M. Askari, H. Pakarzadeh, and F. Shokrgozar, "High Q-factor terahertz metamaterial for superior refractive index sensing," J. Opt. Soc. Am. B 38, 3929-3936 (2021)
Terahertz (THz) science and technology have received increasing demand in recent decades for sensing applications. Additionally, THz metamaterials have presented unique characteristics for sensing applications owing to their adjustable resonance frequency. In this paper, we propose a polarization independent metasurface structure serving as a refractive-index-based sensor at THz frequencies. The unit cell is composed of two perpendicularly intersecting metallic bars at the middle of which a cylindrical metallic element is placed. The structure represents a sharp resonance with a high Q-factor of 475 in the absorption profile, which, in some cases, can even reach values as high as 3052.75, revealing confinement of the profile in a narrow band around the resonance frequency of 1.75 THz. Furthermore, the highest values of sensitivity and the figure of merit turn out to be, respectively, 7000 nm/RIU and 19.44 1/RIU, both of which can even take higher values of 11500 nm/RIU and 89.15 1/RIU, respectively, in some special cases of structural dimensions. Finally, the sensor is successfully employed for detecting crystalline sugar molecules.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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Resonance Wavelengths When Media of Different Refractive Indices Are Presumably Considered around the Structure (See Fig. 5)a
RW
SRW
FWHM
S
n
(µm)
(nm)
(nm)
(nm/RIU)
FOM
Q-Factor
1
170.64
-
470
-
-
363.08
1.01
170.68
40
470
4000
8.5
363.14
1.02
170.74
60
470
6000
12.77
363.27
1.03
170.81
70
440
7000
15.9
388.2
1.04
170.87
60
420
6000
14.29
406.83
1.05
170.91
40
410
4000
9.76
416.85
1.06
170.98
70
400
7000
17.5
427.45
1.07
171.05
70
360
7000
19.44
475.14
1.08
171.11
60
370
6000
16.22
462.46
1.09
171.17
60
370
6000
16.22
462.62
n, refractive index; RW, resonance wavelength; SRW, shift in resonance wavelength; FWHM, full width at half maximum; S, sensitivity; FOM, figure of merit.
Table 2.
Sensitivity Values of Some Terahertz Sensors Reported in Other Works
Values of Sensor Parameters for Different Metallic Layer Thicknesses of , 10, 15, 20, 25, 30 µm, with Other Structural Parameters Kept Constant at Values in the Fig. 1 Captiona
t µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
5
171.848
171.9
52
1600
5200
3.25
107.44
10
171.109
171.169
60
818
6000
7.33
209.25
15
170.918
170.96
42
485
4200
8.66
352.49
20
170.98
171.05
70
360
7000
19.44
475.14
25
171.172
171.221
49
254
4900
19.29
674.1
30
171.266
171.297
31
318
3100
9.75
538.67
RW, SRW, S, and FOM are the same as those defined in the Table 1 caption.
Table 4.
Different Sensor Parameters for Diverse Substrate Layer Thicknesses of , 80, 120, 160, 200, 240 µm, with Other Structural Parameters Kept Constant at Values in the Fig. 1 Caption
µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
40
170.98
171.05
70
360
7000
19.44
475.14
80
171.122
171.137
15
285
1500
5.26
600.48
120
168.667
168.689
22
271
2200
8.12
622.47
160
169.715
169.737
22
163
2200
13.5
1041.33
200
170.944
170.974
30
56
3000
53.57
3052.75
240
167.296
167.3
4
85
400
4.7
1968
Table 5.
Sensor Parameters for Different Values of 10, 20, 30, 40, 50 µm, for Parameter , with Other Structural Parameters Kept Constant at Values in the Fig. 1 Caption
w µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
10
166.181
166.296
115
129
11500
89.15
1289.12
20
168.207
168.322
115
248
11500
46.3
678.72
30
170.007
170.067
60
400
6000
15
425.17
40
170.98
171.05
70
360
7000
19.44
475.14
50
171.444
171.481
37
215
3700
17.21
797.59
Table 6.
Sensor Parameters for Diverse Values of the Radius of Cylindrical Element , 50, 60, 70, 80, 90 µm, with Other Structural Parameters Kept Constant at Values in the Fig. 1 Caption
R µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
40
172.62
172.66
40
220
4000
18.18
784.81
50
172.7
172.74
40
280
4000
14.29
616.93
60
170.98
171.05
70
360
7000
19.44
475.14
70
168.96
169
40
900
4000
4.44
187.78
80
168.62
168.62
20
3020
2000
0.66
55.83
Table 7.
Details on Sensing Performance of the Structure in Cases of Using Different Refractive Indices from 1.7 to 1.79, in Steps of 0.01, for the Medium Filling the Analyte Layer
n
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM
Q-Factor
1.7
159.385
-
134
-
-
1189.44
1.71
159.451
66
166
6600
39.76
960.55
1.72
159.517
66
134
6600
49.25
1190.43
1.73
159.585
68
151
6800
45.03
1056.85
1.74
159.651
66
149
6600
44.29
1071.48
1.75
159.734
83
166
8300
50
962.25
1.76
159.817
83
166
8300
50
962.75
1.77
159.9
83
158
8300
52.53
1012.02
1.78
159.985
85
183
8500
46.45
874.23
1.79
160.068
83
183
8300
45.36
874.69
Table 8.
