Longitudinal coherence of organic-based microcavity lasers

Longitudinal coherence of organic-based microcavity lasers

Andrea Camposeo,1* Luana Persano,1 Pompilio Del Carro,1 Dimitris G. Papazoglou,2,3 Andreas Stassinopoulos,2,4 Demetrios Anglos,2 Roberto Cingolani,1and Dario

Pisignano1,5 1National Nanotechnology Laboratory of INFM-CNR,

c/o Distretto Tecnologico, Università del Salento, via Arnesano, I-73100 Lecce (Italy) 2 Institute of Electronic Structure and Laser, Foundation for Research and Technology – Hellas, P.O. Box 1385,

71110, Heraklion, Greece 3 Materials Science and Technology Department, University of Crete, P.O. Box 2208, 71003, Heraklion, Greece

4 Department of Physics, University of Crete, P.O. Box 2208, 71003, Heraklion, Greece 5 Scuola Superiore ISUFI, Università del Salento, via Arnesano, I-73100 Lecce (Italy)

*Corresponding author: [email protected]

Abstract: We report on the measurement of the longitudinal coherence of organic microcavity lasers based on a conjugated polymer. By using a modified Michelson interferometer configuration enabling single-shot measurements of the coherence length, the transition from spontaneous emission to lasing is investigated. The measured coherence length grows upon increasing the pumping fluence, saturating around 45 μm above threshold. At large fluences, possible thermal and photo-oxidation processes occurring in the gain medium limit the further increase of the coherence length. Our results are important for understanding lasing emission in organic microcavities and optimizing the device design and performances.

©2008 Optical Society of America

OCIS codes: (030.1640) Coherence; (140.3945) Microcavities; (160.4890) Organic materials

References and links

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Adv. Mater. 12, 1655-1668 (2000). 11. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, P. L. Burns, R. H. Friend, and A. B.

Holmes, “Light-emitting diodes based on conjugated polymers,” Nature 347, 539-541 (1990). 12. L. Persano, A. Camposeo, P. Del Carro, E. Mele, R. Cingolani, and D. Pisignano, "Very high-quality distributed

Bragg reflectors for organic lasing applications by reactive electron-beam deposition," Opt. Express 14, 1951-1956 (2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-5-1951.

#92127 - $15.00 USD Received 25 Jan 2008; revised 13 Mar 2008; accepted 5 May 2008; published 27 Jun 2008

(C) 2008 OSA 7 July 2008 / Vol. 16, No. 14 / OPTICS EXPRESS 10384

13. L. Persano, E. Mele, A. Camposeo, P. Del Carro, R. Cingolani, and D. Pisignano, “Absolute luminescence efficiency and photonic band-gap effect of conjugated polymers with top-deposited distribute Bragg reflectors,” Chem. Phys. Lett. 411, 316-320 (2005).

14. L. Persano, A. Camposeo, P. Del Carro, E. Mele, R. Cingolani, and D. Pisignano, “Low-threshold blue-emitting monolithic polymer vertical cavity surface-emitting lasers,” Appl. Phys. Lett. 89, Art. N. 121111 (2006).

15. L. Persano, P. Del Carro, E. Mele, R. Cingolani, D. Pisignano, M. Zavelani-Rossi, S. Longhi, and G. Lanzani, “Monolithic polymer microcavity lasers with on-top evaporated dielectric mirrors,” Appl. Phys. Lett. 88, Art. N. 121110 (2006).

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17. V. M. Papadakis, A. Stassinopoulos, D. Anglos, S. H. Anastasiadis, E. P. Giannelis, and D. G. Papazoglou, “Single-shot temporal coherence measurements of random lasing media,” J. Opt. Soc. Am. B 24, 31-36 (2007).

