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Optical bistability and four-wave mixing with a single nitrogen-vacancy center coupled to a photonic crystal nanocavity in the weak-coupling regime

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Abstract

We explore optical bistability and degenerate four-wave mixing of a hybrid optical system composed of a photonic crystal nanocavity, a single nitrogen-vacancy center embedded in the cavity, and a nearby photonic waveguide serving for in- and outcoupling of light into the cavity in the weak-coupling regime. Here the hybrid system is coherently driven by a continuous-wave bichromatic laser field consisting of a strong control field and a weak probe field. We take account of the nonlinear nature of the nitrogen-vacancy center in the Heisenberg-Langevin equations and give an effective perturbation method to deal with such problems in the continuous-wave-operation regime. The results clearly show that the bistability region of the population inversion and the intensity of the generated four-wave mixing field can be well controlled by properly adjusting the system practical parameters. The nanophotonic platform can be used to implement our proposal. This investigation may be useful for gaining further insight into the properties of solid-state cavity quantum electrodynamics system and find applications in all-optical wavelength converter and switch in a photonic crystal platform.

© 2014 Optical Society of America

1. Introduction

In recent years, cavity quantum electrodynamics (CQED) studies light-matter interactions inside a resonator, which provides an ideal platform for quantum optics and few-photon nonlinear optics [1]. Tremendous progress has been made by coupling single quantum dipole emitters to different cavities [25]. Among them, nanoscale photonic crystal (PC) cavities may be promising due to its highly confined ultrasmall mode volume V in the order of the qubic wavelength and ultrahigh quality Q-factor (i.e., a high Q/V ratio) [69]. Moreover, waveguides can be easily incorporated into PCs, thus complex chip-scale systems of coupled PC waveguides and cavities can be produced [1012]. Hence these PC waveguide-coupled nanocavity systems are compatible with large scale integration for the development of complex devices on-a-chip.

On the other hand, nitrogen-vacancy (NV) centers consisting of a substitutional nitrogen atom and an adjacent vacancy have recently emerged as an excellent test bed for solid-state quantum physics experiments and quantum information processing because they possess a combination of the excellent ground-state spin coherence, the NV’s level structure, and the stability of the diamond host lattice [1321]. In nano-size diamond, the NV centers can be integrated into various other systems, for example, plasmonic elements, cells, or planar PC structures [2224]. Combining high-Q PC cavities and NV centers in diamond represents a promising solid-state CQED system, and attracts much attention. Several attempts have been pursued to couple NV centers to cavities such as microsphere resonators [2527], microtoroids [2832], microdisk [33], or PC nanocavities [3444] both theoretically and experimentally. For example, recent experimental investigations by Sar et al. have demonstrated deterministic coupling of single NV centers to high-quality PC nanocavities [34]. Barth et al. have addressed controlled coupling of a single-diamond nanocrystal to a planar PC double-heterostructure cavity [36]. Wolters et al. have developed a scheme to enhance the zero phonon line emission from a single NV center in a nanodiamond via coupling to a PC nanocavity [37]. Yang et al. have investigated quantum state transfer and quantum correlation in a composite system consisting of two NV centers embedded in two spatially separated single-mode nanocavities in a planar PC [38, 42].

Among CQED platforms, the response of the single quantum dipole emitter (e.g., NV defect center) coupled to a PC nanocavity via a nearby photonic waveguide is generally described by the Heisenberg-Langevin equations. As shown in [4547], the Heisenberg-Langevin equations are nonlinear, and it is very difficult to get an analytic solution to these equations. The weak-excitation approximation is widely used in these CQED research, where the dipole emitter will remain mostly in its ground state, i.e., σ̂11(t) ≈ 1 and σ̂22(t) ≈ 0 with σ̂11 and σ̂22 being the population operators of the emitter ground and excited states. By assuming this so-called low-excitation regime (no more than one photon in the system), we can approximate 〈σ̂z(t)〉 ≈ −1/2 for all time (i.e., we can substitute σ̂z(t) with its average value of −1/2), and thus linearize the operator equation (see Eqs. (13), (14) and (15) below). Here, σ̂z(t) = [σ̂22(t) − σ̂11(t)]/2 is the operator of population inversion between the ground state |1〉 and the excited state |2〉 (see Fig. 1). The Heisenberg-Langevin equations are reduced to a set of linear equations. Such a procedure by ignoring the nonlinear nature of the quantum emitter has been commonly adopted in many previous studies [30, 32, 4860]. However, when optical driving power is intermediate (not too low and not too high), the weak-excitation approximation fails and the nonlinear terms need to be considered (see Fig. 2 below).

 figure: Fig. 1

Fig. 1 Schematic structure of optical coupeld system, which is composed of a two-level NV center, a single-mode PC nanocavity and a nearby photonic waveguide serving for in- and outcoupling of light into the nanocavity. The single-mode PC nanocavity containing the NV center is evanescently coupled to the row defect waveguide with the coupling strength κe. The |1〉 ⇔ |2〉 transition of the NV center is coupled to the mode ĉof the PC nanocavity with the coupling strength gcav. κi is the intrinsic loss rates of the nanocavity mode excluding coupling to the NV center and the waveguide. A bichromatic field consisting of a cw control laser and a cw probe laser is injected into the waveguide via grating couplers (see Refs. [67, 68]). Sin and Sout denote the input and the output field in the waveguide. The bubble shows the detailed structure of quantized energy levels and the coupling scheme of the cavity mode for the two-level NV center. |1〉 and |2〉 denote the ground and excited states of the NV center, respectively. A small red sphere shows the position of the NV center.

