Lecture Physics A2: Quantum Nature of Light (Quantum Optics) - PhD. Pham Tan Thi

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Lecture Physics A2: Quantum Nature of Light (Quantum Optics) - PhD. Pham Tan Thi present the content wave characteristics, electrically charged particles and electromagnetic waves, effect on electric charges, accelerated charges (electrons, protons) produce,...

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Nội dung Text: Lecture Physics A2: Quantum Nature of Light (Quantum Optics) - PhD. Pham Tan Thi

Quantum Nature of Light (Quantum Optics) Pham Tan Thi, Ph.D. Department of Biomedical Engineering Faculty of Applied Sciences Ho Chi Minh University of Technology Courtesy of N. Brunner and J. Simmonds
Light behave like a wave or a particle (This is called the duality of the behavior of light)
Wave Characteristics
Electrically Charged Particles and Electromagnetic Waves Electrons have (-) charge Protons have (+) charge Both have electric fields +- attract ++ and -- repel • The changing position of a charged particle creates “waves” called electromagnetic waves • The electromagnetic waves travel through empty space eventually interacting with a distant charged particle. • Visible light is an electromagnetic wave.
Magnetism (Eﬀect on electric charges) Moving electric charges also produce magnetic fields. Example: electric current passing through a coil. Another interesting example: the Earth’ magnetic field is produced by the spinning of charges in the liquid metal core of the Earth. Conversely, magnetic field force charged particles to move…
Accelerated Charges (electrons, protons) Produce (Ripples in the Electromagnetic Field) An electromagnetic wave is composed of two oscillating fields, and electric field and a magnetic field perpendicular to each other Antenna receives electromagnetic signal from TV station
Wavelength of Electromagnetism means COLOR
The Temperature Scale (Conversion from wavelength/energy to temperature) The scale mostly used in sciences, physics and astronomy is Kelvin. The unit is Kelvin (K).
Thermal Radiation • The fundamental sources of all electromagnetic radiation (EMR) are electric charges in accelerated motion. • All bodies emit electromagnetic radiation as a result of thermal motion of their molecules. This radiation, called thermal Reference radiation, #1 of diﬀerent is a mixture wavelengths. Reference #2 • Thermal radiation is emitted by all objects An electric heating element emits primarily infrared radiation. But if its above absolute zero (-273.15°); but some temperature is high enough, it also of objects is in visible. emits a discernible amount of visible light.
What is a Blackbody A blackbody is an idealized object that absorbs all EMR that falls on it - no EMR passes through it and none is reflected (i.e. perfect emitter and perfect absorber). Because no light (visible EMR) is reflected or transmitted, the object appears black Leave appears green because green color is reflected to human eye
Blackbody in Lab Experiment • An object of controlled temperature T contains a cavity, joined to the outside by a small hole. • If the hole is small enough, the radiation in the cavity comes to equilibrium in the walls. • The hole allows a small fraction of the radiation to pass to a spectrometer — the radiation coming out has the same spectrum as what is inside. • The radiancy is the power emitted per unit area per increment of wavelength and so has unit of Wcm-3
Blackbody in Lab Experiment • The spectral radiance from the hole is independent of the material used and only depends on the temperature.
Rayleigh - Jeans Approximation for Blackbody
Kirchhoﬀ’s Law of Thermal Radiation in Thermal Equilibrium
Ultraviolet Catastrophe
Quantum Theory 1. Stefan-Boltzmann’s Law 2. Wien’s Law 3. Planck’s Theory 4. Planck’s Formula
Stefan - Boltzmann’s Law The total emitted radiation (Mλ) from a blackbody is proportional to the fourth power of its absolute temperature. M = T4 where σ is the Stefan-Boltzmann constant, 5.6697 x 10-8 Wm-2K-4 —> the amount of energy emitted by an object such as the Sun or the Earth is a function of its temperature —> This can be derived by integrating the spectral radiance over entire spectrum Z 1 2⇡ 2 k 4 4 L= L d = 2 3T OR M = ⇡L = T 4 0 5c h
Wien’s Displacement Law In addition to computing the total amount of energy existing a theoretical blackbody such as the Sun, we can determine its dominant wavelength (λmax) based on Wien’s displacement law: k max = T where k is the is the Wien’s displacement constant = 2.898 x 10-3 Km, and T is the absolute temperature in K i.e. there is an inverse relationship between the wavelength of the peak of the emission of a blackbody and its temperature. Therefore, as the Sun approximates a 6000 K blackbody, its dominant wavelength (λmax) is 0.48 µm
Planck’s Quantum Theory Max Planck found a correct law for the black body radiation by assuming that each oscillator can only exchange energy in discrete portions or quanta The energy exchanges between radiation and matter must be discrete and energy of radiation E = nhν Average energy per standing wave h⌫ "= eh⌫/kT 1 Planck’s modifications 8⇡h ⌫ 3 d⌫ u(⌫)d⌫ = 2 h⌫/kT c e 1 h = 6.626 x 10-34 J.s is Planck’s constant
Einstein’s Theory of Photoelectric Eﬀect (Provides a direct confirmation for the energy quantization of light) Heinrich Hertz in 1887 later on, it was explained by Albert Einstein in 1905 It was during this study of electromagnetic waves that he saw the if one sends ultraviolet light onto metals, you’ll get sparks coming oﬀ. The phenomenon of ejection of electron from the surface of a metal when light of a suitable frequency strikes it is called photoelectric eﬀect. The emitted electrons are called photo-electrons. Experiment: The time between the incidence and emission of a photoelectron is very small, < 10-9 s. Classically: raise intensity => more electrons independent of frequency