# Converting microwave and telecom photons with a silicon photonic nanomechanical interface

Sep 8, 2020

### Transducer theory

The transducer consists of one microwave resonator and one optical cavity, both parametrically coupled via the vacuum coupling rates g0,j with j = eo to the same mechanical oscillator as shown in Fig. 1a and b. The intrinsic decay rate of the optical (microwave) resonator is κin,o (κin,e), while the optical (microwave) waveguide–resonator coupling is given by κex,o (κex,e) resulting in a total damping rate of κj = κin,j + κex,j and coupling ratios ηj = κex,j/κj. The mechanical oscillator with intrinsic decoherence rate γm and frequency ωm is shared between the optical cavity and the microwave resonator and acts as a bidirectional coherent pathway to convert the photons between the two different frequencies8,21,22,23. In the interaction frame, the Hamiltonian describing the conversion process is (see Supplementary Note 1):

$${hat{H}}_{{rm{int}}}=sum _{mathrm{j} = {rm{e}},{rm{o}}}left(hslash {G}_{mathrm{j}}({hat{a}}_{mathrm{j}}^{dagger }hat{b}+{hat{a}}_{mathrm{j}}{hat{b}}^{dagger })+{hat{H}}_{{rm{CR}},mathrm{j}}right),$$

(1)

where ({hat{a}}_{mathrm{j}}), ((hat{b})) with j = eo is the annihilator operator of the electromagnetic (mechanical) mode, and ({hat{H}}_{{rm{CR}},mathrm{j}}=hslash {G}_{mathrm{j}}({hat{a}}_{mathrm{j}}hat{b} {e}^{2i{omega }_{{rm{m}}}t}+,text{h.c.})) describes the counter-rotating terms which are responsible for the coherent amplification of the signal. ({G}_{mathrm{j}}=sqrt{{n}_{{rm{d}},mathrm{j}}}{g}_{0,mathrm{j}}) is the parametrically enhanced electro- or optomechanical coupling rate where nd,j is the intracavity photon number due to the corresponding microwave and optical pump tones. For a red-detuned drive in the resolved-sideband regime 4ωm > κj we neglect ({hat{H}}_{{rm{CR}},mathrm{j}}) under the rotating-wave approximation and the Hamiltonian (1) represents a beam-splitter like interaction in which the mechanical resonator mediates noiseless photon conversion between microwave and optical modes. Note that near-unity photon conversion ({zeta }_{{rm{RS}}}=4{eta }_{{rm{e}}}{eta }_{{rm{o}}}{{mathcal{C}}}_{{rm{e}}}{{mathcal{C}}}_{{rm{o}}}/{(1+{{mathcal{C}}}_{{rm{e}}}+{{mathcal{C}}}_{{rm{o}}})}^{2}) can be achieved in the limit of ({{mathcal{C}}}_{{rm{e}}}={{mathcal{C}}}_{{rm{o}}}gg 1) with ({{mathcal{C}}}_{mathrm{j}}=4{G}_{mathrm{j}}^{2}/({kappa }_{mathrm{j}}{gamma }_{{rm{m}}})) being the electro- or optomechanical cooperativity, as demonstrated between two optical24 and two microwave modes25,26, respectively.

### Transducer design

We realize conversion by connecting an optomechanical photonic crystal zipper cavity27 with two aluminum coated and mechanically compliant silicon nanostrings28 as shown in Fig. 1c. The mechanical coupling between these two components is carefully designed (see Supplementary Note 2), leading to a hybridization of their in-plane vibrational modes into symmetric and antisymmetric supermodes. In case of the antisymmetric mode that is used in this experiment, the strings and the photonic crystal beams vibrate 180° out of phase as shown by the finite-element method simulation in Fig. 1d. The photonic crystal cavity features two resonances at telecom frequencies with similar optomechanical coupling strength. The simulated spatial distribution of the electric field component Ey(x, y) of the higher frequency mode with lower loss rate used in the experiment is shown in Fig. 1e. The lumped element microwave resonator consists of an ultra-low stray capacitance planar spiral coil inductor29 and two mechanically compliant capacitors with a vacuum gap of size of ~70 nm. This resonator is inductively coupled to a shorted coplanar waveguide, which is used to send and retrieve microwave signals from the device. The sample is fabricated using a robust multi-step recipe including electron beam lithography, silicon etching, aluminum thin-film deposition, and hydrofluoric vapor acid etching, as described in detail in ref. 30.

### Transducer characterization

Standard sample characterization (see Supplementary Notes 3 and 4) reveals an optical resonance frequency of ωo/(2π) = 198.081 THz with total loss rate κo/(2π) = 1.6 GHz and waveguide coupling rate κex,o/(2π) = 0.18 GHz leading to a coupling efficiency of ηo = 0.11. When the optical light is turned off, the microwave resonance frequency is ωe/(2π) = 10.5 GHz with coupling efficiency ηe = 0.4 and κex,e/(2π) = 1.15 MHz. The mechanical resonator frequency has a value of ωm/(2π) = 11.843 MHz with an intrinsic decoherence rate γm/(2π) = 15 Hz at a mode temperature of 150 mK. The achieved single-photon-phonon coupling rates are as high as g0,e/(2π) = 67 Hz and g0,o/(2π) = 0.66 MHz.

