Very high-quality distributed Bragg reflectors for organic lasing applications by reactive
electron-beam deposition Luana Persano, Andrea Camposeo, Pompilio Del Carro, Elisa Mele, Roberto Cingolani
and Dario Pisignano NNL, National Nanotechnology Laboratory of Istituto Nazionale di Fisica della Materia-Consiglio Nazionale delle
Ricerche (INFM-CNR), c/o Palazzine Garrisi, Università degli Studi di Lecce, via Arnesano, I-73100 Lecce [email protected]
Abstract: We report on the realisation of a few pairs dielectric Distributed Bragg Reflectors fabricated by reactive electron-beam deposition, with state-of-the-art performances, such as very high reflectance (up to about 99.4%), wide stop band (up to 160 nm) in the visible range, and smooth interfaces (roughness as low as 1.5 nm). As a demonstrator of the very high quality of the mirrors we realized a polymer-based vertical microcavity laser by an imprinting-like approach. The device exhibits laser action at 519 nm, indicating low-loss dielectric reflectors grown by electron-beam techniques as promising tools for organic solid-state lasers.
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1. Introduction
Distributed Bragg Reflectors (DBRs) on transparent substrates are the cornerstone of microcavities for the realization of surface light-emitting devices [1]. The growing demand for large bandwidth and high data-rates, memories, and imaging in optical communications, drives the modern technology to an increasing demand of double-side emitting and low-cost scale-manufacturing optical devices [2]. In this framework, the feasibility of processing and testing, together with the emission along a direction perpendicular to the substrate, make vertical cavity surface-emitting lasers (VCSELs) the main optical components for implementing faster and cheaper system architectures. The performance of surface-emitting optoelectronic devices is dramatically limited by the availability of very high quality DBRs forming the resonator needed for the lasing action [3, 4]. A wide stop-band, high reflectance, and high stability under pumped operation regimes are in fact required to the DBRs, to guarantee effective performance of the device. While successful devices have been realized by epitaxial semiconductor heterostructures [5], high-quality dielectric stacks are required for fabricating efficient and stable vertical microcavities employing polymers [6-8] as active materials. In this work, we report on the fabrication of high-reflectance TiO2/SiO2 mirrors by reactive electron-beam deposition (REBD). The reflectors exhibit reflectance as large as (99.4 ± 0.1) % and a width of the reflectance band in the range 100-290 nm, with very low value of the surface root-mean square (rms) roughness, thus improving the uniformity of the subsequently deposited organic film. This prevents corrugation points to occur, which can decrease the effectiveness of the photon coupling with the cavity modes. As a demonstrator, we realized an organic-based VCSEL by sandwiching an approximately half wavelength-thick polymer film between two DBRs by an imprinting-like assembly approach. The realized device exhibits remarkable performances, namely single-mode laser action at 519 nm, with a threshold excitation fluence of 260 μJ/cm2.
2. Experimental method
The growth of the TiO2 and SiO2 layers was performed on Corning glass substrates (10 x 10 mm2) by electron-beam evaporation using a Temescal Supersource 2 electronic gun system operating at 9 kV. The chamber was evacuated at a base pressure of 1-2×10-6 mbar, and the films were deposited in oxygen atmosphere, starting from 99.9% purity TiO2 tablets and SiO2 disks (Leybold, Germany) as source materials. The working pressure was kept constant during the growth at 2.4×10-4 mbar and a temperature of the chamber of 260° C was provided by an infrared halogen lamp. The optical characteristics of the single-layer film and of the reflectors were determined by transmission measurements performed using a Cary 5000 spectrophotometer (Varian, Australia). The absolute specular reflectance of the DBRs was determined by a VW-set-up (Fig. 1(a), (b)), employing an He-Ne stabilized laser, exhibiting a power stability better than 0.1% as light source. The angle of incidence of the laser on the sample was 8.5°. In the V configuration, the sample is not mounted and a movable mirror is positioned in ‘reference’ position. The overall reflectance of the two fixed mirrors and of the moveable mirror is acquired. In the ‘measurement’ position (W configuration), the moveable mirror is positioned behind the mounted sample and the light beam undergoes a double reflection on the sample surface in two different points. The ratio between the ‘measurement’ and the ‘reference’ values of the reflected light collected by the photodiode is equal to the
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square of the sample reflectance, regardless of the reflectance of the mirrors employed. The final value of the sample reflectance was obtained by a wide statistics on many mirror samples and on many points on the single mirror. The surface morphology and the cross-section of the evaporated mirrors were inspected by Atomic Force Microscopy (AFM, NanoScope from Digital Instruments, FL), operating in contact mode under ambient conditions, and by scanning electron microscopy (Raith 150-plus, working at an acceleration energy of 20 keV). The laser device was realized by spin-casting a chloroform solution of the conjugated copolymer, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-diphenylene-vinylene-2-methoxy-5-{2-ethylhexyloxy}-benzene)] (PFV, American Dye Source, Canada) on the top of each mirror, and pressing them together (1.5 MPa) by a commercial precision press (PW100 P/O/Weber, Germany). The printing process was carried out in air heating the hot plates at 150°C for 10 minutes. The sample was finally cooled in air maintaining the pressure on the two mirrors. The two polymer films merged together uniformly, without trapping air, so no interference fringes were formed. The obtained device was excited by the third harmonic (λ =355 nm) of a 3 ns Q-switched Nd:yttrium–aluminum–garnet laser (Spectra-Physics, repetition rate of 10 Hz), and the laser emission at room temperature was dispersed by a monochromator and detected by a Si charge coupled device, providing a spectral resolution of about 0.1 nm. All measurements were carried out in air. For modelling and refractive index calculations, we used a transfer-matrix software for the optical design and analysis of thin films (TfCalc 3.5, from Spectra Inc., OR).
