1 00:00:00,000 --> 00:00:10,000 [MUSIC] 2 00:00:19,456 --> 00:00:22,765 Hi, and welcome to Coursera's Nanotechnology: the Basics. 3 00:00:22,765 --> 00:00:24,231 I'm Daniel Mittleman. 4 00:00:24,231 --> 00:00:25,913 This is week four, and the topic of 5 00:00:25,913 --> 00:00:28,639 week four is nanophotonics, which is the intersection 6 00:00:28,639 --> 00:00:32,935 of the world of nanoscience with the world of photonics, which is the study of light. 7 00:00:32,935 --> 00:00:35,614 In these series of lectures, we're going to talk about 8 00:00:35,614 --> 00:00:37,324 several different ways that, that 9 00:00:37,324 --> 00:00:39,960 intersection leads to interesting phenomena. 10 00:00:39,960 --> 00:00:41,560 In the first lecture here, I want to give a little bit 11 00:00:41,560 --> 00:00:44,800 of an introduction about what is light, and why is it that 12 00:00:44,800 --> 00:00:47,340 light plus nano leads to something interesting. 13 00:00:47,340 --> 00:00:50,390 And what are a few of the situations where that becomes really relevant. 14 00:00:50,390 --> 00:00:53,950 So let's start out with what does light have to do with nano objects. 15 00:00:53,950 --> 00:00:54,440 Right? 16 00:00:54,440 --> 00:00:57,660 Well in order to understand that we really need to understand what's light. 17 00:00:57,660 --> 00:00:59,420 So, this is the first picture that 18 00:00:59,420 --> 00:01:02,530 illustrates what's light, light is an electromagnetic wave. 19 00:01:02,530 --> 00:01:04,590 Its an isolating electric field and 20 00:01:04,590 --> 00:01:07,130 magnetic field that propagates through space. 21 00:01:07,130 --> 00:01:09,670 And there are many different forms of light. 22 00:01:09,670 --> 00:01:09,920 We can 23 00:01:09,920 --> 00:01:12,990 talk about radio waves, which have a very long wavelength. 24 00:01:12,990 --> 00:01:16,060 We can talk about gamma rays, which have a very short wavelength. 25 00:01:16,060 --> 00:01:19,250 And all of these various other terms, that fall in between which 26 00:01:19,250 --> 00:01:21,530 you've heard of, which chained are 27 00:01:21,530 --> 00:01:24,660 simply electromagnetic waves with different wavelengths. 28 00:01:24,660 --> 00:01:26,750 The visible region of the spectrum is here. 29 00:01:26,750 --> 00:01:28,400 It's sort of in the middle of this picture. 30 00:01:28,400 --> 00:01:31,280 That's the tiny slice you can actually see with your eye. 31 00:01:31,280 --> 00:01:34,450 And that corresponds to wavelengths between 400 and 700 nanometers. 32 00:01:36,470 --> 00:01:39,680 So, if that's the wavelength of light, then that immediately 33 00:01:39,680 --> 00:01:42,670 raises the question, what has light got to do with nanoparticles? 34 00:01:42,670 --> 00:01:45,135 Because there's clearly a size mismatch between a 35 00:01:45,135 --> 00:01:47,590 nanoparticle and a light wave, in the same 36 00:01:47,590 --> 00:01:49,350 kind of ratio that there's a size mismatch 37 00:01:49,350 --> 00:01:52,350 between a skier and the mountain she's skiing on. 38 00:01:52,350 --> 00:01:52,510 Right? 39 00:01:52,510 --> 00:01:57,210 A nanoparticle is a 100 times smaller than the wavelength of visible light. 40 00:01:57,210 --> 00:01:59,220 So then what does light got to do with nanomaterials? 41 00:01:59,220 --> 00:02:02,000 It seems like a really logical question to ask. 42 00:02:02,000 --> 00:02:03,770 So, in order to understand that, let's first 43 00:02:03,770 --> 00:02:06,220 start out by talking about what is light. 44 00:02:06,220 --> 00:02:09,370 The properties of light are governed by Maxwell's equations, which were first 45 00:02:09,370 --> 00:02:13,560 written down in the 1860s and are still considered exactly correct today. 46 00:02:14,570 --> 00:02:16,760 they fit very nicely on a coffee mug. 47 00:02:16,760 --> 00:02:18,610 but we're not going to try and solve these equations. 48 00:02:18,610 --> 00:02:20,460 It's a little bit beyond the scope of the class. 