What makes laser light special? Lasers. Lasers. Laser history. Lasers are everywhere! Review of atom discharge lamps. So how do we make laser light?

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1 Lasrs Lasrs What is diffrnt/spcial about lasr light. How dos a lasr work. - rviw atomic discharg lamps. - how light intracts with atoms - how ths idas ar usd to mak lasr. "You know, I hav on simpl rqust. And that is to hav sharks with frickin' lasr bams attachd to thir hads! - Dr. Evil Phys 00, Day 5: Lasrs -Blm 4. Rmindrs Projct titl du tomorrow on DL (quiz) Lab nxt wk No HW ovr th wknd Rading quiz on Tusday (4.) Lasr history Thory dvlopd in 90s/0s by Einstin t al. First lasr (losr) in 960 noon thought it would hav practical us Today lasrs of all shaps ar sizs ar vrywhr Lasrs ar vrywhr! Uss: Barcod scannrs Mtal cutting Nuclar fusion mimicking th sun! Ey surgry Pop concrts What would mak Dr. Hodby MOST WORRIED about hr ysight? a. Shining a lasr into hr y bcaus it is a mor dangrous color b. Shining a lasr into hr y bcaus it has mor powr in th bam c. Shining a lasr into hr y bcaus powr is concntratd to a much smallr spot and could hurt hr rtina d. Nothing worris th Prof. Powr: 0.00W lasr pointrs 0,000W mtal cuttrs W at National Ignition Facility Othr proprtis: All kinds of colors Continuous bams 0.000,000,000,000,00 sc pulss 4 What maks lasr light spcial? Lasr light is diffrnt from light from a flashlight bcaus: All th photons in th bam ar absolutly idntical - xactly th sam color (monochromatic) - All th photons ar going in xactly th sam dirction Lasr light is much mor ordrly than ordinary light and so can b controlld much bttr: - Focusd to a smallr spot - Snt in a paralll bam that dosn t sprad out - Each photon has xactly th sam nrgy and so intracts with mattr (atoms) in xactly th sam way So how do w mak lasr light? Continuum of lctron nrgy lvls: a) In mtal (bulb filamnt), b) Fr lctron (sun) - If hot, jump btwn all diffrnt lvls. - Emit all colors. Rviw of atom discharg lamps Enrgy lvls in isolatd atom: - Excitd by lctron collision or photon absorption - Emits only crtain colors Larg - voltag + - Cathod mits - - Acclrat to anod (gain KE) - Bash gas atoms -Excit lctrons in atoms -Crtain wavlngths mittd whn atoms rturn to ground stat 6

2 Discharg lamp rviw - Atoms lazy- always want to go back to lowst nrgy stat. - Hav to gt rid of nrgy, snd it off as light. Exampl of absorption and mission of light Glow-in-th-dark toys (phosphorscnc). Fast lctron or right color light hits atom in ground stat (lowst possibl. Excitd atom. Atom back to low nrgy ON or Light mittd OFF Spontanous mission Liftim of an xcitd stat: th avrag tim lctrons spnd in th xcitd stat (somwhr btwn < 0-9 s and > 000 s) 8 7 Invstigat light from diffrnt sourcs with diffraction gratings. Diffraction gratings sparat th diffrnt colors from a light sourc First look at fluorscnt lights Hold grating only by dgs...oil from hands ruins grating. Hold clos to y Look at light and you should s rainbows in priphral vision (svral ordrs) Rotat grating if rainbows in vrtical dirction. Nxt look at th non discharg lamp through th grating What do you xpct to s? a) A spctrum lik that from th fluorscnt lights b) Just a fw discrt colors in ach ordr c) On color (sam as lamp) in ach ordr d) No dflctd light Light st ordr nd ordr 9 Small dtour: Spctra of matrials what it is good for Evry matrial (atom, molcul) has a charactristic mission spctrum ( fingrprint ). Thus by looking at th spctrum of an unknown matrial you can figur out its composition (cool!). This is how w knw th rings of Saturn ar mostly composd of watr ic bfor w actually snt thr a satllit to look closly. Nxt look at (rflction of) lasr pointr through grating What do you xpct to s? a) A spctrum lik that from th fluorscnt lights b) Just a fw discrt colors in ach ordr c) On color in ach ordr d) No dflctd light Astronomrs larn about th Univrs by obsrving light from distant astronomical objcts, lik stars or galaxis. Light contains information, and sinc it is much asir to obsrv a star than it is to travl to on, thr is clarly a bnfit to bing abl to undrstand what th light is tlling us! Fig. A fals color imag of th watr contnt in Saturn s rings. (Takn in UV and not visibl). Mor blu (turquois) mans mor watr, rd mans lss watr. Not th fin structur of th rings.

