The influence of structure and environment on spectroscopic and lasing properties of dye-doped glasses

July 1997

ELSEVIER

Optical Materials 8 (1997) 43-54

a enals

The influence of structure and environment on spectroscopic and lasing properties of dye-doped glasses Raz Gvishi *'1, Gary Ruland, Paras N. Prasad Photonics Research Laboratory, Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260-3000, USA Received 25 November 1996; revised 31 January 1997; accepted 29 April 1997

Abstract A new class of hemicyanine dyes was studied as laser dyes. From the influence of structure and solvent effects on the spectroscopic and lasing properties, a basic understanding of the involved processes was obtained. The studied dyes were found to have two distinguishable mesomeric forms, one predominant in the ground-state, and the other in the excited state, leading to a large Stokes shift. The dyes exhibited low fluorescence quantum yields, which were attributed to the presence of a counter iodide ion, which increases singlet-to-triplet intersystem crossing, and to a twisted intramolecular charge-transfer (TICT) of the amino moiety. However, significant lasing efficiencies were observed under pulsed pump conditions, possibly because the stimulated emission competes with the nonradiative processes. The laser losses are mainly due to the cavity. Solvent effect studies showed that the chromophore is very sensitive to hydrogen bonding donor (HBD) solvents. The dye-doped sol-gel composite glass exhibits a behavior close to that of water, suggesting that the dye is attached to the silica skeleton of the composite glass through a hydrogen bonding. Energy transfer between two dyes codoping a multiphasic composite glass was found to be insignificant; therefore, this composite exhibited simultaneous lasing from both dyes. Codoped tunablity was achieved through the range of both dyes, from 560 to 610, nm with an average efficiency of 7%. The lasing properties of this lasing medium was studied and compared to reference dye solutions. © 1997 Elsevier Science B.V.

1. Introduction Dye lasers are an important class of lasers that are attractive due to their tunability over a wide range of wavelengths [1]. Even though other options are now available to achieve tunable lasing over some wavelength ranges, dye laser are still important for high power, short pulses or the so-called solid-state dye lasers [2,3]. Until recently, liquid dye lasers were the

* Corresponding author. t Present address: Arava Laser Laboratory, Temed Industrial Park, Mishor Yamin, D.N. Arava 86800, Israel.

main system used to achieve tunability in the visible. However, solid state dye lasers have advantages over liquid dye lasers by being nonvolatile, nonflammable, non toxic, compact, and mechanically stable [4-8]. The problem of heat dissipation, however, poses a serious impediment for their utilization in applications that require high powers under either cw or pulsed high repetition rate operation. In liquid dyes, on the other hand, a flowing solution or a jet is a practical means of solving the heat dissipation problem. In both cases photostability is a property of prime importance for selecting a laser dye [2]. In order that a chromophore will be appropriate as a lasing medium, it should meet several basic re-

00925-3467/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 5 - 3 4 6 7 ( 9 7 ) 0 0 0 3 5 - 9

44

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

quirements [9,10]: wide and strong absorption band in the UV-IR, high quantum yield fluorescence, wide fluorescence band allowing tunability over a significant region, high photostability and thermal stability. The first properties can be found in aromatic molecules. Rigid molecules are usually more photostable and have higher quantum yields. Fluorescence can be quenched, however, by nonradiative decay due to bond rotation, stretching of hydrogen bonds, intercrossing system process (S-T), formation of mesomeric forms in the excited state, and absorption in the excited state. So far, hundreds of organic molecules have been identified as fair candidates for laser dyes. However, only limited number are commercially used, and they cover the entire spectrum range the UV to the IR. However, there is still a need to search for new and improved dye materials. In order to achieve optimal performance from a laser dye it is important to understand the parameters which influence the spectroscopic and lasing properties of the dye. This paper presents studies of the influence of structure and environment on the spectroscopic and lasing properties of a new class of Hemicyanine dyes [11,12]. Hemicyanine dyes containing long alkyl chains have drawn attention in recent years. The interest originates from the possible optoelectronic and molecular electronic application by incorporation into Langmuir-Blodgett (LB) films [13-15]. For lasing applications hemicyanine dyes containing short alkyl chains such as amino-styryl-pyridinium derivatives (ASPD), which were previously synthesized in our laboratory are more suitable [16]. One of the synthesized chromophores, trans-4-[P-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium with the counter ion being either tetraphenylborate or iodide, (hereafter ASPT and ASPI, respectively) have been demonstrated as efficient laser dyes in the orange-red region, under one or two-photon excitation pumping, in liquid solution and in solid matrices (such as sol-gel glass PMMA composite, Vycore glassPMMA composite, polymer rod and polymer epoxy film) [17-19]. The main features observed for this chromophore are: large Stokes shift (118 nm in ethanol) and low fluorescence quantum yield (7 X 10 -3 in ethanol); on the other hand, they exhibit a large two-photon absorption cross-section (about 2-3 orders of magnitude greater than the corresponding