18. M. Born and E. Wolf, Principles of Optics (Cambridge U. Press, 1999). 19. M. Nomura, S. Iwamoto, N. Kumagai, and Y. Arakawa, “Temporal coherence of a photonic crystal nanocavity

laser with high spontaneous emission coupling factor,” Phys. Rev. B 75, Art. N. 195313 (2007). 20. For sake of comparison, we recall that the maximum coherence length here measured is an order of magnitude

lower than the value reported in Ref. [16] for a phenyl-substituted poly-(p-phenylenevinylene) polymer placed in an external cavity. In that system, a higher Q-factor (~1500) was achieved, leading to a cavity photon lifetime around 0.5 ps and hence to a longer coherence length, since lc∝τc.

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1. Introduction

Microcavities (MCs) are a class of optical devices in which an active medium (atoms [1], molecules [2], organic [3] or inorganic [4] semiconductors) is positioned or deposited between two highly reflective mirrors, with spacing (d) of the order of the wavelength (λ) of the e.m. field. Since the discovery of the possibility of enhancing or inhibiting the spontaneous emission (SE) of materials embedded in MCs [5], these systems attracted a lot of interest both from the fundamental and the practical viewpoint. For instance, in semiconductor MCs the strong coupling between excitons and photons leads to the formation of cavity polaritons [6,7], which exhibit intriguing polarization properties [8].

A remarkable interest towards these devices is motivated by the possible accomplishment of laser action with very low thresholds. This was first demonstrated at optical frequencies by a microcavity with d=λ/2, in which SE was strongly overcome by stimulated emission (STE) [2], thus resulting in virtually zero-threshold lasing emission. In the last decade, a lot of attention was directed to organic semiconductor MCs [3, 9], in which the active layer is generally composed by various types of conjugated polymers. These materials posses a number of attractive properties such as (i) ease of processing, (ii) high photoluminescence (PL) efficiency and STE cross section (about 10-15 cm2) [10], (iii) possibility of chemically tuning the emission wavelength, and (iv) bright electroluminescence obtained in diode architectures [11]. In particular, our group developed a technique for fabricating highly reflective (>99%), dielectric distributed Bragg reflectors (DBRs) [12] directly on top of the conjugated polymer layer, without degradation of the optical properties of the organics [13], thereby demonstrating the realization of monolithic MC lasers [14, 15]. By this approach, single-mode emission tunable in the whole visible range with threshold excitation fluences down to about 1 µJ cm-2, and operation lifetimes larger than 1.5×104 pumping pulses were achieved [14].

Notwithstanding some pioneering experiments on polymer lasers exploiting external cavities [16], the experimental analysis of the coherence properties of organic-based MCs is to date unexplored. Although a laser coherence length can be estimated by the spectral linewidth, only the direct measurement by interferometric techniques allows an accurate analysis of the coherence properties, since this approach does not require any assumption concerning the spectral lineshape of the emissive source. Furthermore, determining the dependence of the coherence length on the MC operation parameters (and particularly on the pumping fluence) would allow a better understanding of the lasing performances and an optimal design of the



organic-based MCs. In this work we investigate the longitudinal coherence length (lc) of polymer-based MC lasers, by using a modified Michelson interferometer that enables single-shot measurements. We measure a coherence length up to 45 μm, mainly limited by the cavity photon lifetime and by possible thermal and photo-oxidation processes affecting the gain organic medium. The rapid increase of the coherence length, as measured by interferometry, from 25 to 40 μm is found to be the more valuable quantitative signature of the onset of lasing in the MC devices.

2. Fabrication of MCs and experimental set-up for measurements

The polymer MCs are realized by a low-temperature reactive evaporation of dielectric DBRs which is described in detail elsewhere [12-15]. Briefly, the bottom DBR, deposited onto Corning quartz substrates, is composed by 8 λ/4 pairs of SiO2 (d1 = 103 nm) and TiO2 (d2 = 67 nm) layers, used as low- (n1 = 1.4) and high- (n2 = 2.2) refractive index medium, respectively. The oxide films are deposited by electron-beam evaporation from 99.9% purity TiO2 tablets and SiO2 disks (Leybold, Germany) at a working pressure of 2.4×10-4 mbar in a controlled oxygen atmosphere. A 10-3 M chloroform solution of poly[2-methoxy-5-(2-ethylhexyl- oxy)-1,4-phenylene-vinylene] (MEH-PPV, American Dye Source) is spin-cast (4000 rpm, thickness ~190 nm) on the bottom DBR. The top mirror, composed by 11 λ/4 pairs of SiOx (d1

= 83 nm) and TiOx (d2 = 58 nm) films is then evaporated on the polymer layer at low temperature (T< 80 °C).

