- Email: [email protected]
Invisibility of plasmonic nanoparticles in THz region
Materials Letters 173 (2016) 149–152
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Invisibility of plasmonic nanoparticles in THz region Yanping Jin a, Qian Wang a, Chih-Hao Yu b, Xinlong Xu a,n a State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, China b Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
art ic l e i nf o
a b s t r a c t
Article history: Received 29 November 2015 Received in revised form 11 February 2016 Accepted 1 March 2016 Available online 2 March 2016
Platinum (Pt) nanoparticles with diameters d less than 10 nm are almost invisible to THz radiation. However, they demonstrates high absorption in infrared (IR) and visible region. Mie scattering theory under the assumption of d far less than λ, suggests the prominent inhibition of absorption and scattering due to the long wavelength character and the extremely large imaginary part of the dielectric function of Pt in THz region. The results are useful for THz photonics as dichromic filters for THz telecommunication. & 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic composites Nanoparticles Spectroscopy
1. Introduction Plasmonic nanoparticles such as Ag, Au, and Cr, etc. have shown much interest in both optical physics such as transmission enhancement [1] and invisibility [2], and applications such as plasmonic biosensors [3]. Cloaking with plasmonic nanoparticles [2,4] is also a fascinating example of prominent reduction of dipolar scattering by cancellation of the overall dipole moment. Although theoretical calculation suggests invisibility of THz or infrared with nanoparticles [4], few experiment results have been demonstrated due to the difficulty in fabrications. It is noticed that the important factor is the diameter of nanoparticles (d) with the probing wavelength (λ), which is a key issue in all the application with the socalled sub-wavelength pointing at the range of 1 < λ /d < 10. While in the range of λ /d > 10, all the secondary wavelets are approximately in phase and more interesting physics such as controlling electromagnetic waves on the nanoscale to achieve new functionalities would be possible [5]. In this letter, platinum (Pt) nanoparticles were synthesized and dispersed homogeneously in gelatin. Broadband spectroscopic responses ranging from THz region to visible region indicate that the absorption decreases with the increasing of wavelength and plasmonic particles are almost invisible to THz radiation against our intuition with Drude theory. Mie scattering theory under the assumption of d far less than λ suggests prominent inhibition of absorption and scattering due to the long wavelength character and the large imaginary part of permittivity (ε) in THz region. This observation could be used for THz photonics such as dichromic n
Corresponding author. E-mail address: [email protected] (X. Xu).
http://dx.doi.org/10.1016/j.matlet.2016.03.001 0167-577X/& 2016 Elsevier B.V. All rights reserved.
filters for both THz telecommunication and optical-pump-THzprobe applications [6].
2. Experimental Synthesis of monodisperse Pt nanoparticles was achieved by reduction of Pt acetylacetonate (Pt(acac)2) in the presence of oleic acid and oleylamine [7]. Fig. 1 demonstrates that the X-ray diffraction (XRD) with two peaks at 39.85 and 46.35 degrees, which correspond to (111) and (200), respectively. Pt particles are spherical in shape with the average diameter of 7.2 nm according to the Scherrer equation. This size also matches the observation from the transmission electron microscopy (TEM) image (insert in Fig. 1). To disperse the Pt particles efficiently, Pt particles are mixed with gelatin (Sigma Aldrich) in distilled water and sonicated for 15 min. The Pt-gelatin solution was deposited on substrate, desiccated in air and then lifted off to provide free-standing Pt-gelatin films. The weight ratio of Pt to gelatin is changed gradually from 0% to 14.5% w/w. To cover a broadband electromagnetic wave region, visible spectrometer (Perkin-Elmer Lambda 9), infrared (IR) spectrometer (Nicolet 6700), and THz time domain spectroscopy (THz-TDS) [8,9] are employed. THz wave was generated by GaAs photoswitch and detected by ZnTe crystal from a Ti: sapphire laser with a central wavelength of 800 nm.