values for Rhodamine dyes) and significant lasing efficiency (up to 13.5%) under either one- [11] or two-photon [17,18] pulse excitation. The objective is to understand the parameters responsible for the observed spectroscopic properties, and direct the design of new dyes with improved lasing properties.

2. Experimental 2.1. Sample preparation The investigated compounds are hydroxyaminostyryl-pyridinium derivatives (ASPD), which were synthesized in our laboratory. The molecular structure of these dyes is shown in Fig. 1. The procedure used to prepare the solid sample of dye-doped composite glasses was described previously by Gvishi et al. [19,20], which is based on the composite glass preparation method of Pope et al. [21]. Briefly, highly porous silica-gel bulk glasses were prepared by the sol-gel process. It is followed by impregnation of bulk glasses with methylmethacrylate (MMA), which diffuses into the bulk glass pores and polymerizes in situ. The final glass was cleaned and polished. The results is a nanostructure composite glass of high optical quality. It can be doped by two (or more) different dyes, each of which residing in different phases of the matrix (the silica phase, the PMMA phase and the interfacial phase), to make multifunctional bulk materials for photonics [12,22,23]. ASPT and ASPI were doped in the inteffacial phase by placing the porous glass in an ASPTcyclopentanone solution (2.1 X 10-3M) or ASPIethanol solution (4.4 X 10-3M), respectively. After the glass was completely impregnated by the solution, it was removed, and placed on a hot plate at 45°C for several hours until the solvent evaporated, leaving the dye on the surface of the pores. This procedure resulted in a concentration of ~ 1.5 X 10-3M ASPT and ~ 3.1 X 10-3M ASPI (this concentration represents the volume of the pores of the glass, ~ 70%, assuming that no chromophore evaporate ion occurred.

2.2. Measurements The absorption spectra were obtained using a Shimadzu W-Vis 260 spectrophotometer with a reso-

45

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

lution of 1 nm. The solution spectra were obtained using quartz cuvettes with 1 cm path length. The emission spectra were collected on a Shimadzu RF-5000U spectrofluorophotometer (90 geometry) or on a SLM 48000 spectrofluorimeter using a Xenon arc lamp as the excitation source. For the solution measurements a fluorometric quartz cuvette was used. For the composite glass, the emission was collected from the glass surface (90 ° geometry) due to the signifcant primary absorption. The resolution of the spectrofluorometers is 2 nm. Fluorescence spectra were background-subtracted and corrected for the detector and monochromator transmission response. Fluorescence quantum yields were ob-

tained using a comparative method, which is discussed in more detailed elsewhere [12]. The system used in our lasing performance studies (both the solution and the composite glass) was a frequency-doubled Quanta-Ray DCR Nd:YAG Qswitched laser with a repetition rate of up to 30 Hz, producing 8 ns pulses at 532 nm.

3. Results and discussion Fig. 2 presents the W-visible absorption and emission spectra of ASPI in ethanol solution (2.6 X 10-6M). The emission spectrum exhibits a mirror

BEPh],

i o

H3 c - +

OR

'----"

~

~

N

/ CH20H

CH2CH2OH

ASPT

ASPI

I°

--

H3C-- N

)

X>

~

N~_)/

/CH3

'-=='="

CH3

/~k

/ CH3

AB[Ph]PI / 'OCH3

[Ph](AB) 2 I" I"

,___4/OCH 3

~

+

NhI(I'D2 Fig. 1. Molecular structure of ASPI, ASPT, AB[PhlPI, [Ph](PI)2 and [Ph](PI)2.