49 00:02:20,460 --> 00:02:22,300 Instead I'm simply going to jump straight to the answer. 50 00:02:23,630 --> 00:02:24,650 And here's the answer. 51 00:02:24,650 --> 00:02:27,170 This is really what Maxwell's equations tell us that 52 00:02:27,170 --> 00:02:27,730 light is. 53 00:02:27,730 --> 00:02:30,180 Light is actually, not just one wave, but two. 54 00:02:30,180 --> 00:02:33,010 It's an electric field, which is shown here in red, 55 00:02:33,010 --> 00:02:35,000 and a magnetic field, which is shown here in blue. 56 00:02:35,000 --> 00:02:38,830 They oscillate at the same frequency, they propagate together through space. 57 00:02:38,830 --> 00:02:41,700 There's always both of them, never just one. 58 00:02:41,700 --> 00:02:44,550 So in order to talk about what is light we need some 59 00:02:44,550 --> 00:02:48,880 terminology which will appear throughout the entire week of lectures this week. 60 00:02:48,880 --> 00:02:50,810 So let's start defining some terms. 61 00:02:50,810 --> 00:02:52,810 The first and most important one is the wavelength. 62 00:02:52,810 --> 00:02:56,300 The wavelength of the light is symbolized by the Greek letter lambda and you will 63 00:02:56,300 --> 00:03:00,890 see that in every lecture this week so please get use to lambda being wavelength. 64 00:03:00,890 --> 00:03:03,110 What we mean by the wavelength is simply the distance from 65 00:03:03,110 --> 00:03:07,530 the peak of one oscillation to the peak, the next peak over. 66 00:03:07,530 --> 00:03:11,700 So it's simply the distance in space that a wave oscillates. 67 00:03:11,700 --> 00:03:14,898 The oscillation distance, the peak-to-peak distance. 68 00:03:14,898 --> 00:03:17,980 As I've mentioned, for visible light that corresponds to somewhere 69 00:03:17,980 --> 00:03:21,520 between around 400 and 700 nanometers, roughly speaking. 70 00:03:21,520 --> 00:03:22,660 For infrared, it's larger. 71 00:03:22,660 --> 00:03:25,120 For ultraviolet, it's smaller, and so on. 72 00:03:25,120 --> 00:03:27,410 The other important quantity here is the frequency, which 73 00:03:27,410 --> 00:03:30,180 has also got a frequency, that's the Greek letter nu. 74 00:03:30,180 --> 00:03:33,060 And the frequency is simply, if you sit at a point in space, and 75 00:03:33,060 --> 00:03:36,900 the wave goes by you, how many times do you see a cycle per second? 76 00:03:36,900 --> 00:03:38,410 How many cycles per second? 77 00:03:38,410 --> 00:03:40,360 So that's the frequency of a wave. 78 00:03:40,360 --> 00:03:43,010 So those two numbers turn out to be related by this simple 79 00:03:43,010 --> 00:03:45,350 equation here, which is a good one to remember. 80 00:03:45,350 --> 00:03:48,450 Lambda times nu equals c, c is the speed of the wave. 81 00:03:48,450 --> 00:03:52,290 If the speed of the wave is a constant number, then lambda, 82 00:03:52,290 --> 00:03:54,950 the frequency times the wavelength has to be equal to that constant. 83 00:03:54,950 --> 00:03:56,480 So if the frequency goes up, the wavelength has 84 00:03:56,480 --> 00:03:59,590 to go down, such that their product remains constant. 85 00:03:59,590 --> 00:04:02,780 Of course, for light waves in empty waves, the speed of the wave is a 86 00:04:02,780 --> 00:04:07,530 fixed constant of the universe, it never changes, and that's a good number to know. 87 00:04:07,530 --> 00:04:08,180 That's the 88 00:04:08,180 --> 00:04:09,700 speed of light in empty space. 89 00:04:09,700 --> 00:04:11,440 That's really the speed limit of the universe. 90 00:04:11,440 --> 00:04:15,353 Nothing, no energy, no information, no objects 91 00:04:15,353 --> 00:04:18,462 can ever travel faster than that speed. 92 00:04:18,462 --> 00:04:20,098 Okay, so what are some other properties of light? 93 00:04:20,098 --> 00:04:21,832 Well clearly light carries energy. 