3. Results and discussion Fig. 2(a) shows the visible absorption spectrum changes with the Pt concentration. It is evident that the optical density can
150
Y. Jin et al. / Materials Letters 173 (2016) 149–152
Fig. 1. X-ray diffraction (XRD) and TEM image of Pt nanoparticles are indicated and the size of nanoparticles are 7 nm.
approach 8, when the weight of concentration goes up to 14.5%, which demonstrates a good tunable neutral density filter for visible applications. Pt as noble metal with an electron
configuration [Xe]4f145d96s1 gives essentially a negative real part and a large positive imaginary part of ε due to the abundance of electrons with the plasma frequency at 9.59 eV [10]. Even though the diameter of the nanoparticles is approximately 50 times less than the wavelength, the free electrons are still an efficient absorber for visible light. Fig. 2(b) shows the dependence of average absorption with the Pt concentration. It is reasonable that near the plasma region, the absorption increases with the increasing of Pt concentration. To avoid the strong absorption peaks from gelatin in IR region, region out of the gelatin absorption are chosen in Fig. 2(c). It is demonstrated that the optical density for 14.5% drop from 8 in visible region to 1.2 in IR region. This is in contradiction with the metallic Drude response because free electrons in metal due to Drude absorption show increasing absorption with the increasing wavelength under the plasma frequency region. The average IR absorption in Fig. 2(d) still shows the increasing dependence with the increasing concentration of nanoparticles but the slope become smoother compared with that in visible region. With the wavelength going down further to THz region, the ratio of d and λ can reach 4 × 10−4 . Fig. 3(a) shows the absorption change with the variation of Pt concentration in THz region. There is no evidence of increasing absorption with the increasing of Pt concentration. Fig. 3(b) shows the average absorption change as a function of Pt concentration. There is almost no absorption in THz
Fig. 2. (a) Visible absorption with the variation of the weight concentration of Pt nanoparticles (lines from bottom to up with colors black, red, blue, cyan, magenta, dark yellow, and navy blue standing for 0, 1.5, 3.2, 4.2, 6.1, 8.4, 14.5% w/w, respectively) in gelatin. (b) Average visible absorption as a function of Pt concentration. (c) IR absorption shows the change with the variation of Pt concentration with the colors same as in (a). (d)Average IR absorption show the change as a function of Pt concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Y. Jin et al. / Materials Letters 173 (2016) 149–152
151
Fig. 3. (a) Absorption change with the variation of Pt concentration in THz region. Lines with colors black, red, blue, cyan, magenta, dark yellow, and navy blue stand for 0, 1.5, 3.2, 4.2, 6.1, 8.4, 14.5% w/w, respectively. (b) Average absorption change as a function of Pt concentration. (c) THz transmission spectrum of dense Pt nanoparticle powder. Dots are experimental data and red line is the 1/f fitting. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
region for Pt, with the optical density drop from 1.2 in IR region to 0.32, which is mainly due to the absorption from gelation. Are Pt nanoparticles really transparent to THz radiation? We further demonstrate the THz radiation transmission through 1 mm dense Pt powder as shown in Fig. 3(c). As we can see from Fig. 3(c), the absorption for dense Pt powder is also high in THz region. The transmission is heavily featuring a 1/f type frequency dependence (red line in Fig. 3(c)), which is similar to the results by metallic nano slit operating beyond the skin-depth limit [5]. This feature is due to the transmission enhancement contributed from particleparticle electromagnetic interaction due to the high density of Pt nanoparticles. These suggest that dense Pt powder with a metallic response is quite different from the isolated Pt nanoparticles and the particle-particle gaps can act as a nanogap-capacitor charged by THz-induced current for dense Pt powder. Why are the isolated Pt nanoparticles invisible to THz wave? We consider the optical antenna effect [11], which can be calculated by Mie theory with an analytical solution to the sphere particle. The linear dissipation optical properties including the absorption, scattering ( β ), and the sum of them can all be expressed in a simple term of extinction coefficient (α ). From the Mie scattering theory [12] under the approximation of d much smaller than λ, the metallic nanoparticles can be viewed as radiative dipoles and α can be expressed as:
