We report the first experimental realization of all-optical electromagnetically induced transparency (EIT) via a pair of coherently interacting SiO2 microcavities in a one-dimensional SiO2/Si3N4 photonic crystal consisting of a distributed Bragg reflector (DBR). The electromagnetic interactions between the coupled microcavities (CMCs), which possess distinct Q-factors, are controlled by varying the number of embedded SiO2/Si3N4 bilayers in the coupling DBR. In case of weak microcavity interactions, the reflectivity spectrum reveals an all-optical EIT resonance which splits into an Autler-Townes-like resonance under condition of strong microcavity coupling. Our results open up the way for implementing optical analogs of quantum coherence in much simpler one-dimensional structures. We also discuss potential applications of CMCs.
© 2014 Optical Society of America
Optical microcavities continue to play a pivotal role in photonics, owing to their wide-ranging applications . An emerging application of microcavities is the all-optical realization of quantum coherence effects such as electromagnetically induced transparency (EIT), for which coupled whispering-gallery and ring cavities have been applied previously [2–4]. EIT is an example of the unusual response of an atomic medium to coherent electromagnetic fields as absorption of light that is resonant to atomic transitions ceases under EIT conditions. This behavior arises as a result of coherence between atomic states, obtained by coupling these states to coherent light sources . In initial experiments exploring optical analogs of EIT, incident light is coupled to a pair of interacting whispering-gallery cavities and circulates in two-dimensional ring-like orbits. In these experiments, controlled coupling between the cavities is found to be vital for attaining EIT-like photonic resonances [2–4]. Owing to their inherent simplicity, it is of interest to investigate these analogous coherence effects in one-dimensional (1D) microcavities as well. Although theoretical investigations of 1D CMCs have been carried out in the past, optical analogs of quantum coherence effects have not been discussed in this context [6,7]. Here we report the first experimental realization of all-optical analog of EIT using 1D coupled photonic crystal microcavities. We also demonstrate tuning of EIT analog phenomena by controlling electromagnetic interactions between the two microcavities.
The structures studied experimentally consist of two half-wavelength thick SiO2 microcavities, sandwiched between SiO2/Si3N4 distributed Bragg reflectors (DBRs) in a 1D photonic crystal. The two cavities possess distinct quality (Q) factors with the low-Q cavity, located closer to the surface, being referred to as the first cavity in this article while the high-Q cavity, which is situated closer to the substrate, is identified as the second cavity (see Fig. 1(a)). The incident light enters the first cavity through the top DBR mirror and the subsequent coupling of light to the second cavity as well as interaction between fields localized in the two cavities is mediated by the middle DBR. Therefore, by varying the number of SiO2/Si3N4 bilayers of the middle (coupling) DBR, the coupling between the two cavities is controlled experimentally. We first provide a brief summary of EIT effect in atoms and its analogy to the optical experiments carried out here. This is followed by a description of experimentally realized CMC structures and the optical response of these structures. Following the discussion of experimental results, we briefly discuss new applications of CMCs.
Figure 1(b) shows a three level Λ atomic configuration for realizing EIT where the two lower energy states are coupled to two coherent light sources, usually referred to as the control (c) and the probe (p) beam, and typically characterized by their Rabi frequency (j = p, c), which is a measure of interaction strength between an applied field of amplitude Ej and an atomic transition that is characterized by the electric dipole matrix element . The control (probe) beam selectively couples the state () to the excited state , while transition between the two lower states is considered to be dipole forbidden. Owing to a strong control field, the excited state may split into dressed states , where the energy difference between the split states is given by . Excitation of level electrons may become inhibited if the splitting is larger than lifetime broadened linewidth of the excited state . Alternatively, excitations from low energy states may cease due to Fano-like destructive interference between transitions occurring along two pathways. The atomic medium thus becomes transparent over a narrow frequency range and a narrow transparency peak appears within the usual absorption dip. By increasing the control field intensity, the splitting between the dressed states may be enhanced, which leads to an increase in the amplitude as well as broadening of the EIT transparency peak. Certain features of a model developed previously for establishing analogies between atomic and all-optical EIT in ring cavities are applicable to the present case . In this model, the top DBR is analogous to the probe beam Rabi frequency, while the middle or coupling DBR plays the role of the control beam Rabi frequency. Accordingly, a variation of coupling via the middle DBR has an effect which is akin to the one obtained by varying the control beam Rabi frequency.
