17th University Conference on Glass Science – June 2005

17th University Conference on Glass Science – June 200517th University Conference on Glass Science – June 2005

Rui M. Almeida* and Ana C. Marques

Departamento de Engenharia de Materiais / ICEMS, Instituto Superior TécnicoAv. Rovisco Pais, 1049-001 Lisboa, Portugal

[email protected]

* Presently on sabbatical leave at Lehigh University, PA.

RareRare--earth photoluminescence in solearth photoluminescence in sol--gel derived gel derived confined glass structures confined glass structures

An IMI Video Reproduction of Invited Lectures from the 17th University Glass Conference

mailto:[email protected]


Outline

Silicate glasses as Er3+ hosts: effects of TiO2 and HfO2 additions on the structure and photoluminescence (PL) spectra

Effects of Ago nanoparticles on the Er3+ PL intensity

Er3+ - doped sol-gel (SG) planar waveguides

Er3+/ Yb3+ - doped Fabry - Perot microcavities (by SG)

Conclusions


Possible RE ion (active) dopants for glassy hosts

4-levelquasi 3-level (4)

3-level

quasi 2-level (3)

second window (~ 1.3 µm) third window (~ 1.5 µm)


The Erbium Doped Fiber Amplifier (EDFA) operates as a 3- level system when pumped at 980 nm and as a quasi 2-level system when pumped at 1480 nm to the long-lived (metastable) 4I13/2 level.

The signal photons at ~ 1.5 µm stimulate emission of Er3+ at ~ 1520-1620 nm, amplifying the original signal, while maintaining its frequency and phase.

4I11/2

4I13/2

4I15/2

Electronic energy levels of Er3+

(Adapted from: Introduction to DWDM Technology, S.V. Kartalopoulos, IEEE Press, 2000)


Er3+ PL spectrum @ ~ 1.5 µm

The 4I13/2 → 4I15/2 emission of Er3+ spans the third window of fiberopticcommunications. In the case of Al/P doped silica glass fibers, it fully covers the C band, from 1530-1565 nm. (FWHM ~ 50 nm).

The Dense Wavelength Division Multiplexing (DWDM) systems also include the S (short, 1450-1530 nm) and L (long, 1560-1620 nm) wavelength ranges.

(Adapted from: Rare earth doped fiber lasers and amplifiers, ed. M.J.F. Digonnet, Marcel Dekker, 1993)


Planar waveguides

Lightn2

A planar dielectric waveguide has a central rectangular region ofhigher refractive index n1 than the surrounding region which hasa refractive index n2. It is assumed that the waveguide isinfinitely wide and the central region is of thickness 2a. It isilluminated at one end by a monochromatic light source.

n2

n1 > n2

Light

Light Ligh

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)


y

E(y)m = 0 m = 1 m = 2

Cladding

Cladding

Core 2an1

n2

n2

The electric field patterns of the first three modes (m = 0, 1, 2)traveling wave along the guide. Notice different extents of fieldpenetration into the cladding.© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Modes in an optical planar waveguide, propagating along the z direction.

There is confinement only along the y direction, only.y

z

x

(Evanescent field increases with mode order and λ).


Silicate glasses as Er3+ hosts

Incorporation of Er3+ ions in silica-based sol-gel planar waveguides, for integrated optical amplification @ 1.5 µm:

Er3+ can also act as a sensitive probe of the structure of the host material:

the disordered structure of glass and amorphous filmsleads to site-to-site variations in the local bonding

inhomogeneous broadening of spectral features

possible splitting of Stark components when Er3+ is in the neighborhood of crystallites within the glass matrix(Strohhofer et al. 1998)


Transitions between individual Stark components of different J multipletscan be observed as discrete lines in RE-doped crystals, but not in glasses. Crystalline hosts provide high cross sections at nearly discrete wavelengths; glass hosts have lower cross sections over a broad range of wavelengths.

(Note: the areas are off-scale and should be the same under both curves).

