chapter 8. semiclassical radiation theoryoptics.hanyang.ac.kr/~choh/degree/quantum...
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Nonlinear Optics Lab. Hanyang Univ.
Chapter 8. Semiclassical Radiation Theory
8.1 Introduction
Semiclassical theory of light-matter interaction (Ch. 6-7)
- Ignores the quantum-mechanical nature (ex : quantum fluctuations) of the EM field
- Treats the matter quantum-mechanically through the Schrodinger equation
=> Semiclassical theory is not perfect to describe fully the light-matter interactions
(ex : spontaneous emission), but successful to describe all of atomic radiation
when the number of photons are much larger than unity.
In this chapter, we will complete our development of the semiclassical theory ;
Maxwell-Bloch equation.
Nonlinear Optics Lab. Hanyang Univ.
8.2 Optical Bloch Equation
: An equivalent set of vector equations of the density-matrix equations ;
)(2
A 21
*
12222111111 i
)(2
)A( 21
*
122221222 i
)(2
)( 1122
*
1212
ii
)(2
)( 11222121
ii
(6.5.14)
(6.5.17)
)(2
1122
*
1212
ii
)(2
11222121
ii
)(2
21
*
1211 i
)(2
21
*
1222 i
(6.5.2) : without relaxation
: with relaxation
Nonlinear Optics Lab. Hanyang Univ.
For two-level atomic system, 12211
Define,
1221 u
)( 1221 iv
1122 w
(8.2.1)
1) In the case of no relaxation
(6.5.2) => Δvdt
du
χwΔudt
dv
χvdt
dw
(8.2.2)
Nonlinear Optics Lab. Hanyang Univ.
Consider a fictitious space with unit vectors,
and define a “coherence vector” (Pseudo spin, Bloch vector),
and a “torque vector” (Axis vector),
3̂,2̂,1̂
wvu 321S ˆˆˆ
S
Q
31Q ˆˆ
(8.2.2) =>
SQS
dt
d(8.2.5)
Nonlinear Optics Lab. Hanyang Univ.
※ QThe effect of is only to rotates about the direction of
It cannot lengthen or shortenS Q
S
※ 0)(222
SQSS
Sdt
d
dt
dS
2
111122
2
221221
2
1122
2
1221
2
1221
2222
24
)()()(
wvuS
11)(
)(2)(4
22*
11
*
22
2*
11
*
11
*
22
2*
22
*
21
*
12
cccc
cccccccccccc
: Conservation of probability in the two-level atom.
(in the absence of collision or relaxation)
※ 1122 w : Degree of inversion
)1,0(1
)0,1(1
1122
1122
w
w : population is entirely in the upper level
: population is entirely in the lower level
Nonlinear Optics Lab. Hanyang Univ.
Ex) const. ,resonance)(at 0
1Q ˆ : Bloch vector rotates about axis 1̂
1̂2̂
3̂
cos
sin
w
v
(8.2.2c) => dt
dθ
sinsin
dt
dt
tw
tv
cos
sin
1122
2211 1
w
2
1,
2
12211
ww
)cos1(2
1
)cos1(2
1
22
11
t
t
: Same form to (6.3.18)
when 0
Nonlinear Optics Lab. Hanyang Univ.
Ex) const. ,resonance)(at 0
t
dttt0
'')()( : “area” of the pulse
)()()ˆ()(
00 tEtEet
εr21
where,
[ pulse]
If , external wave inverts the atomic population from the lower to the upper level.
Nonlinear Optics Lab. Hanyang Univ.
If E0 is time-dependent, ※
1) The rotating wave approximation in going from (6.3.13) to (6.3.14) fails
if itself contributes rapid temporal variation.
=> We must assume that E0(t) is a slow varying time function.
2) If not just the amplitude but the phase of E0 changes in time, we can no longer
assume that an adjustment of the wave function phase will make (t) real.
=> , E should be complex.
[Remarks]
Nonlinear Optics Lab. Hanyang Univ.
2) In the case considering the relaxation processes
(6.5.14), (6.5.17) =>
wiivui
wii
iiviu
))((
)(2
)()(22
21
1122212112211221
}{222 21
*
1222212221221122 iAw
)( 21 ivu
)put,( 212
ivuivuw
211222 ,,2
1 By definitions,
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)]()([2
)1(1 *
1
ivuivui
wT
w
where,
12
2121
1
2
111
1
TT
AT
: longitudinal lifetime of spin
: transverse lifetime of spin
)2( 12 TT
2̂
3̂
1̂
1T
2T
Nonlinear Optics Lab. Hanyang Univ.
8.3 Maxwell-Bloch Equations
Atomic state under the influence of an external perturbation (EM wave or light)
can be described by Schrodinger equation. But, in order to describe the atomic state
exactly, we need additional equation to express the behavior of the light by the
Interaction with the atoms. = Maxwell equation !