α=
18πNVεm3/2 ε2 . λ ⎡⎣ ε1 + 2εm ⎤⎦2 + ε22
The scattering coefficient
β=
24π
3
NV 2εm2 4
λ
(1)
β can be written as:
ε2 . ⎡⎣ ε1 + 2εm ⎤⎦2 + ε22
(2)
where N is the number of particles per unit volume, V the volume of a particle, and εm the dielectric function of the surrounding environment. ε1 and ε2 are the real and imaginary part of the dielectric function of nanoparticles. Qualitatively, the absorption increases with the increasing of Pt concentration, which is
consistent with the visible and IR experiments. The absorption and scattering value will also be influenced by two factors. One is the wavelength and the other is the ε1 and ε2. To implement the calculation quantitatively, Drude and Lorentz terms are adopted for the description of dielectric function of Pt particles:
ε (ω) = 1 −
k
Ωp2 Vf
ω2 + iω (Γ0 + A d )
+
∑
2 j = 1 ωj
f j ωp2 − ω2 + iωΓj
(3)
The first two terms of Eq. (3) are the delocalized components, which are contributed by free electrons. The last term is the localized component, which is ascribed to the interband response of Pt. When d is less than 10 nm, which is sufficiently smaller than the electron mean free path l. The term AVf/d is due to the quantum size effect. A is near to 1 [13,14] and is related to the morphology of particles. Vf ≃ 0.2 × 106 m/s is the Fermi velocity of Pt [15]. To simplify the calculation, the dielectric dispersion of gelatin has not been taken into account due to the fact that εm is much smaller than ε2. All the other values of parameters are taken from reference [10]. Fig. 4 shows the calculated extinction spectrum (a) and scattering spectrum (b) in visible, IR, and THz region. The scattering is quite small compared with the absorption, which suggests the secondary wavelets are approximately in phase due to the deep subwavelength characteristics of particles. Another feature is that both absorption and scattering are exponential decreasing with the increasing of wavelength, which is consistent with the experimental results. In (Eqs. (1) and 2), λ is one of the key issues, and the wavelength of THz wave is much longer than that of visible light. Another factor is due to the difference of ε in visible region, with 9.3 þ 15.5i for 550 nm and 3057.6þ74,814.8i for 1 THz. The negative real part of the dielectric is related to the enhancement of electromagnetic field, and the higher value of that in THz suggests the more efficient electromagnetic enhancement. As is shown in (Eqs. (1) and 2), the larger ε2 compared with ε1 and εm , the less absorption and scattering. Unlike the isolated Pt particles, the dense Pt powder deviates from Mie theory is observed
152
Y. Jin et al. / Materials Letters 173 (2016) 149–152
prominently reduced, which makes the novel metallic nanoparticles ‘transparent’ in THz region. The results raise the interesting questions about the theory of coherent subwavelength transmission and the benefit of using the metallic nanoparticles as a component for photonic devices such as dichromic filters for either THz telecommunication or optical-pump-THz-probe experiments, which may isolate THz signal from optical signal.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11374240), Doctoral Program of Higher Education of China (No. 20136101110007).
References
Fig. 4. Calculated extinction spectrum (a) and scattering spectrum (b) in visible, IR, and THz region.
apparently due to the strong particle-particle electromagnetic interaction as shown in Fig. 3(c).
