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We investigate the photosensitive and thermo-optic nonlinear properties of chalcogenide glass photonic crystal (PhC) cavities at telecommunications wavelengths. We observe a photosensitive refractive index change in AMTIR-1 (Ge33As12Se55) material in the near-infrared, which is enhanced by light localization in the PhC cavity and manifests in a permanent blue-shift of the nanocavity resonance. Thermo-optic non-linear properties are thoroughly investigated by i) carrying out thermal bistable switching experiments, from which we determined thermal switching times of 63μs and 93μs for switch on and switch off respectively and ii) by studying heating of the cavity with a high peak power pulsed laser input, which shows that two-photon absorption is the dominant heating mechanism. Our measurements and analysis highlight the detrimental impact of near-infrared photosensitivity and two-photon absorption on cavity based nonlinear optical switching schemes. We conclude that glass compositions with lower two-photon absorption and more stable properties (reduced photosensitivity) are therefore required for nonlinear applications in chalcogenide photonic crystal cavities.
©2010 Optical Society of America
1. Introduction
Chalcogenide glasses have recently emerged as a promising alternative to semiconductor materials for integrated photonic devices due to their large, ultrafast, Kerr non-linearity (100 – 1000 times silica), negligible free carrier effects and moderate two-photon absorption (TPA) in the near-infrared [1–6]. This has led to demonstrations of various nonlinear processes e.g. all-optical signal regeneration [7], frequency conversion [8] and demultiplexing [9] in planar waveguide structures. Chalcogenide glasses display another interesting property: they are well known to be photosensitive to bandgap light (typically visible wavelengths) [3, 10, 11], leading to a permanent change of the refractive index. This effect has been exploited, for example, to create strong Bragg gratings [12], to post-tune optical components such as distributed feedback lasers [13] and quantum cascade lasers [14] and to create and tune defect structures in 2D planar photonic crystals (PhCs) [15–17].
2D planar PhC structures fabricated in chalcogenide glass [18–20] combine the desirable nonlinear properties of these materials with strong confinement of light, making them an attractive platform for compact, ultrafast all-optical switches [21, 22]. Nanocavity switches have been demonstrated in semiconductor materials based on thermal [23] and free carrier nonlinearities [24, 25], which are limited to speeds of a few MHz and ~tens of GHz, respectively. In contrast, chalcogenide nonlinear devices rely on the near instantaneous Kerr nonlinearity, potentially reaching faster switching speeds that are limited only by the bandwidth of the cavity. In addition, the moderate two-photon absorption of chalcogenides, which exhibit non-linear figures of merit (FOM = n2/βλ where n2 is the intensity dependent refractive index and β is the TPA coefficient) in the range of 1 up to >10 [4, 26, 27], offers superior performance to most semiconductors, in particular silicon with a FOM ~0.35 [28] at 1540nm. A well known general criterion for efficient Kerr based all-optical switching requires a FOM > 2 independent of geometry [29]. However, TPA can impose a further practical limitation on nanocavity based switches due to TPA induced heating, which can become significant in these strongly confined geometries. For practical implementations of cavity based Kerr nonlinear switches, the required FOM will therefore also strongly depend on the thermal properties of the cavity.
In this work we investigate the photosensitive and thermo-optic properties of an AMTIR-1 (Ge33As12Ge55) chalcogenide glass PhC cavity at telecommunications wavelengths. We found that AMTIR-1 films are photosensitive in the range of 1540nm – 1550nm, exhibiting a permanent negative index change which manifests as a blue-shift of the cavity resonances. The magnitude of the index change depends linearly on fluence, which indicates that the photosensitive process results from linear absorption, which is similar to observations made in the As2S3 glass [30, 31]. The thermo-optic properties of the cavity were characterized firstly by observing thermal bistability, yielding an estimation of the thermal response of the structure. Next, by probing the cavity under pulsed excitation, we observed that the primary heating mechanism at higher peak powers was the moderate two-photon absorption. These results highlight the impact of even moderate two-photon absorption in cavity based nonlinear optics, as well the requirement for improved material stability (reduced photosensitivity) for practical device implementations.
