Coupling Microwave to Optical Photons Using Multi-Terminal Acoustic Waves
Studying acoustic waves on solid surface, or in bulk, opens exciting new avenues for quantum research. Experiments in this field often depend on cumbersome analog setups with power splitters, phase shifters, and tunable attenuators to reach the required signal stability and frequency coverage on multiple channels. The Zurich Instruments SHFSG+ Signal Generator in turn offers all these functionalities directly out of the box, and comes with a powerful graphical software to streamline measurement tasks.
Acoustic waves generate dynamic, nanoscale strain and electric fields as they propagate through piezoelectric materials, offering a versatile, contactless means to control quantum states in low-dimensional semiconductor systems, quantum dots, and other solid-state platforms [1,2]. Because acoustic waves can be engineered to interact coherently with charge carriers, spins, or opto-electronic excitations, they facilitate essential functionalities such as controlling and coupling quantum bits for quantum sensing applications.
Recently, we had the opportunity to visit the Paul Drude Institute for Solid State Electronics, which has the core competence in controlling acoustic fields in electronic devices. We conducted two experiments using the SHFSG+ Signal Generator, showcasing the use of acoustic excitations as a versatile tool. In contrast to the conventional use of power splitters, phase shifters and tunable attenuators, the SHFSG+ makes the studied experiments more accessible by providing multiple synchronized channels and a precise digital control of the phase and amplitude of the microwave signals. In the first part of the blog post, I will describe an experiment to generate complex surface patterns by means of overlapping surface acoustic waves (SAWs). In the second part, I will dive into the coupling between microwaves and optical photons, with the coupling mediated by acoustic fields. The two experiments were performed on two consecutive days within 24 hours, which was only possible due to the well-aligned goals and deep expertise between the researchers at the PDI Berlin and Zurich Instruments.
Generation of complex surface patterns
A precise, coherent control of SAWs is crucial for their application in the field of quantum control. In the first experiment, we will showcase the capability of controlling and overlapping SAWs with the SHFSG+ Signal Generator.
Spatial overlaps of SAWs allow us to realize complex dynamic strain patterns on the surface of solid-state devices. With precisely controlled phase differences between three interfering standing SAWs, one can realize an acoustic vortex, an advanced dynamical phenomenon which can be used to modulate light that is reflected on the device surface. Acoustic vortices are swirling patterns of motion where the elements of the material rotate around a central axis.
Experimental Setup
The device under test (DUT) is a sapphire substrate with sputtered ZnO. Planar interdigitated transducers (IDTs) are fabricated on top of ZnO. The six IDTs are arranged in a circle such that they form three SAW cavities rotated by 120 degrees relative to each other (see Figure 1(a)). By exciting IDT pairs with specific phase differences, we can generate complex interference patterns. The experiment is carried out at room temperature.
The IDTs are connected to the wave outputs of the SHFSG+ Signal Generator. SAWs are excited by applying microwave signals to the IDTs. Applying continuous microwave signals (CW signals) at the device resonance frequency f0 = 610.5 MHz addresses the fundamental vibrational mode forming standing wave patterns. The standing wave patterns are electrically characterized by means of scanning laser interferometry, which allows us to map the amplitude of the local SAW field with sub-micrometer resolution [3]. The reflected laser beam is converted into electrical signals and measured. The output range of the microwave channels is set to 5 dBm, which is then amplified with room-temperature amplifiers with a nominal gain of 16 dB.
Results
We first create a simple pattern by applying in-phase CW signals to two pairs of the IDTs. The color plot in Figure 1(b) shows the measured amplitude in a selected area. The bright spots in the plot are regions with maximum amplitude and correspond to maximum surface displacement of the standing waves (anti-nodes). The anti-nodes as well as the nodes repeat periodically over the entire measured map. Acoustic waves are generated along two directions, thus the anti-nodes and nodes form a diamond-shaped pattern. The positions of the nodes are indicated with white dashed lines for reference in Figure 1(b).
Now, we create a more complex pattern by applying CW signals to all three IDT pairs. More specifically, we set the microwave phase difference between two pairs to be 120 degrees, so that they are 0 degrees on IDT pair 1, 120 degrees on IDT pair 2, and 240 degrees on IDT pair 3. The measured amplitude for this setting is plotted in Figure 1(c). Features are again periodic but clearly differ from those in Figure 1(b). When looking at the bright features, they are broadened and form “shrimp-like” shapes. A white dashed circle is added to the plot to outline one of the “shrimp-like” shapes. The occurrence of such a feature is in agreement with the swirling motions of the surface material element. The minimum surface displacement in the center is the center of the vortex. The exact time evolution of the swirling surface element can be reconstructed by measuring both the amplitude and phase. Further fine calibration of the microwave amplitude and phase difference can optimize the shape of the vortices.
Coupling of microwaves to optical photons
In the second part of the blog post, we discuss a path forward for the coupling of microwave signals to optical photons. A coherent coupling between Gigahertz (GHz) microwave signals and optical photons is particularly intriguing in the context of quantum technologies. On the one hand, microwave photons are natural carriers of information in several widely adopted quantum processor types. On the other hand, optical photons, when transmitted through optical fiber, benefit from low loss and high bandwidth. A coherent link between them will enable a long-distance transfer between distributed quantum processors, making existing quantum communication and computing platforms more versatile.
Exciton-polaritons are light-matter composite bosons arising from the strong coupling between excitons and photons. These particles are interesting as candidates for microwave-to-optical transducers [2]. As demonstrated in the previous experiment, acoustic waves on solid-state devices can coherently couple to microwaves. What remains is the demonstration of a coupling mechanism between acoustic fields to optical photons. Here, the polariton can aid with its electronic component. Hence, the goal of this experiment is to show an acoustically mediated coupling between the microwave to optical photons.