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 figure: Fig. 2

Fig. 2 The steady-state population inversion σ̄z as a function of optical driving power Pc for four different values of Δ2. The other system parameters used for the simulation are chosen as gcav/2π = 2.25 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, and δ = 0. Curves A–D are for Δ2 = 0, 100, 140, and 180 MHz, respectively.

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In the present work, using the perturbation method we take into account the nonlinear terms such as higher-order moments −2gcavĉσ̂z, gcavĉσ̂12, and gcavĉσ̂21, where gcav is the coupling strength between the NV center and the cavity mode and ĉis the annihilation operator of the PC cavity mode, and study nonlinear optical response of such a hybrid system in the continuous-wave-operation regime. We find that these nonlinear terms can give rise to some interesting phenomena of the PC cavity-waveguide system in the experimentally achievable parameter range, such as optical bistability (OB) for the population inversion and the nonlinear degenerate four-wave mixing (FWM) process. Besides, the OB and FWM works in the weak-coupling regime where the coupling strength between the NV center and the cavity is smaller than the cavity decay rate and this condition allows more a practical parameter range for solid-state materials on a compact integrated nanophotonic platform.

It should be pointed out that many investigations of the OB and FWM effects in various CQED systems have been reported (for a review, see [61]). The OB of a Fabry-Perot resonator was analyzed experimentally [62] and theoretically [63] in the context of an ensemble of laser cooled atoms, where the input-output response of the atom-cavity system is measured. However, the unusual dependence of the population inversion related to the OB and degenerate FWM effect on optical driving power was not considered before based on the weak coupling between a single NV center and a PC nanocavity. Additionally, we analyze the nanophotonic platform for possible experimental realization of these effects in the continuous-wave-operation regime. We note that the bichromatic driving of a solid-state cavity QED system has been proposed and analyzed by the Vučković group [6466] from different perspectives. However, in these studies, they looked experimentally and theoretically into the problem of the spectral features from the cavity emission.

The rest of the paper is organized into four parts as follows. In Sec. II, we present the theoretical model and establish the corresponding evolution equations for the cavity and the NV center. In Sec. III, we discuss in details optical bistable behaviors of the papulation inversion in the steady-state case of the coupled NV center-cavity system. In Sec. IV, under the driving of a strong control field and a weak probe field, we analyze the nonlinear four-wave mixing process using the perturbation method. Finally, we conclude in Sec. V.

2. Proposed model and evolution equations

2.1. Hamiltonian

A schematic description of optical coupled system under investigation is illustrated schematically in Fig. 1. In the present scheme, an NV center is confined in a single-mode PC nanocavity, and the nanocavity is side-coupled to an optical waveguide serving for in- and outcoupling of light into the nanocavity, with a geometry similar to the one reported in Refs. [34,35,67]. Here, the nanocavity is formed by imbedding spatial point defects into a PC platform with the cavity frequency tunable by changing the geometrical parameters of the defects. The cavity is assumed to have a single mode that couples only to the forward propagating fields [48, 49]. The waveguide modes are formed by row defects. The NV center is a point defect in the diamond lattice, which consists of a substitutional nitrogen atom (N) plus a vacancy (V) in an adjacent lattice site. The NV center addressed in this study is negatively charged with two unpaired electrons located at the vacancy, usually treated as electron spin-1 system [13]. The spin-spin interaction leads to an energy splitting Dgs = 2.88 GHz between the ground states of electronic spin triplet |3A2, ms = 0〉 and |3A2, ms = ±1〉 (the quantum number ms is the electron-spin projection along the NV axis). The optical transitions in the NV center are spin-conserving. Thus, we can choose the electronic states |3A2, ms = 0〉 and |3E, ms = 0〉 of the NV center as the ground state |1〉 and the excited state |2v, respectively. Using the two-dimensional (2D) planar photonic crystal structure, the initial input field, which we call the driving field, is guided by the waveguide in the crystal plane. The |1〉 ⇔ |2〉 transition of the NV center is coupled to the cavity mode with resonance frequency ωcav and coupling strength gcav (also called vacuum Rabi frequency). For dipole-type NV center-field mode interaction inside the cavity, this coupling strength is related to the transition dipole moment μ21 and the location r of the NV center and effective cavity mode volume Veff, etc. The Hamiltonian of a cavity-coupled NV center in the presence of a driving field is given by [67, 68]