4F3/2 → 4I11/2

Nd3+

(Adapted from: Rare earth doped fiber lasers and amplifiers, ed. M.J.F. Digonnet, Marcell-Dekker, 1993)


Nanostructuring of planar waveguidesincorporation of Ago nanoparticles intoEr3+-doped silica-based waveguides

surface plasmon resonanceof noble metal nanoparticles

• enhancement of third-order optical nonlinear susceptibility (χ(3)) of the composite

• increase in local electric field strength at (and near) the metal particles

Local field factor:

floc = Eloc / E =

= 3εg / (εm+2εg)

@ SPR freq.: floc >> 1

(Yamane and Asahara,Glasses for photonics)


Viable systems for 1.5 µm applications:

Er3+-doped SiO2 - TiO2 long fluorescence lifetimes

(Almeida et al. 1999, Orignac et al. 1999, Almeida et al. 2003)

Er3+-doped SiO2 - HfO2 intense Er3+ PL signal

(Gonçalves et al. 2003)

Significant refractive index contrast (waveguiding)Transparency over a wide wavelength rangeLed to the preparation of good optical quality waveguides

Both have:

11


Sol-gel processing of Er3+-dopedsilica-based films and waveguides

TEOS + EtOH + H2O+Hydrolysis catalyst (HNO3)pH = 2

Pre-hydrolyzed TEOS solution

SiO2 – TiO2 solution

EtOH + TiPOT

SiO2 – HfO2 – TiO2

EtOH + HfOCl2

Ageing for 8h

Spin-coating

Stirring60 ºC, 1 hour

Er(NO3)3

SiO2 – HfO2 – TiO2– ErO1.5

(RE = nEtOH/nTEOS+TPOT = 18)

(R = nH2O/nTEOS+TPOT = 4)

SiO2-HfO2-TiO2-ErO1.5

(100-x-y)S-yH-xT-Er

x = mol% TiO2

y = mol% HfO2

e.g.: 75S-20H-5T-Er

Heat treatment @ 900 oC

Hydrolysis:

M-OR + H2O → M-OH + HOR

Condensation:

2 M-OH → M-O-M + H2O

M-OH + M-OR → M-O-M + HOR


Sol-gel processing of Er / Ag-doped, silica-based glasses and waveguides

TEOS + EtOH + H2O+Hydrolysis catalyst (HNO3)pH = 2

Pre-hydrolyzed TEOS solution

SiO2 –TiO2 solution

EtOH + TiPOT

SiO2 – TiO2 – ErO1.5

Ageing for 8h

Spin-coating

Stirring60 ºC, 1 hour

Er(NO3)3

SiO2 – TiO2 – ErO1.5 - Ag

AgNO3

Immersion of the gel in a AgNO3+Er(NO3)3 solution,

before h.t. PORE-DOPING

SOL-DOPING

SiO2 –TiO2 or SiO2 dried gels

Ageing for 8h

Thermal ↓ Ago @ 700 oC Thermal ↓ Ago @ 900 oC


Waveguiding

2 cm

He-Ne laser light (@ 633 nm) being guided by a SiO2-TiO2-Al2O3 optical planar waveguide on SOS, by means of prism coupling. Loss ~ 0.2 dB/cm.


Polarized Waveguide Raman Spectroscopy


Waveguide Raman spectroscopy (WRS): structural study of the films

waveguiding configuration (butt coupling)

λexc = 488 nmparallel polarization (HH)

Ti-O-HfHf-O-Hfbending modes

120 - 300 cm-1

200 400 600 800 1000 1200 1400

100HfO2

100SiO2

1201100

295

639

970

940

399

800

197

144125

600

*

S5T20H

*

S25H

S10T15H

Inte

nsity

(a.u

.)

Raman shift, cm-1

S25T

*

460

I125 / I144 :

tanatase ~ 3-4 nm


X-ray diffraction

and

Er3+ photoluminescence (PL)


Pumping of Er3+ ions

Absorption spectrum of Er3+ in a silicate glass.

Argon ion laser

(Adapted from: Rare earth doped fiber lasers and amplifiers, ed. M.J.F. Digonnet, Marcel Dekker, 1993)


1450 1500 1550 1600 1650

75S-25H-ErFWHM =50 nm

FWHM =15 nm10S-90H-Er

Wavelength (nm)

PL

Inte

nsity

(a.u

.)

Wavelength (nm)

1450 1500 1550 1600 1650

75S-10H-15T-Er FWHM = 33 nm

FWHM = 33 nm75S-15H-10T-Er

1450 1500 1550 1600 1650

75S-25T-Er FWHM = 48 nm

Wavelength (nm)

Er3+ (4I13/2-4I15/2) PL spectra and XRD patternsfor silica-based planar waveguides (0.5% Er)

2 0 3 0 4 0 5 0 2 0 3 0 4 0 5 0 2 0 3 0 4 0 5 0

T + E H OT + E H O

M

T

7 5 S -2 5 H

Inte

nsity

(a.u

.)

