Generally, IDT is used to excite the SAWs. However, this requires IDT fabrication using photolithography.

A point source of sound on a crystal surface can produce  ripple patterns because the speed of sound depends on direction, highly concentrating the acoustic energy in certain directions. This is known as phonon focusing.

Another method to generate and detect the surface waves in metal films on transparent substrates using ultrashort light pulses of duration about 1 picosecond (10-12 s). A blue light pulse excites the sound waves through thermal expansion by illumination through the transparent substrate.

The wavelength of the surface waves is determined by the optical spot size on the surface, a few microns, corresponding to ultrasonic frequencies in the 100 megahertz to 1 gigahertz range (108 to 109 cycles/second).

Further reading: 

https://en.wikipedia.org/wiki/Surface_acoustic_wave

http://kino-ap.eng.hokudai.ac.jp/ripples.html

Making using of the photo acousltic effect

You can generate acoustic wave using non contact excitation using pulsed lasers.

You need to have special mode locked pico second lasers.

Both BAW's and SAW's are generated and can cause interference, and and direction control seems to be a little problem.

Read the beautiful book "Advances in Acoustic Microscopy- Volume 1" edited by Andrew Briggs, you will get a lot of useful information. 

there are lot of interesting papers using such a photo acoustic technique to generate SAW's

K. Sreenivas

Read this recent paper - here they make use of aluminium foil electrodes to generate a MHz-order SAW (lamb-wave mode). No equipment, other than a piece of piezoelectric material (here LN) and a signal generator is required, though the ability to locally define the field aperture is lost as compared to a conventional IDT structure.

http://pubs.rsc.org/en/content/articlehtml/2014/lc/c4lc00182f

What wavelengths / frequencies do you want to achieve, regarding the SAW?

Dear Andreas,

The range of frequency should be around 2-30 MHz

And the media inside the PDMS channel is water

Regards,

Yannapol S.

So, you only need rough structures that should be easily achievable with optical lithography (even without a clean room).

However, I see another important issue: SAWs are a solution of the wave equation for substrates with a thickness much larger than the SAW wavelength. Even for 30MHz (130µm wavelength @ LiNbO3), a standard substrate with 500µm thickness is just 3x thicker, which may be regarded as still sufficient in a first approximation. For 2MHz, you would need a substrate with at least 5mm thickness. Otherwise, you will excite plate waves, not SAWs...

Hi Andreas,

Why is the thickness of substrate dependent on the frequency so much? Why are they so correlated?

Best, 

Xiangchao

Hi Xiangchao,

it seems i forgot to answer your question - or just didnt see it.

For the excitation of SAW, the boundary condition of a substrate thickness larger than the SAW wavelength has to be fulfilled. Thats the only frequency dependence in respect to the substrate thickness. Approx. 2.5x Lambda should be sufficient for most cases. If the substrate is thinner, the chip backside has to be taken into account and will contribute to the wave excitation on the front side. For very thin substrates, you'll excite plate waves. In this case, the frequency and excitable wave modes strongly depend on the substrate thickness. There is a similar discussion here: https://www.researchgate.net/post/For_what_thickness_of_plate_Rayleigh_wave_will_become_Lamb_wave

Best, Andreas

Hi Andreas,

You can use an Angle Beam Transducer & Wedge (Olympus). According to the manufacturer, specialized transducers can work up to 225 MHz. Depending on the sound velocity in your transducer (often chosen to match that of water), you can calculate the Rayleigh angle to generate a Rayleigh wave.

If your material is thinner than a couple of wavelength, you will indeed generate a guided which is not a Rayleigh wave (most common type of SAW) but instead a more general Rayleigh-Lamb (RL) wave. Unlike pure Rayleigh waves, RL waves are dispersive so you will need to adjust the angle of the transducer for each new frequency.

Good luck!

Antoine