Editing Ultrafast monochromator

An '''ultrafast monochromator''' is a [[monochromator]] that preserves the duration of an [[ultrashort pulse]] (in the [[femtosecond]], or lower, time-scale).<ref name="PolettoFrassetto2012">{{cite journal|last1=Poletto|first1=Luca|last2=Frassetto|first2=Fabio|last3=Villoresi|first3=Paolo|title=Ultrafast Grating Instruments in the Extreme Ultraviolet|journal=IEEE Journal of Selected Topics in Quantum Electronics|volume=18|issue=1|year=2012|pages=467–478|issn=1077-260X|doi=10.1109/JSTQE.2011.2135338|s2cid=26107534 }}</ref><ref name="TschentscherPatthey2017">{{cite journal|last1=Tschentscher|first1=Thomas|last2=Patthey|first2=Luc|last3=Brenner|first3=Günter|last4=Frassetto|first4=Fabio|last5=Kuhlmann|first5=Marion|last6=Ploenjes|first6=Elke|last7=Poletto|first7=Luca|editor-first1=Thomas |editor-first2=Luc |editor-last1=Tschentscher |editor-last2=Patthey |title=Grating monochromator with ultrafast response for FLASH2 at DESY
|volume=10237|year=2017|pages=1023716|issn=0277-786X|doi=10.1117/12.2264885|series=Advances in X-ray Free-Electron Lasers Instrumentation IV|s2cid=56251242 |url=http://bib-pubdb1.desy.de/record/396797/files/1023716.pdf}}</ref>  Monochromators are devices that select for a particular wavelength, typically using a [[diffraction grating]] to [[dispersion (optics)|disperse]] the light and a slit to select the desired wavelength; however, a diffraction grating introduces path delays that measurably lengthen the duration of an ultrashort pulse.   An ultrafast monochromator uses a second diffraction grating to compensate time delays introduced to the pulse by the first grating and other dispersive optical elements.

==Diffraction grating==
Diffraction gratings are constructed such that the angle of the incident ray, ''θ<sub>i</sub>'', is related to the angle of the ''m''th outgoing ray, ''θ<sub>m</sub>'', by the expression

:<math> m \lambda = d ( \sin{\theta_i} - \sin{\theta_m})</math>. 

Two rays diffracted by adjacent grooves will differ in path length by a distance ''mλ''. The total difference between the longest and shortest path within a beam is computed by multiplying  ''mλ'' by the total number of grooves illuminated. 

For instance, a beam of width 10&nbsp;mm illuminating a grating with 1200 grooves/mm uses 12,000 grooves. At a wavelength of 10&nbsp;nm, the first order diffracted beam, ''m'' = 1, will have a path length variation across the beam of 120&nbsp;μm. This corresponds to a time difference in the arrival of 400 femtoseconds. This is often negligible for picosecond pulses but not for those of femtosecond duration.

==Applications==
A major application is the extraction, without time-broadening, of a single high-order harmonic pulse out of the many generated by an ultrafast laser pulse interacting with a gas target.{{cn|date=December 2017}}

==See also==
*[[Ultrashort pulse]]
*[[DESY]]

==References==
{{reflist}}
{{Cite book
 |last        = Palmer
 |first       = Christopher
 |title       = Diffraction Grating Handbook
 |edition     = 8th
 |publisher   = MKS Newport
 |year        = 2020
 |url         = https://www.gratinglab.com/Information/Handbook/Handbook.aspx
 }}

[[Category:Optical devices]]


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