2. Experimental setup and methods
2.1 Sample fabrication and cavity formation
The PhC samples used in this work were fabricated from 300 nm thick chalcogenide glass (AMTIR-1, Ge33As12Se55) films using e-beam lithography and ICP etching techniques. The films were deposited by thermal evaporation and were not thermally annealed. Following the patterning, the samples were wet etched with HF to remove the silica underlayer, leaving a suspended membrane in air. The films were patterned into a hexagonal lattice of air holes with a lattice period of a = 530nm and a hole radius of 0.29a. ‘W1’ waveguides were formed in the sample by omitting a single row of holes along the nearest neighbor direction (Γ-Κ direction) of the hexagonal lattice [32]. The waveguides were 120 periods long with 20 rows on each side. The initial refractive index of the AMTIR-1 slab was nslab ~2.69.
Following the PhC fabrication a high-Q photo-induced double-heterostructure type cavity was formed in the waveguide as described in [17]. Briefly, double heterostructure type cavities confine light by the ‘mode gap’ effect [33], which is created by locally increasing the effective index of a ‘W1’ waveguide. In the photo-induced double heterostructure this is achieved by locally modifying the refractive index [34], i.e. by selectively illuminating the structure with visible light (633nm). The cavity we used had an estimated cavity length of ~4.5μm, giving an estimated cavity volume of ~2(λ/n)3 from comparison with simulations. The intrinsic Q-factor of the relevant cavity mode was measured to be ~90 000.
2.2 Experimental apparatus
A basic schematic of the experimental setup is shown in Fig. 1 . Light was coupled into the PhC cavity by evanescent coupling with a tapered fiber [18, 35–37]. The coupling strength, and hence the loaded Q-factor of the cavity mode, was controlled by using a highly curved taper and placing it in contact with the PhC but with an offset from the cavity region (as shown in Fig. 1). The curve of the taper meant that the coupling occurred where the taper was raised from the PhC surface, so the overlap between the taper and cavity modes, or effectively the coupling strength, was determined by the offset. The coupling conditions were very stable due to having the taper in contact with the sample. A variety of modes in the PhC were visible in the transmission spectrum of the taper, including those not associated with the cavity, however these modes did not appear at the same wavelength as the cavity mode we studied and so did not affect our measurements. The taper was formed into a loop [37] which had a diameter of 76μm while the taper waist had a diameter of approximately 1.4μm. The taper had a broadband loss of approximately 20%.
Fig. 1 A schematic view of our experimental setup with a tapered fiber coupled to the PhC cavity. The cavity is depicted by the darker region of the PhC. The input light source was modulated with a Mach-Zehnder modulator and then passed through a circulator so that both the reflection and transmission from the PhC cavity could be measured. Light was coupled into the PhC cavity by evanescent coupling with a tapered optical fiber. The coupling condition between the fiber taper and the cavity mode was controlled by changing the displacement of the taper from the cavity region along the waveguide axis.
The input light source was either a mode locked figure of 8 type erbium-doped fiber laser, which was used to investigate the photosensitive response of the chalcogenide PhC at telecommunications wavelengths (section 3) and the thermo-optic response at high peak powers (section 4.2), or a CW laser that was slowly modulated for investigating thermal bistable switching (section 4.1). The pulsed laser had a pulse width of 1.2ps and a repetition rate of 9MHz. The input power was controlled with a variable attenuator and an electro-optic modulator with an extinction ratio of >30dB. The modulator was used in section 4 to modulate the CW laser (section 4.1) and to chop the pulse stream from the pulsed laser into a series of pulse bursts, reducing the effective duty cycle (section 4.2). The polarization of the input light was set so that the light was in the TE state at the PhC cavity and a circulator was used so that the transmitted and reflected light could be measured.
