Porous Gel-Silica Optical Matrices

Porous Gel-Silica Optical Matrices

9 Porous Gel-Silica Optical Matrices Previous sections described the heterogeneous chemistry involved in control ofthe ultrastructure and texture of ...

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9 Porous Gel-Silica Optical Matrices

Previous sections described the heterogeneous chemistry involved in control ofthe ultrastructure and texture of Type VI porous gel-silica. Figure 9-1 a summarizes the range of surface area per unit volume, volume fraction of pores (Fig. 9-1b), and radius of pores (Fig. 9-1c) as a function of the processing temperatures of the gel-silica matrices. The textural features of various optical applications of the porous matrices are also indicated in Fig. 9-1. Depending upon the chemistry used in making the initial sol, discussed earlier, the surface area of the porous silica matrices can be varied from 800 m2/g to 150 m2/g (Fig. 9-1 a). The volume fraction of porosity can be varied from nearly 0.9 to 0.2 (Fig. 9-1 b) and the mean pore radius varied from 1.2 nm to > l 0 nm (Fig. 9-1 c). As described earlier, the surface area and volume fraction of porosity is a function of stabilization thermal treatment. The larger pore matrices can be heated to higher temperatures without altering the volume fraction of interconnected porosity which is a vital attribute for making environmentally stable commercial porous composites. Table 9-1 summarizes the specifications of commercial Gelsil | porous silica glass manufactured by Geltech, Inc. under license from the University of Florida.

97

98

Sol-Gel Silica



Z~

A

='EIO

1.2 nm pores

I1. <

,,,~ rr" 0

-..--

--..-

e

........... .~,, \

3.s 9 5.0 6.2 .8.0

. . . . . . . . . . . . .

|

o U) LU rr 0

-

~ l

9 ~

a)

1.0

. . . . . . . . . . . - 10.0 nm pores

Ia.

(•

0 . 8 - - 8.o - 5 o 0.6

. . . . . . ~

_

" ,~. ~

Transpiration

r////////////,~ .'J'~,,-..-''~

c o o l e d o p tic s

-315 "_--_------ ~ - " , 12

"

"---------

~ . . . .

"

"

~

q

~.

"-,

0.2

0

0

'~ ~' ~

,

4_'~,~

0.4 --

-~

ii

,b

10.0

m

Matrices for dye lasers, scintillators, photopolymers, liquid crystals, NLO p o l y m e r s

1

~ ' ~ , , ' 4 " , "~ " ~ o f

Laser densification Type VI gel-silica

,

b)

10 --

E ?-

10.0 nm pores . . . . . . . . . . .

__8.0

8

-----------

.

.

.

.

.

v

O3 6

--"

6.2

.

5.0

I,.

rh

,< nuJ cc 0 Iz.

4

.

.

.

.

.

.

.

.

.

.

.

.

.

.

. . . . . . . . . . . . . . .

3.5 m

m

m

~ m

m

m

marne

aline

m

m

m

m

N

m e

m

,/,"

2 00

c)

200

400

600

800

TEMPERATURE

1000

1200

As-cast net-shape Type V,,n,;,. gel-silica

1400

(C)

Figure 9-1. Variation of texture with temperature for alkoxide-derived gel silica optics.

Porous Gel-Silica Optical Matrices

99

Table 9-1. Specifications of Gelsil | Porous Glass Nominal Pore Diameters

]

I ,ooA

25. A

I ,ooA

Physical Properties I

,

,,

,

0.4

0.7

BET Surface Area (m2/g)'

610

580

Bulk Density (g/cc'i

1.2

o.9

Total Pore Volume (cc/g)

,,,

,

I.I

0'9 '.

.

420

.

.,

Specific Volume (cclg)

'

0.6 |

i.1

1.4

1.6

Transparent

Transparent

Transparentto Translucent

Translucent

0.3 pm

0.3 pm 2'14 .m

0.5 pm zt4 .m

"0.8 ,

.

o17 ,,

240

,

9

Optical Properties Appearance UV Cutoff (50~,)

.

IR Cutoff (50%)

.

