D. Ciplys

Electromechanical coupling coefficient for surface acoustic waves in GaN-on-sapphire

The electromechanical coupling coefficient for surface acoustic waves propagating in GaN-on-sapphire structure has been evaluated using in situ measurements of the SAW attenuation during the evaporation of a metal film on the GaN surface. The extracted values for samples with GaN layer thicknesses of 2.2-2.4 μm were in the range of 0.06-0.09% at SAW frequencies of 200-300 MHz.

Diffraction of guided optical waves by surface acoustic waves in GaN

We report on acousto-optical diffraction of the guided optical modes (λ=633 nm) in GaN layers grown on (0001) sapphire substrates. The diffraction of both transverse electric (TE) and transverse magnetic (TM) modes has been observed. The dependence of the diffraction efficiency on the surface acoustic wave (SAW) power is in good agreement with Raman-Nath theory. The values of acousto-optic figure of merit, M2, and effective photoelastic constants p have been determined. For the SAW propagating in [1100] direction, the extracted values are M2=1.54×10−15 s3/kg, p=0.061 (TE modes) and M2=1.95×10−15 s3/kg, p=0.066 (TM modes). These values of M2 correspond to 1 and 1.3 of the acousto-optic figure for fused quartz. The obtained results demonstrate the potential of GaN-based structures for the development of blue–ultraviolet acousto-optical devices.

Surface acoustic waves in graphene structures: Response to ambient humidity

We report on the response to the ambient air humidity of a surface acoustic wave (SAW) in YZ LiNbO3 substrate with the 200 nm-thick multi-layer graphene sheets deposited by chemical reduction from graphene oxide. With increasing relative humidity (RH), the SAW velocity decreases and the SAW attenuation increases. At humidity values below 60–65%, the SAW velocity linearly depends on RH with the slope proportional to the SAW frequency. This is attributed to mass loading of the SAW propagation surface due to moisture adsorption by the graphene layer. The normalized fractional change in SAW velocity is 4.23·10−2 ppm/(MHz·%RH). At higher humidity values, the velocity decrease becomes super-linear due to the change in elastic graphene properties. An exponential growth of the humidity-induced SAW attenuation by the graphene film with frequency (from 1 dB/cm at 57 MHz to 7 dB/cm at 286 MHz) has been observed. The time response to abrupt humidity changes is different for the transmitted SAW phase velocity and attenuation pointing out to different physical mechanisms responsible for humidity-induced SAW attenuation and velocity variations.

Optical Guided Modes and Surface Acoustic Waves in GaN Grown on (0001) Sapphire Substrates

The propagation of guided optical modes and surface acoustic waves in GaN layers grown on sapphire by MOCVD technique is experimentally studied. The effective refractive indices of guided optical modes are measured using the prism coupling technique, and the thickness and refractive indices of the layer are evaluated. The surface acoustic wave velocities are measured by acoustooptic diffraction technique, revealing a strong acoustic dispersion. The evaluation of GaN layer thickness from the SAW measurements is demonstrated.

Acoustoelectric effects in reflection of leaky acoustic waves from LiTaO3 crystal surface coated with metal film

The strong variations in amplitude and phase of leaky acoustic wave excited on one surface of YX lithium tantalate crystal plate and reflected from the opposite parallel surface have been observed during thermal evaporation of copper film onto the reflection surface. These variations are caused by the acoustoelectric interaction in the reflection region, and they are described in terms of surface acoustic wave velocity and attenuation dependence on the film sheet conductivity. The electromechanical coupling constant value of 5% has been extracted from amplitude measurements, and that of 4% from the phase measurements.

Guided-wave acousto-optic diffraction in AlxGa1−xN epitaxial layers

The acousto-optic (AO) diffraction of guided optical waves from surface acoustic waves in AlxGa1−xN layers grown on sapphire substrates by Migration Enhanced Metalorganic Chemical Vapor Deposition (MEMGCVD™) technique was studied at the optical wavelengths of 442 and 633nm and acoustic wavelength of 16μm. In the near-to-Bragg diffraction regime, the diffraction efficiency from 90% to 95% was attained at SAW powers of 0.28 and 0.72W for the blue and red light, respectively. The simulation using photo-elastic and electro-optic constants reported in literature revealed the prevailing contribution of the photo-elastic effect to the AO diffraction. The calculated SAW power required to attain the diffraction maximum was about seven times larger than the measured values. This discrepancy implies that the photo-elastic constants of AlN and GaN available from literature are underestimated. The increase in the diffraction efficiency with the decreasing optical wavelength is in a good agreement with the theoretical ...

Surface acoustic wave response to optical absorption by graphene composite film

Propagation of surface acoustic waves in YZ LiNbO3 overlaid with graphene flakes has been investigated and its optical response to illumination by 633-nm light from a He-Ne laser was studied. The heating of the sample surface caused by optical absorption by the graphene led to a downshift in the transmitted SAW phase caused by the wave velocitys dependence on temperature. The proposed simple model based on optothermal SAW phase modulation was found to be in good agreement with the experimental results.

Detection of CO 2 absorption in graphene using surface acoustic waves

CO2 absorption mechanism in sheets of graphene layers prepared by thermal exfoliation of graphene oxide was investigated using surface acoustic waves. In this experiment, LiNbO3 SAW delay-line device was spray coated with graphene flakes. The transmission at SAW frequencies was attenuated by more than 12.7 dB due to deposition of graphene. A maximum phase change ∼34 degrees was observed as the CO2 pressure inside the chamber increased from 0.01 Torr to 700 Torr, which is at least 10 times than the phase shift observed on free surface of LiNbO3 under similar conditions. Transient measurements on the graphene-coated sample indicate a phase change of 4 degree because of 0.43 mg/cm3 of CO2 absorption and subsequent full recovery after 20 min of CO2 purging.

Acousto-optic interaction with leaky surface acoustic waves in Y-cut LiTaO3 crystals.

The acousto-optic interaction with leaky surface acoustic wave radiation into the bulk of YX-cut LiTaO(3) crystals has been investigated. The light incidence and diffraction angles corresponding to the strongest acousto-optic interaction were calculated and measured as functions of the acoustic wave frequency. The dependencies of the diffracted light intensity on the amplitude of radio-frequency voltage applied to the interdigital transducer (IDT) were studied. Our acousto-optic measurements revealed generation, by the IDTs, of slow shear bulk acoustic waves propagating at different angles depending on their frequency. A secondary acousto-optic interaction from the bulk waves radiated by the receiving IDT has been studied.

Velocity and attenuation of surface acoustic waves in proton-exchanged 128 degrees -rotated Y-cut LiNbO3

The velocity and attenuation of surface acoustic waves (SAW) in a 128 degrees -rotated Y-cut lithium niobate crystal with a proton-exchanged optical waveguiding layer on its surface have been studied. The measurements were carried out using the acoustic-optical diffraction method in the SAW frequency range 30-600 MHz. The SAW propagation was mainly along the X axis, though some velocity measurements were carried out for the normal direction. Strong velocity decrease and attenuation increase due to the proton exchange were observed. The acoustic dispersion and losses grow with the waveguide depth and SAW frequency, and are strongly affected by the post-exchange annealing. The results are explained by the formation of a HxLi1-xNbO3 layer on the LiNbO3 with many defects.