=h¯ωNVσ^22+h¯ωcavc^c^+ih¯gcav(c^σ^21c^σ^12)+ih¯κe[Sin(t)c^Sin*(t)c^],
where the rotating-wave approximation (RWA) and the electric-dipole approximation (EDA) have been made. In the above equation, the first, second and third terms account for the energy of the coupled cavity-NV center system and the fourth term represents the driving of the cavity by an external laser field [67]. In the derivation of the Hamiltonian, the energy of the ground state |1〉 is set as zero to allow for simplicity. h̄ωNV is the energy of the electronic state |2〉, that is to say, ωNV is the frequency of the NV center’s optical transition between the ground state |1〉 and the excited state |2〉. The symbols σ̂mn = |m〉〈n| (m, n = 1, 2) for mn, are the electronic transition or projection operators between the states |m〉 and |n〉 and σ̂mm = |m〉〈m| (m = 1, 2) represent the electronic population operators involving the levels of the NV center [see also the bubble of Fig. 1]. ĉand ĉ are the bosonic annihilation and creation operators of the cavity mode. The parameter κe is the coupling rate between the cavity and the forward propagating mode of the waveguide, and Sin(t) is the driving laser propagating in the waveguide. For the driving laser field, these two cases are considered individually in the following.

Transforming the Hamiltonian (1) into the rotating frame at the frequency of the input laser field ωl by using [46, 47]

free=h¯ωl(σ^22+c^c^),
U(t)=eifreet/h¯=eiωlt(σ^22+c^c^),
rot=U(t)U(t)iU(t)U(t)t=U(t)(free)U(t),
we can derive the resulting effective Hamiltonian as follows:
rot=h¯Δ1σ^22+h¯Δ2c^c^+ih¯gcav(c^σ^21c^σ^12)+ih¯κe[Sin(t)eiωltc^Sin*(t)eiωltc^],
where Δ1 = ωNVωl and Δ2 = ωcavωl are respectively the detunings of the NV center resonance frequency ωNV and the PC cavity resonance frequency ωcav from the laser field ωl.

Case (i): When we consider a monochromatic continuous-wave (cw) driving field with the carrier frequency ωp and the field amplitude sp, i.e., a cw probe field Sin(t) = spept, we can choose ωl = ωp (in a frame rotating at the probe frequency ωp). In this case, the effective Hamiltonian can be expressed by

rot=h¯Δ1σ^22+h¯Δ2c^c^+ih¯gcav(c^σ^21c^σ^12)+ih¯κe(spc^sp*c^),
with Δ1 = ωNVωp and Δ2 = ωcavωp. In the above, sp is the field amplitude of the monochromatic driving laser propagating in the waveguide, which is normalized to a photon flux at the input of the cavity and directly related to the power propagating in the waveguide by Pp=h¯ωpsp2.

Case (ii): When we consider a bichromatic cw driving field, i.e., a control field and a probe field Sin(t) = scect + spept, where ωc (ωp) are the carrier frequency of the control (probe) field and sc (sp) is the field amplitude, we can choose ωl = ωc (in a frame rotating at the control frequency ωc). Consequently, under the above transformation, the resulting Hamiltonian can be given by

rot=h¯Δ1σ^22+h¯Δ2c^c^+ih¯gcav(c^σ21c^σ^12)+ih¯κe[(sc+speiΩt)c^H.C.],
where H.C. represents the Hermitian conjugate. We have defined Δ1 = ωNVωc, Δ2 = ωcavωc, and Ω = ωpωc, respectively. Note that, if δ = ωNVωcav denoting the NV center-cavity detuning, then Δ1 = Δ2 + δ. In the above, sc and sp are the field amplitudes of the bichromatic driving laser propagating in the waveguide, which are normalized to a photon flux at the input of the cavity and directly related to the power propagating in the waveguide by Pc=h¯ωcsc2 and Pp=h¯ωpsp2, respectively. Unlike case (i) above, the time factors in Eq. (7) cannot be eliminated.

2.2. Heisenberg-Langevin equations

Our analysis is based on the Heisenberg-Langevin equations that are derivable from the Hamiltonian of Eq. (7). Including losses in both the cavity and NV center, as well as cavity excitation, we apply the Heisenberg-Langevin formalism to attain the Heisenberg-Langevin equations of motion as follows

