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Observation of Zitterbewegung in photonic microcavities

We present and experimentally study the effects of the photonic spin-orbit coupling on real space propagetion of polariton wavepackets in planar semiconductor microcavities and polaritonic analogs of graphene. In particular, we demonstrate the appearance of an analog Zitterbewegung effect, a term which translates as 'trembling motion' in english, which was originally proposed for relativistic Dirac electrons and consists of the oscillations of the center of mass of a wavepacket in the direction perpendicular to its propagation. For a planar microcavity we observe regular Zitterbewegung oscillations whose amplitude and period depend on the wavevector of the polaritons. We then extend these results to a honeycomb lattice of coupled microcavity resonators. Compared to the planar cavity such lattices are inherently more tuneable and versatile, allowing simulation of the Hamilitonians of a wide range of important physical systems. We observe an oscillation pattern related to the presence of the spin-split Dirac cones in the dispersion. In both cases the experimentally observed oscillations are in good agreement with theoretical modelling and independently measured bandstructure parameters, providing strong evidence for the observation of Zitterbewegung.


INTRODUCTION
The study of analogs to effects appearing in the domain of high energy physics is among the trends of the modern condensed matter physics. In this connection, Fabry-Perot optical microcavities, structures where the internal spinor wavefunction can be directly imaged via photon tunnelling through the mirrors, are of particular interest. Such cavities generally support ballistic propagation of two polarisation states of light and have an inherent polarisation-wavevector coupling, the TE-TM splitting, which is an equivalent of spin-orbit coupling 1 . They may also support birefringent polarisation splitting, all of which features have allowed observation of a range of important physical effects such as optical spin-Hall effect 2 , emergence of monopoles 3 and onset of the non-Abelian gauge fields 4,5 . In semiconductor microcavities, quantum wells (QWs) can be added resulting in formation of composite part-light part-matter quasiparticles called exciton-polaritons. Their unique properties, related to the extremely small effective mass (about five orders of magnitude less then the mass of a free electron), high sensitivity to magnetic fields, giant nonlinear optical interactions, and possibility for optical amplification, have allowed demonstration of optical condensates with macroscopically large coherence length (in the mm scale) 6 , formation of acoustic black holes and Hawking effect 7 , observation of anomalous Hall drift 8 , and many other effects. In this work we use such semiconductor microcavities but focus on the properties coming from the photonic constituent of polaritons, and so we demonstrate the relevant fundamental principles using highly photonic (> 98%) polaritons. Nevertheless, the properties associated with the matter component can easily be * p.m.walker@sheffield.ac.uk added by tuning the energy separations between cavity photons and QW excitons, and open further perspectives to study nonlinear phenomena associated with the spin degree of freedom.
Furthermore, there has been much recent work on engineering the bandstructure of microcavities by imposing a laterally varying optical potential [9][10][11] . This has allowed study of flat bands in Lieb lattice potentials 10 , topological physics [12][13][14] and engineering of Dresselhaus spin-orbit coupling for photons 15,16 . Such lattices, as well as being substantially more tuneable via the added in-plane degrees of freedom, can be used to build photonic analogs of a wide variety of physically important Hamiltonians based on Bose-Hubbard type models 17 . Combined with the other favourable properties of polaritons discussed above this creates a very wide perspectives for polaritonic simulation.
One of the textbook examples of quantum relativistic effects is Zitterbewegung. It consists of an oscillatory motion of a propagating wavepacket transverse to its ballistic trajectory despite the absence of transverse forces 18 . It was first predicted by Schrodinger for the motion of free electrons governed by the Dirac equation 19 and appears due to interference between positive and negative energy states of a spinor (two component) system, enabled by the coupling of internal (spin) and external (momentum) degrees of freedom.