3. Photosensitive response at telecommunications wavelengths
It is well known that deposited films of chalcogenide glass can display a large photosensitivity to light near the electronic band edge of the material [3, 16], which is typically in the visible part of the spectrum (approx 600nm for AMTIR-1 [4]), and we directly exploited this effect to create the cavity under test. The photosensitive index change can be large (~0.1) because the films are deposited in a non-equilibrium, meta-stable state. The glass can rearrange towards the bulk structure with energy, and absorption in the tail of the band edge can provide the energy for this structural rearrangement. However, it has also been found that at least some compositions can display photosensitivity at telecommunications wavelengths as well, even far away from the band edge [30, 31]. This effect is particularly significant in cavity geometries, because light is recycled within small volumes.
We investigated the photosensitivity of the AMTIR-1 PhC cavity at telecom wavelengths using the mode locked fiber laser. The resonance wavelength of the cavity mode was monitored as a function of time and the measurement was repeated at different power levels (at a constant peak to average power ratio of ~8x104 to 1). The measurements were performed with a loaded Q-factor for the cavity resonance of ~20 000, although this may be an underestimate as it is close to the measurement limit of our optical spectrum analyzer (max Q ~25000). A Q of 20 000 at a wavelength of 1547nm gives a line width of 0.077nm (FWHM) for the cavity resonance. Given that the on-resonance coupling efficiency was ~40% and that the FWHM of the laser spectrum was 2.3nm, we estimate that up to 1.3% of the incident power was coupled to the cavity resonance. However, due to the uncertainty in the Q-factor and the location of the taper transmission loss, we label our graphs with the input power at the taper.
An example of the photosensitive resonance shift is illustrated in Fig. 2(a) , where the resonance shifts to shorter wavelengths as a function of time for 7.57μW average power. Figure 2(b) shows how the resonance wavelength shifted as a function of time for a range of input powers. The red lines represent linear fits to the data for each average power level. Since a single cavity was used to perform all of the measurements, the initial wavelength decreased for each measurement series. The fitted slope of each data series is plotted as a function of average power in Fig. 3 , and the data shows a linear relationship between the average power and the rate of index change. From this data we determine the rate of the resonance shift due to the index change is −1.15x10−5nm/s/μW, in terms of input average power and approximately −8.8x10−4nm/s/μW, in terms of average power coupled to the cavity (assuming 1.3% coupling).
Fig. 2 (a) The shift of the resonance is shown in the spectra recorded at various times while the cavity was excited with the near IR pulsed source at an average input power of 7.57μW. (b) Cavity resonance position as a function of time at various input power levels. The lines show a linear fit to the data at each power level.
Fig. 3 The rate of index change as a function of input power shows a linear relationship. This indicates that the photosensitive process results from linear absorption in the regime of peak power we measured.
The linear behavior of the photosensitive index change as a function of power leads us to conclude that the photosensitivity results from linear absorption. The linear absorption at around 1550nm may be due to absorption by sub-gap defects, as suggested by Hu et al for the As2S3 chalcogenide composition [31]. Although we did not find any evidence of nonlinear absorption affecting the photosensitivity, TPA could still contribute to the photosensitivity at higher power levels. At the highest power level used for the photosensitive measurements (4.58W peak power input and ~60mW estimated in the cavity) we measured only a small amount of TPA (see section 4.1).
Although the photosensitivity of AMTIR-1 chalcogenide to visible light was advantageously exploited to create PhC cavities, the photosensitivity of this material to near-infrared light, far away from the absorption band-edge, is highly detrimental for device applications. This effect is particularly significant in resonant structures, where light is strongly confined within a small volume. For nonlinear device applications, it may be possible to anneal the glass to stabilize the material with respect to the as-deposited state. However, such a process would remove the possibility of crating or tuning cavities by photoillumination under visible light. In AMTIR-1 this would also result in a reduction in the refractive index of >0.1, which would be detrimental to the bandgap confinement in the PhC cavity. We therefore conclude that photoinduced PhC cavities in highly photosensitive ChG glasses are not suitable for nonlinear applications. The composition of this glass should be carefully chosen to favour photosentivity or not, depending on the application in mind, restricting the use of photosensitive ChG cavities for linear or reconfigurable optical devices, rather than nonlinear device applications.
