Lower limit of the lasing threshold in an organic microcavity

Bernd Schutte*, Hannes Gothe, Markas Sudzius, Vadim G. Lyssenko, Susanne I. Hintschich,Hartmut Frob, and Karl Leo

Institut fur Angewandte Photophysik, Technische Universitat Dresden, 01062 Dresden,Germany.

ABSTRACT

The application of organic materials as solid state lasers critically relies on a low lasing threshold. We investigatethe characteristics of emission from an organic vertical cavity surface emitting laser. The microcavity studied hereconsists of two highly reflective distributed Bragg reflectors enclosing a wedge-shaped active layer of Alq3:DCM.Lasing of the DCM molecules is induced via two different pump regimes, either exciting Alq3 at a wavelengthof 400 nm or pumping directly into the absorption band of DCM at 532 nm. By a variation of the pump beamposition with respect to the microcavity surface, we demonstrate a continuous wavelength tuning in the organicmicrocavities in a range of 55 nm. The continuously variable cavity thickness allows us to study the thicknessdependence of the input-output characteristics in a single sample. These data are obtained at a certain emissionwavelength, λ, close to the maximum of the gain spectrum, for a number of cavity thicknesses, which correspondto different multiples of λ/2. For a decreasing thickness of the active layer, one-dimensional optical confinementis expected to result in an increased spontaneous emission factor. On the other hand, the loss rate through themirrors increases with decreasing thickness resulting in a minimum threshold value for an active layer thicknessof approximately 3/2 λ. This lower threshold limit is set by nonradiative losses as well as residual absorption.

Keywords: Organic VCSEL, microcavity, laser threshold, spontaneous emission

1. INTRODUCTION

The demonstration of the first organic light emitting diodes1, 2 stimulated the development of lasing in opticallypumped organic thin films.3, 4 Organic semiconductors are attractive candidates as laser materials with favorableproperties, including cost efficiency, mechanical flexibility, and a wide gain spectrum which can be exploited forlarge tuning ranges.5–7 The first lasers based on dye molecules were investigated in the 1960s,8, 9 but suffered fromrapid material degradation due to a limited photochemical stability. The situation substantially improved bythe implementation of organic vertical cavity surface emitting lasers (OVCSELs) which promise applications infibre communications and integrated optical circuits.10 By the restriction of light propagation in one dimension,a low threshold intensity of 20 µJ/cm2 was reported for the system of Alq3:DCM.11

For the use in integrated devices, the development of an electrically pumped organic laser is required.12, 13

Since organic materials suffer from low charge carrier mobilities, the currently obtained threshold values have tobe reduced. A promising approach is the three-dimensional confinement that has been demonstrated in inorganicsemiconductors by micropillars14, 15 and mode mapping of photonic dots16 as well as organic photonic dots.17

In this paper, we demonstrate the existence of a minimum for the threshold value in dependence on thethickness of the active layer. Therefore, we analyze a standard set of rate equations in the steady-state regimefor the inversion and photons in the lasing mode. In spite of its simplicity, it has proven to describe lasingthreshold dependencies in different optical cavities rather well.18–20 The approach holds even in the case ofstrongly nonstationary systems, e.g. in optically pumped microcavities by ultrashort excitation pulses.11

* bernd.schuette@iapp.de

Organic Optoelectronics and Photonics III, edited by Paul L. Heremans, Michele Muccini, Eric A. Meulenkamp, Proc. of SPIE Vol. 6999, 699930, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.781138

Proc. of SPIE Vol. 6999 699930-12008 SPIE Digital Library -- Subscriber Archive Copy

Figure 1. Schematic picture of the wedge-shaped microcavity.

