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Electronic Kerr effect in a polyfluorene derivative is used to reversibly switch near infrared probe beam resonantly coupled to a hybrid polymer-silica microspherical resonator. NIR pumping at 780 nm in pulsed laser regime is used for non-linear switching of the WGM resonances that shift as much as 2 GHz for 50 mW of average pump power, compared to a shift of 250 MHz for the same average pump power at CW regime. The absence of temporal drift and the magnitude of this shift confirm the Kerr nature of the switching, ruling out thermooptical effects.
©2011 Optical Society of America
1. Introduction
Whispering gallery mode resonators (WGMR) have been extensively studied in a large variety of geometrical shapes and in a wide range of promising studies and applications, both fundamental like cavity quantum electrodynamics and practical like low threshold lasers and sensors [1]. The sensitivity and versatility of WGMR are due to their high quality factor (Q-factor) and ultracompactness [2]. Optical microcavities confine light at resonant frequencies into small volumes for extended periods of time enhancing the light-matter interaction. This allows an easier study of non-linear optical phenomena in microspherical WGMR like cascading Raman laser [3], sum frequency generation [4], third harmonic generation [5], surface second harmonic generation [6] and self modulation [7].
Silica microspheres are surface microcavities that exhibit one of highest Q-factors up to date (109) where whispering gallery modes (WGM) can be excited by a tapered fiber [1,2]. Resonance wavelength spectra depend on the size, shape, and refractive index of the microspheres and the surrounding material. Due to the high Q-factor, the all-optical switch at low powers is feasible [8] and has been theoretically proposed in microspheres coated by a Kerr material [9] and demonstrated by means of thermooptic effect [10]. Recently, fast switching dynamics was reported in microspheres coated by a photochromic protein that undergoes a trans-cis isomerisation [11,12]. Other versions of WGMR switches have also been proposed and investigated in the past, being the majority of them theoretical [13,14]. The main advantage of using the electronic Kerr effect for all-optical switching of WGMR is that large refractive index changes can be easily obtained on picosecond time scales using intensities well below the damage thresholds of the polymers [15,16].
For optically induced refractive index changes, a strong third order optical nonlinearity of the involved materials are required; the material refractive index n and absorption coefficient α, then, depend on the light intensity I in the material as n = n0 + n2I with n2 ∝ Re (χ(3)), where n2 is the nonlinear refractive index and χ(3) is the third order nonlinear optical susceptibility; and α = α0 + βΙ with β ∝ Im (χ(3)) where β is the nonlinear absorption coefficient. Two different types of situations can be realized in which the switching of the observed signal can be achieved. In one of the possible configurations, only one light beam is present which undergoes self-switching between high and low intensity levels [17,18]. In the second configuration, referred as the all-optical switching, two light beams of different wavelengths with intensities Iprobe and Ipump are present and the resonant conditions for the probe beam are influenced by the pump beam [10–12].
π−conjugated polymers are exciting nonlinear-optical materials. Actually, their suitability for nonlinear optical devices has been well-known for more than a decade [14]. Materials of this group combine structural flexibility, relative ease of preparation, high χ(3) values and high photostability. This is valid in particular for poly(p-phenylenevinylene) (PPV) [16] and polyfluorene derivatives [19]. Inorganic all-switching devices are still limited by weak non-linearity, slow dynamics and difficulty of discrimination between thermal and Kerr non-linearity at room temperature [17,20], but the use of organic-inorganic hybrid systems can offer significant advantages over systems that are completely inorganic [21,22].
In this paper, we demonstrate the first evidence all optical switching of WGM resonances excited in silica microspheres based on third order Kerr non-linearity in a thin cladding polymer layer. For this work, we have chosen as a Kerr-material a polyfluorene derivative, PF(o)n, functionalised at the C9 position of the fluorine ring with two pendant octyl chains for attaining adequate solubility in common organic solvents and mesogenic behaviour. Details of the synthesis and of the linear optical characterization of PF(o)n have been published elsewhere [23]. The nonlinear coefficients of this material are studied for the first time in this paper.
A schematic studied device in this paper is shown in Fig. 1 . Light of the intensity I probe is coupled in and out of the Kerr-polymer coated WGMR by a coupling tapered fibre. At the same time, the WGMR is illuminated by light of intensity I pump, according to the setup. Both light beams with intensities I probe and I pump interact with the microresonator and the WGM excited by the probe beam are switched by the pump beam.