94 00:04:21,832 --> 00:04:25,275 We know this because if we stand outside on a sunny day we warm up. 95 00:04:25,275 --> 00:04:29,481 That's energy being carried from the sun to our skin by a light wave. 96 00:04:29,481 --> 00:04:31,249 to talk about the energy in a light wave, 97 00:04:31,249 --> 00:04:33,630 we have really two different ways to do this. 98 00:04:33,630 --> 00:04:36,690 Number one is to talk about the energy of a single photon. 99 00:04:36,690 --> 00:04:40,830 And, a photon of course is the quantum version of what a light wave is. 100 00:04:40,830 --> 00:04:44,540 It's the smallest fundamental unit of a light wave, if you like. 101 00:04:44,540 --> 00:04:47,850 It's the indivisible quantum, of a light wave. 102 00:04:47,850 --> 00:04:50,600 and the energy of a single photon is simple h nu. 103 00:04:50,600 --> 00:04:52,500 Nu is the frequency that I just talked about and 104 00:04:52,500 --> 00:04:57,100 h is Planck's constant, which is another fundamental universal constant. 105 00:04:57,100 --> 00:04:58,820 So, if you have a larger 106 00:04:58,820 --> 00:05:01,180 frequency light wave, that corresponds to a 107 00:05:01,180 --> 00:05:04,890 smaller wavelength, and a higher photon energy, right? 108 00:05:04,890 --> 00:05:06,950 The photon energy is proportional to its frequency. 109 00:05:08,410 --> 00:05:10,060 Another way to talk about the energy carried 110 00:05:10,060 --> 00:05:12,170 by a light wave as a more macroscopic quantity. 111 00:05:12,170 --> 00:05:14,310 This is the intensity of a light wave. 112 00:05:14,310 --> 00:05:17,110 It's the total power falling on the surface area. 113 00:05:17,110 --> 00:05:21,540 So it's the energy per area per time. And energy per time is power, right? 114 00:05:21,540 --> 00:05:23,850 So the units are watts per square meter. Really the 115 00:05:23,850 --> 00:05:26,670 correct terminology is not intensity but irradiance, that's 116 00:05:26,670 --> 00:05:28,820 the correct term, but people are often sloppy 117 00:05:28,820 --> 00:05:30,850 and say intensity when they mean irradiance, so 118 00:05:30,850 --> 00:05:32,790 we just have to get used to that. 119 00:05:32,790 --> 00:05:35,890 Some good examples to just give you some scale of numbers. 120 00:05:35,890 --> 00:05:38,130 If you stand outside on a sunny day and the sun is straight 121 00:05:38,130 --> 00:05:41,460 overhead, then the intensity of the light 122 00:05:41,460 --> 00:05:44,410 is approximately a kilowatt per square meter. 123 00:05:44,410 --> 00:05:47,020 The world's most powerful lasers are much bigger numbers 124 00:05:47,020 --> 00:05:49,270 than that, they're more like 10 to the 20th watts 125 00:05:49,270 --> 00:05:51,047 per square meter, and that's obviously something you 126 00:05:51,047 --> 00:05:52,260 would not want to stand in front of. 127 00:05:54,010 --> 00:05:58,610 Okay, so how can we make light talk to objects on the nano scale? 128 00:05:58,610 --> 00:06:00,580 Well, one clear way that we can think about this 129 00:06:00,580 --> 00:06:03,040 is by simply focusing the down to a tiny spot. 130 00:06:03,040 --> 00:06:03,280 Right? 131 00:06:03,280 --> 00:06:04,890 And by putting a lens in front of a laser 132 00:06:04,890 --> 00:06:08,060 beam, we focus that laser beam down to a tiny spot. 133 00:06:08,060 --> 00:06:10,810 Right, or by using a magnifying glass in front of the sun. 134 00:06:10,810 --> 00:06:14,380 We get a little spot of light. So how big is the spot that we can make? 135 00:06:14,380 --> 00:06:15,830 Right? So, let's think about that. 136 00:06:15,830 --> 00:06:17,620 If we take a lens which has a certain diameter 137 00:06:17,620 --> 00:06:21,460 D, and the light wave completely fills the lens, right? 138 00:06:21,460 --> 00:06:23,638 And then a focal length f, so at one 139 00:06:23,638 --> 00:06:27,160 focal length distance away, we will get a tiny spot. 140 00:06:27,160 --> 00:06:28,920 So, how big is the spot? 141 00:06:28,920 --> 00:06:31,030 Well, the calculation's a little sophisticated, a little 142 00:06:31,030 --> 00:06:33,220 complicated, so let's just jump to the answer. 