### Conversion measurements

To perform coherent photon conversion, red-detuned microwave and optical tones with powers Pe(o) are applied to the microwave and the optical resonator. These drive tones establish the linearized electro- and optomechanical interactions, which results in the conversion of a weak microwave (optical) signal tone to the optical (microwave) domain measured in our setup as shown in Fig. 1f. We experimentally characterize the transducer efficiency by measuring the normalized reflection Sjj2 (j = eo) and the bidirectional transmission ζ : = SeoSoe coefficients as a function of signal detuning δj. As shown in Fig. 2a, for drive powers Pe = 601 pW and Po = 625 pW with drive frequencies ωd,j and detunings Δj = ωj − ωd,j of Δe = ωm and Δo/(2π) = 126  MHz leading to intracavity photon numbers of nd,e(approx) 9 × 105 and nd,o(approx) 0.2 with cooperativities ({{mathcal{C}}}_{{rm{e}}}approx 0.57) and ({{mathcal{C}}}_{{rm{o}}}approx 0.9), the measured total (waveguide to waveguide) photon transduction efficiency is (approx)1.1% corresponding to 96.7% internal (resonator to resonator) photon transduction efficiency over the total bandwidth of Γconv/(2π) (approx) 0.37 kHz. In the case of κo > 4ωm and κe < 4ωm, the bandwidth is given by ({Gamma }_{{rm{conv}}}approx ({{mathcal{C}}}_{{rm{e}}}+1){gamma }_{{rm{m}}}) because the nonsideband resolved optomechanical cavity does not induce mechanical broadening. The signal tone adds 17(10−3) photons to the microwave resonator (optical cavity).

Here we use a self-calibrated measurement scheme that is independent of the gain and loss of the measurement lines as described in ref. 31 and we only take into account transduction between the upper two sidebands at ωd,j + ωm as shown in Fig. 1b. Neglecting the lower optical sideband that is generated due to the nonsideband resolved situation κo/4ωm(approx) 30 reduces the reported mean bidirectional efficiencies by (sqrt{2}) compared to the actually achieved total transduction efficiency between microwave and optical fields. The observed reflection peaks indicate that both resonators are undercoupled, equivalent to an impedance mismatch for incoming signal light. All scattering parameters are obtained from measured coherent tones whose linewidths are given by the chosen resolution bandwidth and the stability of the heterodyne setup. While this does not explicitly show long term phase stability of the conversion we find that these results are in excellent agreement with our coherent conversion theory model (solid lines) with γm as the only free fit parameter.

Figure 2b shows the total transduction efficiency for different pump power combinations with microwave and optical pump powers ranging from 30 to 953 pW and 48 to 1561 pW, respectively. Figure 2c, d shows the efficiency versus Po (Pe) for fixed microwave (optical) pump power Pe = 601 (Po = 625) pW. As expected, the transduction efficiency rises with increasing pump powers and reaches a maximum of ζ = 1.2%. The internal transduction efficiency is significantly higher (ζ/(ηoηe) ≤ 135%) because both the microwave resonator as well as the optical cavity are highly undercoupled with coupling ratios of ηo = 0.11 and ηe ranging between 0.07 and 0.18 when both pumps are on. The increase in the intrinsic loss rate of microwave κin,e and mechanical resonator γm at higher pump powers are shown in Fig. 2e and f caused by considerable heating related to (especially optical) photon absorption. This results in the degradation of the microwave and mechanical quality factors and consequently reduces the waveguide coupling efficiency, the cooperativities and the total transduction efficiency (see Supplementary Note 5).

### Sideband resolution and amplification

In the nonsideband resolved limit the contribution of the counter-rotating term of the Hamiltonian ({hat{H}}_{{rm{CR}},{rm{o}}}) is nonnegligible, resulting in a transduction process that cannot be fully noise- free. This interesting effect can be correctly described by introducing an amplification of the signal tone with (in the absence of thermal noise) quantum limited gain ({{mathcal{G}}}_{{rm{o}}}) (see Supplementary Note 1). In contrast, the microwave resonator is in the resolved-sideband condition 4ωm > κe, so that the signal tone amplification due to electromechanical interaction is negligible ({{mathcal{G}}}_{{rm{e}}}simeq 1). This results in the total, power independent, bidirectional conversion gain of ({mathcal{G}}={{mathcal{G}}}_{{rm{e}}}{{mathcal{G}}}_{{rm{o}}}simeq {{mathcal{G}}}_{{rm{o}}}), which turns out to be directly related to the minimum reachable phonon occupation:

$${langle nrangle }_{min }=frac{{({Delta }_{{rm{o}}}-{omega }_{{rm{m}}})}^{2}+{kappa }_{{rm{o}}}^{2}/4}{4{Delta }_{{rm{o}}}{omega }_{{rm{m}}}}={{mathcal{G}}}_{{rm{o}}}-1,$$