2.14 pm

Mechanical Properties Vicker's Hardness ikg/mm 2) Impact S'trength (MPa)

F'

74 '

50 0.06 "

0.4

0.8

J

-

---

Impregnation GELTECH:s porous glass can be readily impregnated with polymers, dyes, catalysts or other materials with particle sizes small enough to reside in the pores. Absorption/ltandling GELTECH's porous glass will absorb up to 30% of its dry weight vapor under ambient conditions and may turn yellow on standing due to the absorption of atmospheric organic materials. It is recommended that samples be stored in protective packaging. ,

,

Composition > 99% Silica Cation Impurities ~1 ppm ,,

Availability

,,.

.

..,

,..

,

.

=.

,L

,.

.i

i

.

i

PartDimensions- Up to 50 mm (2") in diameter (dependent on pore size requirements) ,

_ !

,

Pore Diameter- 25 to 200 A (controllable range) ii

,

..

,

,

.

,

i

As indicated in Fig. 9-1a, one of the important applications of the net shape porous matrices is for fabrication of optical composites where the interconnected porosity is impregnated with an optically active second phase. [8~ There are generally two methods used to make optical composites, as summarized in Fig. 9-2. In method A, pioneered by Avnir, [137] Reisfeld [138] and colleagues, and Zink and Dunn, [139][14~ the second phase component is added during process step 1. Gelation occurs in step 3 and the optically or biologically active component is completely encapsulated within

100

Sol-GelSilica

the gel network and in some cases can be incorporated chemically as part of the network. The advantage of this method is its simplicity and the integral chemical environment of the impregnate and its host matrix. The disadvantages are (i) the range of pH that can be used in the sol-gel system may be limited in order to avoid degradation ofthe second phase, especially if it is a polymer or biological molecule. Innovative chemistry has been used by the teams cited above to circumvent this problem. (ii) Composites made by method A are restricted in their subsequent thermal and environmental exposures because ofthe sensitivity of organic species to thermal or chemical degradation. Process Step

1

METHOD B (Adsorption or Impregnation Processing)

METHOD A (Sol Processing)

Ii.....(Hydrolysis uixi,g

25 ~

Ii

II,,, C~176

<1 Hr

..L

<1 Hr

.,

!"'

_

asting ,, ~ : ,, ,,

Mixing _L

__

i/c ..... 9

0->48

Gelation

Hr

" !1II........... (_P~176176176 ........

100 ~ 12->48

Hr

I

.L il

Aging 11.( S_----9 yneresis)

Hr

....

.L

_

9

Aging (Syneresis)

ov

....

-

~

"

Structural evolution of the gel by polycondensation, dissolution and ~ redeposition.

_L

Drying '_L

II StabilTizaion

'~176 "''on': ,_L

l

l,orou' ..tr,x

loo->95o ~ 12->72

nonx,

Ii 9 )1 Sol solidifies by cross Gelation linking of fundamental I (Polycondunsation particles.

.. 24->72

oc.

molds of desired product shape/surface replication.

asting d.

~::

.L

25~

---1 Produces colloidal

(Hydrolysis & / suspensions (soil, used Conden:)dtion) i for films, coatings...

&

Hr

L 29.

II ,o~,~ o.,..,~ J.

_

11 Or;'"'

25 ~ <1 Hr OPTICAL ![, COMPOSITE : 1 Figure 9-2. Optical composites processing.

,

..--...i..-. ~ .~-.~

OPTICAL I[ COMPOSITE "

Removes any remaining surface species without significant structural alteration. Type VI gel-silica.