The set-up used to characterize the devices is depicted in Fig. 1. The samples are excited by the third harmonic of a Q-switched Nd:YAG laser (Spectron Laser Systems, λ=355 nm, pulse duration 10 ns), with pulse energy controlled through a variable attenuator and measured by an energy meter. The light emitted by the microcavity samples is collimated by a 4× objective. Part of such collimated emitted light is sent through a beam-splitter to a fiber coupled spectrograph (PTI model 01-002AD; grating: 300 lines/mm; spectral resolution around 1 nm) equipped with a diode array detector (OMA-III system, EG&G PARC model 1406/1420UV) for spectral analysis. The longitudinal coherence length of the organic laser devices is measured using a recently developed single-shot technique described in detail in Ref. [17]. The apparatus is based on a Michelson interferometer with tilted mirrors (Fig. 1)

Fig. 1. Scheme of the setup employed for measuring lc. The MC devices are pumped by a ns Nd:YAG laser (3rd harmonic), which is focused by a lens (L1) of focal length, f=30 mm, onto a spot of about 0.4 mm diameter on the sample. The MC emission is collected and collimated by a 4× objective (OBJ). A beam-splitter (BS) directs part of the collimated beam through a lens (L2) into an optical fiber (FBR) connected to a spectrometer. The beam transmitted by BS is sent to the interferometer, and the fringes pattern is imaged through a lens (L3, f=78 mm) on an intensified CDD camera. A long-pass filter (FLT) is used to cut the excitation light. Glass plates (GPs) are used for phase shifting part of the beam traveling along the interferometer and reflecting on the M1 and M2 mirrors.



Fig. 2. (a) Interference pattern from the organic-based MCs (excitation fluence = 1.2 mJ/cm2) as captured by the ICCD detector. The image consists of two parts: the upper part (ii) corresponds to light that passed through tilted GPs, and compared to the lower part (i), corresponds to an OPD increased by ~16 µm. (b) Measured degree of coherence γ(τ) as a function of the optical path difference. The γ(τ) values corresponding to parts (i) and (ii) of Fig. 2a overlap in the OPD interval of 16-24 µm.

generating two spatially separated replicas of the source. The two replicas illuminate an intensified charge coupled device (ICCD) detector (Andor Technology, mod. DH520-18F, 1024×256 pixels), thus providing interference fringes. Due to the symmetry of the configuration, the optical path difference (OPD) and, consequently, the visibility of the resulting interference fringes is varied over the direction of the line that connects the two replicas of the source. By this technique, we are able to capture the variation of the fringe visibility as a function of the OPD in a single interference image, without performing any kind of mechanical scanning and, consequently, directly measuring the longitudinal coherence of a single laser pulse.