In addition to free relativistic electrons the effect is predicted for electrons in crystals with Rashba and Dresselhaus spin-orbit coupling [20][21][22] . The predicted high frequency and low amplitude of the oscillations for the vacuum case, and the difficulty of observing single electrons in solids, make experimental observation highly challenging 18 . This has led to a search for analagous systems in which the observation can be made. Gerrtisma et. al. 23 performed a quantum simulation of the Dirac equation using a single trapped ion. Measurement of the trans-verse position was made indirectly since most observables cannot be directly measured in ion trap experiments. High frequency oscillating currents were also observed in the motion of spin-polarised electrons in a doped semiconductor device 24 . In optics, the Zitterbewegung was observed in arrays of coupled waveguides where the internal degree of freedom mimicking spin was introduced by having two slightly different waveguides per unit cell 25 . In that case the energy separation of positive and negative components was fixed by the geometry of the waveguides rather than being continuously tuneable. Thus different lattices had to be used to examine different parameters. Furthermore, the Zitterbewegung had to be detected indirectly using the fluorescence induced by the intensity of the light inside the waveguides. Microcavities, by contrast, allow direct imaging and excitation of the internal wavefunction and addressing of different energy separations using pump laser incidence angle and frequency.
So far Zitterbewegung has not been observed in the microcavity structures where polaritons can be formed and which allow the wide range of optical analogs discussed above, although it has recently been theoretically predicted in both planar microcavities 26 and honeycomb microcavity lattices at wavevectors close to the Dirac point 16 . In this work we bridge this gap between theory and experiment and report observation of Zitterbewegung in both types of structures. For the planar cavity we demonstrate tuning of the Zitterbewegung period by varying the incidence angle and propagation direction in the birefringent cavity. In the case of wavevectors near the Dirac point in a honeycomb lattice the ability to engineer the bandstructure allows observation of smaller period Zitterbewegung while retaining an observable amplitude. The Zitterbewegung essentially arises from interferences between the two components of the spinor wavefunction. The propagating beam has finite spatial width and hence finite width in momentum (angular) space. Since the polarisation states depend on the momentum through the spin orbit coupling, different angular components of the beam have different relative amplitudes of the spinor components, and these can evolve with propagation. As discussed above we experimentally demonstrate the essential principles of the effect in the highly photonic regime, but polaritons can be made more excitonic by simple tuning of the photon-exciton detuning, which opens up wider perspectives.

RESULTS
We begin with the case of the planar microcavity. In these structures a wavelength thickness cavity layer is enclosed between two Bragg mirrors (periodically repeating stacks of quarter-wave layers of two different materials), as illustrated in the schematic in Fig 1(a). In our case the cavity is made of GaAs, the mirror materials are GaAs and Al 0.85 Ga 0.15 As, and the structure was grown by molecular beam epitaxy. Three In 0.04 Ga 0.96 As QWs Points with total intensity (Ix + Iy) less than 0.20 of the peak have been set to white since the polarisation degree is not well defined for low intensities.
are embedded in the cavity. The energy detuning between the QW excitons and the the cavity photons is more than 20 meV.
