:YAG redirects here. For the IATA airport code see Fort Frances Municipal Airport. For the YMCA sponsored program see YMCA Youth and Government.

Name	Yttrium aluminium garnet
Category	synthetic mineral
Formula	Y3Al5O12
Color	usually colorless, but may be pink, red, orange, yellow, green, blue, purple
System	cubic
Cleavage	none
Fracture	conchoidal to uneven
Mohs	8.5
Luster	vitreous to subadamantine
Polish	vitreous to subadamantine
Refractive	1.833 (+/- .010)
Opticalprop	Single refractive
Birefringence	none
Dispersion	.028
Pleochroism	none
Fluorescence	colorless stones - inert to moderate orange in long wave, inert to weak orange in short wave; blue and pink stones - inert; yellow-green stones - very strong yellow in long and short wave also phosphoresces; green stones - strong red in long wave, weak red in short wave
Gravity	4.5 - 4.6
References

Yttrium aluminium garnet (YAG, Y₃Al₅O₁₂) is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminium composite, the other two being yttrium aluminium monoclinic (YAM) and yttrium aluminium perovskite (YAP). YAG is commonly used as a host material in various solid-state lasers. Rare earth elements such as neodymium and erbium can be doped into YAG as active laser ions, yielding and lasers, respectively. Cerium-doped YAG (YAG:Ce) is used as a phosphor in cathode ray tubes and white light-emitting diodes, and as a scintillator.

Gemstone YAG

YAG for a period was used in jewelry as a diamond and other gemstone simulant. Colored variants and their doping elements include:

Most Nd:YAG lasers produce infrared light at a wavelength of 1064 nm. Light at this wavelength is rather dangerous to vision, since it can be focused by the eye's lens onto the retina, but the light is invisible and does not trigger the blink reflex. Nd:YAG lasers can also be used with frequency doubling or frequency tripling crystals, to produce green light with a wavelength of 532 nm or ultraviolet light at 355 nm, respectively.

The dopant concentration in commonly-used Nd:YAG crystals usually varies between 0.5 and 1.4 molar percent. Higher dopant concentration is used for pulsed lasers; lower concentration is suitable for continuous-wave lasers. Nd:YAG is pinkish-purple, with lighter-doped rods being less intensely colored than heavier-doped ones. Since its absorption spectrum is narrow, the hue depends on the light under which it is observed.

Nd:Cr:YAG

YAG doped with neodymium and chromium (Nd:Cr:YAG or Nd/Cr:YAG) has absorption characteristics which are superior to Nd:YAG. This is because energy is absorbed by the broad absorption bands of the Cr³⁺ dopant and then transferred to Nd³⁺ by dipole-dipole interactions. This material has been suggested for use in solar-pumped lasers, which could form part of a solar power satellite system.

Er:YAG

Erbium-doped YAG (Er:YAG) is an active laser medium lasing at 2940 nm. Its absorption bands suitable for pumping are wide and located between 600 and 800 nm, allowing for efficient flashlamp pumping. The dopant concentration used is high: about 50% of the yttrium atoms are replaced. The Er:YAG laser wavelength couples well into water and body fluids, making this laser especially useful for medicine and dentistry uses; it is used for treatment of tooth enamel and in cosmetic surgery. Er:YAG is used for noninvasive monitoring of blood sugar. The mechanical properties of Er:YAG are essentially the same as Nd:YAG. Er:YAG operates at wavelengths where the threshold for eye damage is relatively high (since the light is absorbed before striking the retina), works well at room temperature, and has high slope efficiency. Er:YAG is pale green.

Yb:YAG

Ytterbium-doped YAG (Yb:YAG) is an active laser medium lasing at 1030 nm, with a broad, 18 nm wide absorption band at 940 nm. It is one of the most useful media for high-power diode-pumped solid state lasers. The dopant levels used range between 0.2-30% of replaced yttrium atoms. Yb:YAG has very low fractional heating, very high slope efficiency, and no excited-state absorption or up-conversion, high mechanical strength and high thermal conductivity. Yb:YAG can be pumped by reliable InGaAs laser diodes at 940 or 970 nm.

Yb:YAG is a good substitute for 1064 nm Nd:YAG in high-power applications, and its frequency-doubled 515 nm version can replace the 514 nm argon lasers.

