We describe an approach to optical non-reciprocity that exploits the local helicity of evanescent electric fields in axisymmetric resonators. By interfacing an optical cavity to helicity-sensitive transitions, such as Zeeman levels in a quantum dot, light transmission through a waveguide becomes direction-dependent when the state degeneracy is lifted. Using a linearized quantum master equation, we analyze the configurations that exhibit non-reciprocity, and we show that reasonable parameters from existing cavity QED experiments are sufficient to demonstrate a coherent non-reciprocal optical isolator operating at the level of a single photon.
© 2014 Optical Society of America
Under most circumstances, Maxwell’s equations of electromagnetism suggest that the propagation of light obeys a principle of reciprocity: transmission from point A to point B is the same as that from B to A . In many cases breaking of this directional symmetry is desirable, most notably to protect sensitive optical elements from scattered light (optical isolators) or to implement direction-dependent logic (circulators). Increasing focus on all-optical fiber networks and photonic integrated circuits [2–5] for replacing electronic circuits for low-power opto-electronic networks  highlights the need for new implementations of non-reciprocal devices such as optical isolators and optical circulators in the context of integrated photonics. Furthermore, applications for non-reciprocal behavior have recently emerged in quantum information processing and many-body physics simulation using photons. In this context, direction-dependent phase shifts at the single photon level [7–11] are important components for simulation of quantum Hall states with light, suggesting exciting applications for non-reciprocal elements in quantum optical networks.
The promise of both classical and quantum photonics  drives exploration of new mechanisms for optical non-reciprocity at micron and sub-micron scales in which control of light in wavelength-scale environments with high device densities can be achieved. Traditional non-reciprocal optics exploits magnetic materials  and magneto-optical phenomena to break directional symmetry. Despite recent research exploring alternative realizations based on diverse mechanisms such as cavity nonlinearities [13–15], mode conversion , spin polarization , coupled atoms and quantum dots [18,19], and optomechanics [11,20], widespread implementation of non-reciprocity in integrated micro- and nano- photonics remains challenging. Major hurdles include shrinking the magnetic or nonlinear medium to the micro- or nano-scale useful for integrated optics  and reducing dependence on power, which can limit isolation performance in the single photon quantum regime . Satisfying these demands requires novel approaches to breaking directional symmetry for non-reciprocal integrated photonics.
Here, we investigate non-reciprocal optical transmission in a hybrid device consisting of a spin-split quantum dot (QD) located in the evanescent field of a whispering gallery mode (WGM) microresonator in a regime of coherent light-matter interactions. We argue that direction-dependent, non-reciprocal behavior can manifest in an axisymmetric geometry from the local helicity of the electric field in the evanescent region where an emitter couples to light [21–23]. Extending the approach for a linear waveguide in , we show that inducing a spin splitting in a V-type three-level system with an applied magnetic field can achieve optical isolation using a coherent dynamical model in a low-power limit [11, 23–25]. This approach demonstrates how to exploit the local helicity of evanescent electric fields for optical non-reciprocity of coherent photons, similar to recent approaches using nonlinear optomechanical coupling .
2. Model and approach
We consider a system of an axisymmetric microcavity evanescently coupled to a V-type three-level single quantum dot (QD) [18, 19], as illustrated in Fig. 1(a). The microcavity is evanescently coupled to a waveguide so that propagating waves from one direction predominantly traverse the resonator clockwise and those from the other direction couple to counter-clockwise propagating modes. The microcavity mediates an interaction between this input field and the QD exciton that can depend on the direction of light propagation through the photonic system. For guided modes in such a microcavity, the electric fields of the two counter-propagating modes possess opposite rotational helicity at each location. This helicity can drive polarization-sensitive optical excitations, as has been demonstrated in atomic cavity QED and atom trapping [21–23]. By inducing an energy splitting between the two polarization-sensitive QD excited states, i.e. with a magnetic field, non-reciprocal transmission through the optical system can be realized [Fig. 1(b)]. A similar idea exploiting the helicity of the electric field of waveguides was explored in a proposal by Shen et al. . In contrast to that approach, which assumed a distinct single photon propagating in the waveguide, our model treats a coherent input field, which can be useful for practical modeling of the dynamics that would be observed in a standard photon counting measurement.