4. Conclusions We have measured the absorption of Pt nanoparticles embedded in gelatin using THz-TDS, IR, and visible spectroscopy. Our measurements clearly indicate that the absorption decreases with the increasing of wavelength and Pt is almost invisible in THz region. Mie scattering theory suggests dipolar scattering is
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
K.J. Chau, G.D. Dice, A.Y. Elezzabi, Phys. Rev. Lett. 94 (2005) 173904. A. Alu, N. Engheta, Phys. Rev. E 72 (2005) 016623. A.J. Haes, S.L. Zou, G.C. Schatz, R.P. Van Duyne, J. Phys. Chem. B 108 (2004) 109. M.G. Silveirinha, A. Alu, N. Engheta, Phys. Rev. B 78 (2008) 075107. M.A. Seo, H.R. Park, S.M. Koo, D.J. Park, J.H. Kang, O.K. Suwal, et al., Nat. Photonics 3 (2009) 152. X.L. Xu, P. Parkinson, K.C. Chuang, M.B. Johnston, R.J. Nicholas, L.M. Herz, Phys. Rev. B 82 (2010) 085441. C.H. Yu, N. Caiulo, C.C.H. Lo, K. Tam, S.C. Tsang, Adv. Mater. 18 (2006) 2312. X. Xu, K. Chuang, R.J. Nicholas, M.B. Johnston, L.M. Herz, J. Phys. Chem. C 113 (2009) 18106. X. Xu, L. Song, Y. Shi, Y. Yang, S. Xie, W. Li, Chem. Phys. Lett. 410 (2005) 298. A.D. Rakic, A.B. Djurisic, J.M. Elazar, M.L. Majewski, Appl. Opt. 37 (1998) 5271. P. Bharadwaj, B. Deutsch, L. Novotny, Adv. Opt. Photonics 1 (2009) 438–483. C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light By Small Particles, Wiley, New York (1983), pp. 82–129. P. Taneja, P. Ayyub, R. Chandra, Phys. Rev. B 65 (2002) 245412. U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. D. Dye, J. Ketterson, G. Crabtree, J. Low Temp. Phys. 30 (1978) 813.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Invisibility of plasmonic nanoparticles in THz region Yanping Jin a, Qian Wang a, Chih-Hao Yu b, Xinlong Xu a,n a State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, China b Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
art ic l e i nf o
a b s t r a c t
Article history: Received 29 November 2015 Received in revised form 11 February 2016 Accepted 1 March 2016 Available online 2 March 2016
Platinum (Pt) nanoparticles with diameters d less than 10 nm are almost invisible to THz radiation. However, they demonstrates high absorption in infrared (IR) and visible region. Mie scattering theory under the assumption of d far less than λ, suggests the prominent inhibition of absorption and scattering due to the long wavelength character and the extremely large imaginary part of the dielectric function of Pt in THz region. The results are useful for THz photonics as dichromic filters for THz telecommunication. & 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic composites Nanoparticles Spectroscopy
1. Introduction Plasmonic nanoparticles such as Ag, Au, and Cr, etc. have shown much interest in both optical physics such as transmission enhancement [1] and invisibility [2], and applications such as plasmonic biosensors [3]. Cloaking with plasmonic nanoparticles [2,4] is also a fascinating example of prominent reduction of dipolar scattering by cancellation of the overall dipole moment. Although theoretical calculation suggests invisibility of THz or infrared with nanoparticles [4], few experiment results have been demonstrated due to the difficulty in fabrications. It is noticed that the important factor is the diameter of nanoparticles (d) with the probing wavelength (λ), which is a key issue in all the application with the socalled sub-wavelength pointing at the range of 1 < λ /d < 10. While in the range of λ /d > 10, all the secondary wavelets are approximately in phase and more interesting physics such as controlling electromagnetic waves on the nanoscale to achieve new functionalities would be possible [5]. In this letter, platinum (Pt) nanoparticles were synthesized and dispersed homogeneously in gelatin. Broadband spectroscopic responses ranging from THz region to visible region indicate that the absorption decreases with the increasing of wavelength and plasmonic particles are almost invisible to THz radiation against our intuition with Drude theory. Mie scattering theory under the assumption of d far less than λ suggests prominent inhibition of absorption and scattering due to the long wavelength character and the large imaginary part of permittivity (ε) in THz region. This observation could be used for THz photonics such as dichromic n
Corresponding author. E-mail address: [email protected] (X. Xu).
http://dx.doi.org/10.1016/j.matlet.2016.03.001 0167-577X/& 2016 Elsevier B.V. All rights reserved.
filters for both THz telecommunication and optical-pump-THzprobe applications [6].