) CH2OIJ

46

R. Gcishi et al. / Optical Materials 8 (1997) 43-54

70000

250 ,°, i

,

60000

200 A

50000

¢-

.o

15o "~

~40000

ILl

:

g

=

t

e-

30000

100 i

<

0 B.

20000 50 10000

I

350

400

450

500

550 600 Wavelength (nrn)

650

700

t 750

0 800

Fig. 2. Absorption (solid curve) and fluorescence emission (dashed curve) spectra of ASPI in ethanol solution ( ~ 2.6 × 10 -6 M).

image symmetry to the absorption spectrum with a Stokes shift of about 117 nm. Large Stokes shifts are attributed to a different charge distribution (or geometry) in the excited state as compared to the ground state [9]. For example, in Coumarin dyes, a nonpolar mesomeric form is predominant in the ground-state, while a polar mesomeric form is predominant in the excited-state. Recently, Fromherz [24] conduct quantum chemical calculations showing that in zwitterionic Hemicyanine dyes the positive charge of the chromophore is displaced upon excitation from the pyridinium moiety towards the amino moiety. Hall et al. [25] report that Hemicyanine dyes have larger dipole moments in the ground state than in the excited state. Following the previous cases, we suggest for ASPI a mesomeric form charged at the pyridinium moiety (structure A, Fig. 3). The latter is predominant in the ground-state, and a charged mesomeric form at the amino moiety (structure B, Fig. 3), which is predominant in the excited-state. The low fluorescence quantum yield observed was attributed to intersystem crossing occurring from the S 1 to the T 1 [12]. To improve our understanding of this chromophore, we investigated the influence of the structure (influence of electron withdrawingdonating groups) and the solvent effect on its spectroscopic properties.

An intramolecular (charge-transfer) process between strongly electron-donating or electronwithdrawing groups and the excited-state of a chromophore results in the loss of electronic excitation [9]. In our case, the studied chromophore (ASPI) contains an amino electron-donating group (one end) and a pyridinium electron withdrawing group (the other end). Therefore, the effect of the electronwithdrawing donating groups was studied by com-

OH

A

"="

--L__J

OH

B Fig. 3. Two mesomeric forms proposed for ASPI and ASPT: Form A, positively charged at the pyridinium moiety; form B, positively charged at the amino moiety.

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

paring the spectroscopic properties of the following modified structures: AB[Ph]PI, [Ph](AB) 2 and [Ph](PI) 2. The first molecule (AB[Ph]PI) based on ASPI was modified by an additional phenylene group. The second ([Ph](AB) 2) is a modified chromophore containing two electron-donating groups and the third ([Ph](PI) 2) is a modified chromophore containing two electron-withdrawing groups. The spectroscopic parameters of the different species are summarized in Table 1. The highlights of the effects compared to ASPI are: a decrease in the molar absorptivity of ASPT, AB[Ph]PI, [Ph](AB) 2 and [Ph](PI)2; a red shift in the absorption spectrum peak position of AB[Ph]PI; as blue shift in the absorption spectrum peak position of [Ph](AB) 2 and [Ph](PI)2; and a smaller Stokes shift for [Ph](AB) 2 as compared to the other species. All these effects were explained [11] by an intramolecular (charge-transfer) process between strongly electron donating or withdrawing group and the excited-state of the chromophore resulting in the loss of electron excitation. The important effect is the change in the fluorescence quantum yield according to the molecular structure from 1% for ASPI up to 16% for [Ph](PI) 2. Quenching of the fluorescence quantum yield can be a result of several processes. One of the possible option is the counter-ion effect. A previous work [24] reported that the counter ion plays a negligible role, and the absorption and emission were found to be almost indistinguishable. This is exactly the case in our study, regarding ASPT and APSI, which differ only by the counter ion (B[Ph]4 and I-, respectively), and exhibit similar peak position of absorption and emission spectra. Nevertheless, their fluorescence quantum yields are completely different. It is reasonable to assume that in our case the effect of I- ions is to increase intersystem crossing from the S~ state to the T l state according to the heavy atom