143 00:06:33,220 --> 00:06:38,230 The answer is that the spot size is twice the f number, times the wavelength. 144 00:06:38,230 --> 00:06:39,510 And here's the f number. The f number 145 00:06:39,510 --> 00:06:42,240 is simply the ratio of the focal length, divided by the diameter. 146 00:06:42,240 --> 00:06:43,250 So, it's a property of the lens. 147 00:06:44,780 --> 00:06:47,580 So, the important thing to realize here is that, that's 148 00:06:47,580 --> 00:06:50,210 the spot, that limits how small a spot size can be. 149 00:06:50,210 --> 00:06:52,430 And the reason is because it's very hard to make 150 00:06:52,430 --> 00:06:55,180 a lens with an f number smaller than a half. 151 00:06:55,180 --> 00:06:56,020 That's really tough. 152 00:06:56,020 --> 00:06:58,417 So that means that the spot size is, practically speaking, 153 00:06:58,417 --> 00:07:00,685 are never going to be much smaller than the wavelength. 154 00:07:00,685 --> 00:07:03,294 Okay, you cant maybe get a little smaller but not a lot smaller. 155 00:07:03,294 --> 00:07:04,734 And that limit is called 156 00:07:04,734 --> 00:07:06,193 the defraction limit. 157 00:07:06,193 --> 00:07:09,765 The defraction limit is, how small can you focus light? 158 00:07:09,765 --> 00:07:12,390 and, and, if it's really limited by the diffraction of waves. 159 00:07:12,390 --> 00:07:14,150 Now diffraction is a phenomena that is a 160 00:07:14,150 --> 00:07:17,040 general property of any wave, not just light waves. 161 00:07:17,040 --> 00:07:19,440 and it's really what causes the limiting, the, what 162 00:07:19,440 --> 00:07:21,900 limits how small we can focus a light wave down. 163 00:07:21,900 --> 00:07:26,440 This is why, for example, when you look through a microscope, you can see cells, 164 00:07:26,440 --> 00:07:27,920 because cells are bigger than the wavelength 165 00:07:27,920 --> 00:07:29,360 of the light you're using to see them. 166 00:07:29,360 --> 00:07:29,820 But you can't 167 00:07:29,820 --> 00:07:33,260 see nanoparticles in an optical microscope, because they're smaller. 168 00:07:33,260 --> 00:07:36,040 So you can't see things smaller than the wavelength, roughly speaking. 169 00:07:37,135 --> 00:07:38,770 Diffraction is a bit of a hard concept to 170 00:07:38,770 --> 00:07:42,360 grasp, so here's a little video that will illustrate a 171 00:07:42,360 --> 00:07:45,160 diffraction phenomenon to show you, kind of, what goes on 172 00:07:45,160 --> 00:07:47,460 when you try and put light through a tiny aperture. 173 00:07:48,600 --> 00:07:50,470 Diffraction is a phenomenon that is 174 00:07:50,470 --> 00:07:53,350 exhibited by all waves, including light waves. 175 00:07:53,350 --> 00:07:55,000 And basically, diffraction is 176 00:07:55,000 --> 00:07:58,960 what happens when a light wave encounters some sort of obstruction or aperture 177 00:07:58,960 --> 00:08:04,990 or scattering site, and then after the scatterer the waves that scatter from 178 00:08:04,990 --> 00:08:06,880 different parts of the object will 179 00:08:06,880 --> 00:08:09,310 interfere constructively and destructively with each other 180 00:08:09,310 --> 00:08:11,420 downstream to create some sort of complicated 181 00:08:11,420 --> 00:08:13,680 pattern, that's known as a diffraction pattern. 182 00:08:13,680 --> 00:08:16,400 So this is illustrated in the picture here, where 183 00:08:16,400 --> 00:08:18,970 our diffracting object is actually a very simple object. 184 00:08:18,970 --> 00:08:20,370 It's just a slit, 185 00:08:20,370 --> 00:08:22,070 which is a pair of metal razor blades 186 00:08:22,070 --> 00:08:24,900 that can be moved closer together and further apart. 