2. Experimental Synthesis of monodisperse Pt nanoparticles was achieved by reduction of Pt acetylacetonate (Pt(acac)2) in the presence of oleic acid and oleylamine [7]. Fig. 1 demonstrates that the X-ray diffraction (XRD) with two peaks at 39.85 and 46.35 degrees, which correspond to (111) and (200), respectively. Pt particles are spherical in shape with the average diameter of 7.2 nm according to the Scherrer equation. This size also matches the observation from the transmission electron microscopy (TEM) image (insert in Fig. 1). To disperse the Pt particles efficiently, Pt particles are mixed with gelatin (Sigma Aldrich) in distilled water and sonicated for 15 min. The Pt-gelatin solution was deposited on substrate, desiccated in air and then lifted off to provide free-standing Pt-gelatin films. The weight ratio of Pt to gelatin is changed gradually from 0% to 14.5% w/w. To cover a broadband electromagnetic wave region, visible spectrometer (Perkin-Elmer Lambda 9), infrared (IR) spectrometer (Nicolet 6700), and THz time domain spectroscopy (THz-TDS) [8,9] are employed. THz wave was generated by GaAs photoswitch and detected by ZnTe crystal from a Ti: sapphire laser with a central wavelength of 800 nm.
3. Results and discussion Fig. 2(a) shows the visible absorption spectrum changes with the Pt concentration. It is evident that the optical density can
150
Y. Jin et al. / Materials Letters 173 (2016) 149–152
Fig. 1. X-ray diffraction (XRD) and TEM image of Pt nanoparticles are indicated and the size of nanoparticles are 7 nm.
approach 8, when the weight of concentration goes up to 14.5%, which demonstrates a good tunable neutral density filter for visible applications. Pt as noble metal with an electron
configuration [Xe]4f145d96s1 gives essentially a negative real part and a large positive imaginary part of ε due to the abundance of electrons with the plasma frequency at 9.59 eV [10]. Even though the diameter of the nanoparticles is approximately 50 times less than the wavelength, the free electrons are still an efficient absorber for visible light. Fig. 2(b) shows the dependence of average absorption with the Pt concentration. It is reasonable that near the plasma region, the absorption increases with the increasing of Pt concentration. To avoid the strong absorption peaks from gelatin in IR region, region out of the gelatin absorption are chosen in Fig. 2(c). It is demonstrated that the optical density for 14.5% drop from 8 in visible region to 1.2 in IR region. This is in contradiction with the metallic Drude response because free electrons in metal due to Drude absorption show increasing absorption with the increasing wavelength under the plasma frequency region. The average IR absorption in Fig. 2(d) still shows the increasing dependence with the increasing concentration of nanoparticles but the slope become smoother compared with that in visible region. With the wavelength going down further to THz region, the ratio of d and λ can reach 4 × 10−4 . Fig. 3(a) shows the absorption change with the variation of Pt concentration in THz region. There is no evidence of increasing absorption with the increasing of Pt concentration. Fig. 3(b) shows the average absorption change as a function of Pt concentration. There is almost no absorption in THz
Fig. 2. (a) Visible absorption with the variation of the weight concentration of Pt nanoparticles (lines from bottom to up with colors black, red, blue, cyan, magenta, dark yellow, and navy blue standing for 0, 1.5, 3.2, 4.2, 6.1, 8.4, 14.5% w/w, respectively) in gelatin. (b) Average visible absorption as a function of Pt concentration. (c) IR absorption shows the change with the variation of Pt concentration with the colors same as in (a). (d)Average IR absorption show the change as a function of Pt concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Y. Jin et al. / Materials Letters 173 (2016) 149–152
151
Fig. 3. (a) Absorption change with the variation of Pt concentration in THz region. Lines with colors black, red, blue, cyan, magenta, dark yellow, and navy blue stand for 0, 1.5, 3.2, 4.2, 6.1, 8.4, 14.5% w/w, respectively. (b) Average absorption change as a function of Pt concentration. (c) THz transmission spectrum of dense Pt nanoparticle powder. Dots are experimental data and red line is the 1/f fitting. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
