# Optomechanical Dirac Physics

###### Abstract

Recent progress in optomechanical systems may soon allow the realization of optomechanical arrays, i.e. periodic arrangements of interacting optical and vibrational modes. We show that photons and phonons on a honeycomb lattice will produce an optically tunable Dirac-type band structure. Transport in such a system can exhibit transmission through an optically created barrier, similar to Klein tunneling, but with interconversion between light and sound. In addition, edge states at the sample boundaries are dispersive and enable controlled propagation of photon-phonon polaritons.

###### pacs:

42.50.Wk, 42.65.SfRapid progress is being made in the field of optomechanics, which studies the interaction of light with nano-mechanical motion (for a recent review, see Aspelmeyer2013RMPArxiv ()). Most current achievements are based on a single vibrational mode coupled to a single optical mode (i.e. a single “optomechanical cell”). A logical next step is to couple many such modes, providing new functionality and generating new physical phenomena. First steps have been taken using setups with a few modes (e.g. for synchronization Zhang2012Sync (); Bagheri2013 (), wavelength conversion Hill2012WavelengthConversion (); Dong2012 (), phonon lasing Grudinin2010 (), or cooling Bahl2012 ()). Going beyond this, we can envisage a periodic arrangement of cells. In that case we will speak of an “optomechanical array”. Optomechanical arrays might be realized on a number of experimental platforms: Microdiscs Ding2010 (); Zhang2012Sync () and microtoroids Armani2007 (); Verhagen2012 () could be coupled via evanescent optical fields Zhang2012Sync (). Superconducting on-chip microwave cavity arrays (of the type discussed in Houck2012 ()) could be combined with nanobeams Regal2008 () or membranes Teufel2011 (). Currently the most promising platform are optomechanical crystals, i.e. photonic crystals engineered to contain localized vibrational and optical modes. Single-mode optomechanical systems based on that concept have been demonstrated experimentally, with very favorable parameters Eichenfield2009 (); Safavi-Naeini2010APL (); Gavartin2011PRL_OMC (); Chan2011Cooling (); SafaviNaeini2014SnowCavity (). Ab-initio simulations indicate the feasibility of arrays Safavi-Naeini2011 (); Heinrich2011CollDyn (); Ludwig2013 (). Given these developments it seems that optomechanical arrays are on the verge of realization. The existing theoretical work on optomechanical arrays deals with slow light Chang2011 (), synchronization Heinrich2011CollDyn (); Holmes2012 (); Ludwig2013 (), quantum information processing Schmidt2012 () and quantum many-body physics Bhattacharya2008 (); Tombadin2012 (); Xuereb2012 (); Akram2012 (); Ludwig2013 () and photon transport Chen2014 (). In this letter, we go beyond these works and illustrate the possibilities offered by engineering nontrivial optomechanical band structures of photons and phonons in such arrays. Specifically, we will investigate an array with a honeycomb geometry. This lattice is the basis for modeling electrons in graphene Neto2009 (), but it has recently also been studied for photonic crystals Peleg2007 (); Polini2013 (), exciton-photon polaritons Jacqmin2014 () and other systems Polini2013 (). It is the simplest lattice with a band structure showing singular and robust features called Dirac cones, mimicking the dispersion of relativistic massless particles. As we will be interested in the long-wavelength properties of the structure, on scales much larger than the lattice spacing, we may call this an “optomechanical metamaterial”. Tunability would be the biggest advantage of optomechanical metamaterials, rivaling that of optical lattices: The band structure is easily tunable by the laser drive (intensity, frequency, phases). Moreover, it can be observed by monitoring the emitted light. Using spatial intensity profiles for driving, one can even engineer arbitrary potentials and hence local changes in the band structure. We predict that these features could be used to observe photon-phonon Dirac polaritons, an optomechanical Klein tunneling effect, and edge state transport.