Assumptions :)(),(ˆ),(1) kztietzt εrE : monochromatic, plane wave propagating along the z-direction
; r,both in field varyingSlow 2) t
,2
2
zk
z
,
kz
t
; varyingslow also ison Polarizati 3) )(
2112 ),(2),( kztietzNetz rP
,2121
t tt
21
2
21
2
Nonlinear Optics Lab. Hanyang Univ.
Maxwell wave equation :
),(),( 21
*
0
tzNik
tzctz
),(
1),(
12
2
2
0
2
2
22
2
tztc
tztcz
PE
)ˆ( *
12
*εr ewhere,
)]()([2
)1(1
))((
)(2
),(
*
1
*
0
2
ivuivui
wT
w
wiivuiviu
ivuNik
tzctz
ivu21
(8.2.18) =>Maxwell-Bloch Equations
Nonlinear Optics Lab. Hanyang Univ.
8.4 Linear Absorption and Amplification
In practice, there may be background atoms in laser active medium,
and we should add the background atom effect to the Maxwell equation, (8.3.6).
),(21
*
0
tzNik
(8.3.1) : background atom effect
Background-atoms are far from resonance and come to steady state extremely quickly,
so we can use the adiabatic result (7.2.1) for 21. And, these atoms are at most only slightly
excited, so that 1,0 1122
)(2/
112221
i
i(7.2.1)
2221
))(2/(2/
ii
i
i/εwhere,
ia
i
c
NN
ik
2
1
2 22
0
*
21
*
0 where,
22
0
2
c
Na
22
0
2
)/(
c
Na
Nonlinear Optics Lab. Hanyang Univ.
(8.3.6) =>
),(),(22
21
*
0
tzNik
tzct
ia
z
[Quasi-steady solution]
0)(2
z
i
z
0,0 1212
ct
))((2
1
22211
iaia
iwhere,
022
**
i
z
i
z0
2
zzzz
**
2||
I|| 2 II
z
ze I(0)I(z)
where, )( 2211 aa
Nonlinear Optics Lab. Hanyang Univ.
1) If the resonant atoms are all in their ground state, 11~1, 22~0
field theofn attenuatio :0 aa
2) If the resonant atoms are all in their excited state, 11~0, 22~1
)0,1(& aaaa
field theofon Aplificati negative large a becan :
* Threshold condition for amplification ; 22 - 11 aa /
Nonlinear Optics Lab. Hanyang Univ.
8.5 Semiclassical Laser Theory
Electric field in a laser cavity ;
ti
mmmm zekt sin)(ε̂E
Lmkm /
),(),( tzEtzm
mE where,
Polarization of m-th mode ;
ti
m
m
m zektzNe sin),(2 )(
2112rP
Maxwell wave equation considering cavity loss ;
zktzNi
zktt
i
m
m
mmm
sin),(
sin)(2
)(
)(
21
*
0
0
Actually, different cavity modes are coupled through the z-dependence of m21z,t,
but in many lasers this coupling is not important.
=> Take the average value of 21t) instead of individual m21z,t).
Nonlinear Optics Lab. Hanyang Univ.
)()(2
)( 21
*
00
tNi
tt
i mm
For quasisteady-state lase operation, and021
)(2/
112221
i
i(7.2.1) : adiabatic approximation
)]()()[(2
)(2/
2)(
112222
0
2
1122
*
00
tttiN
i
iN
i
ti
m
mmm
Nonlinear Optics Lab. Hanyang Univ.
mmm igcit
)()(2
2
1
0
,))(()( 121222
0
2
NNNNc
g
where, g
21
)()(),()( 222111 tNtNtNtN
: Fundamental equations of semiclassical laser theory
)()( 12221111 NNNANN m
)()()( 1222122 NNNAN m
Nonlinear Optics Lab. Hanyang Univ.
In steady state, 0)( tm
)(22
)(0
ig
ci m
gain) (threshold/ 0cg
)(2
21
gc
m
)(2
21
2
21
21
mm
m
gc
gcgc
pulling) (frequency
Cf) section 3.5 : a positive sign of g is now possible !※
Nonlinear Optics Lab. Hanyang Univ.
[Einstein laser model]
mmmmmmc
gc
*
0
** )(1
2
0
2
12
2
))(( mm
mNNc
dt
d
(8.5.13) =>
(8.5.12) => Add a pumping term, K,
and Assume N2>>N1 condition is maintained by the pumping :
KNNAdt
dN 22212
2 )()(
Define atom number : , photon number :VNn 22 Vc
I
c
VVq
20 ||
2/
qqnV
c
dt
dq
0
2
)(
KVnAqnV
c
dt
dn
22122
2 )()(
Results of (1.5.1), (1.5.2)
Einstein laser model