The 1D CMCs appear to be an ideal candidate to explore all-optical manifestation of quantum coherence as they are simple to implement, can be fabricated using standard semiconductor processing techniques, and allow easy integration of a gain medium. Extensive numerical and experimental studies have been devoted towards the study of two or more coupled cavities, and several important applications have been demonstrated, including enhanced spontaneous emission owing to mode-density enhancement at the photonic band edge , enhancement of cavity quantum electrodynamics effects , realization of quantum-optical Josephson interferometer , parametric oscillations in a triple microcavity system , dual wavelength lasing , and realization of photodetectors for high-performance wavelength demultiplexing . Prior work on all-optical EIT considered whispering-gallery resonators [2,4], ring resonators , micro-disk resonators , and slab photonic crystal cavities . These structures represent two-dimensional cavities which require more complex and precise fabrication techniques to accomplish coherent coupling between the two cavities. Furthermore, unlike the 1D photonic crystal cavities, scattering loss cannot be neglected for the two-dimensional cavities.
The all-dielectric CMC samples are realized using plasma enhanced chemical vapor (PECVD) deposition. During the PECVD growth, SiO2 layers were deposited at a pressure of 400 mT with N2O and 2% SiH4 diluted in N2 with flow rates of 180 sccm and 52 sccm, respectively. The Si3N4 layers were deposited at a pressure of 800 mT with N2, NH3, and 2% SiH4 diluted in N2 with respective flow rates of 180, 20 and 88 sccm. Substrate temperature during the deposition of both layers was 250°C. Based on spectroscopic measurements, we estimate the refractive index of the SiO2 layers to be 1.45, and 1.78 for the Si3N4 layers.
The three fabricated CMC samples have similar structures except the coupling DBR which is varied in its reflectivity. The fabricated DBR structures are of the general form = , where H indicates the high-index Si3N4 layer, L describes the low-index SiO2 layer, CL represents the low-index half-wavelength thick SiO2 microcavity, Nj is the number of pairs of quarter-wave Si3N4 and SiO2 layers, and the structure terminates at a silicon substrate. For these samples N1 = 5, N3 = 9, whereas N2 for the samples S1, S2, S3 is 14, 10, and 3, respectively.
The spectral characterization of the samples is carried out using a fiber coupled CCD array spectrometer. Figures 2(a)–2(c) show the measured reflection spectra. The reflectance of sample S1 demonstrates a single resonance at ~593 nm. The reflection spectrum of S2, where the intercavity separation is decreased in comparison to S1, shows appearance of a sharp peak within the resonance dip, a spectral feature that bears a striking resemblance to the EIT lineshape in coherently driven atomic media. Here we note that while EIT in atomic systems results in enhanced transmission, all-optical EIT in 1D coupled cavities leads to increased reflection. In case of sample S3, where the intercavity separation is the least among the three investigated samples, the resonance is split into two dips, each having a Q-factor that exceeds the initial Q-factor in the weak coupling case of Fig. 2(a).