2. SAMPLE AND EXPERIMENTAL SETUP

2.1 Organic VCSEL Fabrication

The microcavity under investigation consists of an active layer which is embedded by two distributed Braggreflectors (DBRs). The first DBR, made of 19 alternating λc/4 layers of silicon dioxide and titanium dioxide,is deposited onto a BK7 glass substrate which is pre-treated by ultrasonic and ethanol cleaning to minimizesurface contamination. Here, λc is the design wavelength of the system, aimed to coincide with the maximumof the active material emission spectrum. For the sample under investigation, it is adjusted to λc = 624 nm.The corresponding refractive indices of the two materials are nSi02 = 1.47 and nTi02 = 2.1, respectively. Theuse of dielectric materials with a large difference in refractive indices allows to obtain high reflectivities with asmall number of periods. Transfer matrix calculations for our sample yield values for the reflectivities of 99.8 %for each DBR. To achieve equal reflectance from both multilayer mirror structures, the second DBR consists of21 layers. The dielectric mirrors are grown by electron beam evaporation (electron beam evaporator ESV 14,Leybold Optics). For this purpose, a vacuum chamber is pumped down to about 10−6 mbar. After heating twocrucibles containing either silicon or titanium by an electron beam, the material is oxidized in flight. The desireddynamic equilibrium is adjusted to 2 · 10−4 mbar.

The guest-host composite of tris-(8-hydroxy quinoline) aluminium (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) is used as the active medium and fabricated via thermal coevap-oration (3xDE-FR/2.1, CreaPhys). The temperature involved in this method is low enough to prevent thedestruction of the molecules, thus reducing molecular fragments with different absorption properties in the or-ganic layer. The system consists of 2 % DCM by mass and has a refractive index of nc = 1.75. For the fabricationof the wedge, a rotating shadow mask is implemented. With the help of a motor, a part of the sample is coveredby the mask. For different parts of the sample, the covering times are different such that a continuous thicknesschange is induced over the sample. A schematic picture of the wedge-shaped microcavity is presented in Fig. 1,with the physical thickness varying from 0.2 to 2.4 µm. This corresponds to an optical thickness variation fromabout λc/2 to 6.7 λc.

2.2 Optical excitation

Two different excitation regimes are used to pump the microcavity optically. In both cases, emission is mainlyobserved from DCM. At a wavelength of 400 nm, Alq3 is excited, followed by a resonant Forster energy transferto DCM. In contrast, pumping at a wavelength of 532 nm excites DCM directly. In this case, Alq3 serves asa matrix material, prohibiting aggregation of the DCM molecules. While the photoexcitation efficiency in thelatter pump regime is lower than that typically obtained when exciting into the absorption band of the Alq3

molecules,21 the photostability of the composite is substantially improved. This is due to the reduced energeticrelaxation and allows excitation far above the lasing threshold.

For the excitation at 400 nm, we use the frequency-doubled output of a regenerative Ti:sapphire amplifiersystem (Spitfire, Spectra Physics). It provides pulses of 100 fs length and an energy of about 75 nJ at a repetitionrate of 1 kHz. The pump spot diameter within the microcavities is approximately 50 µm. The emission signalis collected along the cavity axis with a lens having a numerical aperture of NA= 0.12, then it is spectrallydispersed and recorded by a CCD camera. For the direct excitation of DCM, we use the frequency-doubledbeam of a solid-state laser (FDSS532, CryLas), which has a wavelength of 532 nm. The pulse duration is 1.5 ns

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Figure 2. (a) Spontaneous emission spectrum of the organic microcavity at t = 0, (b) decay kinetics, and (c) effectivequality factors and characteristic decay times of four cavity modes and one sideband mode. The decay times shown arethe fastest decay times obtained from the two-exponential fit (solid lines in (b)) of experimentally measured kinetics. Thewedge-shaped microcavity is excited at 400 nm into the absorption band of Alq3 with 100 fs pulses. The laser beam isfocused to a spot of 50 µm in diameter. The cavity thickness at this specific microcavity position is 2.25 µm.

and the system is operated at a repetition rate of 2 kHz. For the investigatons presented here, the pump spot isfocused to a diameter of 10 µm. For the collection of light in this case, a lens with NA= 0.8 is used.

3. RESULTS

3.1 Photostability of the organic cavity layer

For the investigation of the wedge-shaped microcavity and corresponding threshold considerations, the analysisof the degradation process of the emission intensity is of special importance with respect to the reliability ofthe experimental results. Therefore, we have measured the decay times for spontaneously emitted light afterexcitation at 400 nm. Fig. 2(a) depicts the spontaneous emission spectrum consisting of four cavity modes (m2to m5) as well as one sideband mode (m1) at the time t = 0. In Fig. 2(b), the temporal behavior of the spectrallyintegrated spontaneous emission intensity of the five modes is shown. The experimentally obtained values arefitted by a function with two different exponential decay times, one being much larger than the observation time.