The organization of the paper is as follows. First, we consider the local rotational helicity in the evanescent field of counter-propagating modes in axisymmetric resonators. This result is then used to deduce an effective Hamiltonian for a cavity-QD system, from which transmission spectra of the photonic system are found using a master equation approach. By lifting the degeneracy of the two excited states, optical non-reciprocity will be shown to occur, and the conditions for achieving optical non-reciprocity are analyzed. Finally, we consider a possible cavity QED system in which these parameters for directional asymmetry can be realized.
3. Evanescent helicity in whispering gallery modes
Axisymmetric microcavities support a degenerate pair of propagating modes for every resonant frequency ωC, one propagating clockwise (CW) and the other counter-clockwise (CCW). These modes are labelled by integer-valued azimuthal mode numbers of the same magnitude M but opposite signs: M > 0 for CW and M < 0 for CCW in our geometry [Fig. 3(a)]. Working in cylindrical coordinates and omitting the overall time dependence e−iωCt, the electric field solution of a CW WGM of can be expressed as [26, 27]Fig. 2].
The electric field of the corresponding CCW WGM isEq. (1). Whereas the mode spatial profile of ECW and ECCW are identical by symmetry, the phase of the longitudinal ϕ component has opposite sign for the two modes due to their contrasting direction of propagation. Consequently, the fields possess opposite helicity: the real part of ECCW rotates counter to that of ECW. An interaction sensitive to this electric field helicity will depend on the direction of the propagating light.
We quantify the degree of helicity by first defining a position-dependent unit vector ê⊥(ρ, z) perpendicular to ϕ̂ and parallel to the transverse electric field components. The basis vectors for positive (+1) and negative (−1) helicity can be writtenFigure 2 shows finite-element calculations of the mode profile and helicity for representative TE and TM ring resonator modes.
Because of the symmetries underlying Eqs. (1) and (2), the P values for a pair of counter-propagating WGMs have equal magnitude but opposite sign at each position. Consequently, just one function P(ρ, z) is necessary to describe the mode pair. If we denote the value of P for the CW mode as p, then the P value for the CCW mode is −p at the same position. The components E± for the two modes can be written as a functions of p:
Although the electric field of a WGM possesses local helicity, it is not circularly polarized in the conventional sense. For circularly polarized plane waves, both real and imaginary components of the electric and magnetic fields are transverse to the propagation direction. For a WGM, the helicity axis ê⊙ is transverse to the propagation direction ϕ̂. Despite this distinction, in the dipole approximation, a field E with large P can excite the same optical transitions as circularly polarization. This phenomenon has been observed experimentally for an axisymmetric bottle resonator, in which different Zeeman sub-levels of a Rubidium atom participate in cavity QED ; it has also been inferred from the linewidth broadening of atoms in nanofiber traps [21, 22]. Here, we consider this phenomenon in a fully solid-state integrated photonics implementation consisting of a WGM resonator coupled to a QD with two Zeeman sublevels.