Experimental Setup
The DUT is a vertical semiconductor microcavity (MC), created by epitaxially grown layers of AlGaAs with different Al concentration forming top and bottom so-called distributed Bragg reflectors (DBRs) on a nominally undoped GaAs wafer. The top and bottom DBRs are separated by a spacer layer. A quantum well (QW) region is embedded in the spacer layer, where excitons can be created by laser irradiation. Due to the arrangement of the DBRs, optical fields are vertically confined within the spacer with the anti-node at the QW region. Three bulk acoustic transducers are fabricated in a circular arrangement on the surface of the DUT to inject bulk acoustic waves, see Figure 2(a). The center of the transducer structure is opened for laser irradiation. Furthermore, the device is engineered to have a large overlap between the acoustic and optoelectronic fields in the aperture formed by the transducer structure and at the QW.
A laser (λ = 760 nm) irradiates into the aperture at cryogenic temperature of 10 K such that excitons are generated in the QW. Due to the MC optical confinement, QW excitons couple strongly to the photons leading to the formation of exciton-polaritons. On the microwave side, the bulk acoustic transducers are driven by the SHFSG+ Signal Generator, where microwave CW signals are applied to excite bulk acoustic waves towards the MC. Due to the strong coupling of the acoustic fields with the optoelectronic fields, the spectrum of the polariton resonance can be modified by the microwave signals. Hence, a coupling from the microwave to the optical photons is established, with the coupling mediated by acoustic fields.
Results
Figure 2(b) shows the photoluminescence (PL) signal of the DUT in the absence of acoustic excitations. The spectrum shows a clear resonance at 809.5 nm which corresponds to the exciton-polariton resonance from the MC (top). The spatially resolved map further reveals that the PL signal has a distribution over about 18 µm along the y-axis, consistent with the size of the aperture formed by the transducers (bottom).
A change of the PL signals from their bare values is expected with the application of microwaves. The SHFSG+ Signal Generator is now set to output in-phase 7 GHz microwaves at an output range of 10 dBm on all three transducers. The microwave signals are amplified with room-temperature amplifiers with a nominal gain of 16 dB. Figure 2(c) shows the associated PL signal of the DUT which is clearly broadened and differs from the spectrum of bare polaritons. The spectrum shows a double peak feature with maxima at 808 nm and 811 nm (top). This double peak feature can be observed across the whole one-dimensional scan in the spatially resolved map, and is a result of the acoustic-optoelectronic coupling (bottom). Furthermore, the spectral broadening increases towards the center, where the acoustic waves from all three transducers overlap.
While this experiment demonstrates the coupling between microwave to optical photons using three independently controlled GHz acoustic fields, we have not investigated the effects of microwave amplitudes and phase differences here. Overlapping acoustic waves leads to interference patterns, so a spatial modulation of the polariton spectral line can be controlled by the microwave amplitudes and phase differences. Furthermore, a coherent microwave-to-photon coupling is not evident from this experiment and its demonstration requires further investigation.
Summary
In the first part of the blog post, we demonstrated the generation of complex surface patterns using microwave-induced SAWs. Specifically, we generated an array of acoustic vortices by overlapping SAWs from three directions with controlled phase differences (0°, 120° and 240°). Both the out-of-the-box phase stability between the channels on the SHFSG+ Signal Generator and the independent, electronic control of the microwave phase difference and amplitude allow a convenient setup of the experiment. Due to imperfections in the device and experimental setup, a calibration of the amplitude and phase difference is necessary before the actual pattern generation. This is out of the scope of this blog post.
In the second part of the blog post, we demonstrated a microwave-induced modification of the polariton spectral line, measured by the PL signals in the semiconductor MC. This experiment leveraged the piezoelectric effect to coherently couple microwave to bulk acoustic waves and the generation of exciton-polaritons for the acoustic-optoelectrionic coupling. An interesting quest is to now engineer a system where a few microwave photons can coherently couple to a few polaritons, paving the path for many applications in quantum communication and computation.
The frequency flexibility of DC to 8.5 GHz of the SHFSG+ Signal Generator together with the direct frequency output without mixer calibration eased the change between the SAW (610.5 MHz) and bulk (7 GHz) experiments. In addition, the LabOne graphical interface facilitates a quick testing of acoustic waves with different properties. In the future, it will be interesting to extend the experiment protocols with the pulse sequencing capabilities of the SHFSG+ Signal Generator to enable the study of the dynamics of these hybrid devices.
Acknowledgements
We thank Dr. Alexander Kuznetsov and Mr. Nazim Ashurbekov at PDI Berlin for the possibility of performing these experiments, sharing the data and their valuable feedback regarding the blog post. We also thank Dr. Paulo V. Santos for insights into the physics of their acoustic-optoelectronic hybrid devices. I would also like to thank my colleague Dr. Avishek Chowdhury and Dr. Tim Ashworth for their valuable suggestions and feedback.
References
[1] S. Maity et al. “Coherent acoustic control of a single silicon vacancy spin in diamond”. Nat. Commun., 11, 193, 2020.
[2] A. Kuznetsov et al. “Microcavity phonoritons – a coherent optical-to-microwave interface”. Nat. Commun., 14, 5470, 2023.
[3] Y.-T. Liou et al. “Spatial Analysis of Multi-Frequency SAW Beams Excited by Slanted IDTs on ZnO-SiC Heterostructures”. J. Phys. D: Appl. Phys. 57, 415302, 2024.