and a Gauss part with 6 THz HW@1/e or 5 THz HWHM (Fig. 4). The Voigt

    line is calculated numerically by summing up Lorentz curves in a Gaussian distribution.

    0

    0.2

    0.4

    0.6

    0.8

    1

    602 607 612 617 622 627 632

    Frequency [THz]

    Inte

    nsity

    [a.u

    .]

    Lorentz

    Gauss

    Voigt

    Fig. 4:

    Dotted line:

    Absorption

    spectrum of the

    Tb3+ 7F6 → 4D5

    transition. The

    solid lines show

    fitted Lorentz

    (red), Gauss (blue)

    and Voigt (green)

    curves.

    The dominance of the Gauss part with respect to the Lorentz part in the Voigt curve shows the

    strongly inhomogeneous broadening of this transition.

    The fluorescence emission as a function of excitation wavelength is shown in Fig. 5.

    535 540545 550

    555 560470 nm

    476 nm

    482 nm

    488 nm494 nm

    500 nm

    0

    5

    10

    15

    Cou

    nts [

    108 ]

    Fluorescence Wavelength [nm]

    Excitation Wavelength

    Fig. 5:

    Fluorescence

    spectra as a

    function of

    excitation

    wavelength

    measured in the 10

    at. % Tb3+ sample.

  • 36

    Fig. 5 shows a double peak in the Tb3+ 5D4 → 7F5 transition. This double peak has also been

    shown in other silica glasses [9-11]. The shape of the double peak strongly depends on the

    content of co-doped aluminum or sodium. This can be seen in the figures of [11] where

    especially a high content of sodium leads to a pronounced double peak. Aluminum and/or

    sodium are used as network modifiers in the glass matrix. In [11] it is explained that the

    scission of the glass network generates voids where the Tb3+ ions find a well isolated place.

    This prevents the ions from clustering. The character of the double peak, however, is not

    explained.

    The maximum of the peak at 542 nm slightly shifts with excitation wavelength and Tb3+

    concentration. The shift is shown in Fig. 6.

    470

    475

    480

    485

    490

    495

    500

    541.1 541.4 541.7 542.0 542.3 542.6 542.9 543.2

    Peak Fluorescence Wavelength [nm]

    Exc

    itatio

    nW

    avel

    engt

    h[n

    m]

    2 at. %

    10 at. %

    Fig. 6:

    Shifted 5D4 → 7F5

    fluorescence

    peak when

    excited with

    pump

    wavelengths in

    the range from

    470 nm to 500

    nm.

    The shift shown in Fig. 6 is not an artifact due to the broad excitation and detection

    bandwidth, both of 5 nm. The experiment has also been performed with narrow-band laser

    excitation at 476.5 nm, 488 nm and 496.5 nm measured with a resolution of 0.1 nm. The

    resulting peak positions are given by the circles (2 at. %) and squares (10 at. %). The

    fluorescence wavelength is slightly shifted with respect to the measurements with the LEDs.

    This can be explained by variations in the position of the sample in front of the spectrometer

    slit. Nonetheless the shifts as a function of excitation wavelength and concentration of the two

    experiments are similar.

    The results of the 0.01 at. % sample are practically identical to those of the 2 at. % sample.

    Therefore they are not considered.

  • 37

    The shape of the 542 nm / 548 nm fluorescence double peak changes with excitation

    wavelength. Fig. 7 shows the shape of the green fluorescence peaks of the 10 at. % sample

    excited with the three lines of an Ar-ion-laser at 476.5 nm, 488 nm and 496.5 nm. A shift of

    the 542 nm peak occurs with the change of excitation wavelength, as well as a deformation of

    the green fluorescence double peak. Also higher Tb3+ concentration leads to somewhat longer

    fluorescence wavelength.

    0

    0.2

    0.4

    0.6

    0.8

    1

    535 540 545 550 555 560

    Wavelength [nm]

    Inte

    nsity

    [a.u

    .]