4. Helicity-sensitivity in cavity QED – a linearized master equation approach
We construct a cavity QED model for an axisymmetric microcavity coupled to transitions sensitive to optical helicity. The cavity is modeled as a single resonance of frequency ωC, with two nominally degenerate counterpropagating modes. The system is excited by fields from either direction of frequency ωp, assumed to be close to ωC. Modes propagating forward through the waveguide couple only to one mode of the cavity (the CW mode), while backward propagating modes couple to the other (the CCW mode). The Hamiltonian for this system is
Since the a and b modes are counter-propagating, their respective electric field mode functions have opposite helicity P at all positions. This can be exploited to create non-reciprocal transmission at certain frequencies in the waveguide by having the cavity interact with a system sensitive to the helicity of the electric field. As in  and , we consider a V-type three-level system consisting of a ground state, |g〉, and two excited states, |e1〉 and |e2〉 with transitions to each excited state driven by opposite electric field helicity [Fig. 3(b)]. Such a system could be realized by a quantum dot with a Zeeman-like splitting between spin-polarized excitonic states (Sec. 7). The QD is placed at a point (ρ′, ϕ′, z′) within the evanescent field of the cavity where E is strong and possesses a high degree of helicity (|p| ≈ 1) along the quantization axis of the QD.
Assuming the incident optical power is low and the QD is weakly excited, the two transitions can be approximated as independent (two separate 2-level systems instead of a 3-level system). The Hamiltonian for a QD with zero-splitting excitation energy of ωQD is
The light-matter interaction is described to lowest order by the dipole HamiltonianEqs. (5) and (6) and the boson operators for the optical modes:
We adopt a master equation approach and input-output formalism to account for waveguide coupling and system loss . Using the total Hamiltonian ℋ = ℋC + ℋQD + ℋint, the time evolution of the density matrix ρ for the system is given by the master equation
We seek a steady-state solution to the master equation. Assuming again that the QD is weakly excited by low input power, the fermionic QD operators can be treated as approximately bosonic . From the master equation, we obtain a set of linear equations for the time derivatives of operator expectation values which can then be solved directly for steady-state solutions of the linearized master equation.
We consider the observed optical transmission and reflection in two cases: light input in the forward (a) direction, and light input in the reverse (b) direction. Using the input-output formalism  for this linearized model, when light is input to the waveguide in the forward direction, the normalized forward transmission Tf and the normalized reflection Rf are29] and experiments . The phase difference between g and h, ϑ, has little effect on the conclusions; it is chosen to be π/4 for simplicity.
5. Optical non-reciprocity in axisymmetric cavity QED
To illustrate the directional asymmetry in this model, we first consider an idealized case with no backscattering (h = 0) and perfect mode helicity (|p| = 1). Without these forms of directional mixing, the forward and backward transmissions are fully decoupled; Tf and Tb depend only on the a and σ1 fields, or the b and σ2 fields, respectively. In analogy with the standard Jaynes-Cummings model, each propagating mode hybridizes with its coupled transition to form an independent pair of exciton-polariton eigenstates [Fig. 4(a)]. When the excited state energy splitting δ12 is zero, the energy eigenvalues of the two polariton pairs are equal and the Tf and Tb spectra are characterized by identical dips in the transmission at detuning values of approximately ±g0 [Fig. 5(a)]. There is no difference between forward and backward transmission and the optical transmission is reciprocal.
For nonzero energy splitting δ12, the forward and backward transmission spectra are in general not equal [Fig. 5(b)]. Increasing the spitting causes the cavity modes and relevant QD excited states to go off resonance: one polariton branch becomes more cavity-like and the other more exciton-like. Directional asymmetry in coupling leads to distinct spectral properties from the cavity QED model and non-reciprocal optical transmission.
The emergence of optical non-reciprocity in this model can be explained by time reversal symmetry breaking of the polarization-sensitive Zeeman excited states. If the excited states were time-reversal symmetric, the cavity QED model would be insensitive to direction (Tf = Tb). Since the time reversal symmetry is broken by a magnetic field generating non-zero δ12, the forward and backward directions are inequivalent and optical non-reciprocity is possible. Optical non-reciprocity can be further understood by examining the Hamiltonian under the time reversal operation. Although the magnetic field changes sign under time reversal, the energy splitting between excited states does not since the spins also reverse. The helicity of the WGM field changes sign, however, since the winding motion of the electric field inverts. As a result, p → −p and the interaction Hamiltonian is given by Eq. (13), except with σ1 and σ2 interchanged. As long as δ12 = 0, the Hamiltonians are equivalent and transmission is reciprocal. If there is an energy splitting, however, the symmetry is broken and the two directions are inequivalent.