Fig. 2(c) shows the decay times of the peak intensities of all five modes together with their effective Q factors.The quality factor of a Fabry-Perot type cavity is calculated by Q ≈ λ/∆λ, where λ is the emission wavelengthand ∆λ is the cavity mode full width at half maximum (FWHM). The fact that the effective Q factors shownhere are much smaller than previously reported ones11, 22 is ascribed to two effects. First, the wedged shape ofthe microcavity also introduces different emission properties when compared to planar microcavities, since thecavity thickness changes over the relatively large pump spot diameter of 50 µm which goes along with changingresonance conditions. Fig. 3 shows emission above the threshold for the case of a wedge-shaped microcavity.Four different laser peaks (A to D) are identified which are emitted from different positions from the microcavityand thus separated in the spectrum. An explanation is given by the fact that the pumped area is much largerthan the effective mode area of the induced laser oscillation,23 which leads to the emission of several modesbeing separated in space and hence, also in wavelengths. It can be seen from Fig. 3 that the emission spansa wavelength range of the order of 1 nm. Accordingly, the calculated quality factor is Q ≈ 620. As a secondmechanism, the collection of light into different directions has to be taken into account, which is caused by the

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Figure 3. Excitation of the wedge-shaped microcavity with a spot size diameter of 50 µm at different pump intensitiesshowing four different laser modes (A to D). The total emission spans a range of approximately 1 nm.

Figure 4. Transfer matrix calculation of the transmission as a function of cavity thickness in the wedge-shaped microcavity.The vertical lines mark measured transmission peaks. Best coincidence of calculation and this particular experiment isgiven by the solid horizontal line, yielding a layer thickness of 19.1 qw.

finite NA value of the collimating system. From Fig. 2(c), we deduce a strong correlation between the decaytimes and the effective Q factors. An increased quality factor is accompanied by a stronger enhancement ofelectric field at the emission wavelength within the cavity which supports degradation, i.e. the destruction ofmolecules. Thus, the modes with the highest quality factors, m3 and m4, have the shortest decay times. Theexperiments also showed that the Q factors of the modes m3 and m4 themselves exhibited decreasing values intime. In contrast, no degradation of the quality factors was observed for the modes m1 and m5, while m2 showedslight degradation in the beginning only.

3.2 Wavelength tuning

The inhomogeneous broadening in organic molecules combined with the wedged shape of the active layer canbe exploited for wavelength tuning of the laser emission. First, we determine the thickness of the organic layer.

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Intensfty(a.u.)

,i,I,i,I,

PL

(a.u

.)

Figure 5. Photoluminescence spectrum of a neat Alq3:DCM layer (upper curve) and emission spectra measured at differentpump beam positions on the wedge-shaped microcavity. The intensities of the different laser lines represent the gainspectrum of Alq3:DCM.

Figure 4 shows a transmission spectrum of randomly polarized light which was obtained by transfer matrixcalculations. Depending on the active layer thickness, the transmitted cavity modes shift within the stopbandwhich ranges from approximately 550 to 720 nm. In order to determine the cavity thickness at a specific positionof the microcavity, the transmission spectrum was experimentally obtained after illuminating with white light.In Fig. 4, two transmission peaks that are obtained for this particular measurement, are marked by vertical lines.Both calculation and measured peak positions correspond to an emission angle of 0 ◦. Best agreement with thecalculation is obtained for an optical cavity thickness of 19.1 quarter wavelengths (qw) as indicated by the solidhorizontal line, corresponding to a physical thickness of 1.70 µm.

In the following, the microcavity is excited at a wavelength of 532 nm. Thus, the photostability of the systemis increased, which is favorable with respect to the tuning range. The emission spectra for different positionsof the pump beam on the microcavity are depicted in Fig. 5. There, the sample is moved stepwise along thethickness gradient of the active layer. The lines from 594.8 to 650.0 nm are identified as lasing, while the twooutermost peaks at 591.2 and 653.4 nm correspond to emission below the threshold. Due to the continuousvariation of the active layer thickness, it is possible to choose any wavelength in the 55.2 nm wide tuning range.This range may even be increased, as the photostability of the DCM molecules allows excitation far above thelasing threshold.