4. Thermo-optic non-linear measurements
4.1 Thermal bistable switching
We demonstrated thermal bistable switching and performed time resolved measurements to estimate the thermal response time of the chalcogenide photonic crystal cavity. Measurements were performed using a tunable CW laser that was modulated with a triangular wave at a frequency of 500Hz. This modulation speed was chosen so that the input power changed slowly with respect to the expected thermal response time of the cavity (~100μs). Bistable behavior was observed when the laser wavelength was detuned to the long wavelength side of the resonance by more than 3/2 times the resonance FWHM. The switching time was measured by taking the time between the maximum and minimum of the switching response.
Figure 4(a) shows the resonance used for the bistable switching measurement; the resonance FWHM was 0.148nm, giving a loaded Q-factor of ~104. The input and output powers of the cavity are plotted as a function of time in Fig. 4(b) for a detuning of 0.95 times the resonance FWHM. The measured rise and fall times for the bistable switching were ~63μs and ~93μs respectively. A summary of the thermal hysteresis is shown in Fig. 4(c), which plots output power vs. input power for a range of detunings. The switching contrast was limited to ~5dB by the coupling between the tapered fiber and the PhC cavity. The switching energy was estimated to be in the range of 1-2nJ, which is ~2 orders of magnitude larger than thermal switching energies reported in silicon PhC cavities [23]. We attribute the larger switching energies to the somewhat larger mode volume of our cavity (~2-10 times larger than typical semiconductor cavities) and the low linear absorption of the AMTIR-1, which is the dominant heating mechanism in this power regeme.
Fig. 4 (a): The resonance spectrum for the thermal bistable switching measurement. Resonance linewidth (FWHM) = 0.148nm, λ0 = 1539.597nm (b): Input and output power plotted as a function of time for a detuning of 0.14nm. Switching times were: trise ~63μs, tfall ~93μs (c): Output power against input power for a range of detunings. Thermal hysteresis was observed for sufficiently large detunings.
Compared to thermal switching measurements in semiconductor PhC cavities the switching times we measured are quite slow. For example; switching times of 2μs and 4μs for rise and fall have been reported in InP based photonic crystal cavities [38] and in silicon PhC cavities a thermal response time of 100ns has been reported [23], which, although not directly comparable to the switching time, provides an order of magnitude estimate. We attribute this slower switching time to the much lower thermal diffusivity of AMTIR-1 (1.65x10−3cm2s−1) compared to InP (0.372cm2s−1), or silicon (0.8cm2s−1) at 298K. The long thermal relaxation time of the AMTIR-1 PhC cavities could be detrimental to device applications as it will cause a larger thermal buildup in the cavity for a given absorbed power when compared to semiconductor PhCs. This could affect device stability and limit power handling.
4.2 High peak-power thermal non-linear measurements
Thermal effects in the PhC cavity were also investigated using the pulsed laser source with the aim of measuring the contribution of nonlinear absorption to the heating of the cavity. With the absence of any free carrier effects, nonlinear absorption and the related heating could be the primary detrimental effect for nonlinear switching applications in these structures. Since the spectral width of the laser pulses were much broader than the cavity resonance, the pulsed laser was used like a white light source. The cavity resonance wavelength was measured in the reflection from the cavity.
