Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Modulating Surface Acoustic Wave Generation through Superconductivity

Andrew Christy, Yuzan Xiong, Rui Sun, Yi Li, Kenneth O. Chua, Andrew H. Comstock, Junming Wu, Sidong Lei, Frank Tsui, Megan N. Jackson, Dali Sun, Valentine Novosad, James F. Cahoon, Wei Zhang

Abstract

Surface acoustic waves (SAWs), with their five orders-of-magnitude slower propagation velocity, allow for considerably shorter wavelengths at the same frequency compared to electromagnetic waves. The short wavelengths allow for device miniaturization and on-chip integration. The generic design of these devices involve piezoelectric substrates with comblike arrays of Al or Au electrodes known as interdigitated transducers deposited on the surface. However, Al and Au both have shortcomings at the cryogenic temperatures required for quantum applications, namely the formation of two-level systems and the lack of superconductivity perpetuating Ohmic losses, respectively. In this work, SAWs are generated in the high-MHz to low-GHz range using niobium nitride (NbN) interdigitated transducers (IDTs) and Bragg reflectors. We demonstrate the fabrication of acoustic devices through photolithography and reactive ion etching (RIE). The sharp transition between superconducting and normal states and the corresponding change in SAW transmission allows for fine control of the 'on' (superconducting) and 'off' (normal) states of NbN, with a Δ_T = K separating the transmission minimum and maximum. We demonstrate a 16x difference in transmission between the 'on' and 'off' states of the device. The SAW transmission behavior mirrors the change in resistance of NbN at its Tc. These findings open up new possibilities for the integration of NbN SAW resonators into existing quantum architectures based on NbN and a method for adjusting transmission properties independent of applied voltage.

Modulating Surface Acoustic Wave Generation through Superconductivity

Abstract

Surface acoustic waves (SAWs), with their five orders-of-magnitude slower propagation velocity, allow for considerably shorter wavelengths at the same frequency compared to electromagnetic waves. The short wavelengths allow for device miniaturization and on-chip integration. The generic design of these devices involve piezoelectric substrates with comblike arrays of Al or Au electrodes known as interdigitated transducers deposited on the surface. However, Al and Au both have shortcomings at the cryogenic temperatures required for quantum applications, namely the formation of two-level systems and the lack of superconductivity perpetuating Ohmic losses, respectively. In this work, SAWs are generated in the high-MHz to low-GHz range using niobium nitride (NbN) interdigitated transducers (IDTs) and Bragg reflectors. We demonstrate the fabrication of acoustic devices through photolithography and reactive ion etching (RIE). The sharp transition between superconducting and normal states and the corresponding change in SAW transmission allows for fine control of the 'on' (superconducting) and 'off' (normal) states of NbN, with a Δ_T = K separating the transmission minimum and maximum. We demonstrate a 16x difference in transmission between the 'on' and 'off' states of the device. The SAW transmission behavior mirrors the change in resistance of NbN at its Tc. These findings open up new possibilities for the integration of NbN SAW resonators into existing quantum architectures based on NbN and a method for adjusting transmission properties independent of applied voltage.
Paper Structure (1 section, 2 equations, 4 figures)

This paper contains 1 section, 2 equations, 4 figures.

Table of Contents

  1. Supplementary Material

Figures (4)

  • Figure 1: (a) Schematic illustration of the NbN SAW resonator on lithium niobate. Above the $T_c$ of NbN, SAW transmission is negligible. Below the T$_c$, the superconducting nature of NbN induces strong SAW transmission. (b) Optical, SEM, and AFM images of the SAW device. The electrode edge roughness is due to limited resolution in photolithography. (c) Surface morphology profile of the SAW device scanned by AFM along with a side view of the device design including the thicknesses of the LNO (500 $\mu$m) and NbN (38 nm).
  • Figure 2: (a) Time domain results of the NbN IDT device measured at 11 K. The free space electromagnetic wave(EMW) occurs at 10.7 ns, the single-transit SAW pulse (red) occurs at 178.6 ns, and the round-trip SAW pulse (blue) occurs at 1.541 $\mu$s. The inset shows the device geometry, with dimensions L = 618 $\mu$m, d$_{T}$ = 225 $\mu$m, d = 36 $\mu$m. The IDT dimensions are w = 3 $\mu$m, p = 6 $\mu$m, $\lambda$ = 12 $\mu$m, designed for maximum fundamental frequency amplitude. (b) Frequency domain S$_{21}$ results of the device measured at 11 K for the single-transit SAW pulse (red) and the full cavity SAWs (blue). The fine splitting is due to the cavity that modulates the overall sinc$^2$ profile of the SAW mode. The inset shows the determination of the FSR value of the cavity, 0.733 MHz.
  • Figure 3: (a) S$_{21}$ spectrum of the single-transit SAW at the fundamental frequency over the superconducting transition (11.4 - 12 K). (b) S$_{21}$ of the cavity SAW at the fundamental frequency over the same temperature range. (c) The temperature dependence of the S$_{21}$ at the fundamental frequency (black dots), fitted to a logistic function (red). The blue trace is a separate resistance measurement of a 40-nm continuous NbN film.
  • Figure 4: (a) The line charge model representation of the IDTs in a SAW cavity. (b) Comparison of the normalized experimental data and calculated results from Eq.\ref{['eq01']} with a vertical offset for visual clarity. (c) Simulated frequency domain data, with both single-transit and cavity SAW modes contributions. The inset shows the determination of the FSR value of the cavity, 0.732 MHz.