dc^dt=gcavσ^12(iΔ2+κi/2+κe/2)c^+κe(sc+speiΩt)+f^c,
dσ^11dt=γsponσ^22gcavc^σ^21gcavc^σ^12+f^11,
dσ^22dt=γsponσ^22+gcavc^σ^12+gcavc^σ^21+f^22,
dσ^12dt=(iΔ1+γspon/2)σ^122gcavc^σ^z+f^12,
where σ̂z = (σ̂22σ̂11) /2 stands for the operator of the population inversion. κi is the cavity intrinsic decay rate, which is related to the intrinsic quality factor Qi by κi = ωcav/Qi. κe is the waveguide-cavity coupling rate, which is related to the coupling quality factor Qe by κe = ωcav/Qe [1012]. The total cavity decay rate (cavity linewidth) is κ = κi +κe. γspon is the decay rate of the NV center. The operators fc, 11, 22, and 12 are the quantum noise operators with 〈c〉 = 0, 〈11〉 = 0, 〈22〉 = 0, and 〈12〉 = 0. In this work, we are interested in the mean response of the coupled system [69], so the operators can be reduced to their expectation values, i.e., c(t) ≡ 〈ĉ(t)〉, c*(t) ≡ 〈ĉ(t)〉, σ11(t) = 〈σ̂11(t)〉, σ22(t) = 〈σ̂22(t)〉, σz(t) = 〈σ̂z(t)〉, σ12(t) = 〈σ̂12(t)〉, and σ12*(t)=σ^21(t). In these situations, we reduce the operator equations to the mean value equations and drop the above quantum noise terms because 〈c〉 = 0, 〈11〉 = 0, 〈22〉 = 0, and 〈12〉 = 0. The Heisenberg-Langevin equations then become
dcdt=gcavσ12(iΔ2+κi/2+κe/2)c+κe(sc+speiΩt).
dσ11dt=γsponσ22gcavcσ12*gcavc*σ12,
dσ22dt=γsponσ22+gcavc*σ12+gcavcσ12*,
dσ12dt=(iΔ1+γspon/2)σ122gcavcσz.
The derivation of Eqs. (13)(15) uses the well-known mean-field (or so-called factorization) assumption 〈ÂB̂〉 = 〈Â〉〈〉 [69]. Note that, Eqs. (13) and (14) can be incorporated and rewritten as dσz/dt=γspon(σz+1/2)+gcavc*σ12+gcavcσ12*. This set of coupled equations are ordinary nonlinear differential equations of complex functions instead of operators, and describe the time evolution of the cavity-NV center coupled system.

3. The bistable behavior of the population inversion in the steady-state case

Following standard methods from quantum optics, under the condition that the control laser field is much stronger than the probe laser field, we can use the perturbation method to deal with Eqs. (12)(15). The control laser field provides a steady-state solution (, σ̄12, and σ̄z) of the coupled system with respect to the probe laser field, while the probe laser field is treated as the perturbation of the steady state. In order to linearize the dynamics of the cavity-NV center system, the total solution of the intracavity field, the coherence of the NV center, and the corresponding population inversion under both the control field and the probe field can be written as the forms c = + δc, σ12 = σ̄12 + δσ12, and σz = σ̄z + δσz around the steady state values (, σ̄12, σ̄z). Here δc, δσ12, and δσz describe the fluctuations around the steady state values.

In the following, the steady state without the perturbation will be studied. The steady-state solution of Eqs. (12)(15), in which all time derivatives vanish and sp → 0, can be obtained as

c¯=F1κescF1F22gcav2σ¯z,
σ¯z=12γsponγspon+2gcav2(1F1+1F1*)|c¯|2,
σ¯12=2gcavF1c¯σ¯z,
where F1 = iΔ1 + γspon/2 and F2 = iΔ2 + κi/2 +κe/2.

Since the above coupled equations (16) and (17) are cubic in σ̄z, the system may be characteristic of the OB for a certain parameter range. To demonstrate our numerical results, we briefly demonstrate the experimentally accessible parameters in this hybrid structure. We focus our attention on the dipole transition |1〉 ⇔ |2〉 with a zero phonon line at λ = 637 nm of the diamond NV center. The maximal coupling strength gcavmax between the PC cavity mode and the NV center can be calculated by the relation gcavmax=|E(r)/Emax|3πc3γspon/(2ωc2nencVm) [39,40], where Vm is the volume of the nanocavity electromagnetic mode, γspon the spontaneous decay rate of the excited state |2〉 of the NV center, and c the light speed in free space. nc and ne are the PC nanocavity and diamond nanocrystal refractive indexes, respectively. |E⃗ (r)/E⃗max| stands for the normalized electric field strength at the NV center’s location r. Experimental evidence involving coupling waveguides to similar nanocavities have shown efficiencies of 90% [70]. Experimentally, the coupling parameter κe can be controlled in fabrication, for example, by tuning the waveguide-nanocavity gap [24]. The design with a Q0 factor of 1.5×105 (Q0 here is an order of magnitude less than the value reported from [40], therefore κi = ωc/Q0 = 1.6 GHz) and a mode volume Vm of ∼ 0.52(λ/nc)3 ∼ 4.4 × 10−21 m3 with nc ≃ 3.3 and ne ≃ 2.4 can reach the weak-coupling CQED regime (gcav, κi, κe, γspon)/2π = (2.25, 1.6, 8, 0.013) GHz [40]. In the figures of this paper, unless otherwise stated, we will always consider the above-mentioned parameters.

