1. Introduction

Electromagnetically induced transparency (EIT), a destructive quantum interference effect in multi-level systems, is widely used in slowing/stopping light, magnetometry, lasing without inversion, nonlinear optics and quantum information processing [1–4]. Recently, the all-optical analog to EIT based on coupled microcavities has attracted much more attention [5–15] due to its easy operation, tunable wavelength, engineerable transparency widow via controlling the size of the cavities and easy integration on chip [10–14]. So far these EIT-like effects have been demonstrated in various types of structures, such as directly coupled silica microsphere [8, 9] and optofluidic ring [15] cavities, as well as indirectly coupled silicon microring [10, 12] and photonic crystal [13, 14] cavities. In addition, EIT-like phenomenon based on single microcavity has also been demonstrated [16], resulting from the interference between two whispering-gallery (WG) modes in the polydimethylsiloxane-coated silica microtoroid cavity. However, due to the limitation in fabrication, on-chip EIT-like effect based on coupled microcavities has only been observed in microcavities with the Q factors below 10⁵ and without much tunability [10–14], which severely limits their applications in slowing light and optical information storage.

Here, we experimentally demonstrate an all-optical analog to EIT on chip in directly coupled high-Q silica microtoroid cavities [17] with intrinsic Q factors above 10⁶. In previous works, tuning the transmission spectrum of the all-optical analog to EIT on chip was obtained by fabricating a series of devices with different separations between two microring cavities [10] or thermo-optically tuning via pumping the coupled photonic crystal cavities with two 532 nm lasers [13]. In this work, by tuning the coupling (through varying the distance) between the coupled microtoroid cavities and the frequency detuning between the two coupled WGMs, we have realized precise control over the EIT transmission spectra. In addition, the all-optical analog to EIT is observed when the first microtoroid is either in the undercoupling or in the overcoupling region.

2. Experiment

Figure 1(a) shows a schematic diagram of the experimental setup used to characterize our coupled microcavity system. A narrow linewidth tunable laser (New Focus, model TLB-6328) operated at wavelength of 1550 nm is used to excite the WGMs through a low-loss (<0.5 dB) fiber taper with a diameter of ~1 µm [18, 19]. The transmitted light is measured using a 125-MHz-bandwidth photodetector (New Focus, model 1811) for transmission spectrum measurement. During the experiment, the launched optical power is ensured to be below 1 µW by using a variable optical attenuator to prevent thermal effect, while the cavities are kept in a N₂ purged enclosure to avoid contamination.

 

Fig. 1 (a) Schematic diagram of the experimental setup for measuring the transmission spectrum of the coupled microtoroid cavities. (b) Top-view optical microscope image of the two coupled microtoroid cavities coupled to a tapered fiber. The diameters of the two microtoroids are 60.4 µm and 67.5 µm, respectively. VOA: variable optical attenuator.

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To precisely control the coupling between the first microtoroid (microtoroid 1 in Fig. 2(a) ) to the fiber taper and to the second microtoroid (microtoroid 2 in Fig. 2(a)), we first prepare two microtoroids located at the edge of silicon chips. The fabrication process of the edge-located silica toroids is similar to the one as described in Refs. 20 and 21. Then we place each of the silica microtoroids on a piezoelectric stage to precisely control its position. To change the detuning of the coupled microtoroids, the second microtoroid is mounted on a thermoelectric cooler (TEC) to tune its resonance frequency, while the first one is kept at room temperature. The temperature of the TEC is monitored by a thermistor and actively controlled by a temperature controller with a stability of 0.01 °C. Figure 2(b) presents the dependence (microtoroid 2) of the resonance frequency shift on the increasing temperature with a sensitivity of −3.56 GHz/°C.

 

Fig. 2 (a) Schematic diagram of the coupled microcavity system. (b) Thermal tuning of the second microtoroid cavity. (c) Typically measured and theoretically calculated all-optical analog to EIT spectrum of the coupled microtoroid cavities.

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Figure 1(b) shows the selected two microtoroids used in the experiment with diameters of 60.4 µm and 67.5 µm, respectively. Although they have a large difference in size, the two cavities have their resonant modes close to each other. The initial resonant wavelengths of the two cavities are 1548.1 nm and 1547.7 nm and their intrinsic Q factors are 1.1 × 10⁶ and 4.7 × 10⁶, respectively.