The observed spectral distribution of the laser emission from the microcavity is defined by the degree ofinversion in the system. Since the system is initially overpumped under pulsed optical excitation,24 the spectraldistribution of laser emission from the cavity in the first approximation is proportional to the gain profile ofAlq3:DCM. While it shows a qualitatively similar behavior compared to the photoluminescence spectrum of anAlq3:DCM neat layer (upper curve in Fig.5), the maxima of both are shifted. This effect is ascribed to residualabsorption of both Alq3 and DCM molecules which is a loss for the inversion process in the high energy part ofthe gain spectrum.

3.3 Threshold dependence on cavity thickness

For the investigation of the threshold dependence, we use a system of coupled differential equations (rate equa-tions), which describe the dynamics of the absolute number of inverted states n and number of photons s in thecavity mode:18

dn

dt= p − Afn(1 − β) − An(s + 1)β − nΓ , (1)

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ds

dt= An(s + 1)β − sγ . (2)

Here, p is the pump rate of the inverted states, A and Af are the spontaneous emission rates to the lasing modeand to the free-space modes, respectively. β is referred to as the spontaneous emission coupling factor to thelasing mode, while γ describes the loss rate of the photons in the lasing mode through the cavity mirrors, and Γstands for the nonradiative recombination rate of the inverted states. The analytical steady-state solutions areeasily found. An analysis of the exposure characteristics of the population inversion shows a linear dependencewith the pump rate far below the threshold, whereas above the threshold it is clamped at a constant value. Theintersection of asymptotics of these two regimes leads to the following oscillation condition:

pth = γ

(1 +

Af (1 − β)Aβ

+Γ

Aβ

). (3)

The three terms in the above equation stand for different contributions originating from the system. The firstterm is a fundamental limit for the lasing threshold in a microcavity, which theoretically could be reached in asystem where all inversion recombines to the ground state, emitting a photon to the only mode of the resonator(Γ = 1, β = 1). Such a system is often referred to as ”thresholdless” as it shows no transition from spontaneousemission to the stimulated emission regime if the emission intensity from the microcavity is measured as a functionof the pump rate.25 However, the threshold for the population inversion is clearly defined by pth = γ. The secondterm is an additional rate, which is necessary to compensate optical losses to nonlasing modes in order to keep theinversion population clamped at the lasing threshold level. Moreover, the enhancement of spontaneous emissionrate to the cavity mode (Purcell effect) is also taken into account by introducing different emission rates to thelasing mode and to the free space. The effect is characterized by the Purcell factor Fp = A/Af , which scales thespontaneous emission rates for an emitter in a cavity to the same emitter in free space. Finally, the third term issolely related to the optically active medium and generally stands for all relaxation mechanisms of the inversionwhich do not provide photons to the lasing mode.

Although in the standard set of rate equations, γ has a clear physical definition and stands for the escaperate of photons from the cavity, it can further be generalized to take into account other photon-loss mechanismswhich do not influence the population inversion. For example, residual absorption can be taken into account bysimply adding a term −rs to the second rate equation∗. In that case, from the point of view of rate equations,r is indistinguishable from γ and, therefore, γ can be understood as a rate which generally characterizes photonlosses in the mode of interest as long as s and n are the rates within the cavity.

Figure 6 shows the lasing threshold as a function of optical cavity layer thickness L in a wedge-shaped organicmicrocavity, normalized to the design wavelength of the dielectric mirrors. The pump position on the microcavitywas carefully controlled for each measurement in order to tune the lasing mode close to the peak of the gainspectrum as well as to the design wavelength of the mirrors. The results show that the lasing threshold isreduced with decreasing optical thickness of the cavity if it is above or close to 2.5 times the design wavelength.However, if the cavity thickness is further reduced, the threshold reduction is less pronounced. Moreover, thelasing threshold starts to increase again if the cavity thickness is in the order of the design wavelength of themirrors.