By solving Eqs. (16) and (17) numerically, in Fig. 2 we displays the steady-state value of the population difference σz as a function of optical driving power Pc for four different values of the detuning Δ2 = 0, 100, 140, and 180 MHz when the NV center and cavity frequencies is resonant δ = 0. It is clearly shown that the system exhibits bistable behavior when the driving power is large enough. In this case, σ̄z has three real roots. The largest and smallest roots are stable, and the middle one is unstable, which corresponds to the blue dashed lines in Fig. 2. It is easy to see from Fig. 2 that the bistable threshold is increased gradually and the area of the hysteresis loop becomes narrower as Δ2 increases. In Fig. 3, we show the influence of the cavity-NV center coupling strength gcav on the behavior of OB, while keeping all other parameters fixed. It is found that, with increasing gcav from 1.75, 2.25, 2.75, to 3.25 GHz, the threshold of OB increases progressively and the area of the hysteresis loop becomes wider. In Fig. 4, we show the influence of the cavity-NV center detuning δ on the behavior of OB. The bistable threshold is increased gradually and the area of the hysteresis loop becomes narrower as δ increases from 0, 80, 160, to 190 MHz, which is very similar to Fig. 2.

 figure: Fig. 3

Fig. 3 The steady-state population inversion σ̄z as a function of optical driving power Pc for four different values of gcav. The other system parameters for the simulation are chosen as κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, Δ1 = Δ2 = 100 MHz and δ = 0. Curves A–D are for gcav = 1.75, 2.25, 2.75, and 3.25 GHz, respectively.

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 figure: Fig. 4

Fig. 4 The steady-state population inversion σ̄z as a function of optical driving power Pc for four different values of δ. The other system parameters for the simulation are chosen as gcav/2π = 2.25 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, and Δ2 = 0. Curves A–D are for δ = 0, 80, 160, and 190 MHz, respectively.

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From Figs. 24, we can also note that the value of the population difference σz is confined between −1/2 and 0. For the case of low excitation power, the approximation σz ≈ −1/2 is valid. In this case, the nonlinear nature of the NV center can be neglected safely. Therefore, at low excitation power this approximation matches the actual output quantitatively and successfully explains a lot of theoretical and experimental observations in the linear regime. However, with increasing the driving power Pc, this model fails completely, as the approximation σz ≈ −1/2 becomes invalid. For sufficiently high driving power, however, one can approximate σz → 0, and Eq. (15) reduces to 12/dt = −(iΔ1 +γspon/2)σ12. In view of these factors, one needs to retain the dynamics of the σz term in the Heisenberg-Langevin equation when the monochromatic driving field is intermediate (not too low and not too high).

Similar bistable steady-state behavior of the intracavity photon number has been presented recently in a cavity optomechanical system with a Bose-Einstein condensate [71] or a quantum-degenerate Fermi gas [72]. Our OB system here is totally different from the conventional cavity optomechanical system. In conventional cavity optomechanics, the photon number inside the cavity is determined by the driving laser. In other words, the driving laser field directly affects the coupling between the cavity photons and the mechanical oscillator. However, for the single NV center coupled to the PC cavity, the population inversion of the NV center is dependent on the driving field, which can take the value between −1/2 and 0 via the interaction between the NV center and the cavity mode. The NV center-cavity coupling is independent on the driving field. On the other hand, this OB is a new addition to the family of OB in typical atomic systems in that there are two possible steady states for a single input filed. While the OB in a laser-driven atomic medium has been suggested as a mechanism for an all-optical switching, similarly our OB can provide a candidate for a controlled switching device, since its population inversion for the lower stable branch and the upper stable branch can be manipulated by the driving laser. However, it should be pointed out that different points exist between them. The OB in the atomic medium mainly refers to the absorption or dispersion-type OB in the input-output intensity, while our OB refers to the bistable behavior of the population inversion.

4. The nonlinear four-wave mixing process under the driving of a strong control field and a weak probe field

Now we turn to consider the perturbation made by the probe field. By substituting c = + δc, σ12 = σ̄12 + δσ12, and σz = σ̄z + δσz into Eqs. (12)(15) and retaining only first order terms in the small quantities δc, δσ12, and δσz, we then obtain

dδcdt=gcavδσ12(iΔ2+κi/2+κe/2)δc+κespeiΩt,
dδσzdt=γsponδσz+gcavc*¯δσ12gcavδc*σ¯12+gcavc¯δσ12*+gcavδcσ¯12*,
dδσ12dt=(iΔ1+γspon/2)δσ122gcavc¯δσz2gcavδcσ¯z.

Next we solve the problem for the driving field (in the rotating frame) δsin = speiΩt. For a given Ω = ωpωc, we use the ansatz

δc=c+eiΩt+ceiΩt,
δσz=σz+eiΩt+σzeiΩt,
δσ12=σ12+eiΩt+σ12eiΩt.

If sorted by rotation terms e±iΩt, this yields six equations about c+, c, σz+, σz, σ12+, and σ12− as follows

D1σ12+2gcavc¯σz+2gcavcσ¯z=0,
gcavσ12+D2c=0,
gcavσ12++D3c+κesp=0,
D4σ12++2gcavc¯σz++2gcavc+σ¯z=0,
(γsponiΩ)σz++gcavc*¯σ12++gcavc*σ¯12+gcavc¯σ12*+gcavc+σ¯12*=0,
(γspon+iΩ)σz+gcavc*¯σ12+gcavc+*σ¯12+gcavc¯σ12+*+gcavcσ¯12*=0,
where D1 = iΔ1 + iΩ + γspon/2, D2 = iΔ2 + iΩ + κi/2 + κe/2, D3 = iΔ2iΩ + κi/2 + κe/2, and D4 = iΔ1iΩ + γspon/2, respectively. , σ̄12, and σ̄z are decided by Eqs. (16)(18). It is difficult to obtain the analytical solutions of the above equations (25)(30) except under very special conditions and hence we will resort to numerical solutions of them.