187 00:08:24,900 --> 00:08:29,780 And what you're seeing now, is a video of what that slit looks like, 188 00:08:29,780 --> 00:08:33,810 without the laser beam on it, just so you can picture what's going on here. 189 00:08:33,810 --> 00:08:36,400 So, what I have, is the red laser beam and 190 00:08:36,400 --> 00:08:39,750 the green laser beam pointed together, onto this slit and as 191 00:08:39,750 --> 00:08:41,915 I open and close the slit, you can see the 192 00:08:41,915 --> 00:08:45,930 diffraction pattern changing, because the diffraction pattern depends on the opening 193 00:08:45,930 --> 00:08:47,140 size of that slit. 194 00:08:47,140 --> 00:08:49,890 If I open it a lot, then you'll see 195 00:08:49,890 --> 00:08:52,790 essentially no diffraction pattern because the light goes straight through. 196 00:08:52,790 --> 00:08:55,210 And if I close it down, so that it's a very small slit, 197 00:08:55,210 --> 00:08:59,130 then you could see a pattern of light and dark stripes on the wall. 198 00:08:59,130 --> 00:09:01,630 And if you look closely, you'll notice that those light and 199 00:09:01,630 --> 00:09:05,100 dark stripes are spaced differently for the green and for the red. 200 00:09:05,100 --> 00:09:08,460 Because green light has a shorter wavelength than red light, 201 00:09:08,460 --> 00:09:11,220 it's diffraction pattern is different, because the diffraction pattern depends 202 00:09:11,220 --> 00:09:12,200 on the wavelength of the light. 203 00:09:16,690 --> 00:09:20,720 Now that you've seen that diffraction can lead to complicated light patterns 204 00:09:20,720 --> 00:09:23,140 when you put light through a small aperture, maybe you can begin 205 00:09:23,140 --> 00:09:27,140 to appreciate why it's really hard to, force light through, into a 206 00:09:27,140 --> 00:09:28,520 spot that's much smaller than the 207 00:09:28,520 --> 00:09:31,020 wavelength, because things get pretty complex. 208 00:09:31,020 --> 00:09:32,940 As an aside, it's worth pointing out, that there are 209 00:09:32,940 --> 00:09:36,510 many images we've showed you in this class already of nanostructures. 210 00:09:36,510 --> 00:09:38,500 And you now know that those could have not have possibly 211 00:09:38,500 --> 00:09:41,760 been taken, those images, could not have possibly be taken in 212 00:09:41,760 --> 00:09:43,460 an optical microscope, and now you know why. 213 00:09:43,460 --> 00:09:45,520 Because their, the objects you're looking at 214 00:09:45,520 --> 00:09:48,100 are smaller than the wavelength of visible light. 215 00:09:48,100 --> 00:09:51,350 instead we use typically an electron microscope to image nanostructures 216 00:09:51,350 --> 00:09:54,410 and the reason that works is because electrons behave like waves. 217 00:09:54,410 --> 00:09:56,330 And, the wavelength of those electrons can be 218 00:09:56,330 --> 00:09:58,930 much smaller than the wavelength of visible light. 219 00:09:58,930 --> 00:10:03,494 So we can use electrons to image objects that are smaller than a few hundred nano- 220 00:10:03,494 --> 00:10:03,740 [INAUDIBLE]. 221 00:10:03,740 --> 00:10:06,100 Okay, so can we overcome this diffraction limit? 222 00:10:06,100 --> 00:10:08,030 Can we force light down through a tiny 223 00:10:08,030 --> 00:10:10,840 hole in order to be able to image nanostructures. 224 00:10:10,840 --> 00:10:11,050 Right? 225 00:10:11,050 --> 00:10:12,930 That's a really good question to ask, and there's 226 00:10:12,930 --> 00:10:14,900 a lot of people thinking about how to do that. 227 00:10:14,900 --> 00:10:17,700 Sort of the simplest way you might think about doing that is simply 228 00:10:17,700 --> 00:10:20,190 just what we did before, take our light and focus it down with 229 00:10:20,190 --> 00:10:23,330 the lens, but now at the focus, let's put an opaque screen, a 230 00:10:23,330 --> 00:10:26,945 piece of metal, with the sub wavelength aperture, a little hole in it. 231 00:10:26,945 --> 00:10:27,350 Okay? 232 00:10:27,350 --> 00:10:28,740 And see if we can force the light through 233 00:10:28,740 --> 00:10:30,050 that little hole. 234 00:10:30,050 --> 00:10:33,780 So, interestingly this does work, although typically you don't use 235 00:10:33,780 --> 00:10:35,440 a flat piece of metal with a hole in it, 236 00:10:35,440 --> 00:10:37,560 you use a different geometry which is say, for example 237 00:10:37,560 --> 00:10:41,830 an optical fiber, which has been coated by a metal coating. 