# Lecture 21: Lasers, Schrödinger s Cat, Atoms, Molecules, Solids, etc. Review and Examples. Lecture 21, p 1

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1 Lecture 21: Lasers, Schrödinger s Cat, Atoms, Molecules, Solids, etc. Review and Examples Lecture 21, p 1

2 Act 1 The Pauli exclusion principle applies to all fermions in all situations (not just to electrons in atoms). Consider electrons in a 2-dimensional infinite square well potential. 1. How many electrons can be in the first excited energy level? a. 1 b. 2 c. 3 d. 4 e. 5 Hint: Remember the (n x,n y ) quantum numbers. 2. If there are 4 electrons in the well, what is the energy of the most energetic one (ignoring e-e interactions, and assuming the total energy is as low as possible)? a. (h 2 /8mL 2 ) x 2 b. (h 2 /8mL 2 ) x 5 c. (h 2 /8mL 2 ) x 10 Lecture 21, p 2

3 Insulators, Semiconductors and Metals Energy bands and the gaps between them determine the conductivity and other properties of solids. Insulators Have a full valence band and a large energy gap (a few ev). Higher energy states are not available. In order to conduct, an electron must have an available state at higher energy. Semiconductors Are insulators at T = 0. Have a small energy gap (~1 ev) between valence and conduction bands. Higher energy states become available (due to kt) as T increases. E Conduction band Metals Have a partly filled band. Higher energy states are available, even at T = 0. insulators semiconductors metals Valence band Lecture 21, p 3

4 Lecture 21, p 4

5 Semiconductors Silicon: Z = 14 (1s 2 2s 2 2p 6 3s 2 3p 2 ) 3s, 3p 2s, 2p 1s 4N states 4N states N states 8N electrons fill this band 2N electrons fill this band Total # atoms = N Total # electrons = 14N The 3s/3p band is only half filled (4N states and 4N electrons) Why isn t Silicon a metallic conductor, like sodium? Lecture 21, p 5

6 Energy Bands in Si Z = 14: 1s 2 2s 2 2p 6 3s 2 3p 2 3s, 3p 4N states In Si, the 3s 2 and 3p 2 hybridize to form four equivalent sp 3 hybrid states. These eigenstates are linear combinations of s and p states: 3s 2 3p x 3p y 3p z sp 3 hybridization sp 3 sp 3 sp 3 sp 3 Lecture 21, p 6

7 Silicon (2) (1/4) Si Silicon Unit Cell (1/4) Si There are 2 types of superpositions between neighbors: (1/4) Si Si (1/4) Si 2 - Si atoms/unit cell Antibonding Bonding 3s/3p band Antibonding states Bonding states Empty band at T = 0. (Conduction Band) Energy Gap E gap 1.1 ev Filled band at T = 0 (Valence band) 8 sp 3 -orbitals in one unit cell 8 orbitals form superpositions (4 bonding and 4 anti-bonding combinations) 8 orbitals X 2 spin = 16 states 8 electrons fill the 4 lower energy (bonding) states Lecture 21, p 7

8 Lecture 21, p 8

9 Silicon (3) At T = 0, the bonding states in Si are completely filled, and the anti-bonding states are completely empty. A small (1.1 ev) energy gap separates the bonding and anti-bonding states: Si is a semiconductor. The electrons in a filled band cannot contribute to conduction, because with reasonable E fields they cannot be promoted to a higher kinetic energy. Therefore, at T = 0, Si is an insulator. At higher temperatures, however, electrons are thermally promoted into the conduction band: Resistivity 1 Metal: scattering time gets shorter with increasing T Semiconductor: number n of free electrons increases rapidly with T (much faster than decreases) Temperature, T This graph only shows trends. A semiconductor has much higher resistance than a metal. Lecture 21, p 9

10 Photodetectors Shining light onto a semiconductor can excite electrons out of the valence band into the conduction band. The change in resistance can then be used to monitor the light intensity. Examples: photodiode optical power meter barcode scanner digital cameras (each pixel) photovoltaic solar cell A 3.2 Gigapixel camera Lecture 21, p 10

11 Act 2 The band gap in Si is 1.1 ev at room temperature. What is the reddest color (i.e., the longest wavelength) that you could use to excite an electron to the conduction band? Hint: Si is used in the pixels of your digital camera. a. 500 nm b. 700 nm c nm Lecture 21, p 11