2 θ (d e g re e s )

1 0 S -9 0 H -E r

M + E H O

H T 2T + H T

H T + H T 2 + T

H T + T

H T

H T 2H T + H T 2

H TH T 2

7 5 S -1 5 H -1 0 T

7 5 S -1 0 H -1 5 T


A

7 5 S -2 5 T


T- HfO2 (tetragonal)

M- HfO2 (monoclinic)

A - TiO2 (anatase) ~ 3 - 4 nm

HT - HfTiO4 ~ 4 - 5 nm

HT2 - HfTi2O

EHO - Er2Hf2O7 or Er4Hf3O12

τmeas = 2 ms

τmeas = 4.2 ms

τmeas = 5.6 ms


Effects of Ago nanoparticles on the Er3+ PL intensity

TEM, visible absorption spectroscopy

and

Er3+ PL of the nanocomposites


Sol-doping of planar waveguides

Transmission electron microscopy

Ag-containing sample

Agº nanocrystallites (2r~10-20 nm)

DF

77.25% SiO2 – 19.25% TiO2 – 0.5% Er – 3% Ag

1 h at 600º C80% SiO2 –20% TiO2 – 0.5% Er

3 h at 600º C


Influence of silver on the Er3+ PL intensity: sol doping of waveguides with Ag+

300 400 500 600 700 800 9000.00

0.05

0.10

0.15

0.20

0.25

Abs

orba

nce

Wavelength, nm

2 min at 700º C 6 min at 700º C

78.8% SiO2 - 19.7% TiO2 - 0.5% ErO1.5 - 1% Ag

planar waveguides 4 layers1 min at 600º C / layer

80 SiO2 – 20 TiO2 - 0.5 ErO1.5 - (Ag)

Sol-doping with 1% Ag, 2 min @ 700 oC~ 60 times PL enhancement

λ pump = 514.5 nm

1400 1450 1500 1550 1600 16500

1x104

2x104

3x104

4x104

5x104

6x104

7x104

FWHM=48.0 nm

FWHM=46.3 nm

FWHM=47.0 nm

940 counts

18933 counts

54032 counts

62138 counts

0 Ag - 2 min 700 ºC 0 Ag - 6 min 700 ºC 1 Ag - 2 min 700 ºC 1 Ag - 6 min 700 ºC

PL In

tens

ity

Wavelength (nm)

planar waveguides 4 layers1 min 600ºC / layer

FWHM=47.7 nm

Ago absorption2r = νF

Ag / FWHM (ω)2r ~ 2 – 4 nm


Transmission electron microscopy bulk silica gel samples treated for 5 h at 900º C

(Ag-containing sample, Er1Ag2)

Agº nanocrystallites (2r~10-15 nm)

Pore-doping of bulk silica gels

Er3+-doped bulk silica glass (preparedby pore-doping with a Er(NO3)3 solution)

Er3+ / Ago-doped bulk silica glass (prepared by pore-doping with a

Er(NO3)3 + AgNO3 solution)


Influence of silver on the Er3+ PL intensity:pore doping of bulk silica gels with Er3+ and with Er3+/Ag+

100% SiO2 (bulk samples)

immersion for 30 h in a 0.15M Er(NO3)3 solution

no silverFWHM=33 nm FWHM=24 nm

pore doping with Er and Ag immersion for 30 h in a 0.15M Er(NO3)3 +

0.25 M AgNO3 solution

larger Agº nanopart. ~10-15 nmless silver and smaller nanoparticles, d ~ 3nm

λpump = 488 nm

pore doping with Erimmersion for 30 h in a

0.15M Er(NO3)3 + 0.15 M AgNO3 solution

1450 1500 1550 1600 1650

Wavelength (nm)

Er1Ag1

1450 1500 1550 1600 16500

1x104

2x104

3x104

4x104

5x104

6x104

PL

Inte

nsity

Wavelength (nm)

Er1

1450 1500 1550 1600 1650

Wavelength (nm)

Er1Ag2no silver low silver high silver

11 times stronger

3 times stronger

FWHM=27 nm


400 410 420 430 440 450 460 470 480 490 500 510 520

Abs

orba

nce

(a.u

.)