Model - We consider a 2D honeycomb lattice of identical optomechanical cells, driven uniformly by a laser (frequency ). Each cell supports a pair of co-localized mechanical (eigenfrequency ) and optical (eigenfrequency ) modes interacting via radiation pressure. This geometry could be implemented based on optomechanical crystals, see Figure 1, but also in other physical realizations such as arrays of microdisks, microtoroids, or superconducting cavities. We adopt the standard approach of linearizing the dynamics around the steady-state classical solution and performing the rotating wave approximation, valid for red detuned () moderate driving Aspelmeyer2013RMPArxiv (). In a frame rotating with the drive, the linearized Hamiltonian reads

(1) |

This Hamiltonian describes the non-equilibrium physics of the array of phonon modes (annihilation operator ) and photon modes (), interacting via the linearized optomechanical interaction of strength . The term describes the tunneling of photons and phonons between neighboring sites and with amplitudes and , respectively Heinrich2011CollDyn (); Ludwig2013 (); Safavi-Naeini2011 (). Here, is a multi-index, where , indicate the unit cell, which contains two optomechanical cells on sublattices A/B (denoted by ).

The interaction strength is , where is the bare optomechanical coupling, i.e. the shift of the local optical resonance by a mechanical zero-point displacement, and is the local complex light field amplitude, proportional to the laser amplitude Aspelmeyer2013RMPArxiv (). For completeness, we mention that the operators and in Eq. (1) are assumed shifted, as usual Aspelmeyer2013RMPArxiv (), by and by the radiation-pressure-induced mechanical displacement , respectively. The detuning incorporates a small shift in due to the static mechanical displacement.

The eigenfrequencies of Hamiltonian (1) form the optomechanical band structure, shown in Fig. 2 (a,b) for realistic parameters and a translationally invariant system (). It comprises four polariton bands, constructed out of the original two photon and two phonon bands, giving rise to photon-phonon polariton Dirac cones.

A weak additional probe laser can inject excitations at arbitrary frequency. It can be spatially resolved (via tapered fiber) or momentum-resolved (extended beam). Even without the probe, the momentum-resolved band structure is visible in the emitted far-field radiation in the form of Raman-scattered laser-drive photons, see Fig. 2 (d,e). We incorporate dissipation and noise via the standard input/output theory Aspelmeyer2013RMPArxiv (), taking into account the photon (phonon) decay rate () and the thermal phonon number , see Supplemental Material. We emphasize that the band structure (and transport) could be observed in this manner even at room temperature.

The emergence of the Dirac cones at the Dirac points and follows from the symmetries of the honeycomb lattice Hasan2010RMP (). Without the drive (), the standard scenario for honeycomb lattices applies to photons and phonons separately: Excitations can be on sublattice A or B, corresponding to a binary degree of freedom, . Diagonalizing the Hamiltonian using a plane wave ansatz, one recovers a Hamiltonian for every wave vector . Close to a symmetry point, this reduces to the Dirac Hamiltonian for relativistic massless particles. Around , it has the form , where and is the vector of Pauli matrices . The photon velocity at the Dirac point, , will be generally significantly larger than the mechanical one, , see Fig. 2(c). For nearest-neighbor hopping amplitudes (photons) and (phonons), we find , .

We now consider the interacting case (), turning the Hamiltonian (1) into its first-quantized counterpart in momentum space and expanding it around a symmetry point. The particle type can now be encoded in a second binary degree of freedom, for photons/phonons (with Pauli matrices ). We find the optomechanical Dirac Hamiltonian:

(2) |

This Hamiltonian describes the mixing of two excitations of very different physical origin, with properties that are easily tunable. The terms describe, in this order, an offset between photon and phonon bands, the Dirac part, and the optomechanical interaction (plus a constant offset). Here we defined , , and . The interaction is tunable in-situ via the drive laser intensity (in contrast, e.g., to bilayer graphene systems). Photon-phonon Dirac polaritons feature a dispersive spectrum

(3) |

i. e. the velocity is momentum-dependent and varies on the momentum scale , well within the range of validity of Eq. (2), . This effect comes from the mixing of two Dirac excitations with different velocities.