While the exact dependency presented in Fig. 6 is a specific result of the cavity layer we use, it can qualitativelybe well understood from the analysis of Eq. 3 even without knowing the exact optical properties of the gainmedium. It follows from the oscillation condition that the threshold in the pump rate (as well as in the populationinversion) decreases with an increasing spontaneous emission coupling factor β and emission rate to the lasingmode A. Both of them are dependent on the cavity thickness. Furthermore, the threshold is porportional toγ which in general also depends on the cavity thickness because it is related to the Q factor of the resonator,γ ∝ 1/Q+ r. Nevertheless, all three parameters can be related to the cavity thickness owing to the fact that thespectral position of the lasing mode was kept close to the center of the stopband in all experiments down to an

∗We assume that the number of ground states giving rise to residual absorption is large enough such that no saturationeffects occur in the threshold region.

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Figure 6. Lasing threshold dependence on the normalized optical thickness of the cavity layer L measured in a wedge-shaped organic microcavity. The solid line shows the simulated threshold dependence using Eq. 3 and taking into accountdependencies of the spontaneous emission coupling factor β, the Purcell factor Fp = A/Af , and the photon-loss rate fromthe mode of interest γ on the cavity thickness (see also discussion in the text). The dip in the threshold dependence isexplained by a change of the dominating mechanisms which determine the threshold value. In the case of a thick cavity,these are the spontaneous emission coupling factor β ∝ 1/L2 and the Purcell factor Fp ∝ 1/L, which both increase withdecreasing cavity layer thickness and lead to a reduction of the threshold. On the other hand, in the case of the cavitythickness approaching λc/2, the lasing threshold becomes large as the quality factor Q of the mode is proportional to thecavity thickness L. This leads to the enhancement of the escape rate of the photons from the cavity (γ ∝ 1/Q ∝ 1/L)and, therefore, an increase of the lasing threshold at small cavity thicknesses. The absolute minimum of the thresholdis defined by nonradiative losses in the system (characterized by the nonradiative decay rate Γ) and residual absorption(characterized by the rate constant r).

optical cavity thickness of λc. Keeping in mind that the quality factor Q of the mode is proportional to L andthe mode volume Veff is proportional to L2,23, 26 we obtain the following expressions:

β ∝ 1/Veff ∝ 1/L2 , Fp = A/Af ∝ Q/Veff ∝ 1/L , γ ∝ 1/Q + r ∝ 1/L + r . (4)

Eq. 4 together with the oscillation condition (Eq. 3) are further exploited to analyze the effect of a decreasingactive layer thickness on the pump threshold. The solid line in Figure 6 shows the simulated threshold dependenceon the cavity thickness. Rather good agreement with the experimental data is obtained which allows qualitativeexplanation of the experimentally observed behavior. It follows from the analysis of the oscillation conditionthat the dip in the threshold dependence is due to the change of the dominating mechanisms which determinethe threshold. In the case of a thick cavity, these are the spontaneous emission coupling factor and the Purcellfactor, which both increase with a decreasing cavity layer thickness and thus reduce the threshold. On the otherhand, in the case of the cavity thickness approaching λc/2, the lasing threshold becomes large as the qualityfactor Q of the resonator reduces with the cavity thickness. This leads to the enhancement of the escape rate ofthe photons from the cavity and, therefore, an increase of the lasing threshold at small cavity thicknesses. Still,the minimum threshold is defined by residual absorption and nonradiative losses taking part in the opticallyactive medium.

4. CONCLUSION

In this paper, we have reported on spontaneous and stimulated emission characteristics from wedge-shaped mi-crocavities. We have studied decay times of photoluminescence for different cavity modes and found a strong

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correlation between an increasing quality factor of a certain mode and its decay time which decreases correspond-ingly. Exploiting the wide gain spectrum of the organic composite Alq3:DCM, continuous wavelength tuning ina range of 55 nm was achieved by moving the position of the pump beam with respect to the microcavity surface.In order to investigate the threshold behavior for different cavity thicknesses, we used a standard set of rateequations in the steady-state regime. In contrast to the expected reduction of the lasing threshold with a de-creasing active layer thickness, we have demonstrated the existence of a minimum threshold value for a thicknessof 1.5 λc. For larger thicknesses, the threshold is mainly determined by the spontaneous emission coupling factor,β, and by the Purcell factor, Fp. On the other hand, for cavity thicknesses approaching the design wavelength,the loss rate through the cavity mirrors, γ, is the leading term resulting in an increasing threshold value.

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from the Bundesministerium fur Bildung und Forschung(Grant FKZ: 13N9279).

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