Stimulated emission

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron, causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with a phase, frequency and direction of travel that are all identical to the photons of the incident wave; this is in contrast to spontaneous emission, which occurs at random intervals without regard to the ambient electromagnetic field. The process is identical in form to atomic absorption in which the energy of an absorbed photon causes an identical but opposite atomic transition: from the lower level to a higher energy level. In normal media at thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in the lower energy states than in the higher energy states. However, when a population inversion is present, the rate of stimulated emission exceeds that of absorption, a net optical amplification can be achieved.

Such a gain medium, along with an optical resonator, is at the heart of a maser. Lacking a feedback mechanism, laser amplifiers and superluminescent sources function on the basis of stimulated emission. Electrons and their interactions with electromagnetic fields are important in our understanding of chemistry and physics. In the classical view, the energy of an electron orbiting an atomic nucleus is larger for orbits further from the nucleus of an atom. However, quantum mechanical effects force electrons to take on discrete positions in orbitals. Thus, electrons are found in specific energy levels of an atom, two of which are shown below: When an electron absorbs energy either from light or heat, it receives that incident quantum of energy, but transitions are only allowed between discrete energy levels such as the two shown above. This leads to emission lines and absorption lines; when an electron is excited from a lower to a higher energy level, it's unlikely for it to stay that way forever.

An electron in an excited state may decay to a lower energy state, not occupied, according to a particular time constant characterizing that transition. When such an electron decays without external influence, emitting a photon, called "spontaneous emission"; the phase and direction associated with the photon, emitted is random. A material with many atoms in such an excited state may thus result in radiation which has a narrow spectrum, but the individual photons would have no common phase relationship and would emanate in random directions; this is the mechanism of thermal emission. An external electromagnetic field at a frequency associated with a transition can affect the quantum mechanical state of the atom without being absorbed; as the electron in the atom makes a transition between two stationary states, it enters a transition state which does have a dipole field, which acts like a small electric dipole, this dipole oscillates at a characteristic frequency. In response to the external electric field at this frequency, the probability of the electron entering this transition state is increased.

Thus, the rate of transitions between two stationary states is increased beyond that of spontaneous emission. A transition from the higher to a lower energy state produces an additional photon with the same phase and direction as the incident photon. Stimulated emission was a theoretical discovery by Einstein within the framework of the old quantum theory, wherein the emission is described in terms of photons that are the quanta of the EM field. Stimulated emission can occur in classical models, without reference to photons or quantum-mechanics. Stimulated emission can be modelled mathematically by considering an atom that may be in one of two electronic energy states, a lower level state and an excited state, with energies E1 and E2 respectively. If the atom is in the excited state, it may decay into the lower state by the process of spontaneous emission, releasing the difference in energies between the two states as a photon; the photon will have frequency ν0 and energy hν0, given by: E 2 − E 1 = h ν 0 where h is Planck's constant.

Alternatively, if the excited-state atom is perturbed by an electric field of frequency ν0, it may emit an additional photon of the same frequency and in phase, thus augmenting the external field, leaving the atom in the lower energy state. This process is known as stimulated emission. In a group of such atoms, if the number of atoms in the excited state is given by N2, the rate at which stimulated emission occurs is given by ∂ N 2 ∂ t = − ∂ N 1 ∂ t = − B 21 ρ N 2 where the proportionality constant B21 is known as the Einstein B coefficient for that particular transition, ρ is the radiation density of the incident field at frequency ν; the rate of emission is thus proportional to the number of atoms in the excited state N2, to the density of incident photons. At the same time, there will be a process of atomic absorption which removes energy from the field while raising electrons from the lower state t


BikeMi is a public bicycle sharing system in Milan, Italy. It was launched on 8 December 2008 and is contracted to and operated by Clear Channel on the basis of its SmartBike system; the scheme encompasses 280 stations. The service is active every day from 7 a.m. to midnight and had a daily ridership of about 15,890 in 2013. The operating hours are sometimes extended for special events; the system is based on 1-week or 1-year subscriptions, which allows users to rent a bike. Rentals are free for the first 30 minutes. After that time, in order not to pay more, the bike has to be returned in a station. However, unlimited number of rentals are allowed in a day; the scheme is similar to the Vélib' one. There are at present 43,213 users with an annual subscription. Prices range from €4.50 for a 1-day subscription to €36 for the annual one. An additional fare of €0.50 per half-hour is charged for rentals exceeding 30 minutes and not longer than further 2 hours. After that time a €2.00 per hour fine applies.

List of bicycle sharing systems Official website

Dungaree (fabric)

Dungaree fabric is a historical term for an Indian coarse thick calico cloth. Cotton twill with indigo dyed warp thread is now more referred to as denim, or more blue denim; the word is derived from Dongri, a dockside village near Mumbai. In American English, the term is used for hard-wearing work trousers made from such fabric and in British English for bib overalls in various fabrics, either for casual or work use. By 1891 Kipling was using the word to refer to a kind of garment as well as a fabric. Although dungarees now refers to denim, it is unclear whether traditional dungaree was a precursor to denim. In the late 17th century, most dungaree produced in was either washed and bleached, or dyed after weaving. Denim refers to cotton twill which may be undyed, or dyed after weaving. Denim may be 3x1 twill, it is unclear. In United States, the mill at Shady Lea, North Kingstown, Rhode Island, was built in the late 1820s by Esbon Sanford to manufacture a cotton-wool blend twill fabric called Kentucky Jean, resembling a cross between burlap and the dungaree fabric of today