47

effect [9], leading to a lower fluorescence yield. Another mechanism that may influence the fluorescence quantum yield is the rotation around the C - N bond connecting the amino group to the chromophore, forming a twisted intramolecular chargetransfer (TICT) [26]. The rotation of the amino group out of the chromophore plane may lead to a lower fluorescence quantum yield. Indeed, we observed a higher fluorescence quantum yield for [Ph](PI) 2, which has no amino moiety. Solvent effects influence the Stokes shifts of the dyes. A common way of studying solvent effects is to measure the Stokes shifts as a function of the solvent orientational polarizability [27]. to a first approximation, the Stokes shift is a linear function of the orientational polarizability and is given by the Lippert expression 2(/**

-/z)2Af

hca 3

(1) where ~a is the wavenumber of the absorption peak, ve is the wavenumber of the emission peak, (A~) v = (~a-~e)v is the Stokes shift of the molecule in the vapor state. The orientational polarizability (A f ) is defined as Af=f(¢)-f(n), where f ( e ) = ( ¢ 1 ) / ( 2 ¢ + 1 ) and f ( n ) = ( n 2 - 1 ) / ( 2 n 2 + 1 ) ; ¢ is the low-frequency relative dielectric constant, and n is the optical refractive index in the visible. /, and /~* are the ground and excited-state dipole moments, respectively, h is Planck's constant, c is the speed of light and a is the characteristic dimension of the probe molecule. The solvent effect was studied in the ASPT solutions (10 /,M) in benzene, chloroform, cyclopentanone, ethanol and water [28]. The observed values of absorption maxima, emission maxima, calculated

Table l Spectroscopic parameters of (aminostyryl)-pyridinium derivative dyes in ethanol solution Dye

absma x (nm)

Molar abs ( M - I c m - i)

ASPI ASPT AB[Ph]PI [Ph](PI) 2 [Ph](AB) 2

483 486 496 464 411

~ ~ ~ ~

65000 + 2% 43500 30000 18000 18000

emma x (nm)

AA (nm)

q~

600 603 606 580 490

117 117 117 117 79

~ ~ ~ ~ ~

0.007 0.035 0.016 0.160 0.054

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

48

Compo=dte g l a ~ " - " - ' ~

o 7

8 °

•

X

ai P.HEM A~

¢3_ ¢0

4

1/) 0

¢0

0.00

I

I

I

I

0.07

0.14

0.21

0.28

Orientational

Polarizability

0.35

(~j')

Fig. 4. Lippert plot: (Stokes shifts versus ambient orientational polarizability) for ASPT/ASPI. The numbers of next to the full circle symbols ( 0 ) correspond to the numbering in Table 2, identifying the solvent used. The open squares symbols ( n ) represents the solid matrix.

Stokes shift, dielectric coefficients, and refractive indices are summarized in Table 2. The results presented in Table 2, are used again in a Lippert plot. The Lippert plot for ASPT in liquid solvents (filled points) is shown in Fig. 4. The data indicates that the chromophore is a solvatochromic dye. The Lippert plot obtained does not display a linear dependence. From the absence of negative solvatochromism in the absorption, yet due to the existence of a large Stokes shift, we can conclude that for ASPT there are two distinguished mesomeric forms. One is predominant in the ground-state and the second is pre-

dominant in the excited-state. The two forms do not differ much in polarity. The later conclusion is in agreement with our earlier assumption of the structure of the mesomeric forms displayed in Fig. 3. The departure from a linear Lippert plot is an indication of specific interactions (for example, hydrogen bonding, electron withdrawing-donating) between the dopant and the solvent. In our case, it is clear that departure from linearity is a result of a strong hydrogen bonding. The sensitivity of ASPI/ASPT to hydrogen bonding donating solvent (HBD) is understandable, since this chromophore differs from other Hemicyanine dyes by a substitution of a hydroxyl group on the amino moiety. The influence of the hydroxyl group is significant in water, where the I-IBD effect decreases the energy of the ground state, leading to a significant blue shift in the absorption, but also decreases the energy of the excited state, resulting in an extraordinarily large Stokes shift. The additional points (empty squares) in solid matrices (composite glass and poly-HEMA) are also shown in Fig. 4, and the relevant data are summarized in Table 2. The ASPT in poly-HEMA (data #6 in Fig. 4) exhibits a somewhat larger Stokes shift than expected by the orientational polarizability of the matrix. This can be explained by HBD effect created by the hydroxyl groups of the polymer. In the sol-gel composite glass (data #7 in Fig. 4) the chromophore exhibits a much larger Stokes shift as compared to the one expected from the macroaverage orientational polarizability of the matrix, similar to that of water. In this case, the dye was impregnated using an ethanol solution, and deposited on the pore surface to form an interfacial phase. Therefore,