The accessible OPD range is limited by the size of the ICCD detector. In order to enhance this range, we slightly modify the setup described in [17] by inserting two identical glass plates (GPs) of 300 μm thickness in each arm of the interferometer (Fig. 1). The GPs are arranged to cover only one half of the interferometer mirrors (M1 and M2 in Fig. 1), and they are both imaged on the ICCD detector by a lens, L3. Using this arrangement, the GPs overlaying images cover only the upper half of the detector, whereas the lower half is illuminated by light passing through the interferometer without being affected by the GPs. A longer OPD is therefore experienced by light collected by the upper half of the ICCD due to the slight variation of the effective thickness of the tilted GPs. The resulting OPD can be adjusted to any desired value by properly tilting each of the plates. Hence, we are able to simultaneously record, in one single ICCD image, the interference fringes corresponding to two ranges of OPD. Figure 2(a) shows a typical interferogram captured by the ICCD from a single MC emission pulse. The thin horizontal line between the two parts of the interferogram (black arrow in Fig. 2(a)), is the image of the GPs edges. The lower part of the image in Fig. 2a (labelled as (i)) corresponds to light that was not affected by GPs, and covers an OPD range from -4 μm to 24 μm, whereas the upper part of the image (labelled as (ii)) corresponds to light which propagated through the GPs, and covers an OPD range from 16 μm to 44 μm. In the image of Fig. 2(a) the fringes OPD is increased upon moving from left to right and, due to the finite coherence length of the MC light, the interference fringes fade out correspondingly.

The OPD in the ICCD images is first calibrated by using interferograms of laser sources



of known wavelength. Furthermore, the extra OPD inserted by the two GPs is measured by the interferometer mirror displacement needed to compensate it in such a way that the zero OPD, corresponding to the maximum fringes visibility, appears in the upper part of the image in Fig. 2(a). The degree of coherence, γ(τ) (τ= OPD/c, where c is the speed of light) measured from such interferograms, is defined [17, 18] as:

1 2

1 2

( ) ( ) ( )( )

2 ( ) ( )

I I I

I I

τ τ τγ ττ τ

− −= (1)

where I indicates the intensity distribution of the overall interferogram, and I1 and I2 are the intensity distributions of each interferometer arm, captured with the other arm blocked. The γ(τ) values are calculated by separately applying Eq. 1 to each pixel in the upper and lower parts of Fig. 2(a), and the results for each part are averaged along the direction of the interference fringes. The plot of the measured degree of coherence for the typical interferogram of Fig. 2(a) is given in Fig. 2(b). The two independent γ(τ) curves corresponding to the upper and lower part of Fig. 2(a) perfectly overlap on the common OPD range (16-24 μm). The coherence length lc is defined as the full width at half maximum (FWHM) of the envelope of the ⏐γ(τ)⏐curve [18]. In our case, the ⏐γ(τ)⏐envelope is calculated [17] by applying low pass filtering to the measured⏐γ(τ)⏐values. This procedure provides a coherence length, corresponding to Fig. 2(b), lc = (40±5) μm. The uncertainties on the lc values, related to the shot-to-shot fluctuation of the MC emission, are determined by the standard deviation of a set of at least five consecutive measurements.

3. Results and discussion

The MC transmittance exhibits a cavity mode centered at λ=590 nm with a FWHM, Δλ=6 nm, corresponding to a quality factor (Q) of 100 (i.e. to a cavity photon lifetime, τc ≅ 30 fs). Upon pumping the cavities by intense UV laser pulses, the PL reduces to a single peak, centered at λL=589 nm. The emission linewidth drops by 75% upon increasing the excitation fluence, Eexc, from 0.3 to 0.6 mJ/cm2 (inset of Fig. 3(a)). However, the dependence of the output intensity on the pump fluence does not show a clear threshold behavior, although an appreciable change of the slope of the input-output characteristics is observed for Eexc = 0.60 mJ/cm2 (Fig. 3(a)). Figure 3(b) shows the measured coherence length as a function of the pump fluence. lc increases from (25±4) μm (at Eexc = 0.27 mJ/cm2) to (40±5) μm (at Eexc = 0.6 mJ/cm2), and then saturates. By comparing the lc values below and above threshold, a 2-fold increase is found, which is comparable to results measured in photonic crystal devices operating with InAs quantum dots [19]. The coherence length of a reference bare MEH-PPV film in the same range of Eexc, where amplified spontaneous emission (ASE) occurs, is (5.5±0.5) μm and is almost independent of the pumping fluence (Fig. 3(b)). Hence, lasing corresponds to an increase of the coherence length by almost an order of magnitude with respect to out-of-cavity STE [20]. Furthermore, the measured lc shows saturation around 45 μm for pumping fluences above threshold. This behavior, which differs from the expected linear increase commonly observed in laser devices [19], can be explained since our measurements are performed in air conditions and without sample temperature control. Both thermal and photo-oxidation processes can affect the spectral properties of the organic gaining medium, thus causing chirping of the emission wavelength (leading to multimode emission) and quenching of the emission intensity [14, 21], whose effects are increasingly important as the temporal duration of the pumping pulse is increased (in our case 10 ns, instead of 100 fs – 2 ns as in other reports [14-15]). For instance, thermal chirping can lead to a 10-fold decrease of the coherence length of light emitted from inorganic MCs upon increasing the pulse width from 5 ns to 50 ns [21]. Furthermore, the lifetime of organic-based MC devices is limited by the operation in the presence of oxygen, due to the UV-assisted photo-oxidation processes that decrease the emission efficiency of the gain medium [15]. These effects can be minimized by