Nd:Ce:YAG

Neodymium-cerium double-doped YAG (Nd:Ce:YAG, or Nd,Ce:YAG) is an active laser medium material very similar to Nd:YAG. The added cerium atoms strongly absorb in the ultraviolet region, and transfer their energy to the neodymium atoms, increasing the pumping efficiency; the result is lower thermal distortion and higher power output than Nd:YAG at the same pumping level. The lasing wavelength, 1064 nm, is the same as for Nd:YAG. The material has a good resistance to damage caused by UV from the pump source, and low lasing threshold. Usually 1.1-1.4 % of Y atoms are replaced with Nd, and 0.05-0.1% with Ce.

Ho:Cr:Tm:YAG

Holmium-chromium-thulium triple-doped YAG (Ho:Cr:Tm:YAG, or Ho,Cr,Tm:YAG) is an active laser medium material with high efficiency. It lases at 2097 nm and can be pumped by a flashlamp or a laser diode. It is widely used in military, medicine, and meteorology. It works well at room temperature, has high slope efficiency, and operates at a wavelength where the threshold for eye damage is relatively high. When pumped by a diode, the 781 nm band is used. Other major pump bands are located between 400 and 800 nm. The dopant levels used are 0.35 atom.% Ho, 5.8 atom.% Tm, and 1.5 at.% Cr. The rods have green color, imparted by chromium(III).

Tm:YAG

Thulium-doped YAG (Tm:YAG) is an active laser medium that operates between 1930 and 2040 nm. It is suitable for diode pumping. A dual-mode Tm:YAG laser emits two frequencies separated by 1 GHz.

Cr⁴⁺:YAG

Chromium (IV)-doped YAG (Cr:YAG) provides a large absorption cross section in the 0.9-1.2 micrometer spectral region, which makes it an attractive choice as a passive Q-switch for Nd-doped lasers. The resulting devices are solid-state, compact and low-cost. Cr:YAG has high damage threshold, good thermal conductivity, good chemical stability, resists ultraviolet radiation, and is easily machinable. It is replacing more traditional Q-switching materials like lithium fluoride and organic dyes. The dopant levels used range between 0.5 and 3 percent (molar). Cr:YAG can be used for passive Q-switching of lasers that operate at wavelengths between 1000 and 1200 nm, such as those based on Nd:YAG, Nd:YLF, Nd:YVO₄, and Yb:YAG.

Cr:YAG can be also used as a laser gain medium itself, producing tunable lasers with outputs adjustable between 1350 and 1550 nm. The Cr:YAG laser can generate ultrashort pulses (in the femtoseconds range) when it is pumped at 1064 nm by a Nd:YAG laser.

Cr:YAG has been demonstrated in an application of non-linear optics as a self-pumped phase-conjugate mirror in a Nd:YAG "loop resonator". Such a mirror provides compensation of both phase and polarization aberrations induced into the loop resonator.

Dy:YAG

Dysprosium-doped YAG (Dy:YAG) is a temperature-sensitive phosphor used in temperature measurements. The phosphor is excited by a laser pulse and its temperature-dependent fluorescence is observed. Dy:YAG is sensitive in ranges of 300-1700 K. The phosphor can be applied directly to the measured surface, or to an end of an optical fiber.

Sm:YAG

Samarium-doped YAG (Sm:YAG) is a temperature-sensitive phosphor similar to Dy:YAG.

Tb:YAG

Terbium-doped YAG (Tb:YAG) is a phosphor used in cathode ray tubes. It emits at yellow-green color, at 544 nm.

Ce:YAG

Cerium(III)-doped YAG (Ce:YAG or YAG:Ce) is a phosphor, or a scintillator when in pure single-crystal form, with wide range of uses. It emits yellow light when subjected to blue or ultraviolet light, or to x-ray light. It is used in white light-emitting diodes, as a coating on a high-brightness blue InGaN diode, converting part of the blue light into yellow, which then appears as white. Such an arrangement gives less than ideal color rendering. The output brightness decreases with increasing temperature, further altering device color output.

Ce:YAG is also used in some mercury-vapor lamps as one of the phosphors, often together with Eu:Y(P,V)O₄ (yttrium phosphate-vanadate). It is also used as a phosphor in cathode ray tubes, where it emits green (530 nm) to yellow-green (550 nm) light. When excited by electrons, it has virtually no afterglow (70 ns decay time). It is suitable for use in photomultipliers.

Ce:YAG is used in PET scanners, high-energy gamma radiation and charged particle detectors, and high-resolution imaging screens for gamma, x-rays, beta radiation and ultraviolet radiation.

Ce:YAG can be further doped with gadolinium.