Fig. 1 Experimental pump-and-probe set-up. Left hand side inset: optical image of the WGMR. Right hand side inset: typical resonance.
2. Experimental setup and results
Figure 1 shows the pump-and-probe experimental set-up. Microspherical WGMR of typically 250 μm in diameter, as shown in the left hand side inset in Fig. 1, have been fabricated directly on the tip of a standard telecom fiber by using a fiber fusion splicer (FITEL S182K), and stored under vacuum, in order to avoid contamination [24]. Afterwards, a thin film of PF(o)n (0.1 mg/ml in toluene) was deposited on the microsphere surface by dip coating; the microspheres were then let to dry until total solvent evaporation. All reactions were performed under a chemical hood. The Q values were obtained by measuring the resonance linewidth of the WGM modes around 1.55 μm: they were higher than 108 for bare microspheres and higher than 106 after polymer coating [24]. On the right side of Fig. 1, a typical resonance for a coated WGMR is shown. The thickness of the polymer layer is estimated to be <100 nm [24].
The transmission spectrum of the microspherical WGMR was observed using a tunable external-cavity laser emitting in the 1500-1670 nm wavelength region, with 300 kHz linewidth (Tunics Plus), acting as a probe. The probe laser could be finely swept at very low frequency around a resonance by a few GHz. Light of the intensity I probe was coupled in and out of the WGMR by means of an in-house fabricated tapered fiber with the minimum waist of about 3 μm in diameter. The light transmitted through the coupler-WGMR system was monitored at the output of the taper using an amplified InGaAs photodiode detector connected to an oscilloscope. PF(o)n - coated microspheres were then illuminated by light of intensity I pump, according to the pump-and-probe setup. The pump laser was an ultrafast Ti:Sapphire laser (Mira 900F, Coherent). The ultrafast radiation was coupled to a single mode fiber (SM800, Fibercore) with a lensed distal end, placed in front of the microsphere at the distance of about 1 mm. The lensed fiber end was obtained by using the fiber splicer at lower power level than for the microsphere fabrication. The pump beam out of the lensed tip was illuminating an hemisphere of the WGMR. The measurements were performed in a controlled environment in order to avoid contamination and oxidation of the polymer.
Figure 2 shows the linear absorption spectrum (maximum absorption at λabs = 379 nm) and the chemical structure of PF(o)n. The absorption band is rather broad and originates from a series of inhomogeneously broadened vibronic transitions from the ground S0 to the first excited electronic state S1. The exciton decay time is of the order of 1 ns [19].
In order to characterise the third order non-linear properties of the PF(o)n, we performed a closed-aperture Z-scan measurement [25,26] with a Ti:Sapphire laser at 800 nm and 130 fs pulse duration. This Ti:Sa laser wavelength was chosen in order to excite a two photon resonance in the polymeric coating of the WGMR. A laser beam of about 0.15 W average power was focused on the sample with a lens of 60 mm focal length and an beam spot of about 50 μm. The sample was moved along the beam propagation direction, in the vicinity of the lens focal plane, so that the pump intensity was changing continuously. Therefore, the nonlinear correction to the refractive index, n2I, was being changed and determined the conditions for the beam propagation transmitted through a diaphragm. The measured quantity is the normalized transmittance, i.e. the ratio T = T(z)/T(∞), the latter being the transmitted intensity obtained when the sample was far from the lens focal plane, so that the nonlinear effects can be neglected. Figure 3 shows the plot of the normalized transmission coefficient as a function of the sample position. One can see that the model fit is in full agreement with the Z-scan experimental data of a film of PF(o)n. The measured n2 and β coefficient are n2 = 0.2⋅10−11 cm2/W and β = 7⋅10−7 cm/W. We referred the Z-scan measurement to the standard of n2 of 200 μm thick microporous silicon films in the off-resonant conditions. Porous silicon films were obtained by electrochemical etching technique of a standard Si(001) wafer and deposited on a glass substrate after the separation from the Si substrate, as described in [26]. The transmission Z-scan measurements were performed in the same experimental condition as for the case of the PF(o)n film, so that the values of the nonlinear refraction coefficients normalized on the thickness of the corresponding films can be compared. The obtained n2 values are consistent with those reported in the literature [27].
Fig. 3 Closed aperture Z-scan measurement of a film of PF(o)n. The solid line corresponds to the theoretical fit which is in very good agreement with the experimental data (circles).