Resonance Frequencies of the Structure in Cases of Using Different Sugar Molecules around the Sensor, along with Absorption Coefficients and Complex Refractive Indices of Molecules at Corresponding Resonance Frequencies
Sugar Molecule
Resonance Frequency (THz)
Absorption
Complex Refractive Index
Aspartame
1.983
0.4928
Cellulose
1.99
0.5611
D-Glucose
1.995
0.8923
Fructose
1.977
0.7
Sucrose
2.002
0.5722
Tables (8)
Table 1.
Resonance Wavelengths When Media of Different Refractive Indices Are Presumably Considered around the Structure (See Fig. 5)a
RW
SRW
FWHM
S
n
(µm)
(nm)
(nm)
(nm/RIU)
FOM
Q-Factor
1
170.64
-
470
-
-
363.08
1.01
170.68
40
470
4000
8.5
363.14
1.02
170.74
60
470
6000
12.77
363.27
1.03
170.81
70
440
7000
15.9
388.2
1.04
170.87
60
420
6000
14.29
406.83
1.05
170.91
40
410
4000
9.76
416.85
1.06
170.98
70
400
7000
17.5
427.45
1.07
171.05
70
360
7000
19.44
475.14
1.08
171.11
60
370
6000
16.22
462.46
1.09
171.17
60
370
6000
16.22
462.62
n, refractive index; RW, resonance wavelength; SRW, shift in resonance wavelength; FWHM, full width at half maximum; S, sensitivity; FOM, figure of merit.
Table 2.
Sensitivity Values of Some Terahertz Sensors Reported in Other Works
Values of Sensor Parameters for Different Metallic Layer Thicknesses of , 10, 15, 20, 25, 30 µm, with Other Structural Parameters Kept Constant at Values in the Fig. 1 Captiona
t µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
5
171.848
171.9
52
1600
5200
3.25
107.44
10
171.109
171.169
60
818
6000
7.33
209.25
15
170.918
170.96
42
485
4200
8.66
352.49
20
170.98
171.05
70
360
7000
19.44
475.14
25
171.172
171.221
49
254
4900
19.29
674.1
30
171.266
171.297
31
318
3100
9.75
538.67
RW, SRW, S, and FOM are the same as those defined in the Table 1 caption.
Table 4.
Different Sensor Parameters for Diverse Substrate Layer Thicknesses of , 80, 120, 160, 200, 240 µm, with Other Structural Parameters Kept Constant at Values in the Fig. 1 Caption
µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
40
170.98
171.05
70
360
7000
19.44
475.14
80
171.122
171.137
15
285
1500
5.26
600.48
120
168.667
168.689
22
271
2200
8.12
622.47
160
169.715
169.737
22
163
2200
13.5
1041.33
200
170.944
170.974
30
56
3000
53.57
3052.75
240
167.296
167.3
4
85
400
4.7
1968
Table 5.
Sensor Parameters for Different Values of 10, 20, 30, 40, 50 µm, for Parameter , with Other Structural Parameters Kept Constant at Values in the Fig. 1 Caption
w µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
10
166.181
166.296
115
129
11500
89.15
1289.12
20
168.207
168.322
115
248
11500
46.3
678.72
30
170.007
170.067
60
400
6000
15
425.17
40
170.98
171.05
70
360
7000
19.44
475.14
50
171.444
171.481
37
215
3700
17.21
797.59
Table 6.
Sensor Parameters for Diverse Values of the Radius of Cylindrical Element , 50, 60, 70, 80, 90 µm, with Other Structural Parameters Kept Constant at Values in the Fig. 1 Caption
R µm
RW (µm)
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM (1/RIU)
Q-Factor
40
172.62
172.66
40
220
4000
18.18
784.81
50
172.7
172.74
40
280
4000
14.29
616.93
60
170.98
171.05
70
360
7000
19.44
475.14
70
168.96
169
40
900
4000
4.44
187.78
80
168.62
168.62
20
3020
2000
0.66
55.83
Table 7.
Details on Sensing Performance of the Structure in Cases of Using Different Refractive Indices from 1.7 to 1.79, in Steps of 0.01, for the Medium Filling the Analyte Layer
n
RW (µm)
SRW (nm)
FWHM (nm)
S (nm/RIU)
FOM
Q-Factor
1.7
159.385
-
134
-
-
1189.44
1.71
159.451
66
166
6600
39.76
960.55
1.72
159.517
66
134
6600
49.25
1190.43
1.73
159.585
68
151
6800
45.03
1056.85
1.74
159.651
66
149
6600
44.29
1071.48
1.75
159.734
83
166
8300
50
962.25
1.76
159.817
83
166
8300
50
962.75
1.77
159.9
83
158
8300
52.53
1012.02
1.78
159.985
85
183
8500
46.45
874.23
1.79
160.068
83
183
8300
45.36
874.69
Table 8.
Resonance Frequencies of the Structure in Cases of Using Different Sugar Molecules around the Sensor, along with Absorption Coefficients and Complex Refractive Indices of Molecules at Corresponding Resonance Frequencies
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