By properly tuning the positions of the two cavities (i.e. the coupling of the first cavity to the fiber taper and the second cavity) and the temperature of the second microtoroid (i.e. the detuning between the two cavities), an all-optical analog to EIT is clearly observed in this system. Figure 2(c) depicts a typical all-optical analog to EIT spectrum of the coupled cavities when the separation between the cavities is 0.98 µm and the temperature of the second toroid is 64.58 °C. The transmission spectrum of the coupled microcavity system can be well analyzed by the coupled mode theory [22]. The measured spectral width of the transparency window is as narrow as 200 MHz with a maximum transparency of 87%. The linewidth of the transparency window is much narrower than the ones achieved in the previous on-chip structures [10, 13] and is close to the one obtained in the directly coupled microsphere cavities [8, 9].

To investigate the controllability of the EIT spectrum, we first tune the transmission of the coupled microcavity system via changing the coupling between the two cavities. As shown in Fig. 3(a) , when the distance between the cavities is decreased, the transmission spectrum is changed from Fano resonance to analog to EIT and then Fano resonance again. The top curve in Fig. 3(a) is the transmission spectrum of the first microtoroid in the absence of the second microtoroid. The loaded Q-factor is 0.94 × 10⁶, indicating that the first microtoroid is undercoupled. The temperature of the second microtoroid is 64.11 °C, corresponding to a frequency detuning of −0.44 GHz. We then make the second microtoroid couple to the first one and tune the distance between them. During the process, the calculated coupling rate is increased from 0.485 GHz to 1.18 GHz when the distance is decreased from 1.21 µm to 0.96 µm. It is worth to mention that the detuning is also changed due to the temperature increase of the first toroid when the second microtoroid is moved towards it. In contrast to the directly coupled microsphere cavities [8] where the EIT-like effect is only observed when the first cavity is undercoupled. For the smaller mode volume and hence larger evanescent field around the microtoroid cavities, the all-optical analog to EIT is also observed when the first toroid is overcoupled in our system due to the strong coupling between the two microtoroids. As shown in Fig. 3(b), the loaded Q-factor of the first toroid is 0.33 × 10⁶ (overcoupled) and the temperature of the second microtoroid is 65.17 °C. By changing the distance between the coupled microtoroids, the transmission spectrum of this coupled system can also be controlled.

 

Fig. 3 Measured and theoretically calculated transmission spectra of the coupled microtoroid system for various coupling rates of the two coupled WGMs when the first toroid is undercoupled (a) and overcoupled (b). The top curves are the transmission spectra of the first microtoroid coupled to a tapered fiber waveguide in the absence of the second microtoroid. The loaded Q factor of the first microtoroid is 0.94 × 10⁶ for (a) and 0.33 × 10⁶ for (b). The temperature of the second toroid is 64.11 °C for (a) and 65.17 °C for (b), respectively. The coupling between the two coupled WGMs is controlled by changing the distance between the two cavities.

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We then fix the coupling (distance) between the two microtoroid cavities and measure the dependence of the transmission spectrum on the detuning of the cavity mode by changing the temperature of the second microtoroid. Figure 4 shows a series of transmission spectra of the coupled microtoroid system by increasing the temperature of the second microtoroid from 64.74 °C to 64.94 °C. The calculated tuning rate of the frequency detuning between the two microtoroids on the temperature is around −3.49 GHz/°C, in good agreement with the measured results (Fig. 2(b)). During this process, the coupling between the two coupled WGMs is also increased due to the thermal expansion of the TEC and the heated silicon chip.

 

Fig. 4 Measured and theoretically calculated transmission spectra of the coupled microtoroid system for various frequency detunings of the coupled WGMs. The top curve is the transmission spectrum of the first microtoroid coupled to the tapered fiber waveguide in the absence of the second microtoroid. The tuning is controlled by change the temperature of the second microtoroid.

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3. Conclusion

We have experimentally demonstrated an all-optical analog to EIT on chip based on two directly coupled silica microtoroid cavities at the wavelength of ~1550 nm. By optimizing the frequency detuning and the coupling between the coupled microtoroid cavities, we have obtained a narrow (200 MHz) spectral width of the transparency window with a maximum transparency near 90%. All-optical analog to EIT is observed when the first cavity is either undercoupled or overcoupled to the tapered fiber. The transmission spectrum is precisely controlled by changing the positions and the frequency detuning between the two cavities. Since the edge-located microtoroids with Q factors higher than 10⁷ [20, 21] and the normal (non edge-located) microtoroids with Q factors higher than 10⁸ [17] have already been fabricated, we expect to achieve narrower EIT-like spectral width in future by further optimizing the fabrication process and stability of the TEC heater. We believe that this all-optical analog to EIT on chip can be very useful in optical sensor [23], cavity optomechanics [20, 24], and quantum information processing [25, 26].