The output field transmitted through the coupled system can be obtained using the standard input-output relation Sout(t)Sin(t)=κec [73, 74] for the propagating field, where Sout (t) is the output field amplitude. We have

Sout(t)=(sc+κec¯)eiωct+(sp+κec+)ei(ωc+Ω)t+κecei(ωcΩ)t=(sc+κec¯)eiωctControlfield+(sp+κec+)eiωptProbefield+κecei(2ωcωp)tDegenerateFWMfield,
with the coefficients sc+κec¯, sp+κec+, and κec being the optical responses at the control frequency ωc, the probe frequency ωp, and the generated frequency 2ωcωp, respectively. The transmission of the probe laser field is defined as tp=1+κec+/sp. Some previous works have used |tp|2 to study dipole-induced transparency (DIT) [48]. Also, it is easy to see from Eq. (31) that the output field contains two input components (the control field ωc and the probe field ωp) and one new degenerate FWM component at frequency ωf = 2ωcωp [75]. Then, the normalized FWM intensity in terms of the probe field, which is also called the FWM efficiency, can be defined as
IFWM=|κecsp|2.

In what follows, we present a discussion about the intensity of the FWM. In Fig. 5, we plot the normalized intensity of the generated FWM field as a function of the detuning Δ2 between the cavity field and the control field for Pc = 30 pW and Pp = 3 pW, respectively. It can be seen that there is a peak value located at Δ2 = 0, which is caused by the effect of DIT [48]. At the same time, there is a transparency window in the transmission spectrum of the probe field as shown in Fig. 5. The normalized intensity of the generated FWM field versus the power of the control driving field Pc is shown in Fig. 6. We observe that, due to the nonlinear nature of the NV center, the intensity of the generated FWM first increases to a maximum point, then decreases rapidly with Pc increasing. Finally it approaches to a zero steady-state value for sufficiently high driving Pc.

 figure: Fig. 5

Fig. 5 The normalized FWM intensity versus the detuning Δ2. The other system parameters for the simulation are chosen as Pc = 30 pW, Pp = 3 pW, gcav/2π = 2.25 GHz, Ω/2π = 1 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, and δ = 80 MHz, respectively.

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 figure: Fig. 6

Fig. 6 The normalized FWM intensity versus optical driving power Pc. The other system parameters for the simulation are chosen as Pp = 3 pW, gcav/2π = 2.25 GHz, Ω/2π = 1 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, Δ2 = 0, and δ = 80 MHz, respectively.

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The reason for the above results can be interpreted by analyzed the nonlinearity of the system in the following three steps:

  1. (i) For the low-power driving field (no more than one photon in the system), the NV center will remain mostly in its ground state |1〉, and we can approximate σ̄z → −1/2 as can be verified in Figs. 24. In this case, the nonlinear nature of the NV center is negligible and can be neglected.
  2. (ii) However, with increasing the driving power, the nonlinear nature of the NV center gradually boosts up and needs to be taken into account [45]. Furthermore, the approximation σ̄z → −1/2 becomes invalid.
  3. (iii) For sufficiently high driving power, we can approximate σ̄z → 0. Similarly, the nonlinear nature of the NV center is negligible. According to Eqs. (16)(18), we have the results σ̄12 = 0 and c¯=κesc/F2. Inserting them into Eqs. (25)(30) and performing some algebra, it is found that the expression c = 0 ⇒ IFWM = 0 which is in good agreement with Fig. 6 for sufficiently high driving power Pc.

Before ending this section, we would like to address more technical aspects about the NV center in this hybrid system. We consider a simplified model for the NV center consisting of a ground-state level (|1〉) and an excited-state level (|2〉). For a more general treatment, the NV center has a relatively complicated structure of excited states [13], which includes six excited states defined by the method of group theory as |A1=12(|E,ms=+1|E+,ms=1), |A2=12(|E,ms=+1+|E+,ms=1), |Ex〉 = |X, ms = 0〉, |Ey〉 = |Y, ms = 0〉, |E1=12(|E,ms=1|E+,ms=+1), and |E2=12(|E,ms=1+|E+,ms=+1) with |E+〉, |E〉 being orbital states with angular momentum projection ±1 along the NV axis, and |X〉, |Y〉 being orbital states with zero projection of angular momentum. The spin-orbit interaction splits the pair (|A1〉, |A2〉) from the others by at least about 5.5 GHz. The spin-spin interaction increases the energy gap and produces a gap of 3.3 GHz between |A1〉 and |A2〉. At the same time, the ground states |3A2, ms = 0, ±1〉 are associated with the orbital state |E0〉 with zero projection of angular momentum (for simplicity, the spatial part of the wavefunction is not explicitly written). The optical transitions in NV centers are spin-conserving, but could change electronic orbital angular momentum depending on the photon polarization [20]. That is to say, spin-conserving transitions between the ground state and six different excited states can be driven optically. In the limit of zero strain, the |A2〉 state is robust due to the stable symmetric properties, and decays to the ground-state sublevels |3A2, ms = −1〉 and |3A2, ms = +1〉 with radiation of σ+ and σ circular polarizations, respectively. On the one hand, as we all know, only when the frequency of the cavity mode and the transition frequency of the NV center between two levels are same (resonance) or close (near resonance), a strong coherent interaction of the cavity mode with the NV center appears. Therefore, in the case of a single-mode cavity field, the NV center can be regarded as two-level system owing to a large level space. On the other hand, by properly choosing the polarization of the cavity mode according to selection rules [20], the other levels can be well decoupled. Therefore, we believe that the present model can provide a qualitative illustration of the observable OB and FWM properties.