Table 2 SSpectral properties of ASPT in various solvents and solid matrices a No.

Solvent

Ex. max

Em. max

A~

Af

n

e

1 2 3 4 5 6 7 8

benzene chloroform cyclopentanone ethanol water poly-HEMA composite glass TMOS sol-gel

477 483 480 486 457 485 450 450

559 568 590 603 598 577 586 586

3075 6098 3884 3992 5159 3287 5157 5157

0.002 0.148 ~ 0.250 0.289 0.320 0.020 0.191 ~ 0.277

1.446 1.437 1.359 1.332 1.496 1.472 1.444

4.806 ~ 17 24.30 78.54 2.45 7.8 ~ 55

a Data from Ref. [II].

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

11

we can assume that the microenvironment experienced by ASPT/ASPI is environment of a pure silica. Taking this into account, and using the orientational polarizability of pure silica in Lippert's expression, one gets a much better fit to the solvent data, as shown by data # 8 in Fig. 4. From the similarity in the behavior of the dye-doped sol-gel composite glass and the dye-water solution, we conclude that in the composite glass as well, the HBD effect is dominant, indicating that the dye is attached to the silica skeleton through hydrogen bonds. In contrast, for two dyes, PRODAN [20] and RPD [29], acting as dopants in a composite glass the Strokes shift fitted that expected by the macroaverage orientational polarizability of the matrix (shown in Fig. 5). In these cases, the dyes were initially impregnated in the glass via the mehtylmetracrylate monomers, and located in the polymer phase. Therefore, the microenvironment experienced by the dyes were a combination of PMMA and sol-gel-like environments [20]. One-photon lasing of ASPI in an ethanol solution and in a composite glass was demonstrated in a cavity that consists of a ~ 100% reflecting metallic

PRODAN

A

•7

9 Sol-Gel

~" [] •6

05

7 4~3

02 5 o?, O )<

o0

Composite

t3 .,~--~ PMMA

I

3 1.3

cCO

D~,~

1

I

I

B

I

Red Pm'ytimids Dye o8

1.2 •

I1)

o 09

•7

6

1.1 o5

1.0

• 4

0.9

p =

Cornpos~e

PM~

3

0.8 0.0(

I 0.07

I 0.14

Orientational

I 0.21

PoladzabUity

I 0.28

0.35

(at)

Fig. 5. Lippert plot: (Stokes shifts versus ambient orientational polarizability) for PRODAN (A) and red perylamide dye (B).

30%

49

T t i

I i

25% ~d~

20% ¢-

._ u 15% o.

o

10%

/

5%

O%

I

[

I

I

I

3

4

5

6

7

COncentration (M x 10 "3)

Fig. 6. Lasing slope efficiency as a function of concentration in ethanol solution. ASPI is represented by triangles: Rhodamine-6G is represented by diamonds. The dyes were transversely pumped with an 8 ns pulsed frequency-doubled Nd:YAG laser at 532 nm operating at 30 Hz repetition rate.

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

50

Table 3 Spectroscopic and lasing parameters for ASPI and Rhodamine-6G in solution and in composite glass Solution state ASPI

Absorbance m a x i m u m ( n m ) Molar absorbtivity Fluorescence/maximum Fluorescence F W H M ( n m ) Stokes shift (nm) Quantum yield Laser efficiency (%) Laser tunability range ( F W H M ) (nm)

Composite glass

R h o d a m i n e - 6 G A S P I / R h o d a m i n e - 6 G ASPI

483 530 6.8 x 10 -4 1.05 × 1 0 - 5

**+ , , ,

600

562

595

38

25

40

118 32 6.5 x 10 -3 0.9 13.5

25.2 562-584

.