At the Dirac points, the band structure comprises two pairs of cones split by . Sweeping the laser detuning from positive to negative values, the upper cones evolve from purely optical (velocity ), over polaritonic (slope ) to purely mechanical (velocity ). Since the helicity, , is conserved, bands of equal helicity feature avoided crossings, while bands of different helicity cross, see Fig. 2(d,e).

Edge states - The physics of edge states is significantly modified by inhomogeneous optomechanical couplings that can be tailored via the laser intensity but also naturally occur in a finite system under uniform drive. There, the coupling is smaller at the edges than in the bulk, see Fig. 3(a). In an infinite strip with zigzag edges this leads to a band of polariton edge modes with tunable velocity. That is because edge states with momenta closer to the Dirac points have larger penetration lengths (compare Fig. 3(b)) and thus explore regions of stronger optomechanical coupling, making their energy momentum-dependent (Fig. 3(d)). In contrast, no transport occurs at the edge of graphene since it supports a flat band of edge modes Neto2009 ().

The photonic local density of states (LDOS) is experimentally accessible via reflection/transmission measurements, e.g. with a tapered fiber probe brought close to the sample. The LDOS on site , , characterizes the probability to inject a photon with frequency . Figure 3(c) shows the LDOS for sites in the bulk (gray) and at the edge (black line). Typical features, like the vanishing DOS at the Dirac points, are smeared out slightly by dissipation. The edge states show up as two peaks. For weak coupling one would naively expect a single edge state peak broadened by dissipation. However, figure 3(e) shows a peak with a narrow dip on top. This can be understood as optomechanically induced transparency Aspelmeyer2013RMPArxiv (), an interference effect visible for . We note that the gradient in leads to the formation of additional bands of edge states, cf. close-up in Fig. 3(d).

Edge state transport – The zigzag edge forms a polariton waveguide for excitations injected by a local probe at the edges. Its group velocity is tunable in-situ via the laser amplitude. Although the edge states are not protected by a band gap, the transmission remains mainly along the edge, see Fig. 4(a). Figure 4(b) depicts the optical transmission vs. the propagation distance and the probe frequency. For small probe frequencies there are no edge states, thus the response is local and weak. Increasing the probe frequency makes edge states resonant, leading to transmission along the edge. For a given probe frequency, two edge modes are resonant, with a quasimomentum difference . This explains the interference pattern, with transmission minima at . The mechanical transmission mirrors the optical one () for strong coupling, and there is no transport for weak coupling (a flat edge state band).

Optomechanical Klein tunneling – The in-situ tunability of optomechanical metamaterials allows to create arbitrary effective potential landscapes simply by generating a spatially non-uniform driving laser profile. This can be nicely illustrated in a setup that permits the study of Klein tunneling, the unimpeded transmission of relativistic particles through arbitrary long and high potential barriers. Electrons in graphene realize a special variant of this Young2009 (). Here, we show that the backscattering of Dirac polaritons impinging on an optomechanical barrier is suppressed. Moreover, photons can be converted into phonons (and vice versa) while being transmitted.

To create a barrier for Dirac photons propagating in the array, we make use of the distinctive in-situ tunability of optomechanical metamaterials. As shown in Fig. 5(a), when a region of width is illuminated by a strong control laser (of detuning ), a position-dependent optomechanical coupling is created. This region represents a barrier for Dirac photons injected by a probe laser at another spot. We first solve the scattering problem within the Dirac Hamiltonian (2) in the presence of a barrier with infinitely sharp edges: for and otherwise. We consider a right-moving photon with quasimomentum perpendicular to the barrier, . Backscattering is forbidden, because the helicity is conserved and only the right-moving waves [bold lines in Fig. 5(b)], have positive helicity . Thus, the wave is entirely transmitted. Beyond the barrier, it is a superposition of photons and phonons:

(4) |

where . Note that can be interpreted as the probability that the photon is converted into a phonon, with ensuring conservation of probability. Matching the solutions of the Dirac equation in the different regions, we find

(5) |

where are the two momenta of the right-moving polaritons in the interacting region, at the probe frequency. In a more accurate description, we compute numerically the stationary light amplitude and the mechanical displacements using the full Hamiltonian (1) and including also dissipation, see Supplemental Information. We assume the probe laser to be injected at a finite distance from the barrier, in a Gaussian intensity profile, see Fig. 5(a). The solution, depicted in Fig. 5(c), shows all the qualitative features predicted using the effective relativistic description of Eq. (5). Inside the barrier, photons are converted back and forth into phonons. Phonons reach higher probabilities, since their speed is smaller (), and their decay length is shorter (for realistic parameters ). We deliberately chose a steep barrier (on the scale of the lattice constant), to illustrate a small Umklapp backscattering to the other Dirac point (tiny wiggles for ). The ratio of the phonon current to the complete current at , , serves as an estimate for the phonon transmission probability . Figure 5(d) shows the optical and mechanical transmission against the barrier height, which can be tuned via the control laser. The fact that the numerical results with dissipation differ from the theoretical expectation (grey line: is mostly due to . Having a large mechanical quasimomentum, , diminishes slightly the quality of the Dirac approximation.

Experimental realizability - The strong coupling regime, , is routinely reached on several optomechanical platforms, including optomechanical crystals. It is also crucial to avoid a phonon-lasing instability, which requires (see Supplemental Information). In principle, can be made small by design (e.g. distance between sites Heinrich2011CollDyn (); Ludwig2013 (); Safavi-Naeini2011 ()), although disorder effects become more pronounced at smaller . In , even for frequency fluctuations of the order of , the Anderson localization length is several hundred sites, safely exceeding realistic sample sizes. Disorder which is not smooth on the scale of the lattice constant may still induce Umklapp scattering between different Dirac points. Numerical simulations indicate that the Klein tunneling is robust for disorder strengths of of .

Outlook - Optomechanical metamaterials will offer a highly tunable platform for probing Dirac physics using tools distinct from other systems. Future studies could investigate the rich nonlinear dynamics expected for blue detuning, which would create novel particle pair creation instabilities for a bosonic massless Dirac system. Pump-probe experiments could reveal time-dependent transport processes. Novel features can also be generated by modifying the laser drive, e.g. optical phase patterns could produce effective magnetic fields and topologically nontrivial band structures Peano2014 (), and a controlled time-evolution of the laser would allow to study adiabatic changes, sudden quenches and Floquet topological insulators Oka2009 ().

We acknowledge support via an ERC Starting Grant OPTOMECH, via the DARPA program ORCHID, and via ITN cQOM.

Supplemental material for the article: “Optomechanical Dirac Physics”

[5mm] M Schmidt, V Peano and F Marquardt

[1mm] University of Erlangen-Nürnberg, Staudtstr. 7, Institute for Theoretical Physics, D-91058 Erlangen, Germany

Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1/Bau 24, D-91058 Erlangen, Germany

## Classical stationary solutions

In a frame rotating with the driving, the equations of motion for the classical fields (averaged over classical and quantum fluctuations) of an optomechanical array read

(S.6) |

Here, is the laser amplitude and is the (bare) detuning. Notice that, in deriving the above equations, we have just incorporated a general coherent coupling to the standard equations for single uncoupled optomechanical cells Aspelmeyer2013RMPArxiv (). Implicitly, we have assumed that the dissipation is caused by independent fluctuations on the different lattice sites. For an infinite array one can readily find a stationary solution of the classical equations (S.6) using the mean field ansatz, and . The resulting equations have the same form as the equations for single-mode optomechanics Meystre1985 ()

(S.7) |

As in the standard case, the radiation pressure induced mechanical displacement translates into a shift of the optical mode eigenfrequencies, . In the main text, we incorporate this shift in the effective detuning . An additional shift of the mechanical and optical eigenfrequencies is induced by the coupling to the neighboring sites, and (for nearest neighbor hopping and ). For a finite array the stationary fields and are not independent of . In this case, we solve the classical equations (S.6) numerically.