The single and almost critically-coupled resonance dip related to sample S1 (Fig. 2(a)) shows that nearly all incident photons which are coupled to the dual cavity structure are transmitted since the resonances of the two cavities are tuned to the same wavelength and thus resonant photons can easily tunnel from the low-Q to high-Q microcavity and finally escape the CMC structure in transmission. However, as the separation between the two microcavities is reduced, the probability of reflection increases since resonant photons now have to tunnel through a smaller number of bilayers in reflection. Therefore, a narrow peak appears within the resonance dip appearing in the reflection spectrum, which indicates increased reflectance, as illustrated in the case of sample S2 (Fig. 2(b)). This is the all-optical analog of EIT arising from a 1D CMC structure. The amplitude of EIT peak continues to increase as the separation between the two microcavities is further reduced. Eventually the resonance dip will split into two dips, which is exactly what is observed for sample S3 (Fig. 2(c)). These spectral features are very similar to the Autler-Townes effect in atomic physics , and therefore the doublet appearing in Fig. 2(c) represents the 1D all-optical analog of Autler-Townes effect. However, it is important to note that Autler-Townes effect is a dynamic effect related to ac splitting of atomic levels in an external field which drives the population of atomic levels at the Rabi frequency. We also note that EIT and Autler-Townes lineshapes have distinct origins . Therefore, it is more appropriate to view the splitting in sample S3 as optical analog of an enhanced Aulter-Townes-like splitting of EIT resonance owing to the high strength of the coupling field.
As pointed out in the illustration of Fig. 1(a), CMCs also enable occurrence of all-optical analog of electromagnetically induced absorption (EIA) , which is a distinct quantum coherence effect since in this case constructive interference occurs between transitions occurring along two pathways. Owing to finite substrate thickness, it was not possible to measure the transmitted signal. However, the transfer matrix method (TMM) based calculated transmission spectrum of sample S2 clearly shows appearance of a narrow dip within the usual transmission peak (Fig. 3). This resonance represents the 1D all-optical analog of EIA. The calculated profile is obtained by first fitting a theoretical model to the measured spectrum of sample S2 and the extracted DBR parameters are then used to calculate the transmission spectrum. Therefore, all-optical EIT and EIA resonances may be realized simultaneously at the reflection and transmission ports of the dual cavity system, respectively. Our calculations of the dispersive response of the CMC system reveal that under conditions resulting in all-optical EIT and EIA, subluminal group delays are obtained for the reflected pulses, while superluminal group velocities are attained for the transmitted pulses. This is in direct analogy to EIT and EIA mediated slow and fast light propagation in atomic systems.
It is well-known that microcavity-mediated sharp resonances may be used to achieve subluminal group velocity for an incident optical pulse. Like the EIT resonance in an atomic medium, the photonic EIT resonance can further enhance the slowing down characteristics of such a medium . This slow light scheme is distinct from other coupled resonator slow wave structures such as the one described in Ref. 20, which slow down an optical pulse owing to a large number of resonant circulations inside each cavity of a coupled cavity array. Increasingly, the slow light phenomenon is applied to enhance performance of devices such as interferometers , gyroscopes , and solar cells . Furthermore, owing to spatial compression of an optical pulse in a slow light medium, its energy density is increased, which enables realization of non-linear optical effects at low light intensities for applications such as low-power all-optical switching  and enhanced stimulated Raman scattering . Our simulations show the feasibility of realizing CMCs where a larger probability exists for localizing photons in the second cavity due to a corresponding large intracavity field. If the DBR is composed of material such as silicon  which exhibits free carrier dispersion effect , then on illuminating the bottom DBR with a short wavelength laser pulse, photons can be transferred adiabatically to the first cavity due to suppression of the second cavity’s field as the bottom DBR is no longer resonant to the incident photons. Such an adiabatic photon transfer scheme is important for future applications in quantum information processing . Finally, such CMC systems embedded with active media can also be used to realize bistable lasing and entangled photon sources . Using several CMCs, and through careful coupling between them, one can also study optical analogs of collective phenomena that occur in electronic systems.
In summary, we have experimentally demonstrated realization of all-optical EIT using a 1D coupled cavity system. By controlling the coherent interactions between the two cavities, we showed tuning of coupled-cavity spectral features. Our theoretical calculations confirm that all-optical analog of EIA may also be achieved in 1D coupled microcavities such as those studied here.
Research at CUNY was supported partially through PSC CUNY Research Grant. AN acknowledges support through a Higher Education Commission development grant.
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