Porous Gel-Silica Optical Matrices

101

Method B potentially circumvents both of these problems. [118] A porous matrix, prepared by steps 1-6, is described in this review, stabilized to a specific ultrastructure, texture, and pore surface chemistry and then impregnated with the optically or biologically active second phase (Fig. 9-2, Step 7). Either an aqueous or organic media can be used as a vehicle for the second phase with very little limitation on pH since the silica matrix is resistant to liquids with pH = 2-10. Controlled evaporation of the carrier liquid occurs in step 8 which results in an optical composite. Properties of the composite depend upon (i) volume fraction of pores, (ii) pore radius, (iii) size and information of the impregnate molecule, (iv) thermal history of the matrix, and (v) residual amount of media left after drying; i.e., thickness of bound and free water layers (discussed in Ch. 5). The kinetics of impregnation to form a composite and subsequent chemical behavior of a composite depends greatly on pore radius of the matrices, as illustrated in Figs. 9-3, 9-4 and 9-5 from Jim Kunetz' study of restricted diffusion in porous gel-silica matrices. [141] Figure 9-3 shows the large effect of pore size on the rate of water penetration into porous gel-silica matrices at ambient temperature and pressure. The small pore matrices restrict transport considerably. Partitioning of charged ions, such as Cr 3§ also occurs when they diffuse into the porous matrices, as illustrated in Fig. 9-4. The partitioning effect is a function of both pore size of the matrix and the charge on the silica surface sites of the pore. The isoelectric point of the gel-silica matrices was determined by Kunetz [141]to be in the range ofpH = 2.5 to 3.5. Consequently, chromium solutions with pH < 2.5 result in a net positive surface charge and give rise to the large partitioning effect. Figure 9-5 summarizes the effect of pore size on the effective diffusion coefficient ofthe gel-silica matrices. The data used to compute the diffusion coefficients were normalized for the concentration partitioning effect. The range of values shown in Fig. 9-5 for each pore size corresponds to different volume fractions of porosity. The results demonstrate that for matrices with greater than approximately 30 A average pore radius there is minimal restricted diffusion. Also, for large pore matrices with rp in the range of 90 A there is very little effect of volume fraction of porosity on transport of water and chromium ions. In marked contrast, matrices with pores in the range of 13 to 16 A radius show an order of magnitude restricted diffusion and substantial pore volume effects.

1.2

E CI O3 (/3 Ld Z v 0.8 L3 -r i-z

~95 A

~35

g

~ 0.6 r~ FLIJ Z L.I a

m17

0.4

N _J r~

o 0.2Z

O~

0.00

j

5,00

,

10.lO0

15,00

,

20.00

I

25~00

30.00

4"t (see^0.5) Figure 9-3. Rate of penetration of water into porous gel-silica matrices as a function of average pore radius.

1.6 1.4

0,01 M 1.2 0.5 M

~ ]

..................................................................X----I-~o........................................................................................................................................

x JQ (J

~0.8

I B

9 o

0.6-

[]

o

0.1M unb0xed

0.4- i

IEPrange Iiliiiiiiiiiiiii~i~~iiiiiiiiiii!iii

0,2

J!~i~:~iii~i:i:~i~:~:~i:~!!::~:i!ii~i!i~i~!~i~:~:i:i:!:!!i:ii~i

)

0.5

1

1,5

I []

2

13 A PR 9

2.5 pH 30 APR X

3

3.5

4

4.5

95 A PR I

Figure 9-4. Effect of pH in the partitioning of Cr3+ in porous gel-silica matrices of differing pore sizes.

5

I E-05

Bulk Diffusion Coefficient X

X X

9

9

9

X

X

,.%uEIE-06 E

FD

9

E] [:3 EJ

~P

I r_]

I E-07

1'0

2'0

3'0

13 A PR I 16 A PR

4'0

5'0

Pore RQdius (,&)

6'0

X 27 A PR A

7'0

100 A PR

8'0

9'0

I

100

Figure 9-5. The effective diffusion coefficient of chromium ion in porous gel-silica as a function of average pore radius corrected for effect of concentration partitioning.

