lecture one of laser and photonics course on optics

photonics

1. PHN-315
L1
CWS + Attendance = 25+5 = 30 Marks
MTE = 20 Marks
ETE = 50 Marks

2. •For today’s ( 19 July 2024) lecture content, you can go through the
slides here.
•You may refer to book “Photonics –Optical electronics in Modern
Communications” , 6
th
Ed. By Yarivand Yeh, page numbers 211 to
213.
•Some of the content may be further elaborated in the next class.

3. 3
Radiative Process & Atomic Transitions
•Ifanatomicsystemisingroundstate(E
1)andissubjectedtotheradiationofanincidentphotonthen
theincidentphotonwillgetabsorbedbytheatomicsystem,provided
ℏ⍵=�
2−�
1 …(1)
whereE
2=upperstateorelevatedstateofatomicsystem
ℏ=
h
2π
and ⍵=2πf
•Let??????
1&??????
2arewavevectorscorrespondingtoupperandlowerstateofatomicsystemthen:
∆??????=??????
1−??????
2=??????
??????
Where??????
??????
iswavevectorofphoton.
•Theatomicsystemundergoestransitionwhenelectronabsorbsphoton,thus??????
1&??????
2arewavevector
correspondingtoelectronsinlowerenergystate(E
1)andupperenergystate(E
2)respectively.

4. 4
Radiative Process & Atomic Transitions
∆??????=??????
1−??????
2=??????
??????
??????
1=??????
2+??????
??????
Generally, ??????
2≫??????
??????
??????
�=??????
�
•Suchtransitions are called direct or vertical transitions.

5. 5
Radiative Process & Atomic Transitions
•Ifanelectronisalreadyinstate2att=0,thereisafiniteprobabilityperunittimethatitwillundergoa
transitiontostate1andwillemitaphotonofenergy:
�=�
2−�
1=ℏ⍵
•Aboveprocessishappeningwithoutanyincidentradiationanditisknownasspontaneousemission.

6. 6
Radiative Process & Atomic Transitions
•Ifanatomicsystemisalreadyinstate2att=0andatthesametimeaphotonisincidentonitthen
atomicsystemmaygotostate1,emittingaphotonofenergyequaltoenergyofincidentphoton(in
additiontospontaneousemission).
•Suchaprocessiscalledstimulatedemission.

7. 7
Summary of three basic optical processes

8. 8
Atomic Polarizability & Dielectric Constant
•Considertransmissionofalightbeamthroughatransparentisotropicmedium.
•Theelementarychargedparticlesinsideeachatomofthemediumaredisplacedfromtheirequilibrium
positionsundertheinfluenceofelectricfieldoflightbeam.
•Inmostdielectrics,thechargeseparationisdirectlyproportionaltothestrengthofelectricfieldof
opticalbeam.
•Theinduceddipolemoment??????canbewrittenas:
Ԧ??????= ??????�
Where??????=atomicpolarizabilityormolecularpolarizability.
•Thedirectionofchargeseparationisinthedirectionofelectricfield.
•Foratomicsystemwithsphericalsymmetry,thepolarizabilityreducestoascalarandcanbederived
usingasimpleclassicalelectronmodel.

9. 9
Atomic Polarizability & Dielectric Constant
•Thedielectricconstantofanymediumisdecidedbythemannerinwhichtheatomsareassembledin
thatmedium.
•Assumingthecaseofagasmediumandtaking‘N’asthenumberofatomsperunitvolume,then
polarization:
??????=??????Ԧ??????
??????=????????????�
Also ??????=ɛ
0χ�=????????????� …(1)
where χ= electric susceptibility of medium
ɛ
0= permittivity of vacuum
Electricdisplacementvector�isoftenwrittenas
�=ɛ
0�+?????? …(2)

10. 10
Atomic Polarizability & Dielectric Constant
•From(1)&(2) �=ɛ
0�+ɛ
0χ�
�=ɛ
0(1+χ)�
ɛ�=ɛ
0(1+χ)�
ɛ=ɛ
0(1+χ)
•Fromequ.(1), ??????=ɛ
0χ�=????????????�
χ=
????????????
ɛ
0
Therefore;ɛ=ɛ
01+χ=ɛ
01+
????????????
ɛ
0
…(3)