238 00:10:41,830 --> 00:10:47,320 And then that tapers down to a tiny spot tiny tip and then at 239 00:10:47,320 --> 00:10:50,350 the tip you have a little bitty opening where the light can leak out. 240 00:10:50,350 --> 00:10:53,665 And if that opening is small, smaller than the wavelength, 241 00:10:53,665 --> 00:10:56,135 and you hold that tip very close to the surface 242 00:10:56,135 --> 00:10:59,450 you want to image, again on the nanometer scale close, then 243 00:10:59,450 --> 00:11:02,375 you can make images of that surface which are with 244 00:11:02,375 --> 00:11:06,578 a resolution, a spatial resolution that is smaller then the wavelength. 245 00:11:06,578 --> 00:11:08,173 Because the size of the hole is what 246 00:11:08,173 --> 00:11:11,212 determines your resolution, not the wavelength of the light. 247 00:11:11,212 --> 00:11:14,294 So this can be done, it works its a technique known 248 00:11:14,294 --> 00:11:19,185 as near field scanning optical microscopy usually abbreviated NSOM, like this, and 249 00:11:19,185 --> 00:11:21,715 here you're seeing a couple of examples. 250 00:11:21,715 --> 00:11:25,189 This is an NSOM image showing individual molecules in florescence. 251 00:11:25,189 --> 00:11:28,669 And this is an NSOM image showing the light emission from a smaller laser 252 00:11:28,669 --> 00:11:30,931 which is not coming uniformly from everywhere 253 00:11:30,931 --> 00:11:33,430 on the laser, they are clearly hot spots. 254 00:11:33,430 --> 00:11:37,110 So there's, clearly NSOM is a powerful technique for being able to image 255 00:11:37,110 --> 00:11:42,030 things with light, of visible wavelengths, on a scale smaller than the wavelength. 256 00:11:42,030 --> 00:11:44,400 The best, the world's record that I'm aware of, 257 00:11:44,400 --> 00:11:47,150 the best resolution for NSOM is about lambda over 30. 258 00:11:47,150 --> 00:11:49,770 More typical is probably lambda over 10. 259 00:11:49,770 --> 00:11:53,310 So, NSOM works well for imaging below the wavelength, but you're really 260 00:11:53,310 --> 00:11:55,880 not going to get below it, I mean, you can't do lambda over 1,000. 261 00:11:55,880 --> 00:11:59,270 So we still can't image individual nanoparticles using NSOM. 262 00:12:00,720 --> 00:12:03,540 So, what else is interesting about nano plus photons? 263 00:12:03,540 --> 00:12:05,640 Well, I want to talk about three 264 00:12:05,640 --> 00:12:09,290 situations in which that combination is important. 265 00:12:09,290 --> 00:12:13,790 Number one is that the sensing of things on the nano scale is often transduced. 266 00:12:13,790 --> 00:12:16,190 The signals are often transduced with light. 267 00:12:16,190 --> 00:12:17,730 And we've seen examples of this before. 268 00:12:17,730 --> 00:12:19,820 Here's one example, which is the example of 269 00:12:19,820 --> 00:12:22,540 atomic force microscopy, where the signals are let, 270 00:12:22,540 --> 00:12:28,560 read out by bouncing a laser off of the structure that has a nano component to it. 271 00:12:28,560 --> 00:12:31,920 So, that's one example of transducing a signal from the nano 272 00:12:31,920 --> 00:12:34,860 scale to a place where we can read it using light. 273 00:12:34,860 --> 00:12:36,660 And that, there are numerous examples of that. 274 00:12:38,690 --> 00:12:42,320 a second idea, is another one we've encountered before which is that the 275 00:12:42,320 --> 00:12:44,980 optical properties of materials often change 276 00:12:44,980 --> 00:12:47,140 when we make those materials nano sized. 