The polarisation and angle dependent reflection of the cavity mirrors leads to a wavenumber dependent energy splitting (TE-TM splitting) of the cavity modes having electric and magnetic fields transverse to the wavevector. This combines with a slight optical birefringence 27 resulting in a complicated splitting between linear polarization states. For small in-plane wavevector components the Hamiltonian of the system in the basis of circular polarized states reads 28 : where m is the effective mass of the polaritons, k x and k y are the in-plane wavevector components of the photons in the sample reference frame where x is the fast axis (see Supplementary Fig. 1), k 2 = k x 2 + k y 2 , and parameters Ω and β describe the values of the k-independent optical birefrigence and TE-TM spliting respectively. The parameter β is related to the difference of the longitudinal and transverse masses of the photons m l and m t as 29 The corresponding dispersions of the two photon branches split in linear polarisations read: where ϕ is the in-plane angle between the wavevector k and the x axis of the crystal. With these definitions, the energy of the TE polarised mode (electric field perpendicular to k ) increases faster with k than the TM mode for positive β, and at k = 0 the mode polarised along x has higher energy for positive Ω. Note, that the combination of birefrigence and TE-TM splitting leads to a clear in-plane anisotropy of the dispersions, which cross for ϕ = 0 at k = Ω/ (2β). This will now be seen experimentally as we present the basic characterisation of the sample. The experiments in this paper were performed at approximately 10 K temperature in a continuous-flow coldfinger cryostat. The energy vs. wavevector dispersion relations E(k y ) at k x = 0 were measured by angle and polarisation resolved photoluminescence (PL) spectroscopy and can be seen in Fig. 1(c) and (d). Note that x and y are coordinates in the laboratory reference frame (see Fig. 1(a)). In both figures the angle θ on the horizontal axis gives the wavevector k y = k 0 sin θ, k x = 0, where k 0 = 2π/λ. Fig. 1(c) shows the case where the sample is rotated such that k = k yŷ is parallel to y (ϕ = 90 • ). Two branches with different polarisation are visible. As expected there is a splitting at k = 0 resulting from the birefringence and the splitting increases with k due to the the TE-TM splitting. Fig. 1(d) shows the case where the sample is rotated such that k = k yŷ is parallel to x (ϕ = 0 • ). In this case the dispersions cross at 5.7 • (k = 0.73µm −1 ), again as expected. Note that at the crossing point the polarisation is random resulting in zero polarisation degree. For each measured dispersion we extract the energy at the peak intensity for each angle and for each polarisation and fit the resulting energy vs. angle curves to find the parameters describing the sample. We find Ω = 43 ± 19 µeV, β = 33.6 ± 3.5 µeV µm 2 . The energy at k = 0 (averaged between the two dispersion) is 1.4531 eV and 2 / (2m) = 947.5 µeV µm 2 .
We now proceed to measure the Zitterbewegung effect in the sample. We excited the cavity resonantly using a tuneable continuous wave Ti:Sapphire laser as illustrated in Fig. 1(a). The laser energy and incident angle were set to match points on the dispersion allowing efficient injection of photons into the cavity. The laser spot incident on the sample was circular with full-width-athalf-maximum (FWHM) 15 µm. The size of the Fourier transform of this spot, e.g. its size vs. wavenumber k was sufficient to efficiently excite both polarisation branches. The excitation polarisation was circular to ensure equal  Fig. 1(c). (c) and (d) are for the case where the principle axis x is aligned along y and correspond to the dispersion curve in Fig. 1(d). Blue points correspond to the experimental data, red solid line gives the theoretical fit. excitation of both branches. Light was collected from the opposite side of the sample to the excitation (transmission geometry) and the total intensity recorded by a thermo-electrically cooled CCD camera. An example of the recorded intensity pattern is shown in Fig. 1(b). The highest intensity part near y = 10 µm is the incident laser spot. Since we excite at finite angle where the group velocity (slope of the dispersion) is finite the photons in the cavity propagate away from the excitation spot in the positive y direction, decaying by emitting photons through the Bragg mirrors towards the detector. For any value of y we can take a slice along x and find the value of x at which the intensity is maximum. To minimise the effects of scatter of the data points it is better to define the center of intensity according to In Fig. 2 we plot x c (y) for different incidence angles and orientations of the sample. Figs. 2(a) and (b) show the cases for 8 • and 12 • angles of incidence respectively and where we inject light along the y axis of the cavity (ϕ = 0). The blue points give the experimentally extracted trajectory of the wavepacket. Clear oscillations of x c are visible with increasing distance y away from the excitation spot. For the Zitterbewegung effect the period of the oscillation L corresponds to the frequency separation of the positive and negative branches and is expected to vary with excitation angle 26 . Qualitatively, at larger angle we expect the TE-TM splitting between the branches to increase leading to higher frequency (shorter period) oscillations. As expected we see in the experiment that the period becomes shorter at higher angles.