The results of the pump-and-probe WGM optical switching are shown in Fig. 4 , where a frequency shift of up to 2 GHz is obtained in pulsed regime of the pump laser. The error bars are the result of the statistical averaging of the data over a series of measurements, which proves a high stability of the suggested device, being the largest contribution to the experimental error the fluctuation of the pump power due to the instability of the pump laser.The pulse width was τ = 500 fs after propagation through the fiber; the average pump power was up to 50 mW. In order to discriminate the thermal shift from the Kerr shift, we have performed the pump and probe measurements in the CW and pulsed regimes for the same range of the average pump power. It can be seen from Fig. 4 that an optically induced shift of WGM of up to 250 MHz is obtained in the CW pump regime, which is nearly an order of magnitude smaller as compared to the pulsed probe regime. Such a difference of the values of the shift induced optically by the power of the pump radiation is an indicator of the nonlinear-optical mechanism of the shift.
3. Discussion
Let us consider the possible mechanisms of the observed optical switching of the WGMR in polymer-coated microspheres. The intense pump radiation can cause thermal WGM shift due to the temperature dependence of the refractive index as well as due to the thermal expansion of both the polymer and the silica core of WGMR [1]. Taking into account typically long relaxation times of the thermally induced effects as compared with the femtosecond pulse width and the pulse repetition rate of about 82 MHz, it can be assumed that the thermal contribution is practically the same in both regimes and, therefore, the fast electronic Kerr nonlinearity is responsible for the observed difference in the slopes. Indeed, in the CW regime only the thermal effects contribute to the shift, since it depends on the average pump power. In the pulsed regime, however, the peak power is several orders of magnitude higher, 1.25 kW compared to 50 mW. In this case, there is a nonlinear optical contribution and nonlinear-optical refraction index changes induce an additional larger shift of the WGM resonances.
The theoretical value of the temperature-dependent shift can be calculated as follows. The resonance wavelength λ is determined by the stationary condition, l λ = π N d, where l is an integer which defines the order of the longitudinal mode, and N and d are the refractive index and the diameter of the sphere, respectively. An increase of cavity temperature ΔT results in a WGMR diameter increase Δd and in a change of index of refraction ΔΝ. Both changes affect the resonance condition and the stationary condition can be written as:
According to the data available in the datasheets, the thermal expansion and thermo-optic coefficients at room temperature for SMF-28 silica fibers are 5.6·10−7 °C−1 and 1.2·10−5 °C−1, respectively. It can be noted that the index change with temperature accounts for the main part of the resonance shift in CW regime, which is about 13.5 pm/°C at room temperature [1]. The thermal coefficients of the polymer are at the moment unknown. In CW regime, we have observed a maximum shift of about 250 MHz (see Fig. 4), which is one order of magnitude lower than in the pulsed regime: at λprobe = 1600 nm such shift requires an increase of the temperature of about 0.2 °C, in agreement with the experimental results reported in [28].
We can estimate roughly the n2 value of the PF(o)n coating, neglecting the thermal effects and taking into account the stretched pulses after fiber propagation, the repetition rate and the maximum frequency shift, τ = 500 fs, ν = 82 MHz and 2.1 GHz, respectively. The peak power of the laser is about 1.25 kW and the spot size is maximized at 250 μm. A change of the index of refraction ΔΝ will affect the resonance condition and the stationary condition can be written as:
The obtained value is n2 = 0.3·10−11 cm2/W, which is in good agreement with the experimental value obtained from the z-scan experiment n2 = 0.2· 0−11 cm2/W.
4. Conclusions
We have demonstrated an all-optical switching technique based on an hybrid WGMR, namely an high Q silica spherical WGMR coated by a polyfluorene thin layer. It is shown that the outstanding material properties of conjugated polymers offer great potential for realizing nonlinear all-optical switching devices in a hybrid device. Resonant frequency shifts of 2 GHz were observed under pulsed optical pumping. This shift is one order of magnitude higher than in the CW optical pumping case, showing a clear discrimination of χ(3) related detuning at room temperature. The estimated nonlinear optic constant is coincident with bulk measurements.
Acknowledgments
S. Berneschi acknowledges funding by the “Centro Studi e Ricerche Enrico Fermi”. Funding by Russian Foundation for Basic Research (RFBR) and Consorzio “E.I.N.S.T.E.I.N.” is gratefully acknowledged. We are grateful to M. De Santo, F. Cosi and G. Ghini for their assistance and Prof. L. Oriol for providing the polyfluorene sample.
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