277 00:12:47,140 --> 00:12:48,760 And the conical example of this that 278 00:12:48,760 --> 00:12:51,210 we've discussed on several occasions in this class 279 00:12:51,210 --> 00:12:52,810 is the idea of quantum confinement, where 280 00:12:52,810 --> 00:12:55,050 if you take a semiconductor nano particle and 281 00:12:55,050 --> 00:12:57,960 go from a big one to a small one, then you shift the color the 282 00:12:57,960 --> 00:13:01,430 emission, because you've changed the electronic structure, 283 00:13:01,430 --> 00:13:03,880 the electronic level spacings, of the material by 284 00:13:03,880 --> 00:13:06,180 virtue of having, by making it small. 285 00:13:06,180 --> 00:13:07,940 That causes the level spacing to change, and 286 00:13:07,940 --> 00:13:09,890 therefore the color of the light to change. 287 00:13:09,890 --> 00:13:13,230 And we will talk in more detail about this in subsequent lectures this week. 288 00:13:14,410 --> 00:13:16,380 A third idea, which we will also address in some 289 00:13:16,380 --> 00:13:18,910 detail this week, is a little bit different from that one. 290 00:13:18,910 --> 00:13:21,760 Instead of changing the properties of the material, what we're doing is 291 00:13:21,760 --> 00:13:25,410 structuring the material, in order to change the way it interacts with light. 292 00:13:25,410 --> 00:13:26,900 So the important idea here is that when 293 00:13:26,900 --> 00:13:29,130 you structure a material on the nano scale, 294 00:13:29,130 --> 00:13:31,370 you can change the interaction with light 295 00:13:31,370 --> 00:13:34,470 even if the intrinsic material properties are unchanged. 296 00:13:34,470 --> 00:13:37,900 And let me give you a thought experiment to, to illustrate what I mean here. 297 00:13:37,900 --> 00:13:40,890 So let's supposed you're looking at a window, ordinary window pane. 298 00:13:40,890 --> 00:13:43,434 And let's suppose that we replace the pane of glass with a 299 00:13:43,434 --> 00:13:46,890 thinner one and then a thinner one and then a thinner one. 300 00:13:46,890 --> 00:13:48,590 The question is, does the view out the window change? 301 00:13:48,590 --> 00:13:50,820 And the answer is, if you make the glass thin 302 00:13:50,820 --> 00:13:54,320 enough, on the hundreds of nanometers scale, then indeed the view 303 00:13:54,320 --> 00:13:58,080 does change and that's not because the properties of the glass have changed, 304 00:13:58,080 --> 00:13:59,210 it's because of the way that 305 00:13:59,210 --> 00:14:01,980 that nanostructure interacts with light has changed. 306 00:14:01,980 --> 00:14:04,410 And we'll talk about this in subsequent lectures this week. 307 00:14:05,480 --> 00:14:08,650 So the big ideas for today's lecture are, first of all 308 00:14:08,650 --> 00:14:11,530 that light has a characteristic length scale, which is the wavelength. 309 00:14:11,530 --> 00:14:13,490 Please get used to that symbol lambda because it 310 00:14:13,490 --> 00:14:15,250 will, you will see it in every lecture this week. 311 00:14:16,280 --> 00:14:18,210 which is much larger than a nanometer. 312 00:14:18,210 --> 00:14:19,790 On the other hand despite 313 00:14:19,790 --> 00:14:21,745 that the interaction of light with 314 00:14:21,745 --> 00:14:24,010 nanomaterials is interesting for three reasons. 315 00:14:24,010 --> 00:14:29,700 First of all, nano-enabled sensing is often enabled by transduction with light. 316 00:14:29,700 --> 00:14:32,640 Number two, in certain types of materials when you make them 317 00:14:32,640 --> 00:14:35,140 nano, their properties really change, including 318 00:14:35,140 --> 00:14:37,070 their way they interact with light. 319 00:14:37,070 --> 00:14:40,320 And number three, even if their properties don't change, by structuring them on the 320 00:14:40,320 --> 00:14:44,860 nano scale, we can really create new ways for that material to interact with light, 321 00:14:44,860 --> 00:14:46,990 which can give us really new and exciting 322 00:14:46,990 --> 00:14:50,420 possibilities for, for devices and components and structures. 323 00:14:50,420 --> 00:14:50,990 Thanks for listening.