Er3+

PL

inte

nsity

at 1

.5 µ

m (a

.u.)

excitation wavelength, nm

no silver low silver high silver

Ago optical absorption and Er3+ PL intensity at 1.5 µm (bulk glass samples prepared by pore-doping of gels)

Er3+ PL enhancement (~ 11 times) occurs only for resonant excitation (when λexc is absorbed by the Er3+ ions @ 488, 497, 515 and not @ 458, 477 nm)

Local electric field enhancement around Er3+, due to SPR of Ago nanoparticles


Effects of TiO2 and HfO2 additions to SiO2 based optical waveguides, prepared by sol-gel:

Conclusions→ The structure and spectroscopic properties of these RE-doped

sol-gel materials were found to be strongly connected:

- the appearance of hafnia and hafnia-titania mixed crystals in SiO2-HfO2-TiO2 waveguides led to a significant narrowing of the Er3+ PL peak, with the formation of well resolved Stark components.


→ The presence of ~2-15 nm Agº nanoparticles in silicaglasses and silica-titania planar waveguides caused asignificant enhancement of the Er3+ PL at 1.5 µm (~ 3-60 X),irrespective of the method used for Ag or Er3+ incorporation.

→ The dominant mechanism responsible for the observed PL increase in Ag-containing samples is proposed toinvolve a local electric field enhancement around theEr3+ ions, due to SPR of the Agº nanoparticles.

Effects of the incorporation of Agº nanoparticles

into SiO2-based sol-gel optical waveguides:Conclusions


Rare-earth doped Fabry-Perot

microcavities


PHOTONIC BANDGAP STRUCTURES

Also called photonic crystals, the optical analogues ofsemiconductors.

Structures whose refractive index is periodic on a length scale ~ λ inthe optical region of the electromagnetic spectrum.

Light is prevented from propagating by Bragg reflection:

θλ 22 sin2 −= effnd


1-D, 2-D and 3-D photonic crystals

artificial opal

Schematic of 1-, 2-, and 3-D periodic lattices consisting of two materials

of different dielectric constants. The lattice constant is denoted a.


Artificial opal prepared by convective self-assembly

SEM micrographs of an opal made of 460 nm diameter PS spheres, by convective self-assembly: (a) top view of {111} plane; (b) cleaved edge showing {100} and {111} planes of the fcc structure.

(a) (b)


Titania inverted opal

SEM micrographs of a titania / air inverted opal structure, prepared by convective self-assembly of PS templates, at different magnifications.


knc

aa

0=ω

Interference filter as 1-D photonic crystal

Reduced zone scheme for the dispersion relation

Typical filter characteristicswith “stop band”

dielectric band

aλ= 2a

knc

dd

0=ω

Interference filter

(also called a Distributed Bragg Reflector, or simply a Bragg mirror)


Mirror M1

Mirror M2

Cavities

Substrate

Mirror M1

Mirror M2

Mirror M3

D1

D2

b)a)

D

Bragg mirrors: “quarter-wave” stacks of optical thickness: nx = λ/4Microcavities prepared from SiO2 and TiO2 sol-gel layers

Schematics of 1-D PBG structures: (a) microcavity of silica;(b) coupled microcavities of silica.

TiO2 layers are gray and SiO2 layers are the open rectangles.


Microcavity with a TiO2 defect layer

] H ~ 50 nm

} H ~ 100 nm

] L ~ 80 nm

Cross-section FEG-SEM micrograph of a microcavity, with a titania defect layer. (Backscattered electron mode).


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Wavelength (nm)

Tran

smitt

ance

(LH)10 L (LH)10 Q = 284L=SiO2 (nL=1.45, eL=258 nm); H=TiO2 (nH=2.3, eH=163 nm)

Simulation of F-P microcavity by transfer matrix method

Q = λc / FWHM(can lead to RE PL enhancement of Q)

FWHM


Coupled cavities in the Visible region

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

300 350 400 450 500 550 600 650 700 750 800 850 900

Wavelength (nm)

Tran

smitt

ance

CalculationExperiment

(LH)3 L (LH) L (LH)3L=SiO2 (n=1.45, e=100 nm) H=TiO2 (n=2.3 e= 63 nm)


Coupled cavities in the NIR region

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

600 800 1000 1200 1400 1600 1800 2000

Wavelength (nm)

Tran

smitt

ance

%

SimulationExperience

Comparison between experimental and simulated optical transmission of a coupled microcavity with the structure (LH)2L3/2(LH)L3HLH where L stands for SiO2 and H for TiO2.


Antenna (or sensitizing) effect

2H11/2

4F7/2

4F9/2

4S3/2

4I9/2

4I11/22F5/2

4I13/2

980 nm1.53µm

2F7/24I15/2

Yb3+ Er3+

Energy level diagram of Er3+ and Yb3+ ions (“antenna” effect).