Fig. 3. (a) Normalized output intensity vs. excitation fluence. The output intensities are normalized to the maximum measured intensity value. The continuous and dashed black lines are linear fits to the data in the excitation fluence intervals 0.2-0.6 mJ/cm2, and 0.6-1.2 mJ/cm2, corresponding to MC emission below and above the lasing threshold, respectively. Inset: Laser emission spectra acquired at different pump fluence values: Eexc = 0.3 mJ/cm2 (green dashed line), Eexc = 0.6 mJ/cm2 (red continuous line) and Eexc = 1.2 mJ/cm2 (blue dotted line), and normalized to their maximum heights. (b) Measured coherence length, lc (full red circles) of the organic laser devices, and coherence length estimated by the ratio, λL

2/Δλ (open green squares) vs. excitation fluence. The dotted and dashed lines are guides to the eyes. The error bars represent the standard deviation of the measured quantities from shot to shot for a set of at least 5 consecutive laser shots. The measured lc of the reference polymer film (full blue diamonds) is also displayed.

a suitable packaging of the devices, reducing oxygen diffusion in the MC gain medium, and by controlling the device temperature and heat dissipation.

Finally, we study the excitation fluence dependence of the value of the coherence length (≈λL

2/Δλ), estimated by the measured peak emission wavelength and FWHM of our laser devices (Fig. 3). The behavior of λL

2/Δλ is in good agreement with that of lc as measured by single-shot interferometry, although with lower absolute values. Such deviation is expected, since the estimation of lc by the spectral features of the emission is accurate only for single mode, monochromatic Gaussian lines. Such a condition is not satisfied in our case, since both spectral and spatial mode data suggest a multimode emission, especially below threshold, therefore a quantitative estimation of the coherence length can only be achieved by interferometric measurements. In fact, the discrepancy between the spectral and interferometric lc values decreases upon increasing the pumping fluence, namely when the laser emission is forced into fewer modes. The agreement between the trends, exhibited by both the curves (full circles and open squares in Figure 3(b)) upon increasing Eexc, suggests that the narrowing of the emission linewidth can be only used as qualitative indication of the onset of lasing action.

4. Summary

In conclusion, we report a study of the longitudinal coherence of a polymer-based MC laser. By using a Michelson modified interferometer that enables single-shot measurements of lc, we investigate the transition from SE to lasing action. In our monolithic devices, we measure a maximum coherence length of 45 μm for values of pump fluence larger than 0.6 mJ/cm2. Overall, these findings open the way for the achievement of optimal performances in organic-based lasers by an optimal design of the MC.

Acknowledgments

This work was partially supported by the EC through the Research Infrastructures activity of FP6 ("Laserlab-Europe" RII3-CT-2003-506350) and by the Regional Exploratory project PE086. A. S. receives a graduate fellowship through project ΠENEΔ, contract 03E581.



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Longitudinal coherence of organic-based microcavity lasers

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