5. Conclusions

In summary, we have theoretically investigated OB and FWM effects occurring in a hybrid optical system composed of a PC nanocavity, a single NV center and a PC waveguide by taking into account nonlinear terms. It is shown that, in the weak-coupling regime and the experimentally available parameter range, the tunable bistable behavior of the population inversion and the efficient generation of the FWM signal can be achieved due to the nonlinearity of the system. The results obtained here may be useful for gaining further insight into the properties of solid-state CQED systems and find applications in an all-optical wavelength converter and switch in a PC platform.

This photonic device combines the advantages of photons (low decoherence rates and high velocities), waveguides (low-loss and long-range photon transfer with a controllable group velocity), NV centers (long electronic spin decoherence time at room temperature), and PC cavities (small volumes and high-quality Q-factors). In particular, the great attraction of the solid-based CQEDs is that they can be monolithically fabricated and integrated into the large-scale arrays. Finally, it is worth pointing out that the present theory can also be applied to other similar systems, such as a single quantum emitter (e.g., atom, molecule, or quantum dot) positioned close to a microtoroidal resonator [2, 3] with the whispering-gallery-mode fields propagating inside the resonator. Therefore, we believe that our proposal is feasible in experimental realizations and deserves to be tested with the currently available technology.

Acknowledgments

J.L. gratefully acknowledges Xiaoxue Yang for useful advice and discussion. This research was supported in part by the National Natural Science Foundation (NNSF) of China (Grants No. 11104210, No. 11375067, and No. 11275074), the National Basic Research Program of China (Contract No. 2012CB922103), and by the Doctoral Foundation of the Ministry of Education of China (Grant No. 20134103120005).

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Figures (6)

Fig. 1
Fig. 1 Schematic structure of optical coupeld system, which is composed of a two-level NV center, a single-mode PC nanocavity and a nearby photonic waveguide serving for in- and outcoupling of light into the nanocavity. The single-mode PC nanocavity containing the NV center is evanescently coupled to the row defect waveguide with the coupling strength κe. The |1〉 ⇔ |2〉 transition of the NV center is coupled to the mode ĉof the PC nanocavity with the coupling strength gcav. κi is the intrinsic loss rates of the nanocavity mode excluding coupling to the NV center and the waveguide. A bichromatic field consisting of a cw control laser and a cw probe laser is injected into the waveguide via grating couplers (see Refs. [67, 68]). Sin and Sout denote the input and the output field in the waveguide. The bubble shows the detailed structure of quantized energy levels and the coupling scheme of the cavity mode for the two-level NV center. |1〉 and |2〉 denote the ground and excited states of the NV center, respectively. A small red sphere shows the position of the NV center.
Fig. 2
Fig. 2 The steady-state population inversion σ̄z as a function of optical driving power Pc for four different values of Δ2. The other system parameters used for the simulation are chosen as gcav/2π = 2.25 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, and δ = 0. Curves A–D are for Δ2 = 0, 100, 140, and 180 MHz, respectively.
Fig. 3
Fig. 3 The steady-state population inversion σ̄z as a function of optical driving power Pc for four different values of gcav. The other system parameters for the simulation are chosen as κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, Δ1 = Δ2 = 100 MHz and δ = 0. Curves A–D are for gcav = 1.75, 2.25, 2.75, and 3.25 GHz, respectively.
Fig. 4
Fig. 4 The steady-state population inversion σ̄z as a function of optical driving power Pc for four different values of δ. The other system parameters for the simulation are chosen as gcav/2π = 2.25 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, and Δ2 = 0. Curves A–D are for δ = 0, 80, 160, and 190 MHz, respectively.
Fig. 5
Fig. 5 The normalized FWM intensity versus the detuning Δ2. The other system parameters for the simulation are chosen as Pc = 30 pW, Pp = 3 pW, gcav/2π = 2.25 GHz, Ω/2π = 1 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, and δ = 80 MHz, respectively.
Fig. 6
Fig. 6 The normalized FWM intensity versus optical driving power Pc. The other system parameters for the simulation are chosen as Pp = 3 pW, gcav/2π = 2.25 GHz, Ω/2π = 1 GHz, κi/2π = 1.6 GHz, κe/2π = 8 GHz, γspon/2π = 13 MHz, Δ2 = 0, and δ = 80 MHz, respectively.