See also

Yttrium iron garnet

Gadolinium gallium garnet

Micro-Pulling-Down

References

Category:Yttrium compounds Category:Aluminium compounds Category:Synthetic minerals Category:Optical materials Category:Laser gain media Category:Phosphors and scintillators Category:Diamond simulants

Name	Nadarajah Raviraj
Constituency mp	Jaffna District
Parliament	Sri Lanka
Successor	N. Srikantha, TNA
Term start	2001
Term end	2006
Office2	Mayor of Jaffna
Term start2	2001
Term end2	2001
Predecessor2	Pon Sivapalan, TULF
Office3	Deputy Mayor of Jaffna
Term start3	1998
Term end3	2001
Office4	Member of Jaffna Municipal Council
Term start4	1998
Term end4	2001
Birth date	June 25, 1962
Birth place	Chavakachcheri, Ceylon
Death date	November 10, 2006
Death place	Colombo National Hospital, Colombo, Sri Lanka
Nationality	Sri Lankan
Religion	Hindu
Party	Tamil United Liberation Front
Otherparty	Tamil National Alliance
Profession	Lawyer
Alma mater	Chavakachcheri Drieberg CollegeSt. John's College

Nadarajah Raviraj ( ; June 25, 1962 – November 10, 2006) was a prominent Sri Lankan Tamil politician and human rights lawyer. Born in the Jaffna district in the North of Sri Lanka, he served as a Member of Parliament for the Tamil National Alliance, which is backed by the LTTE, until he was shot dead by gunmen in Colombo, Sri Lanka, on November 10, 2006. TNA and others alleged that the Sri Lankan government was involved in this assignation. The Sri Lankan government denied responsibility and strongly condemned his killing.

On November 9, 2006, one day before his assassination, he led a protest demonstration in front of the UNICEF office in Colombo regarding the Vaharai bombing by the Sri Lankan Army of a school that led to the death of 45 civilians.

Biography

Born in Chavakachcheri, Raviraj attended Chavakachcheri Drieberg College and St. John's College, Jaffna. He was enrolled as an Attorney of Law in the Supreme Court of Sri Lanka in November 1988 and appeared in the Sri Lankan Supreme Court, Court of Appeal and in the High Courts of Ampara, Batticaloa, Vavuniya, Trincomalee, Jaffna and elsewhere in the south. His law firm, Raviraj & Associates, specialized in cases under the Prevention of Terrorism Act, and Emergency Regulations. Raviraj also worked as a human rights lawyer for Home for Human Rights in Colombo, from 1984 to 1990 and again from 1993 until 1999.

Political career

He joined the Tamil United Liberation Front in 1987 and became a Central Committee Member of the TULF in 1990, the party's legal adviser in 1998 and a member of political bureau in 2000. He was also appointed as administrative secretary of the party. Raviraj was elected mayor of Jaffna in 1998, a position in which he served until his election to Parliament from Jaffna District in December 2001 as a candidate of the newly formed TNA. He was reelected to parliament in 2004.

During the last few months Raviraj participated in the Civil Monitoring Committee (CMC), working with Mano Ganesan MP (Colombo District), Vasudeva Nanayakara MMC, Dr.Vickramabahu Karunaratne, and Appapillai Vinayagamoorthy to monitor extra judicial killings, abductions and disappearances. Raviraj MP has participated in conflict resolution workshops hosted by the Institute of Federalism in Switzerland, the Konrad Adenauer Foundation in Germany and the Peace Institute in Austria among others. In an interview given to the Asia Tribune on October 14, 2006 he reportedly said of LTTE leader Velupillai Prabhakaran "LTTE leader Prabhakaran's children live a comfortable life abroad, poor Tamil children have to wait in long queues all day without even going to school to get a piece of bread".

Raviraj was always supportive of Tamil self-rule and a leading campaigner for Tamil self-rule. Another MP of the TNA claimed that "This is a clear message to Tamil parliamentarians ... Don't open your mouth" and supported the TNA's allegations on the government. .

Assassination

Raviraj was shot in Colombo, Sri Lanka by 2 assailants riding a motorbike on November 10, 2006 when he was leaving his home. He had just left his home after returning from an interview between 7:00 am to 8:00 a.m(local time) on the Derana TV Channel. He was rushed to the General Hospital in Colombo where he succumbed to his injuries at 9:20am. His bodyguard, Police Constable Lakshman Lokuwella attached to the Jaffna Police Station, was also killed in the shooting.

The shooting occurred around 8.45am as he was proceeding to the Elvitigala Road from Martha Road (6°54'7.11"N 79°52'38.26"E) in his vehicle. The assailants escaped leaving behind a T-56 rifle and a few rounds of ammunition in a bag. The site of the attack was within the Manning Town Complex and was opposite the Sri Lankan Military Police headquarters.

