Photoactive materials are highly promising candidates for novel applications as they enable all-optical control of photonic devices. Photochromic molecules exhibit a reversible change of their dielectric function upon irradiation with light of proper wavelength. The trans- and cis-isomers of azobenzene exhibit different absorption properties due to the effect of the configuration on the polarizability of the molecule. Here, we introduce a novel molecular/semiconductor hybrid device which is fully tunable by all-optical means via the integration of a semiconductor microdisk into a photo-adressable polyelectrolyte material. We demonstrate that such polyelectrolyte superlattices can be used to tune semiconductor photonic resonators with high precision and without any significant degeneration of device performance. Moreover, we demonstrate an all-optically tunable laser based on this hybrid concept.
© 2012 OSA
The functionalization of liquid crystals (LCs) with azobenzene  as a photoactive component has already been demonstrated in a number of applications, ranging from tuning of the optical properties of LC based Bragg reflectors [2, 3], cholesteric LC lasers , or the blue phase photonic band gap [5, 6], to the initialization of the nematic-isotropic phase transition by UV light exposure . Moreover, in functionalized LC films as well as in pure azobenzene films optical anisotropy was induced by UV light irradiation [8–10].
However, most of those devices do not contain active light sources and are hence passive in their operation. Moreover, the integration of a photonic structure with embedded quantum systems into a photoactive matrix has not yet been demonstrated.
Semiconductor microdisks (MD) are optical resonators in which light can efficiently be confined spatially with high quality factors. The electromagnetic field is localized in the periphery of the disk, giving rise to whispering gallery modes (WGM), which are characterized by their radial quantum number n and an azimuthal quantum number m . Recently, it was shown that the optical modes in such semiconductor MDs can be controlled by the use of semiconductor/liquid crystal (LC) hybrid structures [12, 13]. Until now, tunability of such photonic microcavities was achieved electrically  as well as by temperature [14, 15]. Previously, it was also demonstrated that the adsorption of molecules [16, 17] can be utilized in order to achieve a controlled change of the optical modes.
In this paper, molecular/semiconductor hybrid devices that are fully tunable by all-optical means by the integration of a semiconductor MD into a photo-adressable polyelectrolyte superlattice are discussed. Polyelectrolyte (PEL) molecules dissociate in water to polyanions or polycations and can, thus, be deposited on surfaces using layer-by-layer (LbL) techniques. Since polyelectrolyte film deposition relies on the ionic interaction between oppositely charged molecules,  it is possible to use spin assembly for deposition, which is a fast and efficient process that creates very smooth surfaces . We will demonstrate that the coating with these molecular films is a highly controlled process with predictable effect on the photonic modes of the device. Moreover, the devices operate as microlasers before and after coating, and the impact on the intrinsic lasing performance of the devices is only very minute. We demonstrate that the resonant laser wavelength can be tuned all-optically by UV exposure, initializing the trans-cis isomerization.
For the experiments in this study, PEL molecules with photoactive side groups and excellent deposition properties on insulating/semiconducting substrates were required. Therefore, PEI (poly(ethylenimine)) as polycation and PAZO (poly[1-[4-(3-carboxy-4-hydroxy-phenylazo)benzene sulfonamido]-1,2- ethanediyl, sodium salt]) as polyanion were chosen, which are well characterized for deposition on glass substrates [20,21]. The photoisomerization of the azobenzene side groups of the PAZO molecules inside thick PAZO-layers and PEI/PAZO bilayers was already demonstrated [9, 22–24]. Irradiation with UV light causes a strong effect on the absorption (and hence on the complex refractive index as well). Thus, the deposition of monomolecular bilayers on semiconductor photonic devices is expected to have a significant impact on the resonant modes. Such polyelectrolyte/semiconductor hybrid devices have been formed by embedding a GaAs-based microdisk with integrated quantum well inside bilayers of PEI and PAZO molecules using a spin assembly process.
The sample material consists of a GaAs based heterostructure, which is grown epitaxially on a semi-insulating GaAs (001) substrate. A 50 nm buffer layer of GaAs is deposited, followed by a 300 nm layer of silicon-doped GaAs. Afterwards, a sacrificial layer of 350 nm Al0.7Ga0.3As is grown followed by a membrane layer sequence of 160 nm thickness. The membrane consists of 50 nm GaAs, 10 nm Al0.33Ga0.67As followed by 20nm GaAs and the In0.28Ga0.72As quantum well (3 nm). Finally, the membrane is capped by 20 nm GaAs, 10 nm Al0.33Ga0.67As and 50 nm GaAs. The two symmetric layers of Al0.33Ga0.67As are used in order to increase the confinement in the quantum well. The entire layer sequence is shown schematically in Fig. 1.