Realistically, due to the surface roughness of a real cavity and mode scattering from the QD, h ≠ 0 and |p| ≠ 1; directional input does not exclusively couple to just one excited state. Instead of two dips, Tf and Tb exhibit four dips at the eigenenergies of the four exciton-polaritons [Figs. 5(c)–5(d)]. Although still non-reciprocal for non-zero δ12, spectral differences are diminished, particularly near the exciton-like polariton. For zero helicity (p = 0) when δ12 = 0, the model predicts three eigenvalues characteristic of an axisymmetric WGM coupled to a 2-level system, which would manifest as three dips in Tf and Tb [24,25,32]. As |p| increases, the cavity-like polariton in the eigenvalue spectrum divides into two branches for intermediate helicities before reaching the ideal directional degeneracy at |p| = 1 [Fig. 4(b)].
6. Single-mode optical isolation from directional asymmetry
The directional asymmetry shown in Figs. 5(b)–5(d) suggests that this system can function as a coherent optical diode . The requirements for a non-reciprocal system to function as an optical isolator have been discussed recently [33, 34]. First, there must exist a mode of the waveguide for which the backward transmission is near zero while the forward transmission is near unity. This is satisfied here by tuning the parameters so that Tf ≃ 1 and Tb ≃ 0 at the same cavity-probe detuning. The second requirement is that transmission for all backward input modes must be blocked. This requirement is satisfied here by using a single-mode waveguide, as additional frequency modes not resonant with the cavity would be unblocked. This system can be classified as a single-mode optical diode; it provides optical isolation for a narrow-band of frequencies for a particular propagating TE or TM mode.
Here, we consider the system parameters that allow non-reciprocal isolator-like performance. For ideal directional contrast (defined as Tf/Tb), the system parameters must be optimized for the highest forward transmission at the lowest possible value of the backward transmission. Parameters that can be controlled experimentally include: ΔC by tuning the frequency of input light, δ12 by modifying an external magnetic field, and κex by adjusting the distance between the waveguide and the cavity. The remaining parameters, g0, γ, κi, p, and h are considered fixed by the fabrication process and are treated as constants. For this analysis, we seek to maximize Tf/Tb in the three-dimensional parameter space (ΔC, δ12, κex).
6.1. The ideal case
Ideal optical diode behavior requires that for a particular light frequency ωp, Tf ≈ 1 and Tb = 0. To obtain simple expressions for the parameters required for diode behavior, we first focus on the ideal case with no directional mixing, h = 0 and p = 1. The forward and backward transmission can be written analytically asEq. (17), restrictions on two of the controllable parameters, chosen arbitrarily to be δ12 and ΔC, can be derived so that Tb = 0:
With the restrictions on δ12 and ΔC given by Eqs. (18)–(19), Tf can be optimized with the last controllable parameter, κex. By examining the forward transmission as a function of κex with δ12 and ΔC set to enable zero backward transmission, plotted in Fig. 6(a), it can be seen that that high forward transmission is achievable when κex is is in the regime κi < κex < κi + γ/2 even for a low value of the coupling strength g0. Additionally, this regime requires a lower value of δ12 for high Tf than the the the regime κex > κi + γ/2 as can be seen in Fig. 6(b). For the choice of parameters (g0, κi) = (20, 5)γ and ϑ = π/4, the optimum value of κex is 5.2γ, giving a forward transmission of 97.5% with a required δ12 and ΔC of 30.3γ and −13.8γ respectively.