Porous Gel-Silica Optical Matrices

105

Figure 9-6 depicts the Kunetz model [141]for the partitioning and pore size effects on the behavior of an aqueous solution of Cr 3+ions in porous gelsilica matrices. The combined effects of surface charge, thickness of bound water layer, and pore size all influence transport in the system. The chemical interaction of the Cr 3+ ions with the silica network also affect the optical properties of the impregnated composite, as shown in Fig. 9-7. Outward diffusion of the chromium ion from the gel-silica matrix results in a shift of the 575 nm absorption peak to a value of 600 nm (Fig. 9-7a). Theoretical analysis of the system by Kunetz et al. indicates that ligand-ion distance influences the position ofthe absorption peak maximum.[ 142] The theoretical MO calculations of molecular structures such as shown in Fig. 9-7b and 97c show variations in their electronic spectra (Fig. 9-7). The conclusion is that pore surface charge and pore size alter the equilibrium position of the hydrated chromium ion complex and therefore produce a shift in the optical absorption spectra. The schematic structures shown in Fig. 9-6 summarize the results of the experimental and theoretical findings. Similar diffusion results were obtained for Ni, Co, and Nd without the peak shifting or partitioning observed for Cr ions. [141] An important implication of the study of transport in matrices with restricted geometries is that the matrices can be molecularly tailored for specific optical properties by varying size and volume fraction of pores and surface charge on the pores. A large number of optically active organics have been impregnated into Type VI gel-silica optical matrices (Table 9-2). [113][118][131][132]Experimental results from two studies on solid state dye lasers demonstrate the unique features of this method of making an optical composite. Liu et al. [143] investigated the effects of gel-silica texture on the lasing characteristics of a water soluble laser dye, 4 PyPO-MePTS. Table 9-3 summarizes the results obtained from matrices with 12 A, 30 A, and 90 A pore radii. Figure 9-8 compares the UV spectrum for the laser dye in solution (curve a) with the spectrum of the gel-silica matrix (curve b) and the dye impregnated-gel silica composite (curve c). The composite properties are very similar to the individual components. The matrix which has a larger volume fraction ofporosity can accommodate more organic dye molecules in its structure. For example, gel-silica matrices with pores of 12 A average pore radius have a volume fraction of porosity of 52%. Therefore, theoretically, the maximum amount of organic dye solution which can be incorporated will not exceed 52%. In fact, because ofthe size ofthe dye molecules, much less than that amount of dye solution can be doped into the gel-silica matrix.

106

Sol-GelSilica

However, about 64% 4PyPO-MePTS dye solution was determined to be impregnated into the 100 A average pore radius gel-silica matrix which has a volume fraction of 72% porosity. This high value is because of the large volume fraction of porosity and the large pore radius, the pore shape, and the pore size distribution of gel-silica matrices with aging treatments. The gelsilica specimens with a larger pore size have a narrower pore size distribution and a more cylindrical pore shape, as discussed earlier in the section "aging." Studies by King and colleagues at the University of Manchester, England, show that use of these matrices with larger pores can improve the photostability of the dye molecules within the matrix as compared with gel-dye composites made by co-polymerization.[ 144] I

24 angstrom

I

positivesurface pH
negativesurface pH>IEP

9 O

190 angstrom

positivesurface pH
'

Cr 3+ ion hydrated water structural water free water

I

negativesurface pH>IEP

Figure 9-6. Schematicof interactions of aqueous solutions of Cr ions in porous gel-silica matrices.

Porous Gel-Silica Optical Matrices

.4

107

594

c

0 L0 ./:3

.2 575

4-00

6 0 Nanometers

800

(a) Figure 9-7. Ligand-ion distance influences the peak maximum position and pore surface charge and pore size alter the inner coordinated waters equilibrium position. (a) Shift in Cr3+adsorption during diffusion in porous gel-silica, (b) MO model ofSi(OH)4 [Cr(H20)6] 3+ complex; (c) MO model of[Cr(H20)6] 3+molecule; (d) calculated electronic states of various Cr and Cr/SiO 2 model complexes, by Kunetz et al., Refs. 141, 142.

108

Sol-GelSilica

S i!OH)4[c r(H20)6] 3+, Label Bonding Atoms Cr-O 1 Cr-O 2 Cr-O 3 O-H 4 Si-O 5 Muiiiker~"" q 6 Mulliken q 7 Mulliken q 8 Mulliken q 9 .

.

.

.

.

.

.

.

.

.

.

.

(b) Figure 9-7. (Cont'd.)

Value 1.85 A

~.98 A 2.00

A

~.o8 A 1.63 A 4.02 e 6.87 e 2.48 e_ _6.67e

......