.

45

. .

599-635

.

Rhodamine-6G ASPI/Rhodamine-6G

. .

. .

. .

. .

. .

***

** * . .

' ** .

.

.

546

594/552

53

41/43

. .

9.4

598-643

back mirror, and a ~ 70% reflecting dielectric output coupler• The influence of the large Stokes shift in ASPI on its lasing properties is demonstrated in Fig. 6. Fig. 6 shows the lasing slope efficiency of both Rhodamine-6G and ASPI in ethanol as a function of the dye concentration. In contrast to Rhodamine-6G whose lasing efficiency is strongly concentration dependent, the ASPI lasing efficiency was almost constant in the studied concentrations range. The lasing slope efficiency of Rhodamine-6G is due to self absorption, yet this effect is lacing in ASPI. The measurement of the lasing efficiency of ASPI was

. .

.

. .

. .

. .

. .

3.8

7.3

585-606 561-573

564-601

terminated at a concentration of 6.1 x 10-3 M, which is just at the solubility limit of ASPI in ethanol. The observed data are also summarized in Table 3. The output energy versus input pump energy, under excitation with a frequency-doubled Nd:YAG laser (532 nm) is plotted in Fig. 7 for ASPI in ethanol solution and in a composite glass• The lasing thresholds observed were 0.295 mJ/pulse for 3.8 × 10-3M in ethanol and 0.244 rnJ/pulse for 3.1 X 10-3M in a composite glass• Lasing slope efficiencies of ",, 13.5% in ethanol solution and ~ 9% in a composite glass were observed using single photon

0.4

0.35 0.3

•°-•

•°

.• ~t

I~ o ° • • • o ° • ° •

~-025

g

°

°

0.2 .--I

••°

"5 0 1 5 °

"-i

0

°°

•°

•••°

.°•

•••

•••°•

01

•.•a" •.o•

0.05

.il° •

•,•M .1~ °. X

~t

I1."

I 0.5

I 1

I 1.5

I 2

I 2.5

I 3

t 3.5

Pump Lasing (m J/pulse) Fig. 7. Lasing slope efficiency of ASPI in a composite glasses. The least squares best fit line is drawn through the data points, and the

observed efficiency is provided.

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

excitation. While under two-photon excitation (1.06 ~ m ) a lasing energy conversion efficiency of ~ 3.5% was reported [17]. At the threshold, the loss coefficient Cqoss is equal to the amplification coefficient F ( v ) , which is given by [30]

observed for RPD in solution [31] and doped composite glass [5] are relatively small, at most as the order of the error in the above loss assessment. The relatively high lasing slope efficiency of ASPI is surprising considering its low fluorescence quantum yield compared to that of Rhodamine-6G. This result may indicate that the low quantum yield of ASPI is caused by a radiationless energy loss that occurs on the time scale of the excited state lifetime under continuous pumping conditions. We believe that inside the cavity, with the short pulsed pumping used, the dye. is .stimulated to emit a photon at a much faster rate than the nonradiative processes. This phenomenon is well known [1] and has ben demonstrated using dyes that have fluorescence quantum yields as low as 5 × 10 - 4 with a lasing conversion efficiencies of 10 to 20%. Energy transfer studies were done with a mixture of ASPI and Rhodamine-6G in an ethanol solution and a composite glass. The fluorescence emission spectra of ASPI (3.1 X 10 - 3 M ) , Rhodamine-6G (6.5 × 10-SM) and of a mixture solution of ASPI (3.1 × 10 - 3 ) and Rhodamine-6G (6.5 X 10-SM) in ethanol are shown in Fig. 8. These concentrations were chosen to be comparable to the concentrations in the final composite glass. The excitation wavelength used is 509 nm, since the dyes have the same molar

g(v)ANA 2 r'(/))

8 ,./TF/2Tr

(2)