Choosing to find restrictions on a different pairs of parameters other than δ12 and ΔC to satisfy Tb = 0 results in the same conclusion. One can visualize a one-dimensional line in the three-dimensional parameter space spanned by κex, δ12, and ΔC which give Tb = 0. The plots in Fig. 6(b) can be interpreted as the projections of this line on the κex-δ12 plane for various values of g0. Along the line there exists an optimal point at which Tb is maximum and the optical isolation is greatest.
6.2. The non-ideal case
Having established that near-perfect single-mode optical isolation can be achieved with ideally tuned parameters, we briefly consider performance under more realistic, non-ideal conditions (h ≠ 0, |p| ≠ 1). Backscattering by the QD will couple the CW and CCW modes through increased h and degradation of local polarization will manifest as reduced |p|. Although in practice backscattering from a single small QD is not expected to be significant relative to surface roughness , the local polarization can be modified. Here, we show that isolation performance can still be obtained for reasonable variations from ideal parameters. In Figs. 5(c)–5(d), it can be seen that the forward transmission is impaired by mode mixing and non-unity helicity of the electric field, yet the spectra still display a high contrast Tf/Tb. Although the transmission cannot be written in a simple analytic form as Eq. (17), we may use our results for the optimal values of δ12, ΔC, and κex from analysis of the ideal case as initial values in a numerical optimization of the the contrast in the non-ideal case.
Using numerical methods, we find that Tb = 0 is still achievable along a line of values in the space spanned by κex, δ12, and ΔC. Again, there exists an optimal point along this line at the contrast is greatest. In general, for increasing h or decreasing p, the optimal value of κex increases whereas the optimal value of δ12 changes little from the ideal case.
The isolation contrast is plotted in Fig. 7(a) as a function of κex and δ12, with ΔC set to the cavity-like dip in the backwards transmission spectrum. The projection of the line of values that enable Tb = 0 in the κex-δ12 plane can be seen as well as the optimal point where the isolation contrast is greatest. If κex and δ12 can be tuned to within about 30% of their optimal values, isolation of over 30 dB can be achieved with a forward transmission of over 70% [Fig. 7(b)].
7. Feasibility and implementation
The formal conditions for optical isolation in the ideal ring resonator cavity QED system are κex > κi and . These translate to saying that critical waveguide-cavity coupling must be achievable and the light-matter coupling should be in a high cooperativity regime. Strong QD-cavity coupling is not required for optical isolation. Both requirements are achievable with current cavity QED methods . Early work with nanocrystal QDs demonstrated typical coupling ħg > 10 μeV in a WGM cavity and ħγ ∼ 6 μeV , and self-assembled QDs can have even more favorable parameters . These numbers suggest that the required cavity parameters can be achieved with WGM resonators of quality Q ≳ 105, well within the state-of-the-art for lithographically-processed rings [35, 37] and freestanding microresonators .
A more challenging requirement for realizing optical isolation in axisymmetric cavity QED is achieving helicity-sensitive eigenstates dominant over QD anisotropy . Additionally, the splitting between excited states must be on the order of the cavity-QD coupling. For typical QDs, meeting these conditions necessitates a magnetic field on the order of 1 T , aligned with the helicity axis ê⊙ at the site of the QD. Although sufficient fields can easily be applied externally in the lab, a miniaturized integrated photonics approach is less straightforward. Obtaining the required Zeeman splitting may be feasible using diluted magnetic semiconductor QDs [41, 42], which could provide optically-induced splitting for excitons of greater than 10 T .
We have analyzed direction-dependent transmission through a waveguide coupled to an axisymmetric resonator interacting with a helicity-sensitive quantum dot. Lifting the excited-state degeneracy with a Zeeman-type splitting induces non-reciprocal transmission. With properly tuned parameters, optical isolation can be achieved for light in a coherent single-photon regime, which could be useful for coherent optical diodes and phase shifting in quantum photonics.
This work was funded by the Institute for Sustainability and Energy at Northwestern. E. J. L. is part of the NSF IGERT ( DGE-080168), and N. P. S. is an Alfred P. Sloan Research Fellow.
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