Porous Gel-Silica Optical Matrices

[Cr(H20)s] 3+ Label Bonding Atoms 1 'Cr-0 . . . . 2 O-H 3 Mull iken q 4 Mulliken q ii

ml IIIII

Value 1.97'A 0.95 A 3.95 e6.67 eII

(c) Figure 9-7. (Cont'd.)

'

II

-

,~L-

109

110

Sol-Gel Silica

Calculated electronic states of various Cr and Cr/SiO model complexes.

Molecular Structure

Electronic Spectrum (nm) 575 (0)4T2g 420 (0) 4Tlg (F)

[Cr(H20)6]3+#,* [Cr(HEO)5OH]2+ [Si(OH)4Cr(H20)6]3+

599 (0), 599 (0), 590 (0) 593 (0), 576 (0), 568 (0)

[Si(OH)4Cr(H20)5]3+ [Si(OH)30Cr(H20)5] 2+ [3 RingO-Cr(H20)5]2+ [4 RingOCr(H20)5]2+

546 (0), 534 (0), 511 (0) 604 (0), 526 (0), 472 (0) 663 (0.006), 559 (0.002) 629 (0), 594 (0), 583 (0)

389 (0), 387 (0) 449 (0.0033), 431 (0.0022) 425 (0), 402 (0) 455 (0), 388 (0) 475 (0.0010), 470 (0) 455 (0.0015), 459 (0)

267 (0) 4Tlg(P) m

#Literature values of 575,407,265 nm from A. Lever, InorganicElectronicSpectroscopy,Elsevier Science Publishing, NY, NY, p. 419 (1984) *Experimental Values of 575,408,275 nm. OscillatorStrengthsare in parenthesisfollowingthe electronicstate. 0 representsa electric-dipoleforbidden transition.

(d)

Figure 9-7. (Cont'd.)

T a b l e 9-2. O p t i c a l C o m p o s i t e M a d e f r o m T y p e V I Silica ( P o r o u s G e l s i l |

Non-Linear Optical Polymers Phenylenebenzobisthiazole 2-Methyl-4-nitroaniline

Organic Fluors 2-4"-t-Butylphenyl)-5-(4"-biphenylyl)-l, 3, 4-oxadiazole p-Terphenyl p-Quaterphenyl

Wavelength Shifter 3-Hydroxyflavone

Laser Dyes 4 PyPo-Me PTS Rhodamine 6G (590)

Liquid Crystals Photo Cross Linking Polymers

Porous Gel-Silica Optical Matrices

111

lO (a)

(c)

80

w 0 Z <

(n z < n~

(b)

60

40

(]::) 2O

i

200

"

i

i

i

I

600

i

I

i

i

I000

i

i

i

i

1400

WAVELENGTH (nm) Figure 9-8. UV spectra for (a) 6 • 103 M 4PyPO-MePTS dye solution; (b) porous gel-silica matrix (100/It) and (c) gel-silica matrix (100 A) impregnated with 6 • 10-3 M 4PyPOMePTS dye solution.

The solid state dye laser composites made by impregnating Type VI gel-silica matrices with controlled textures with a dilute 4PyPO-MePTS laser dye solution show good lasing action (Table 9-3). Porosity of the gel-silica matrix plays an important role in determining the lasing characteristics. A larger pore size, a larger volume fraction of porosity, a narrower pore size distribution, and a more cylindrical pore shape of the gel-silica matrices all provide a higher gain and total energy output of the laser (Table 9-3).