    488 nm

    476.5 nm

    Fig. 7:

    Shape of the

    fluorescence

    double peak as a

    function of

    excitation

    wavelength for the

    10 at. % sample.

    Excitation is

    performed with

    three lines of an

    Ar-ion-laser. The

    resolution of the

    spectrometer is

    0.1 nm (30 µm

    slit).

    Conclusion

    The absorption in the Tb3+ 7F6 → 5D4 transition has been measured in the range of 470 nm to

    500 nm. The absorption peak is at 485 nm. The spectral absorptance can be described by a

    Voigt line with a Lorentz part of 1.6 THz HWHM and a Gauss part with 5 THz HWHM. 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

    show a slight shift of less than 2 nm of the fluorescence peak at 542 nm with excitation

  • 38

    wavelength. Longer excitation wavelength leads to longer emission wavelength. Also higher

    Tb3+ concentration leads to somewhat longer fluorescence wavelength.

    Acknowledgments

    We would like to thank Th. Feurer for helpful discussions. This work is supported in part by

    the Swiss NCCR program "Quantum Photonics", Subproject 5c "Coherent Control of Matter

    in Photonic Crystal Fibers".

    References

    [1] S. Bjorklund, G. Kellermeyer, C.R. Hurt, N. McAvoy, N. Filipescu

    ”Laser Action from Terbium Trifloroacetylacetonate in p-Dioxane and Acetonitrile

    at Room Temperature”

    Appl. Phys. Lett. 10 (5) 160-162 (1967)

    [2] S. Bjorklund, G. Kellermeyer, N. McAvoy, N. Filipescu

    ”Liquid laser cavities”

    J. Sci. Instrum. 44 947-948 (1967)

    [3] A.A. Kaminskii

    ”Laser Crystals”

    Springer Verlag, Berlin, Heidelberg, p.172 (1981)

    [4] H.P. Jenssen, D. Castleberry, D. Gabbe, A. Linz

    ”Stimulated Emission at 5445 Å in Tb3+:YLF”

    IEEE J. QE-9 (6) 665 (1973)

    [5] C.R. Hammond

    ”CRC Handbook of Chemistry and Physics”

    R.C. Weast editor, 55th edition 1974-1975, The Chemical Rubber Company, p. B33

    (1974)

    [6] R. Renner-Erny, L. Di Labio, W. Lüthy

    ”On the Possibility of a Tb-doped Sol-Gel-based Fibre Laser”

    IAP Scientific report 2005

  • 39

    [7] D. Michel, M. Locher, W. Lüthy, V. Romano, H.P. Weber

    ”Tunability of a Nd3+:Al3+:Sol-Gel Glass Fibre Laser”

    EPS-QEOD Europhoton Conference on solid-state and fiber coherent light sources,

    Lausanne, Switzerland, August 20 - September 3

    Europhysics Conference Abstracts Volume 28C Fib-10055 (2004)

    [8] L. Di Labio, R. Renner-Erny, W. Lüthy

    ”Spectral Measurements of Tb3+:Sol-Gel Glass Fluorescence with fW Power

    Levels”

    IAP Scientific report (2005)

    [9] C. Armelli, M. Ferrari, M. Montagna, G. Pucker, C. Bernard, A. Monteil

    ”Terbium(III) Doped Silica-Xerogels: Effect of Aluminium (III) Co-Doping”

    Journal of Non-Crystalline Solids 245 115-121 (1999)

    [10] A. R. Spowart

    ”Energy Transfer in Terbium-Activated Silicate Glasses”

    Journal of Physics and Chemistry of Solids, 12 3375-3380 (1979)

    [11] K. Itoh, N. Kamata, T. Shimazu, C. Satoh, K. Tonooka, K. Yamada

    ”An Improved Emission Characteristics of Tb3+-Doped Sol-Gel Glasses by Utilizing

    High Solubility Terbium Nitrate”

    J. of Luminescence 87-89 676-678 (2000)