12 Lecture 21, p 12

13 Lasers Photons are emitted when the electrons in atoms go from a higher state to a lower state Conversely, photons are absorbed when the electrons in atoms go from a lower state to a higher state photon Emission photon Absorption Fermions and bosons: Electrons, protons, and neutrons are fermions. Two identical fermions cannot occupy the same quantum state. (exclusion principle) Photons (and many atoms) are bosons. Unlike fermions, bosons actually prefer (to be explained soon) to be in the same quantum state. This is the physical principle on which lasers are based. Lecture 21, p 13

14 Lasers Suppose we have an atom in an excited state. Eventually (at some random time) it will emit a photon and fall to the lower state. The emitted photon will go in a random direction. This is called spontaneous emission. Emission photon Suppose, however, that before the spontaneous emission occurs, another photon of the same energy (emitted by another atom) comes by. Its presence will stimulate the atom to emit its photon. Photon emitted by some other atom Two identical photons! Stimulated emission We now have two photons in the same quantum state: the same frequency, the same direction, and the same polarization. As they travel, they will stimulate even more emission. Lecture 21, p 14

15 Lasers Light Amplification by Stimulated Emission of Radiation Laser operation (one kind): Mirrors at both ends Tube of gas with mirrored ends. Excite the atoms to the upper state (mechanism not shown). 100% Tube of gas 99.99% Spontaneous emission generates some photons. Photons that travel along the axis are reflected. Other directions leak out the sides. Because the amplification process is exponential, the axial beam quickly dominates the intensity. One mirror allows a small fraction of the beam out. Lecture 21, p 15

16 Lecture 21, p 16

17 LASER: Lasers Light Amplification by Stimulated Emission of Radiation What if you don t have an atom that emits the color you want? Make one (e.g., by stressing the crystal lattice). Did you know: Semiconductors don t naturally emit in the red. Charles Henry (UIUC 65) discovered how to combine layers of different semiconductors to make a quantum well (essentially a 1-D box for electrons). By adjusting the materials, one could shift the emission wavelengths into the visible. Nick Holonyak (UIUC ECE Prof) used this to create the first visible LEDs & visible laser diodes.

18 Schrödinger s Cat: How seriously do we take superpositions? We now know that we can put a quantum object into a superposition of states. But if quantum mechanics is completely correct, shouldn t macroscopic objects end up in a superposition state too? This puzzle is best exemplified by the famous Schrödinger s cat paradox: A radioactive nucleus can decay, emitting an alpha particle. This is a quantum mechanical process. The alpha particle is detected with a Geiger counter, which releases a hammer, which breaks a bottle, which releases cyanide, which kills a cat. Suppose we wait until there is a 50:50 chance that the nucleus has decayed. Is the cat alive or dead? Lecture 21, p 18

19 Schrödinger s Cat According to QM, because we can t know (i.e., predict), until we look inside the box, the cat is in a superposition* of being both alive and dead! 1 cat (alive) (dead) 2 And in fact, according to QM, when we look, we are put into a quantum superposition* of having seen a live and a dead cat!! Did you ever perceive yourself as being in a superposition? (probably not ) This paradox leads to some dispute over the applicability of QM to large objects. The experiments are difficult, but so far there is no evidence of a size limit. Where does it end?!? it doesn t end ( wavefunction of the universe ) there is some change in physics (quantum classical) many-worlds interpretation In any event, the correlations to the rest of the system cause decoherence and the appearance of collapse. *More correctly, we, the atom, and the cat become entangled. Lecture 21, p 19

20 Lecture 21, p 20

21 FYI: Recent Physics Milestone! There has been a race over the past ~20 years to put a ~macroscopic object into a quantum superposition. The first step is getting the object into the ground state, below all thermal excitations. This was achieved for the first time in 2010, using vibrations in a small drum : Quantum ground state and single-phonon control of a mechanical resonator, A. D. O Connell, et al., Nature 464, (1 April 2010) Quantum mechanics provides a highly accurate description of a wide variety of physical systems. However, a demonstration that quantum mechanics applies equally to macroscopic mechanical systems has been a long-standing challenge... Here, using conventional cryogenic refrigeration, we show that we can cool a mechanical mode to its quantum ground state by using a microwave-frequency mechanical oscillator a quantum drum... We further show that we can controllably create single quantum excitations (phonons) in the resonator, thus taking the first steps to complete quantum control of a mechanical system. Lecture 21, p 21

22 Entanglement If we have two quantum systems, there are two possibilities for the total quantum state: 1. can be written as a product of the two individual systems: 2. cannot be written as a product: We ve seen several examples before in this course: Atom that just emitted a photon: S. cat: Photon emitted in all directions; atom must recoil in opposite direction. Double slit with quantum which-path detector: Lecture 21, p 22

23 THE END Best wishes in Physics 213 and all your other courses! Lecture 21, p 23

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