3. Results and Discussion
The deposition method and the growth parameters strongly affect the optical and morphological characteristics of the dielectric materials constituting dielectric mirrors. Among the possible growth approaches, such as electron cyclotron resonance (ECR), plasma-enhanced chemical vapour deposition (PECVD) [9], and sol-gel methods [10], the REBD [11, 12] represents a very powerful technique to deposit transparent dielectric films with flat surfaces, controlled thickness, and high reproducibility. By this technique, we realised a large number of mirror structures, whose photonic band-gap is tunable in the whole visible range, from the violet to the near infrared (Fig. 1(c), (d)).
Performing AFM analysis of single TiO2 and SiO2 evaporated layers (Fig. 2(a) and (b),
(b)
(d)
(a)
(c)
Fig. 1. Experimental VW-set-up for measuring absolute specular reflectance, in ‘reference’ position (a) and ‘measurement’ position (b). (c) Different mirrors realised by REBD, whose photonic band-gap is tuned in the visible and near infrared range. (d) Corresponding transmission spectra of the DBRs, having a stop-band centred from 425 to 850 nm.
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respectively), we observed a quite uniform and densely packed grain size distribution centred at about 30 nm and a value of the rms roughness of about 1.5 nm, corresponding, for a mirror with reflection band centred at λ0 = 500 nm, to a peak-valley distance of λ0 /50, for both the films. Such very low values of the achieved roughness are particularly required to the layer in direct contact with the polymer, namely TiO2 (Fig. 2(a)), as the smoothness of the top dielectric surface can improve the homogeneity of the organic film onto the DBR stack, for the subsequent realization of organic-based photonic devices. According to the Cauchy dispersion relation for the wavelength-dependent refractive index [13], we estimated for the evaporated transparent films of titanium and silicon dioxide values of n = 2.24 and 1.44, respectively (at about 500 nm). Both the high values of n and the high packing density of the deposited materials are fully consistent with a bi-oxygen content of the titanium and silicon oxides films [14, 15]. This result indicates that the growth parameters (temperature and oxygen partial pressure) allow one to force the evaporant to recombine reactively with the supplied oxygen, thus restoring the stoichiometric composition of starting materials. The high refractive index contrast (Δn = 0.8) between the two films allows the fabrication of high reflective DBRs evaporating only few TiO2/SiO2 pairs (Fig. 2(c)).
The stack is composed by 7.5 pairs of TiO2/SiO2 layers, which are about 56 and 87 nm thick, corresponding to the quarter wavelength of the designed optical wavelength, λ0, of the reflector. The stop-band of the evaporated DBRs is centred at about 500 nm with a maximum of reflectance of (99.4 ± 0.1) % and full width at half maximum (FWHM) as large as 160 nm (Fig. 3). For sake of comparison, the reflectivity values of our DBRs and of other dielectric mirrors reported in the literatures are shown in Table 1. The mirror losses, L for our realised stacks within the stop-band can be estimated from the energy conservation relationship L =100% - (R + T), were R and T are the reflectance and the transmittance values, respectively. Since T ~ 0.2 % we can deduce for the mirror losses of the value, L ~ 0.4 %.