Where ~'r is the radiative lifetime, n is the refractive index of the matrix, g ( v ) is the emission intensity as a function of the frequency, normalized to a unity integral, and A N is the population inversion density. Following the calculation detailed in Ref. [11], and inserting the experimental values for the emission peak at the wavelength Areax = 610 nm in ethanol, and Amax = 586 nm in a composite glass, one gets Celoss ~ 4 . 1 or 5.9 cm -1, respectively. This values includes all loss contributions such as mirrors ( a m = 0.36 cm -1 and 0.71 c m - l ) , diffraction ( a d = 2 . 8 cm -1 and 4.8 cm-1), dye reabsorption (ara are negligible) and scattering ( a s = 0.08 c m - l and 0.16 c m - 1). From these results it is clear that we obtained good agreement between the total loss observed estimated by the slope efficiency measurements, and the sum of cavity losses. We may then conclude that additional losses, such as excited-state absorption as 1 T

fk

-'"

I

0.9

,

r

0.8

51

'.

:

':,

!

', i

.~ 0.7

,.

-~ 0.6

i

'.

", '.

"~ 0.5 T

'i

,/

0.1

0

,.

'.

\\

',,

x

-,

~'" 515

I

565

615

665

715

765

Wavelength (nm) Fig. 8. Fluorescence emission o f Thodamine-6G (6.5 X 1 0 - S M - - - ), ASPI (3.1 X 10-3M • • • ), and a mixture solution containing Rhodamine-6G and ASPI (6.5 X 1 0 - 5 M / 3 . 1 X 10-3M, respectively, - ). The solvent was always ethanol.

52

R. Gvishi et aL / Optical Materials 8 (1997) 43-54

absorptivity at this wavelength. It is clearly evident, that the emission from Thodamine-6G is completely quenched in the mixture solution when combined with ASPI at such concentrations. The most probable quenching mechanism is F6rster type energy transfer, which is inversely proportional to the sixth power of the distance between the donor and the acceptor [12]. In contrast, for the same dye concentrations in glass, emission from both dyes codoping the same glass were observed without significant quenching. Fig. 9 presents the fluorescence emission of the dye doped composite glasses. The composite glass containing both dyes displays emission from both without significant quenching of emission from either dye. The peaks were reconstructed using a computer code with a Gaussian fit function. The observed fluorescence emission (maxima and width) parameters in the composite glasses are presented in Table 3. The main differences between the solid and liquid hosts is, that in the glass we used is a multiphasic matrix, and the ethanol is a single phase matrix. The multiphasic composite glass contains three different

'

i

0.9

d=3~

1000

,

CNa

(3)

where C is the molar concentration, and Na is Avogadro's number. The distance between Rhodamine-6G and the ASPI molecules, evenly distributed in the mixture solution, is about 70 A, well within the range where F6rster energy transfer can OCCUr.

In glass, we consider the pores shape as cylindrical [12]. When calculating the distance between adsorbed molecules on the pore surface and the molecules in the pores of the multiphasic composite glass, we must consider the ratio between the specific surface area and the pore volume. This reduces

A

. ~x "

,,,: /I \\

' :"/ /

0..

0.7.

i

0.6-

i

,'Y'"

/ /

phases: silica, polymer and interfacial. In this case, the two different laser dyes reside in different phases and, therefore, energy transfer is insignificant. The ASPI is adsorbed onto pore walls of the (interfacial phase) and the Rhodamine-6G enters the polymer phase (PMMA). In a solution, the mean distanced between the solute molecular centers is given by

/

~ I-.

~'

""%

o, i J'i""'"''..I I, r /', :\ °"t',' /" ..\ o.,t / ',, 0.,I, / ',, "

oP ~ 515

\\

",

,

565

.

615 665 Wavelugth(am)

.

.

.

.

.

.

.

.

.

715

.

.

.

.

.

,

765

Fig. 9. Fluorescence emission of Thodamine-6G composite glass (- - -), ASPI composite glass (. • • ), and the composite glass containing both dyes ( - - ) .