The input peak and average powers were here independently controlled by using both the variable attenuator and the modulator. The attenuator was used to control the peak power of the pulses and the modulator was used to control the duty cycle, or effectively the average power. The modulator was set to a frequency of 100kHz and hence a period of 10μs. This is somewhat less than the thermal relaxation time of ~60μs we estimated from the bistable switching times in section 4.1, so we treat average power effects without considering any relaxation dynamics between the pulse bursts. This scheme for controlling the power is depicted in Fig. 5 . We estimate that ~3.9% of the input power was coupled into the PhC cavity.
Fig. 5 A schematic showing how the peak power and duty cycle were varied to maintain the same average power. The shaded areas represent the pulse bursts and both diagrams show the same average power.
Figure 6(a) shows a series of measurements of the cavity resonance where the average input power was kept constant at 5.74μW while the peak power was increased. The measurements show that, as the peak power is increased, the resonance shifts to longer wavelengths and the reflection peak also decreases by ~1.5dB. The shift in the resonance wavelength is plotted as a function of peak power in Fig. 6(b). The figure shows results from three series of measurements with different (fixed) average powers. Each series shows a linear increase in the resonance shift as a function of peak power, excluding linear absorption as the main cause for the observed heating. Note that a linear increase in resonance shift with peak power is expected for the Kerr nonlinearity. However, in our experiment we also expect a linear increase for TPA related heating because we kept the average power fixed as we increased the peak power. To discriminate between these two effects we can observe the behavior as a function of average power and note that the resonance shift is larger for the series with higher average power. The thermal shift should increase with average power while the Kerr shift should not. So we can conclude that the heating of the cavity under pulsed laser excitation is primarily due to TPA and that the TPA related heating is causing the majority of the resonance red shift at higher duty cycles. Still, the Kerr nonlinearity likely provides a residual contribution to the observed red shift. the thermal induced resonance shift should indeed increase linearly with average power, whereas we clearly observe a sub-linear dependence in Fig. 6(b).
Fig. 6 (a): A series of measurements of the cavity reflection spectrum with a fixed average power but increasing peak power. (b): A plot of the resonance shift relative to the cold cavity against peak power for three series of measurements, each with a fixed average power.
AMTIR-1 has a nonlinear figure of merit of 2.4 [4], which just satisfies the criterion that the figure of merit should be >2 for efficient all-optical switching. However, our results clearly show that this moderate level of TPA caused significant heating in the cavity and the resonance shift due to TPA heating predominantly swamped any shift due to the Kerr effect. The TPA induced heating has a significant detrimental effect for nanocavity based Kerr switches, particularly where the combination of material and geometry lead to relatively slow thermal relaxation. For nonlinear cavity based devices, a glass composition with a higher figure of merit will be therefore required.
6. Conclusion
Chalcogenide glasses have been proposed as promising candidates for fabricating nonlinear PhC devices due to their large Kerr nonlinearity and negligible free carrier effects. We found that, despite the beneficial photosensitivity at visible wavelengths, the AMTIR-1 glass is also photosensitive to light at telecom wavelengths, making the material unsuitable for fabricating stable devices unless the photosensitivity can be annealed out. Additionally, we found that TPA makes a significant contribution to thermal heating in the cavity under pulsed laser excitation. This TPA induced heating provides an additional practical limitation for cavity based Kerr switches, beyond the requirement that FOM >2. This limitation is particularly evident in cases such as ours, where the thermal relaxation of the cavity is relatively slow. Clearly, glass compositions with a higher FOM and negligible photosensitivity will be needed for PhC cavity based Kerr nonlinear switching applications. It should be noted that there are already some promising candidates, such as optimized compositions in the Ge, As, Se system of glasses [39] and silver doped chalcogenides [5]. The chalcogenide glasses remain an exciting class of materials for integrated photonic devices; however a careful choice of compositions will be needed in order to maximize their properties for specific applications.
Acknowledgments
This work was produced with the assistance of the Australian Research Council under the ARC Federation Fellowship and Centres of Excellence programs. CUDOS (the Centre for Ultrahigh-bandwidth Devices for Optical Systems) is an ARC Centre of Excellence.
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