The MDs are dry etched to circular patterns using SiCl4 and Ar (flow 4.5 sccm) each, inductive power 60 W, radio-frequency power 23 W, chamber pressure p = 2 mTorr) based anisotropic plasma process resulting to a vertical etching profile. The post is formed using selective etching of the sacrificial layer over the GaAs by HF. The SEM images in Fig. 2 have been taken with an acceleration voltage of 5 kV.
The MD resonator devices used in this study are 2.7μm in diameter and 160 nm thick and contain. The embedded single In0.28Ga0.72As quantum well layer exhibits a photoluminescence at room temperature in the wavelength range of 900 – 1000 nm.
For the polyelectrolyte spin assembly process, the PEI and PAZO molecules are each dissolved in water (1 mM,R > 18 MΩ). Layer deposition is carried out on top of the spinning substrate (5000 min−1,60 s) beginning with the polycation (PEI) solution. Thereafter, the water molecules are removed by drying the sample on a hotplate (110°C) after each spinning step for 60 s (PEI) or 180 s (PAZO). The layer thickness was characterized by ellipsometric measurements using a spectroscopic ellipsometer (λ = 430 ...826 nm). Single-wavelength (λ = 532 nm) imaging ellipsometry was used to monitor the overall film roughness. The parameters Δ and ψ from the ellipsometry study are determined using a fit procedure with a Cauchy model by assuming refractive indices n1 = 3.4 for the GaAs and of n2 = 1.46 for the PEL bilayers . The thickness growth of the PEI/PAZO bilayers on a smooth GaAs (001) substrate and on a GaAs surface patterned with microdisks is shown in Fig. 2(a). The layer growth is found to be linear for both samples, with a slightly higher rate for the MD sample, which is due to the increased GaAs surface roughness after patterning. The measured increase in layer thickness is R = 0.69 nm/bilayer for the GaAs substrate and R = 0.82 nm/bilayer for the MD sample. However, for the first four bilayers, the thickness increase is lower due to the GaAs surface charge which is screened at higher thicknesses. A single-wavelength ellipsometric scan of the surface after deposition of 30 bilayers of polyelectrolyte on GaAs is displayed in the inset of Fig. 2(a). The observed surface is of smooth quality, exhibiting a roughness as low as 2.3 nm.
3. Results and discussion
SEM images of one of the microdisks before coating and after deposition of 95 bilayers of PEL are shown in the insets of Fig. 2(b) together with photoluminescence (PL) spectra of the device. The MDs show several sharp WGMs on top of the quantum well’s spontaneous emission. The quality factors Q = ω/Δω of the WGM are typically around 5000 below threshold and up to 12000 in the lasing regime.
In the course of the deposition process, a change of the resonant mode wavelengths of the MD devices occurs due to the change in the refractive index of the environment of the MDs. To monitor this, a PL spectrum is recorded after each deposition of a PEI/PAZO bilayer in order to study the effect of the growth of the PEL films on the WGMs with increasing thickness [Fig. 3(a)]. It is found that the WGMs shift to larger wavelengths (redshift). At about 60 bilayers, the shift begins to saturate. After 95 bilayers, the device is fully coated, and the total shift in energy is found to be ΔE = −15.1 meV. A theoretical estimate for the shift has been performed by solving the Helmholtz equationFig. 3(b). While this method has the disadvantage of neglecting the out-of-plane effects, it is superior over numeric techniques that require spatial discretization. The latter methods cannot satisfactorily resolve changes in layer thickness on a near-monomolecular scale. The results are plotted in Fig. 3(a) together with the experimental data. For layer thicknesses d < 40 nm, the agreement is excellent. For higher thicknesses, the experimentally observed shift is even higher than the theoretical estimate. Since the calculations consider the change of index of refraction of the disk’s background in z-direction only, the theoretically estimated mode shift of ΔEtheo = −11.5 meV after saturation is less than in the experiment in which the index of refraction also changes in the xy-direction. The resulting mode shift is due to in-plane and out-of-plane contributions. However, the saturation of the calculated mode shift after 80 nm PEL is in good agreement with the experiment. Furthermore, the quality factor of the WGM with increasing coating thickness of the MD is investigated. It is found that there is no significant decrease of quality factor within the resolution limit (ΔEres = 40μeV) of the setup. Apart from the approximation of the spectral shift, the calculation using Helmholtz’s equation is also valuable to see the influence of the coating on the mode volume. For the 160 nm thin membrane used in these experiments, the effect on the mode profile is not very significant. However, with decreasing membrane thickness, the electromagnetic field is less confined in the semiconductor and the decay length in the polyelectrolyte increases. Thus, if it is desirable to increase the effect of the polyelectrolyte film even more, this can be obtained by choosing thinner membranes.
Fig. 4 shows the lasing performance of the microdisk devices before coating and after deposition of 95 bilayers of PEI/PAZO. A superlinear increase of the intensity of one or two modes with increasing excitation power is observed. For these modes, at transparency a decrease of the FWHM of more than a factor of two is found, as expected for lasing cavity modes. Spectra below and above threshold are shown as insets, demonstrating that single mode lasing operation is regularly achieved for the PEL/semiconductor hybrid devices.