We simulated the expected trajectory of the wavepackets using a model of the evolution of the spinor wavefunction accounting for both TE-TM splitting and birefringence (see Supplementary Note 1). The red curves in Figs. 2(a) and (b) show the theoretical evolution of x c vs. y. The parameters Ω = 28.8 µeV·µm 2 and β = 32.45 µeV·µm 2 for the simulation were obtained by fitting the experimental oscillations. Both the period and amplitude of the simulated trajectories are in excellent agreement with the experimental points. Importantly, the values of β and Ω found from fitting the oscillations are in good agreement with the values found independently from fitting the dispersion relations (β agrees within 0.33 of the uncertainty, Ω agrees within 0.75 of the uncertainty), as discussed above. This verifies that the experimentally observed oscillations are in good agreement with the theory describing Zitterbewegung. The amplitude of Zitterbewegung oscillations is in general a complicated function of sample and excitation parameters, but to a good approximation is governed by the experimentally chosen excitation angle 26 and is independent of the parameters β and Ω. The fact that the observed amplitude also agrees well with the model provides further evidence that the oscillations we observe experimentally are indeed the real transverse oscillations of the wavepacket in the cavity, which is the Zitterbewegung.
We then rotated the sample and measured the oscillations for the case where we inject light along the x axis of the cavity (ϕ = 90 • ). As discussed above, this changes the nature of the dispersion, in particular the splitting between the two polarisation branches for a given angle. The data is shown in Figs. 2 (c) and (d) for incidence angles 8.5 • and 10 • respectively. We used exactly the same parameters as in the ϕ = 0 • case to simulate the trajectory of the light and obtain semi-quantitative agreement between the experimental points and the theory, without further fitting. This further verifies that the observed oscillations are consistent with the Zitterbewegung effect. The agreement of the amplitude may be slightly less good than in the ϕ = 0 • case due to proximity to the crossing of the two polarisation branches (Fig. 1(d)) at 5.7 • . At this point there is no splitting and the period becomes singular, with the oscillations being correspondingly more sensitive to the exact parameters at angles close to the singular point.
Finally, the Zitterbewegung effect arises due to interference between the two polarisation components, e.g. it is fundamentally a spinor effect. From theory we expect that the oscillations in the total intensity pattern should disappear if only one polarisation branch is excited. We tested this in two ways, described in more detail in Supplementary Note 2. First, we excited the system with light linearly polarised parallel to one polarisation branch and perpendicular to the other, in comparison to the case of circularly polarised excitation discussed so far. We detected oscillations in the case of cir-  16 . Blue and red curves represent states linearly polarised along x and y respectively. cular polarised excitation but not in the linear polarised case (see Supplementary Figs. 2(a) and (b)). Secondly, using circular polarised excitation, we tuned the excitation angle away from the condition where both branches are equally excited and towards a condition where one branch is preferentially excited. The amplitude of the oscillations was seen to reduce and disappear as the relative amplitude of the modes became unequal resulting in the contribution of the interference between modes to the total intensity reducing. Overall, the simultaneous agreement of oscillation period and amplitude with modelling for parameters that agree with those independently obtained from the dispersion relation, together with the expected dependence on the spinor nature of the incident light provide strong evidence that we observe Zitterbewegung in the planar microcavity structure.