112

Sol-Gel Silica

Table 9-3. Texture Effects on Lasing Characteristics of 4PyPO-MePTS Impregnated (at 45~ Porous Gel-Silica Specimens Sample

Wavelength (nm)

12 A

500 505 510

30 A

500 505 510

90 A

500 505 510

I: [w/amp: lw/o amp: I/Iw/o:

Iw/o amp

I/Iw/o

(~tJ)

[w/amp (~tJ)

<0.4 <0.4 <0.4

3.71 3.91 3.25

9.28 9.78 8.13

5.23 7.34 6.99

7.26 9.29 9.45

6.31 8.36 8.43

15.78 20.90 21.08

0.72 0.79 0.74 <0.4 <0.4 <0.4

Total output energy. Total output with amplification Total output without amplification Gain or amplification factor

Another experiment on solid state dye lasers by Moreshead et al. [1451 compared two host matrices for the dye; (i) rhodamine 6G in a methanol solution impregnated in porous gel-silica matrices with 25 A and 3 7A pore radii, and (ii) rhodamine 6G encapsulated within an ORMOSIL, using Dunn et al. procedure, impregnated in the equivalent porous gel-silica matrices. The results demonstrate the feasibility of using porous silica glass as a host material for making solid state dye lasers. The performance of these materials was shown to be equal to or better than that of other rhodaminebased solid state dye lasers reported to date and have promise as hosts for solid state dye lasers. The following conclusions were drawn from the work. The efficiencies ofrhodamine 6G (also known as rhodamine 590) in the porous glass were comparable to or better than most commonly used hosts (Table 9-4).

Porous Gel-Silica Optical Matrices

113

Table 9-4. Conversion Efficiencies of Dye Doped Sol-Gel Samples When Pumped with a 20-ns 532-nm Source Sample

Efficiency (%)

Dye-Only Doped 50 A Disk

29

Dye+10% ORMOSIL Doped 50 A Disk

25

Dye-Only Doped 75 A Disk #1

37

Dye-Only Doped 75 A Disk #2

39

Dye+10% ORMOSIL Doped 75 A Disk

32

Dye-Only Doped 50 A Small Rod

23

Dye+10% ORMOSIL Doped 50 A Small Rod

27

Dye-Only Doped 75 A Large Rod

32

The conversion efficiencies observed for the rhodamine 590 doped solgel samples in the study (Table 9-4) were encouraging for their use as dye laser gain media. The best ofthe samples, the dye-only doped 75 A disks, gave both absolute and slope efficiencies of 39% due to the low threshold for laser action. This compares favorably to the 33% efficiency achieved with rhodamine 590 in methanol solution. [146] In ORMOSIL, Dunn et al. report a slope efficiency of 30% for rhodamine 590 and a conversion efficiency of 39% for rhodamine B [147] when pumped with the output of a doubled Nd:YAG laser. Using a similar type of sol-gel to that reported here, also doped with rhodamine 590, Charlton et al. [144]reported an efficiency of~3 0% under pumping conditions similar to the ones used in the Moreshead et al. study. [145] Use of an 80% reflectively output coupler may have somewhat reduced the efficiency Charlton et al. could achieve. The only host for rhodamine 590 which has yielded higher efficiencies than those reported by Moreshead et al. is polymethylmethacrylate (PMMA), a solid plastic. Gromov et a1.[148]report conversion efficiencies of 50% for doubled Nd:YAG pumping of PMMA doped with rhodamine 590. However, lifetime of the PMMA samples is significantly lower than those observed for the doped solgel samples.

114

Sol-Gel Silica

Achievable dye laser efficiency also depends on the geometry of the dye-only doped sol-gel sample. For the highest efficiency sol-gel host, the pure sol-gel with 75 A pore size (no ORMOSIL), the best efficiency was achieved by longitudinal pumping of a 63 mm thick disk. Going to a 15 mm rod ofthe same host material reduced the conversion efficiency from 38% to 32% (Table 9-4). The 32% efficiency compares favorably to other hosts. Furthermore, the improved lifetime ofthe rod samples (discussed below) may still make them the geometry of choice. The lifetime ofrhodamine 590 in the porous glass is better than other commonly used hosts, including common solvents, by 1 to 2 orders of magnitude (Fig. 9-9).

1.00 \

>. O r tg Z W F--

~\

,

~,

.75A Dye Only Doped / Rod (5 Hz)

~

075

13. p-

O

~

_\

0.50

\

75A Dye Only Doped

~

_

t-t uJ N ..J

=~ IZ:

0.25 75A Dye+10% ORMOSIL Disk (2 Hz)

0 2:

0

I

0

I

20

I.

.