(2)

induced by optomechanical quantum backaction when the mechanical resonator is decoupled from its thermal bath32. Due to this amplification process the measured transduction efficiency in Fig. 2a is about 110 times larger than one would expect from a model that does not include gain effects for the chosen detuning, and adds the equivalent of at least one half of a vacuum noise photon to the input of the transducer in our case of heterodyne detection (for ηj = 1 and ({mathcal{G}}gg 1)). However, it turns out that this noise limitation, which might in principle be overcome with efficient feedforward9, sideband suppression33,34, or sideband resolution35, accounts for only about 0.1% of the total conversion noise observed in our system. The total transduction (including gain) can be written in terms of the susceptibilities of the electromagnetic modes ({chi }_{mathrm{j}}^{-1}(omega )=i({Delta }_{mathrm{j}}-omega )+{kappa }_{mathrm{j}}/2) and the mechanical resonator ({chi }_{{rm{m}}}^{-1}(omega )=i({omega }_{{rm{m}}}-omega )+{gamma }_{{rm{m}}}/2) as:

$$zeta ={left|frac{sqrt{{kappa }_{{rm{ex}},{rm{e}}}{kappa }_{{rm{ex}},{rm{o}}}}{G}_{{rm{e}}}{G}_{{rm{o}}}{chi }_{{rm{e}}}{chi }_{{rm{o}}}left[-{chi }_{{rm{m}}}+{tilde{chi }}_{{rm{m}}}right]}{1+[{chi }_{{rm{m}}}-{tilde{chi }}_{{rm{m}}}]left[{G}_{{rm{e}}}^{2}({chi }_{{rm{e}}}-{tilde{chi }}_{{rm{e}}})+{G}_{{rm{o}}}^{2}({chi }_{{rm{o}}}-{tilde{chi }}_{{rm{o}}})right]}right|}^{2},$$

(3)

where (tilde{{chi }_{mathrm{j}}}(omega )={chi }_{mathrm{j}}{(-omega )}^{* }).

Equation (3) can be decomposed into a product of the conversion gain ({mathcal{G}}) and the pure conversion efficiency θ, i.e., (zeta :={mathcal{G}}times theta), for frequencies in the vicinity of ωm (see Supplementary Note 1). Equation (2) shows that the signal amplification depends only on the resonator linewidth and the detuning and is not directly related to the (propto {hat{a}}^{dagger }{hat{b}}^{dagger }) interaction term or the pump power31. This can be understood by the alternative interpretation that the gain represents the ratio of the transduced upper sideband to the difference between upper and lower sideband at each cavity. Therefore, it is instructive to measure the transducer parameters as a function of optical pump detuning as shown in Fig. 3a. While changing the optical detuning, we also vary the pump power in order to keep the optical intracavity photon number constant at nd,o = 0.185 ± 0.015. This way it is possible to investigate the influence of Δo at a constant optomechanical coupling ({G}_{{rm{o}}}={g}_{{rm{0,o}}}sqrt{{n}_{{rm{d}},{rm{o}}}}). The measured total transduction efficiency is shown in Fig. 3a and reaches  ζ(,approx,)1% at Δo(approx) 0 for the chosen pump powers in agreement with Fig. 2c, d. We can now separate the measured transduction (Eq. (3)) into conversion gain and pure conversion, as shown in Fig. 3b. The gain shows the expected steep increase at Δo → 0 where the pure conversion θ approaches zero for equal cooling and amplification rates. Around Δo = κo/2 on the other hand, where ({langle nrangle }_{min }) reaches its minimum of roughly κo/4ωm(approx) 30, also the gain reaches its minimum and the noiseless part (at zero temperature) of the total (internal) conversion process shows its highest efficiency of θ = 0.019% (θ/(ηeηo) = 1.6%).

The fitted effective mechanical bath temperature as a function of pump powers is shown in Fig. 4c. It reveals the strong optical pump dependent mechanical mode heating (blue), while the microwave pump (red) has a negligible influence on the mechanical bath. Fig. 4d shows the measured total added noise at the output of the microwave resonator and optical cavity as a function of optical pump power. The noise added to the optical output (blue) increases with pump power due to absorption heating and increasing optomechanical coupling rate Go, while the degradation of the resonator-waveguide coupling efficiency ηe explains the decreasing nadd,e at higher optical powers for the microwave output noise (red), see Fig. 2e. The intersection of the two noise curves occurs at ({{mathcal{C}}}_{{rm{e}}}simeq {{mathcal{C}}}_{{rm{o}}}) with cooperativities Cj as defined above, and shows that the optical and microwave resonators share the same mechanical thermal bath. The power dependence is in full agreement with theory (solid lines) and demonstrates that the thermal mechanical population is the dominating origin of the added transducer noise.