The planar microcavity, with its simple structure and tuneability through excitation angle, is a good platform for proof of principle demonstrations. Lattices formed by etching the microcavity to introduce a lateral pattern allow a wide variety of physically important bandstructures to be simulated and are highly tunable via the lateral patterning. They are thus an important complementary platform which open up a wide perspective for future studied. We therefore studied Zitterbewegung in a honeycomb lattice, a photonic analogue of graphene. The lattice is formed by etching the planar cavity into air post cavities. These so-called pillar microcavities support discrete energy states localised in all three spatial dimension. By overlapping neighbouring pillars (see Fig. 3) we allow photons to tunnel from one pillar to another, allowing the discrete states to hybridise and form energy bands. The structure we study here is a honeycomb lattice, which is a triangular lattice with 2 pillars per unit cell. The diameter of the pillars is 3 µm, the center to center spacing of adjacent pillars is d = 2.8 µm, the lattice periodicity is a = d √ 3, the lattice vectors are a √ 3/2, ±1/2 , and the reciprocal lattice vectors are (2π/a) 1/ √ 3, ±1 . In Fig. 3(a) we show a schematic diagram of the honeycomb lattice structure being excited by the laser beam, resulting in propagation of the light in the cavity accompanied by transverse oscillations. An example of an experimentally measured intensity pattern is shown in Fig. 3(b) and is equivalent to the pattern seen in the planar case ( Fig. 1(b)) except that it results from propagation of photons in the bands of the lattice rather than those of the planar cavity. The bandstructure of the lattice can be seen in the angle resolved photoluminescence spectrum of Fig. 3(c). Three distinct sets of bands separated by band-gaps can be seen. The lowest set of bands, labelled 's bands', are composed of the lowest energy states of the individual pillars, resembling lowest order Hermite-Gauss modes 16 . The next highest set of bands, labelled 'p bands', are composed of the first order Hermite-Gaussian modes of the individual pillars. For the purpose of this work the essential difference between the two sets of bands is that the polarisation dependent tunnelling rates from pillar to pillar are different resulting in different splittings and different group velocities (Fermi velocity) close to the Dirac points (at k x = 0, k y = 4π/ (3a)). We therefore expect different Zitterbewegung period and amplitude close to the Dirac points in either band. The red and black dots on Fig. 3(c) mark the energy and wavevector at which we resonantly excite the bands to measure the Zitterbewegung. More detailed plots of the bandstructure are given in Figs. 3(d) and (e) for the p and s bands respectively. These are obtained from a tight binding model using parameters from Ref. 16 where the same lattice was studied extensively and the model was fit to the dispersion. The blue and red colour of the lines correspond to the polarisation of the states and the black dots denote the energy and wavevector of excitation, slightly detuned from the Dirac points. It can be seen that, as in the planar case, we equally excite two orthogonally polarised modes with finite group velocity. It is this velocity which results in the photons propagating away from the excitation point, as seen in Fig. 3(b).
As in the planar case we use Eqn. 4 to extract x c vs. y from the intensity pattern of the propagating photons. Figs. 4(a) and (b) show the extracted trajectories of the photons for the p and s bands respectively. Oscillations of x c are visible and we can make the qualitative observation that the period of oscillation is shorter for the s-  Fig. 3(d).
(b) Case for excitation of the s-band states, corresponding to black circle in Fig. 3(e).
bands. The period is expected to scale as L = 2π/∆k where ∆k is the splitting in momentum between the states excited at the fixed laser energy. Therefore the shorter period for the s-band is consistent with the larger ∆k near the excitation point in the s-band (Fig. 3(e)) compared to the p-band (Fig. 3(d)). We calculated the theoretical trajectory for the photons in a similar way as for the planar case, but using evolution equations for states near the Dirac point of the honeycomb lattice 16,30 (see Supplementary Note 3). Parameters for the modelling were taken from Ref. 16 where the same lattice was studied extensively and the dispersion relation was fitted. Semi-quantitative agreement is obtained between the theory and experiment showing that the observed oscillations are once again consistent with the expectation for Zitterbewegung.
In summary, we experimentally demonstrate Zitterbewegung in planar microcavity and microcavity honeycomb lattices. Unlike previous demonstrations the Fabry-Perot cavity design allows direct visualisation of the complex spinor field inside the device. Lattice structures allow building photonic analogs to important physical system, such the photonic graphene we study here. In exciton-polariton lattices, identical to our structure apart from smaller energy detuning between photons and quantum well exciton, interparticle interactions and high sensitivity to magnetic fields can easily be added. This work then opens the door to studying a very wide class of photonic analogs of relativistic systems with particle interactions, time-reversal symmetry breaking and dissipative effects.

DATA AVAILABILITY
The data supporting the findings of this study are available from the corresponding author upon reasonable request.