Equations (32)

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= h ¯ ω N V σ ^ 22 + h ¯ ω cav c ^ c ^ + i h ¯ g cav ( c ^ σ ^ 21 c ^ σ ^ 12 ) + i h ¯ κ e [ S in ( t ) c ^ S in * ( t ) c ^ ] ,
free = h ¯ ω l ( σ ^ 22 + c ^ c ^ ) ,
U ( t ) = e i free t / h ¯ = e i ω l t ( σ ^ 22 + c ^ c ^ ) ,
rot = U ( t ) U ( t ) i U ( t ) U ( t ) t = U ( t ) ( free ) U ( t ) ,
rot = h ¯ Δ 1 σ ^ 22 + h ¯ Δ 2 c ^ c ^ + i h ¯ g cav ( c ^ σ ^ 21 c ^ σ ^ 12 ) + i h ¯ κ e [ S in ( t ) e i ω l t c ^ S in * ( t ) e i ω l t c ^ ] ,
rot = h ¯ Δ 1 σ ^ 22 + h ¯ Δ 2 c ^ c ^ + i h ¯ g cav ( c ^ σ ^ 21 c ^ σ ^ 12 ) + i h ¯ κ e ( s p c ^ s p * c ^ ) ,
rot = h ¯ Δ 1 σ ^ 22 + h ¯ Δ 2 c ^ c ^ + i h ¯ g cav ( c ^ σ 21 c ^ σ ^ 12 ) + i h ¯ κ e [ ( s c + s p e i Ω t ) c ^ H . C . ] ,
d c ^ d t = g cav σ ^ 12 ( i Δ 2 + κ i / 2 + κ e / 2 ) c ^ + κ e ( s c + s p e i Ω t ) + f ^ c ,
d σ ^ 11 d t = γ spon σ ^ 22 g cav c ^ σ ^ 21 g cav c ^ σ ^ 12 + f ^ 11 ,
d σ ^ 22 d t = γ spon σ ^ 22 + g cav c ^ σ ^ 12 + g cav c ^ σ ^ 21 + f ^ 22 ,
d σ ^ 12 d t = ( i Δ 1 + γ spon / 2 ) σ ^ 12 2 g cav c ^ σ ^ z + f ^ 12 ,
d c d t = g cav σ 12 ( i Δ 2 + κ i / 2 + κ e / 2 ) c + κ e ( s c + s p e i Ω t ) .
d σ 11 d t = γ spon σ 22 g cav c σ 12 * g cav c * σ 12 ,
d σ 22 d t = γ spon σ 22 + g cav c * σ 12 + g cav c σ 12 * ,
d σ 12 d t = ( i Δ 1 + γ spon / 2 ) σ 12 2 g cav c σ z .
c ¯ = F 1 κ e s c F 1 F 2 2 g cav 2 σ ¯ z ,
σ ¯ z = 1 2 γ spon γ spon + 2 g cav 2 ( 1 F 1 + 1 F 1 * ) | c ¯ | 2 ,
σ ¯ 12 = 2 g cav F 1 c ¯ σ ¯ z ,
d δ c d t = g cav δ σ 12 ( i Δ 2 + κ i / 2 + κ e / 2 ) δ c + κ e s p e i Ω t ,
d δ σ z d t = γ spon δ σ z + g cav c * ¯ δ σ 12 g cav δ c * σ ¯ 12 + g cav c ¯ δ σ 12 * + g cav δ c σ ¯ 12 * ,
d δ σ 12 d t = ( i Δ 1 + γ spon / 2 ) δ σ 12 2 g cav c ¯ δ σ z 2 g cav δ c σ ¯ z .
δ c = c + e i Ω t + c e i Ω t ,
δ σ z = σ z + e i Ω t + σ z e i Ω t ,
δ σ 12 = σ 12 + e i Ω t + σ 12 e i Ω t .
D 1 σ 12 + 2 g cav c ¯ σ z + 2 g cav c σ ¯ z = 0 ,
g cav σ 12 + D 2 c = 0 ,
g cav σ 12 + + D 3 c + κ e s p = 0 ,
D 4 σ 12 + + 2 g cav c ¯ σ z + + 2 g cav c + σ ¯ z = 0 ,
( γ spon i Ω ) σ z + + g cav c * ¯ σ 12 + + g cav c * σ ¯ 12 + g cav c ¯ σ 12 * + g cav c + σ ¯ 12 * = 0 ,
( γ spon + i Ω ) σ z + g cav c * ¯ σ 12 + g cav c + * σ ¯ 12 + g cav c ¯ σ 12 + * + g cav c σ ¯ 12 * = 0 ,
S out ( t ) = ( s c + κ e c ¯ ) e i ω c t + ( s p + κ e c + ) e i ( ω c + Ω ) t + κ e c e i ( ω c Ω ) t = ( s c + κ e c ¯ ) e i ω c t Control field + ( s p + κ e c + ) e i ω p t Probe field + κ e c e i ( 2 ω c ω p ) t Degenerate FWM field ,
I FWM = | κ e c s p | 2 .
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