5th Southeast Symposium on Contemporany Engineering Topics (SSCET)
September 19, 2014 • New Orleans, LA
Using the dielectric of a capacitor
irradiated with a diode laser as a
new optoelectronic switch
Cristian Bahrim
Department of Physics
Joint appointment with the Phillip Drayer Department of Electrical Engineering
Collaborators:





Dr. Wei-Tai Hsu – Former postdoc at the Research
Center for Adaptive Data Analysis at the National
Central University
Nick Lanning – Graduate student at LSU
Don Duplan – Engineering firm in Dallas.
Md Mozammal Raju – EE alumni (Aug. 2014).
Md Khairuzzaman – EE graduate student.
2
Objectives

Accurate measurements of indices of refraction (and relative
permittivity) from the analysis of the polarized light reflected
by the dielectric surface near the Brewster angle.

Best precision:
- for the Brewster angle is 0.001 degrees.
- for indices of refraction is 10-4.

Shift of the photon’s energy from a laser source as perceived
by the dielectric due to an additional (uniform) external
source of energy, U:
E2  E1  U

Analysis of the optical response of a non-magnetic dielectric
materials using a low voltage applied across, while a laser
radiation illuminates the dielectric surface.
3
Measurements of refractive indices
Index of refraction
1. Dispersion - light of different colors travel at different speeds through
the same material.
Minimum deviation method:

Spectrometry
i
Wavelength [nm]
2. Reflection of polarized light
n

sin    min  / 2 
sin  / 2

Si
Sr
90o
The Poynting vector S 
1


E  B of the

EM radiation experiences a discontinuity
at reflection or refraction.
St
4
Our Experimental Method
Based on measurements of the polarized light reflected by a
dielectric surface near the Brewster angle.

The parallel component of
the reflected E-field vanishes.
Plane of incidence
Brewster's law : i  tan1 nd
Precision: 1) The Brewster angle is measured with 0.001 deg precision.
2) The index of refraction is calculated with a precision of 10-4.
5
Disadvantages of MDM:

Uneven dispersion - violet wavelengths are spread out more than
the red ones.

Rayleigh effect - violet-blue wavelengths are scattered more than
the red wavelengths (the violet part of the spectrum appears less
intense than in standard spectrum charts/spectroscopic tables).
Advantages of RPL versus MDM:

It is not restricted to solid materials of triangular shape.

The local non-homogeneity of the material is not a problem. Only a
locally smooth surface is necessary for having specular reflection.

It does not require experimental data exactly at the Brewster angle,
but within a range of about 1°.
6
Interest in the study of polarized light:

Bio-chemistry - Brewster angle microscopy (it is used for physical
and morphological analysis in microbiology).

Spectro-polarimetric astronomical measurements (spectroscopic
analysis of stellar nebulas).

Forensic analysis (detecting latent fingerprints in a crime scene).

Imaging nano-particles.

Material science (reducing the reflectance of materials).

Analysis of gemstones (such as measuring high index of refraction).
7
Basic Physics: Maxwell equations with boundary
conditions for dielectrics: the Fresnel’s equations.

We impose the optical E-field to be continuous across a nonmagnetic dielectric:

Laws of geometric optics:
i  r
ni sin  i  nr sin  r
8
Fresnel’s equations for the parallel and the
perpendicular components of the reflectance

The reflectance R is the ratio of the reflected irradiance to the
incident irradiance (irradiance ~ E2):
2
 Er   nt cos i  ni cos t 
R   

E
n
cos


n
cos

t
t
i 
 i  i
2
2
 Er   ni cos i  nt cos t 
R     

E
n
cos


n
cos

i
t
t 
 i   i

2
The transmittance T is the ratio of the transmitted irradiance
to the incident irradiance:
2
2
 Et  

2ni cos i
T   

E
n
cos


n
cos

t
t
i 
 i  i
2
E 


2ni cos i
T   t   

E
n
cos


n
cos

i
t
t 
 i   i
2
9
Parallel and perpendicular components
of the reflectance:
 tan    t 
R 



tan



t 

2
 sin     t 
R  



sin



t 

2
Total reflectance:
R  R  R
10
Both components of the reflectance normalized
to the total reflectance have a parabolic shape!
@
Brewster
angle
@
Brewster
angle
0
1
11
Reflectance versus the angle of incidence
12
Dipole Oscillator (Lorentz-Cauchy) Model
13
Interpretation of the interaction between light
and atomic dipoles on the dielectric surface.
n
2
 1    C 2  C o 2
1
14
15
Experimental setup with PASCO equipment
16
17
18
Data acquisition with the Data Studio software
Raw data – normalized reflectances
19
Parabolic fit of the raw data
Parallel Component
0.05
0.04
Ratio
Brewster Angle
0.03
0.02
0.01
0
50
52
54
56 58 60 62
Angle (degrees)
64
66
68
20
Resolution (required)

Visible range
Better than 0.01 degrees!
21
Computer-based analysis of raw data
22
Computer-based analysis of raw data
23
Analysis of raw data for flint glass
irradiated with 532 nm
Range of thermal
stability
24
Correction of the wrong data
during the measurement
A small error of 1.5% in the location
of only three experimental data points
leads to about 0.1o shift
in the position of Brewster angle!
25
Advantages of using a computer–based
procedure for collecting and processing
data in real time.

Allows to recognize during measurements
when the surface is overheated.

Allows to re-measure the data which are out
of trend during data acquisition.
26
Results for two glasses irradiated with two lasers
System
MDM
( degrees )
RPL
( degrees )
Index
of refraction
F650
F532
C650
C532
58.210 ± 0.001
58.380 ± 0.002
56.475 ± 0.001
56.555 ± 0.001
58.213 ± 0.003
58.388 ± 0.008
56.470 ± 0.006
56.553 ± 0.003
1.6141 ± 0.0002
1.6252 ± 0.0006
1.5096 ± 0.0003
1.5143 ± 0.0002
Legend: F= Flint; C= Crown; Wavelengths of 650 and 532 in nm.
27
Our apparatus/methodology allows measurements of any
small variation of the indices of refraction.
Influence of an isotropic and uniform
external energy to the
index of refraction of
the dielectric material.
E2  E1  U
U
E1
E2
 o
28
Capacitor-type configuration
The setup used to observe the changes in
the refractive index of a dielectric surface at
the Brewster angle when a capacitor voltage
is applied across the dielectric.
29
30
Shifted wavelengths of the probe laser signal (of 532nm)
at different voltages applied across the capacitor :
Shifted
Capacitor voltage
Vc (Volts)
wavelength
of the probe
signal
λ2 (nm)
3.0
532
0.0000
4.0
518
0.0629
5.0
505
0.1246
6.0
495
0.1742
9.0
475
0.2797
31
RESULTS
nd
 tan  B .
nI
2
nd   r .
r 
P (t)
 1.
 o EI  t 
Degree of polarization
Linear regime
F = -kx
E2  E1  U
At 0 and 3V
r is the same.
Optoelectronic switch
32
ANALYSIS

A capacitor voltage lower than 0.5V aligns the electric dipoles on the
dielectric surface along the E-field of the laser. The polarized dipoles
reduce the net charge on the plates, and implicitly the capacitance.
The E-field of the laser is
polarized at 45 degrees.

The decrease in the electric permittivity is actually the effect of an
increase in the inertial resistance of the dipoles to the alignment under
the influence of the probe laser due to the presence of a relative weak
capacitor voltage.
33
ANALYSIS
34
35