Doped coupled microcavities

Mirror M1

D1 A D1 (Er3+, Yb3+); D2 (Er3+, Yb3+);

B D1 (Yb3+); D2 (Er3+);

C D1 (); D2 (Er3+);

Mirror M2

D2

Mirror M3

[Er3+] = 0.5 mol% in SiO2

[Yb3+] = 2.0 mol% “

(Sample A has 50% more Er3+ than B or C)


Transmission of cavity A

0

10

20

30

40

50

60

70

80

90

100

400 600 800 1000 1200 1400 1600 1800 2000

Wavelength (nm)

Tran

smitt

ance

0º

20º

Incidence angle

990 nm

1565 nm

“Transmission” (1-R%) spectra of cavity A at different incident angles.


1400 1450 1500 1550 1600 16500.0

0.2

0.4

0.6

0.8

1.0 A B C

PL

inte

nsity

(arb

.uni

ts)

Wavelength (nm)

Normalized Er3+ photoluminescence spectra of coupled microcavities, excited at 488 nm, at normal incidence.

A: both cavities co-doped with Er3+ and Yb3+; B: D1 doped with Yb3+ and D2 doped with Er3+; C: only cavity D2 doped with Er3+.

@ 488 nm: no Yb3+ effect

Normalized PL of A, B and C (excited @ 488 nm)


Comparison of Er3+ photoluminescence spectra of coupled microcavities A, B and C, excited at 514.5 nm (TE).

1450 1500 1550 1600 1650

Inte

nsity

[arb

itrar

y un

its]

Wavelength [nm]

λexc = 514.5 nmExcitation = 10º off-normalDetection = 0º

B

A

C

No Yb3+ effect

PL of A, B and C (excited @ 514.5 nm)


A

BC

Comparison of Er3+ photoluminescence spectra of coupled microcavities A, B and C, excited at 980 nm (TE).

1400 1450 1500 1550 1600 1650 1700

Inte

nsity

[arb

itrar

y un

its]

Wavelength [nm]

λexc = 980 nmExcitation = 10º off-normalDetection = 0º

Yb3+ effect ! (in A)

PL of A, B and C (excited @ 980 nm)


Comparison of Er3+ photoluminescence spectra of coupled microcavityA, excited at 514.5 nm and 980 nm (TE).

1450 1500 1550 1600 1650

Inte

nsity

[arb

itrar

y un

its]

Wavelength [nm]

λexc = 514.5 nm and 980 nmExcitation = 10º off- normalDetection = 0º

514.5 nm excitation 980 nm excitation

Yb3+ effect (in A)

PL of A (excited @ 514.5 nm and 980 nm)


1400 1450 1500 1550 1600 1650 1700

Inte

nsity

[arb

itrar

y un

its]

W avelength [nm]

λexc = 514.5 nm and 980 nmExcitation = 10º off- normalDetection = 0º

514.5 nm

980 nm

Comparison of Er3+ photoluminescence spectra of coupled microcavityB, excited at 514.5 nm and 980 nm (TE).

Even @ 980 nm, no Yb3+ effect in B!

PL of B (excited @ 514.5 nm and 980 nm)


Conclusions:

• 1-D single and coupled Fabry-Perot microcavities can be prepared by sol-gel processing, with Q factors up to 35, using silica and (anatase) titania alternating layers.

• The silica cavity (defect) layers can be doped with Er3+ and / or Yb3+

ions.

• The presence of Er in both cavity layers of coupled microcavities led to an increase of the PL signal width and intensity , compared to Er present only in one of the cavities (after correcting for the total Er concentration).

• A significant sensitizing (“antenna”) effect was observed, when exciting Er3+ with 980 nm light in the presence of Yb3+, as long as the two types of ions were present simultaneously in the same cavity.


Acknowledgements

M. Ferrari, M. Montagna, A. Chiasera and L. Zampedri (Univ. of Trento, Italy).

M.C. Goncalves and S. Portal.

Financial support of FCT (Lisboa), under R&D contracts POCTI/CTM/36109, POCTI/CTM/32823 and POCTI/CTM/33307 and FSE/FEDER (EU).

Support of IMI – New Functionality in Glass for sabbatical leave at Lehigh University (RMA, Nov. 04 - July 05).

17th University Conference on Glass Science – June 2005

Documents

Transcript of 17th University Conference on Glass Science – June 2005