Fig. 5 shows the dependence of the threshold excitation power on the thickness of the PEL bilayer film. The observed threshold power increases, while the measured laser emission intensity decreases at the same time. The laser threshold Pth is recorded over the entire coating process [Fig. 5] showing an increase in Pth with increasing layer thickness by a factor of five. This increase in threshold power is not caused by a degradation in the laser device itself. This can be shown, by comparing the pumping power ratio between coated and uncoated device at transparency with the emission intensity ratio at threshold. As these ratios are identical within measurement accuracy, it can be concluded that scattering and absorption losses in the PEL film are the reason for the threshold increase with film thickness growth, leading to a reduction of excitation intensity as well as of emission intensity. Assuming a Lambert-Beer law for the intensity dependence on the film thickness, a scattering/absorption coefficient of α = 3.0 × 105 cm−1 is determined.
It should also be mentioned that the azimuthal quantum number of the lasing mode might change in the course of the coating process. After some coating steps, another mode will become dominant due to the change of the mode’s energy, if there is better overlap with the quantum well’s luminescence resulting in an enhanced mode feeding.
While the coating process only gives rise to a permanent mode shift, the possibility to induce a reversible trans-cis isomerization in the PAZO molecule enables all-optical tuning. For this experiment, the coated sample was exposed to UV light (xenon lamp filtered to λ = 340 nm – 380 nm) at a power density as low as P/A = 0.0127 W cm−2, corresponding to P = 10 mW and an excitation spot diameter of d = 1 cm. The UV light is absorbed by the PAZO molecules changing the equilibrium between the trans and the cis state in favor of the thermally less stable cis state. Due to the different absorption properties of the azobenzene side group’s cis state, the complex index of refraction changes, resulting in a change of the resonant mode frequency of the MDs . Fig. 6 shows the energy shift of two photonic modes of a MD as a function of the UV exposure time. The modes are optically tuned over a range of 5 meV before the saturation limit is reached after an exposure time of 20 h. This exposure time is consistent with the work of Ferreira et al.  and Dante et al.  who also found exposure times of several hours for PAZO containing LbL-bilayers. They explain this behavior with the need of the molecule’s photoactive side groups to produce space in the layer in order to undergo the isomerization, which is hindered by the strong ionic interaction between the polycations and polyanions. Since the spin assembled bilayers studied here are packed even denser than the films investigated in [9,22], it is assumed that the shape modification process of the azobenzene side groups is even stronger hindered by the ionic binding in the layers resulting in the observed long writing times. It should be noted that other photochromic molecules have isomerization mechanisms which are significantly faster, such as diarylethenes. 
To check if the WGM shift is indeed caused by UV light only, the sample was also exposed with a cw laser of wavelength 532 nm and similar power density as applied during the UV irradiation. For this wavelength, no change in the resonant mode energies was observed. The mode also does not shift after hours in the dark. Hence, we conclude that the exposure with UV light changes the equilibrium of the PAZO molecules to the cis state resulting in a slow change of the layer structure, caused by the shape modification process of the molecule’s azobenzene part. The mode tuning is found to be stable even after switching off the UV light. Heating of the device results in restoring the original wavelength.
Dante et al.  measured decreasing absorption in a PAZO layer during exposure with UV light even spectrally far away of the maximum absorption at λ = 360 nm. A decreasing absorption is over the extinction coefficient κ associated with the complex index of refraction ñ = n + iκ. The change of the surrounding index of refraction leads to a stronger confinement of the resonant modes inside the MD and gives thus rise to a blueshift of the frequency of the WGMs, consistent with the results of the experiment.
In conclusion, a novel all-optically tunable photonic resonator is introduced. A redshift of WGMs by ΔE = −15.1 meV is achieved after deposition of 95 bilayers of the polyelectrolytes PEI and PAZO on a semiconductor microdisk using spin assembly. Theoretical calculations are in good agreement with the observed behavior. Ellipsometric measurements indicate that the growth of PEI/PAZO layers is linear on a planar GaAs surface as well as on the MDs. The quality factor of the WGMs is found to be preserved. Nevertheless, the laser threshold increases by a factor of 5 during the coating procedure due to scattering. Under the exposure of UV light, the resonant mode energy shifts by ΔE = 5 meV caused by the isomerization of the PEL molecules. The shift saturates after 20 h and is found to be stable even after hours in the dark. Hence, the use of PEL molecules with a photochromic side group as coating material for the photonic resonator leads to an all-optically tunable laser.
Funding by the BMBF via NanoFutur and nanoQUIT programs ( 03X5509 and 01BM451) and the Deutsche Forschungsgemeinschaft via grant GRK ’Micro- and Nanostructures for Opto-electronics and Photonics’ is gratefully acknowledged.
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