3 How dos a lasr work? Evrything to know about intraction of light and atoms, in asy stps. LASER-- Light Amplification by Stimulatd Emission of Radiation: rpatd cloning of photons to produc LOTS of idntical photons of light. Rquirmnts:. Stimulatd mission (always hav). Population invrsion of bunch of atoms (tricky). Optical fdback (mirror). Absorption of light photon atom Excitd atom. Spontanous mission of light 4. Stimulatd mission of light. First ralizd by A. Einstin An atom has an nrgy sparation btwn ground and first xcitd stat qual to th nrgy of a grn photon. A grn photon hits atom alrady in th first xcitd stat. What happns? a. gts absorbd by th atom, lctron gos to vn highr nrgy lvl. b. gos right on through, atom dos not notic, c. original photon continus and scond idntical on coms out of atom. d. original photon scattrs off in random dirction. Stimulatd mission qustion what will com out on th right? Think bfor you pick a. photon, b. photons, c. photons, d. 4 photons,. 8 5 What happns whn photon travls through discharg tub.. Chanc of stimulatd mission of xcitd atom EXACTLY th sam as chanc of absorption by ground stat atom. Equal chanc bottom atom will absorb photon laving no photon, as top atom will jump down and so hav two photons. To amplify numbr of photons going through th atoms I nd a. mor atoms in lowr nrgy lvl, b. half in lowr, half uppr, c. mor in uppr nrgy lvl, d. a sufficint numbr in uppr lvl and it dos not mattr how many ar in th lowr. glass tub full of atoms (discharg lamp)? How many photons will com out at right compard to numbr in on lft? a. mor, b. lss, c. sam numbr, d. non will com out. 7 8

4 To amplify numbr of photons going through th atoms I nd mor atoms in uppr nrgy lvl than in lowr. - Lowr lvl atoms at photons up, - Uppr lvl atoms clon photons -Equal prob. for absorption/stimulatd mission so amplification or loss proportional to N uppr -N lowr. Population Invrsion N uppr > N lowr photon amplification Gt photon amplification if mor atoms in uppr lvl than lowr This condition is vry unnatural Population invrsion N uppr < N lowr photon loss 9 How do w gt population invrsion? Lvl structur for population invrsion For population invrsion, want: - atoms to pil up in uppr lasr lvl, - kp lowr lasr lvl mpty Nd at last (idally 4) nrgy lvls involvd. Nd rat into uppr lasr lvl fastr than rat out (sp. mission). Try shining lots of light on atoms. If w turn th powr of th light way up, w will gt: a. population invrsion with all atoms xcitd to uppr lvl, b. no atoms in uppr lvl, c. half th atoms in uppr lvl. (absorption or lctron collisions) Quick (collision or sp. mission) Uppr lasr lvl 4 Lowr lasr lvl Quick (collision or sp. mission) pumping procss to produc population invrsion Lvl structur for population invrsion Making a lasr bam What color light could com out on to 4 transition? a. grn, b. blu, c. rd, d. a and b.,. a and c Now w undrstand how to clon photons: - Population invrsion in th lasr mdium - Stimulatd mission Whr dos th first photon com from? - Spontanous mission from on of th xcitd atoms (absorption or lctron collisions) Quick (collision or sp. mission) Uppr lasr lvl Bam dirction 4 Lowr lasr lvl Quick (collision or sp. mission) pumping procss to produc population invrsion Will cloning th photon abov contribut to th lasr bam? a) Ys b) No 4 4

5 Making a lasr bam Making a lasr bam Bam dirction Bam dirction Nd to slct a spontanous photon travlling in dsird bam dirction to sd th bam How? - Put mirrors on ithr nd of tub - Photon travlling EXACTLY along tub axis bouncs back and forth. Snowballs mor and mor idntical photons with ach pass - Othr photons bounc out of sid of tub Nd to slct a spontanous photon travlling in dsird bam dirction to sd th bam How? - Put mirrors on ithr nd of tub - Photon travlling EXACTLY along tub axis and with EXACTLY th right wavlngth bouncs back and forth. Snowballs mor and mor idntical photons with ach pass - Othr photons bounc out of sid of tub How do w gt a bam? mak on mirror a littl laky (partially rflctiv) 6 5 5