## Linearized Langevin equations

In our work, we apply the standard approach of linearizing the dynamics around the classical solutions WallsMilburn_QuantumOptics (), the linearized Langevin equations read

(S.8) |

with the noise correlators

(S.9) |

The output fields are related to the fields in the array and the input fields by the input output relations WallsMilburn_QuantumOptics (),

(S.10) |

Notice that contains also counter rotating terms, These terms have been omitted in Eq. (1). This is the standard rotating wave approximation which applies to any side band resolved optomechanical system driven by a red detuned laser with a moderate intensity, and . In an optomechanical array, the laser should be red detuned compared to the lowest frequency optical eigenmode. Thus, in the regime when Dirac photons and Dirac phonons are resonantly coupled (), we find the additional constraint .

### Photon emission spectrum

In Fig. 2(d,e), we plot the power spectrum of the photons emitted by the array (periodic boundary conditions have been assumed),

(S.11) |

Here, are the annihilation operators of the photonic Bloch modes, [ is the position counted off from a site on sublattice and is the number of unit cells]. In a large array (where finite size effects are smeared out by dissipation), is proportional to the angle-resolved radiation emitted by the array at frequency .

For periodic boundary conditions and nearest neighbor hopping, the Langevin equations (S.8) can be solved analytically (including also the counter rotating terms). By plugging the corresponding solutions into the definition (S.9) and taking into account the correlators Eqs. (S.9), we find

(S.12) |

in terms of the analytical functions

Here, we have introduced the free susceptibilities and . Moreover, and are the spectra of tight-binding photons and phonons on the honeycomb lattice (the photon spectrum is defined in the rotating frame), respectively. They are given by and where .

### Local Density of states and transmission amplitudes

In Fig. 3 and 4 of the main text, we plot the local photonic densities of states (LDOS) on site , and the transmission amplitude relating the emission in the output field at site to an input probe field at sites with frequency , where . These two quantities are directly related to the photonic retarded Green’s function

In fact, the density of state is defined as

(S.13) |

where . Moreover, using Kubo formula and the input output relation Eq. (S.10), we find the photon transmission amplitude to be

(S.14) |

For an infinite strip of width unit cells, it is most convenient to introduce the partial Fourier transform of ,

(S.15) |

Here, is the momentum in the translationally invariant direction (-axis). Formally, we have introduced a finite length of cells and periodic boundary conditions. However, the spurious finite size effects induced by this assumption are smeared out by dissipation for an appropriately large . After taking the partial Fourier transform of the classical displaced fields and , we organize their Fourier components , in a -dimensional vector with equation of motion in the form (when no probe laser is present). The matrix is obtained from the Langevin equations (S.8) by neglecting the counter rotating terms. Thus, the Green’s function is the block of the matrix which acts on the optical subspace of . The LDOS and transmission amplitudes are then readily calculated from Eqs. (S.13-S.15)

## Details of the numerical calculation of the Klein tunneling of photons and phonons

In Fig. 5, we consider an infinite strip with armchair edges and a width of unit cells (in the -direction). Notice that the unit cell of an armchair strip is formed by four sites. Thus, the photon and phonon dynamics is described by the Langevin equations (S.8) with the multi-index , where , , and . The optomechanical barrier created by the strong control laser is translationally invariant in the -direction, with , , and . The probe laser has a gaussian intensity profile in the -direction with average inplane momentum close to the symmetry point, We choose , , , and . The other parameters are given in the main text. The stationary Langevin equations have been solved by computing numerically the Green’s functions for .

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