Fig. 2. (a) Three-dimensional AFM images of the electron-beam evaporated TiO2
(a) and SiO2 (b) films. (c) Cross section of the realized DBR structure. The bright and dark alternating areas correspond to the TiO2 and SiO2 regions respectively.
1 µm
(a) (b)
(c)
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Table 1. Typical figures of merit (number of couples, reflectance and stop band FWHM) of SiO2/TiO2 DBRs reported in recent works.
In the microcavity lasers the cavity length is very short, being of the order of the cavity thickness (about λ/2n, where λ is the wavelength of the emitted radiation and n is the refractive index of the cavity active medium). The gain of an organic active medium (typically characterized by the cross-section of stimulated emission in the range 10-19-10-15 cm2) over such short lengths has to overcome the cavity losses. In order to minimize such losses, organic-based VCSELs strongly benefits from mirrors exhibiting large values of the reflectivity [7]. Our device was fabricated by sandwiching the conjugated co-polymer between two identical mirrors. We point out that our imprinting-like technique for assembling organic-based symmetric resonators allows an unequalled control of the thickness of the active material, and a much better adhesion with respect to previous reports [7, 8]. During the
Fabrication technique
# of couples
Reflectance (%)
FWHM (nm)
Ref.
Electron beam
3.5
85
200
# 10
ECR/PECVD
4.5 86 100 # 7
Electron beam
7.5 99.4 160 This work
Sol-gel
10 98 n.a. # 8
Electron beam
10.5 >99 n.a. # 9
Fig. 3. Transmission spectrum of the DBR mirrors. The continuous line indicates the theoretically predicted transmission, calculated by using the refractive index of TiO2 and SiO2 films determined by the optical measurements on the single layers, whereas the dots represents the experimental data.
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imprinting, the polymer flow is primed by the applied external pressure, as the organic compound is driven above its softness temperature. Because of the flat imprinting surface, a quite large force (in the range of kN) is required to induce the polymer movement and the consequent control of the resulting thickness. The photoluminescence (PL) spectra of the polymer resonator, measured at excitation density slightly and well above lasing threshold are reported in the inset of Fig. 4. For a pump fluence of 520 μJ/cm2, the PL is modulated by the cavity mode, and a peak centred at 519 nm rises from the spontaneous emission background. Well-above threshold, the additional energy supplied by the external pumping reduces the mode competition to only a few of transverse modes, driving the emission into a highly resolved peak at 519 nm, with a FWHM linewidth narrower than 2.5 nm. The dependence of the integrated intensity as function of the pump fluence, reported in Fig. 4, indicates a threshold at a fluence of 260 μJ/cm2. At higher excitation fluences, the output intensity increases linearly with the pump, as expected for laser action. By assuming a stimulated emission (SE) cross section, σSE, of the copolymer we used (at threshold, TH, for lasing) ≥ 10-
16 cm2, which can be straightforwardly achieved by a fs-pulsed excitation [16, 17], we could estimate a significantly lower value for the expected excitation threshold of the microcavity realized with our reflectors. Indeed, considering the gain at threshold as gTH ≅ -1/(2nd) ln (R1R2) ≅ 200 cm-1, where d indicates the thickness of the organic layer, and R1 and R2 are the reflectance values of the mirrors, we would have for the excitation density at threshold: NTH = gTH /σSE ≤ 2×1018 cm-3. Hence, for a 171 nm thick film absorbing 81% of the pump light, we would obtain as value of the excitation fluence at threshold: ETH ≤ 20 μJ/cm2. The difference between such performance for the threshold, achievable by the realized mirrors, and the value that we found here is likely due to a non optimum efficiency of the excitation of the active material under ns-pumping.
4. Conclusions
In conclusion, in this work we reported on the fabrication and characterization of very high quality mirrors realized by REBD of few alternating couples of TiO2/SiO2 layers, with high refractive index contrast. The very high reflectance achieved by the optimized growth process, together with the achieved smooth surfaces, render the fabricated DBRs very suitable for hybrid dielectric/organic devices. As a demonstrator, we realized a microcavity laser showing competitive figures of merit in terms of FWHM linewidth (2.5 nm) and threshold (260 μJ/cm2).
Fig. 4. Laser emission characteristic as a function of the pump excitation fluence (full circles). The solid line is a linear fit to experimental data above threshold. The inset shows PL emission spectra of the laser device measured at pumping fluences of 0.52 (dashed line) and 1.73 mJ/cm2 (continuous line), respectively.
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