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

both dyes it is ~ 37 nm. It is evident that the glass containing both dyes is tunable across the combined tunability ranges, (560-610 nm), whereas in solution, Rhodamine-6G emission has been quenched.

the problem to be two dimensional and the following expression can be used: d =

(4)

CNaPv S.,

53

where C is the molar concentration of the solution used to deposit the molecules, N, is Avogadro's number, Pv is the pore volume, and Sa is the surface area. This calculation reveals that, when compared to the same concentration in solution, the distance between the ASPI and Rhodamine-6G molecules, increases from 70 A to 1 l0 .~. This represents an increase in the distance between the ASPI and Rhodamine-6G molecules by a factor of 1.6 FSrster energy transfer is inversely proportional to the sixth power of distance. Thus, the rate of energy transfer has decreased by more than an order of magnitude! Fig. 10 shows the lasing tunability of the composite glasses. Tunable narrow-band laser outputs were observed in a cavity consisting of a grating as the back reflector, and a ~ 70% reflecting outcoupler. The FWHM of the tunability spectra for the ASPI composite glass ~ 21 nm, and that for Rhodamine6G ~ 12 nm. For the composite glass containing

4. Conclusions We describe the influence of structure and solvent on spectroscopic and lasing properties in a new class of Hemicyanine dyes as laser dyes. The studied chromophores exhibit large Stokes shifts and low fluorescence quantum yields; they, however act as efficient laser dyes under pulse pump condition. From the influence of the structure, we conclude that a mesomeric form charged at the pyridinium moiety is predominant in the ground-state, and that a mesomeric form charged at the amino moiety is predominant in the excited-state. The lowering of fluorescence quantum yield depends on two mechanisms: (i) the present of a counter ion, iodide, which increases the S~-T 1 the intersystem crossing; (ii) a TICT derived from rotation of the amino moiety. Solvent effect study shows that ASPT/ASPI is solvatochromic yet with a nonlinear dependence of

1.2

•

"'" "',..,.. A

\

mm

=',

\

0.8 I f-

l

0.6

t t k I l

(D N ¢0

0.4

l

Z

l I l

0.2 ¸

0 555

565

575

585 595 Wavelength (rim)

605

615

625

Fig. 10. Lasing tunability of Rhodamine-6G composite glass (©), ASPI composite glass ( I ) , and the multiphasic composite glass containing both Rhodamine-6G and ASPI ( • ) . The composite glasses were transversely pumped with a 8 ns pulsed frequency-doubled Nd:YAG laser at 532 nm operating at a 1 Hz repetition, using a grating as the back reflector.

54

R. Gvishi et al. / Optical Materials 8 (1997) 43-54

the Stokes shift on the solvent orientational polarizability. The chromophore was found to be very sensitive to H B D solvents, such as water, which expected since the chromosome is hydroxyl substituted. By analyzing the Stokes shift o f A S H - d o p e d s o l - g e l composite glass we demonstrate, that the dye experiences a microenvironment similar to a pure silica glass, indicating that the dye is attached to the silica surface through hydrogen bonds. Significant lasing efficiency were observed in liquid solution and solid matrices o f A S P I / A S P T , under pulse pump conditions. Although this dye exhibits a low fluorescence quantum yield, the nonradiative transitions are considerably slower than the stimulated in the laser cavity. Thus, they constitute efficient lasing media under pulsed conditions. A multiphasic nanostructured composite glass doped with two dyes was demonstrated as tunable lasing media across the tuning range of both dyes ( 5 6 0 - 6 1 0 nm), avoiding quenching which occurs in solution for the same mixture. The difference between the solution and the composite glass states is due to a significant increase in the distance between the two dyes; a result of the extremely high ratio o f interfacial surface area to pore volume, which lowers the F~Srster energy transfer process. This result represents the fascinating possibilities embodied in the multiphasic composite glasses for fabricating multifunctional devices for photonics, in this case a multidye solid state laser, tunable over a wide wavelength range. In addition, this chromophore has interesting properties such as two-photon absorption induced lasing.

Acknowledgements The authors are indebted to Ms. Chan F. Zhao for synthesizing the trans-4-[P-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium iodide (ASPI). The authors are grateful to Dr. Zeev Burshtein for useful discussions. The current work was partial sponsored by the Air Force Office of Scientific Research and the Polymer Branch o f Wright Laboratory through contract number F49620-93-C0017.

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