0:00:03here is a great in parallel bar
0:00:06if you put a point of the waves in the flat grading which way we'll
0:00:10the waves go
0:00:11the shape of the spiking helmet is actually a clue more about later
0:00:16we experimented with sound in microscopic grading
0:00:20structures like this local phone only crystals was acoustic properties have been engineered to vary
0:00:25periodically in space
0:00:27in the experiment blue laser light of this create tiny time triples by deleting a
0:00:32small ball
0:00:34infrared light pulses detect the ripples
0:00:36this time pitch is very high around one together with amplitude in the peak a
0:00:41meter range
0:00:42with down the detection one and change the timing of the latent vote is to
0:00:46make movies of the ripples using interferometry
0:00:49our grading is made of tiny colour and delay constraints on a silicon flat and
0:00:54this is coded with goal
0:00:56we've by i like the pulses repetitively at the center of a one and you
0:01:01might want square area
0:01:02this movie slowed down a thousand million times
0:01:06let's analyze different frequencies
0:01:09at low frequencies we see roughly circular ripples
0:01:13actually this is reasonable because the sound wave length is about ten microns much larger
0:01:19than the grading by a
0:01:21so this time detective it doesn't see the grading
0:01:24but at high frequencies we see this interesting shaped pattern
0:01:29and the x shape close is at higher frequencies
0:01:32the top row shows that we can get the same effect numerical simulations
0:01:37but bottom row is experiment
0:01:39now that and of life sound wavelength understand the in these images
0:01:45there are two important space is called case phase and group velocity space
0:01:50for a given frequency in case space we plot inverse of the wavelength or rather
0:01:55the wave number in the x and y directions
0:01:58in group a multi-space shown on the right we plot speed of a sound files
0:02:04in this approximate analytical approach at low frequencies we get round shapes in both spaces
0:02:10this means the ripples are roughly circular
0:02:13as the frequency increases we hate of and got in the horizontal direction of direction
0:02:18weights can travel visible through the opening so in case space
0:02:24and this makes to the in group a two d space
0:02:29these two circles position at higher frequencies
0:02:34in case basis weights to vary strongly perpendicular to the turning points in the curves
0:02:40in group a multi-space biz correspond to joining the times of the circles
0:02:45this produces the shape with or
0:02:48if we see all frequencies to get a group of l d space looks like
0:02:52this
0:02:54so now d the relation with the viking helmet
0:02:58imagine a cross section three the helmet here
0:03:01it is just the circle and that gives the round trip will happen
0:03:05what is use like the helmet at higher frequencies that means higher you get too
0:03:10small circles
0:03:12so the way the channel in the direction of these two circles
0:03:16this explains why exchange close is at higher frequencies
0:03:20actually i should rotate the ice of this helmet to correspond to experiment like the
0:03:29the spiky focusing buttons a cold costings
0:03:33you can find them in coffee cups
0:03:35and also when you generate sound waves on real crystals
0:03:40so point so integrating produces an x pattern code by acoustics related to the band
0:03:46is that they should also be seen in a variety of spatially periodic structures and
0:03:51is not just confined to stand
0:03:53it would with electromagnetic or what waves for example
0:03:58i will conclude by saying
0:04:00for that
0:04:02which is the biking for thanks policemen