I

40

I

I

60

I

I

80

LASER SHOTS (xl000)

Figure 9-9. Dye lifetime as a function of sample type.

..

I

I

100

.

120

Porous Gel-Silica Optical Matrices

115

The first series of lifetime measurements performed by Moreshead et HI.[145]examined the role ofsample type on lifetime. The samples were a dyeonly doped 75 A disk, a dye + 10% ORMOSIL doped 75 fit disk, and a dyeonly doped 75 A rod. The disks were 12 mm diameter x 6.3 mm thick, and the rod was 8 mm diameter • 15 mm long. The disk lifetime tests were performed at 2 Hz and the rod lifetime tests at 5 Hz. This difference in repetition rate had a negligible effect on the lifetime of the samples. A plot ofthe dye laser output energy, normalized to an initial value of 1, vs. number of pump pulses is given in Fig. 9-9. The results shown in Fig. 9-9 and Table 9-5 indicate that rhodamine 590 in the sol-gel derived porous silica has a lifetime that is 1 to 2 orders of magnitude better than other hosts, including a standard dye solvent.[ 1451The lifetime per unit volume of pumped dye host increased for the longer rod geometry.

Table 9-5. Comparison of Rhodamine 590 Photostability in Various Hosts .

.

.

.

,,,~,

,

,

, . ~ : -

. . . . .

,,~,

.,,

. . . : - -

_,_

: - :

~

,,,,,

~,

:~_.

,

,,7

. ; . .

,. . . .

;;

;

, ,

_ _

._-

_ _ _

Host

Pump source

Photostability (J/mm3)

Comments

Ethylene glycol

Green lines of cw argon ion laser

3.6

Ref. 22 calculated from reported 10 W*hr/l lifetime

-

'

_

r-

Methanoi . . . . . . . . . . Doubled Nd:YAG

0.3-0.4

..

.

.

.

_

.

_

Re[" 23

..

~--

oRMosIL

Doubled Nd:YAG

-0.3

ORMOSIL

Pulsed Nitrogen Laser

0.11

Ref. 7

6.3 mm thick, 75 A pore, sol-gel disk

Doubled Nd:YAG

23

Moreshead et. HI.

15 mm long, 75 A pore, sol-gel rod

Doubled Nd:YAG

43

Moreshcad et. HI.

t

r i

=~

. . . . . .

,

,,

.

.

.

.

.

.

.

Ref. 5 calculated from reported 0.5 m m x 20 mm transverse pump volume, 95 ul/pulse, 11000 shot i lifetime

116

Sol-Gel Silica

Charlton et al. [144] have reported on doping rhodamine 590 into a solgel host similar to that reported by Moreshead, et al. When the host was then back filled with water they achieved 20,000 shot lifetimes with transverse pumping by 3 mJ of doubled Nd:YAG output. Studies ofrhodamine 590 in ORMOSIL by Altman et al. [147] found a 50% lifetime of 3000 shots when pumping a one centimeter thick plate with 3 mJ at 532 nm. No beam diameter was given so a calculation of lifetimes as given in Table 9-5 were not reported. Gromov et al. [14s] report the lifetime of rhodamine 590 in modified PMMA as being about 800 shots. Data is not given on the volume of the pumped region, so the photostability parameter given Table 9-5 could not be calculated by Moreshead, et al. They do, however, report that this lifetime is for pumping at a fluency of 1 J/cm 2. Since this is comparable to the pump level of 0.7 J/cm 2 used in the Moreshead work, a simple comparison of shots is reasonable. The> 90,000 shot lifetime for rhodamine 590 in the sol-gel rod is clearly superior to the <1000 shot lifetime reported for PMMA. Thus, when doped with rhodamine G, the porous gel-silica glass host gives a broader emission spectrum which may in turn lead to broader tune ability as a dye laser gain medium. Doping the porous glass with only the dye gave better lifetimes and efficiencies than were obtained when the porous glass was doped with both the ORMOSIL and the dye. Absorption/scattering losses in the porous glass are comparable to or better than those reported for the ORMOSILs. Scattering losses can be reduced by the use of an index matching fluid. [1131