Upon a request by the Sri Lankan government, the Metropolitan Police Service (from Scotland Yard) agreed to send a team of forensic experts to assist the investigation into the killing. The investigations have not yet concluded. However, a local human rights organization has accused that the killer is known to the Government and remain free. It further alleged that the killers belonged to paramilitary TMVP cadres. Many accused the government of not showing any interest in finding the real killers behind this assassination.

Rajavarothiam Sampanthan, leader of the TNA, a political party widely believed to be a proxy of the LTTE, blamed the government for the murder and claimed "It is government forces or forces aligned to the government, there can be no question "and argued "This is an attempt to stifle... and silence those who can justifiably espouse the Tamil cause." Selvam Adaikalanathan, a fellow TNA MP said "A democratic voice of Tamils has been silenced in Sri Lanka's capital today," and charged that resourceful Tamil politicians, journalists and activists (see State terrorism in Sri Lanka) are being systematically killed in Sri Lanka. "Raviraj tirelessly worked to improve the conditions of the people living under economic embargo to relieve the humanitarian catastrophe. He had a convincing way of dealing with even the crude bureaucracy of this failed state," Mr. Adaikalanathan said. The LTTE released a statement mentioning "As a parliamentarian he worked tirelessly for the Tamil nation. He spoke loudly to the world about the Sinhala Buddhist chauvinistic state terror. He shattered the false propaganda of the Sinhala state infusing his arguments with his legal expertise." The LTTE's eulogy to Raviraj added "Death never destroys the great souls who lived their life for truth. They will live for ever in our nation’s soul as heroes of our history."

United National Party organizer S.B. Dissanayake, member of the main opposition to President Rajapakse's Sri Lanka Freedom Party blamed the Sri Lankan government for the killing, claiming the blame cannot be passed onto the Karuna group or any other organization. Dissanayake remarked "It was impossible for such assassinations to take place in broad daylight in the city (Colombo) without the knowledge of the state."

The Asian Human Rights Commission expressed shock at the killing of N. Raviraj and remarked that in Sri Lanka, "the rule of law machinery has been in a state of collapse now for a long time it is no surprise that things have gone out of control" and observed "these deaths are just part of a continuous situation of assassinations done by all parties to the conflict" and "the parties to the conflict claim justification for the deaths they cause and there is no regret in any quarter about what is taking place."

The US State Department deplored the murder of the Tamil MP and urged the Sri Lankan government to promptly investigate, arrest and prosecute those responsible, claiming "It is critical that crimes such as the murder of Nadarajah Raviraj do not go unpunished." and the "The people of Sri Lanka have been unjustly robbed of his energy and talent."

The anti-rebel website, Asian Tribune, alleged that the Colombo-based International Center for Strategic Defense (ICSD) in its analaysis has pointed the finger at the LTTE as the prime source for the assassination .

The Criminal investigations Department has announced that three suspects have been held with regard to the assassination and investigations are proceeding .

EPDP Leader and Minister Douglas Devanananda categorically dismissed all allegations leveled against the EPDP regarding the killing of TNA MP Nadarajah Raviraj .

Government's involvement

A local human right organization, University Teachers for Human Rights, claimed that they were near certain that Gotabhaya Rajapaksa, current Permanent Secretary to the Ministry of Defence of the Government of Sri Lanka, was involvement in the assassination. Further, it claimed that Gajanayake ,whose forces work under the Ministry of Defence, was the main organiser behind the assassination.

See also

Human Rights in Sri Lanka

Origins of the Sri Lankan civil war

Black July

Sri Lankan civil war

Sri Lankan Tamils

LTTE

Sri Lankan Army

References

External links

Assassination of an Activist, youtube.com, unionblackcolombo, June 2008

Peace and Conflict Timeline (PACT) - an interactive timeline of the Sri Lankan conflict

Complete text of final interview given by Nadarajah Raviraj

Tragic demise of Nadarajah Raviraj in Colombo

Category:Sri Lankan Tamil politicians Category:Sri Lankan Tamil people Category:Minority rights activists Category:Assassinated Sri Lankan activists Category:Assassinated Sri Lankan politicians Category:Sri Lankan Hindus Category:Deaths by firearm in Sri Lanka Category:People murdered in Sri Lanka Category:Tamil United Liberation Front politicians Category:People from Chavakacheri Category:Sri Lankan terrorism victims Category:1962 births Category:2006 deaths Category:Members of 12th Parliament of Sri Lanka Category:Members of 13th Parliament of Sri Lanka Category:Tamil National Alliance politicians Category:Illankai Tamil Arasu Kachchi politicians