region for Pt, with the optical density drop from 1.2 in IR region to 0.32, which is mainly due to the absorption from gelation. Are Pt nanoparticles really transparent to THz radiation? We further demonstrate the THz radiation transmission through 1 mm dense Pt powder as shown in Fig. 3(c). As we can see from Fig. 3(c), the absorption for dense Pt powder is also high in THz region. The transmission is heavily featuring a 1/f type frequency dependence (red line in Fig. 3(c)), which is similar to the results by metallic nano slit operating beyond the skin-depth limit [5]. This feature is due to the transmission enhancement contributed from particleparticle electromagnetic interaction due to the high density of Pt nanoparticles. These suggest that dense Pt powder with a metallic response is quite different from the isolated Pt nanoparticles and the particle-particle gaps can act as a nanogap-capacitor charged by THz-induced current for dense Pt powder. Why are the isolated Pt nanoparticles invisible to THz wave? We consider the optical antenna effect [11], which can be calculated by Mie theory with an analytical solution to the sphere particle. The linear dissipation optical properties including the absorption, scattering ( β ), and the sum of them can all be expressed in a simple term of extinction coefficient (α ). From the Mie scattering theory [12] under the approximation of d much smaller than λ, the metallic nanoparticles can be viewed as radiative dipoles and α can be expressed as:
α=
18πNVεm3/2 ε2 . λ ⎡⎣ ε1 + 2εm ⎤⎦2 + ε22
The scattering coefficient
β=
24π
3
NV 2εm2 4
λ
(1)
β can be written as:
ε2 . ⎡⎣ ε1 + 2εm ⎤⎦2 + ε22
(2)
where N is the number of particles per unit volume, V the volume of a particle, and εm the dielectric function of the surrounding environment. ε1 and ε2 are the real and imaginary part of the dielectric function of nanoparticles. Qualitatively, the absorption increases with the increasing of Pt concentration, which is
consistent with the visible and IR experiments. The absorption and scattering value will also be influenced by two factors. One is the wavelength and the other is the ε1 and ε2. To implement the calculation quantitatively, Drude and Lorentz terms are adopted for the description of dielectric function of Pt particles:
ε (ω) = 1 −
k
Ωp2 Vf
ω2 + iω (Γ0 + A d )
+
∑
2 j = 1 ωj
f j ωp2 − ω2 + iωΓj
(3)
The first two terms of Eq. (3) are the delocalized components, which are contributed by free electrons. The last term is the localized component, which is ascribed to the interband response of Pt. When d is less than 10 nm, which is sufficiently smaller than the electron mean free path l. The term AVf/d is due to the quantum size effect. A is near to 1 [13,14] and is related to the morphology of particles. Vf ≃ 0.2 × 106 m/s is the Fermi velocity of Pt [15]. To simplify the calculation, the dielectric dispersion of gelatin has not been taken into account due to the fact that εm is much smaller than ε2. All the other values of parameters are taken from reference [10]. Fig. 4 shows the calculated extinction spectrum (a) and scattering spectrum (b) in visible, IR, and THz region. The scattering is quite small compared with the absorption, which suggests the secondary wavelets are approximately in phase due to the deep subwavelength characteristics of particles. Another feature is that both absorption and scattering are exponential decreasing with the increasing of wavelength, which is consistent with the experimental results. In (Eqs. (1) and 2), λ is one of the key issues, and the wavelength of THz wave is much longer than that of visible light. Another factor is due to the difference of ε in visible region, with 9.3 þ 15.5i for 550 nm and 3057.6þ74,814.8i for 1 THz. The negative real part of the dielectric is related to the enhancement of electromagnetic field, and the higher value of that in THz suggests the more efficient electromagnetic enhancement. As is shown in (Eqs. (1) and 2), the larger ε2 compared with ε1 and εm , the less absorption and scattering. Unlike the isolated Pt particles, the dense Pt powder deviates from Mie theory is observed
152
Y. Jin et al. / Materials Letters 173 (2016) 149–152
prominently reduced, which makes the novel metallic nanoparticles ‘transparent’ in THz region. The results raise the interesting questions about the theory of coherent subwavelength transmission and the benefit of using the metallic nanoparticles as a component for photonic devices such as dichromic filters for either THz telecommunication or optical-pump-THz-probe experiments, which may isolate THz signal from optical signal.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11374240), Doctoral Program of Higher Education of China (No. 20136101110007).
References
Fig. 4. Calculated extinction spectrum (a) and scattering spectrum (b) in visible, IR, and THz region.
apparently due to the strong particle-particle electromagnetic interaction as shown in Fig. 3(c).
4. Conclusions We have measured the absorption of Pt nanoparticles embedded in gelatin using THz-TDS, IR, and visible spectroscopy. Our measurements clearly indicate that the absorption decreases with the increasing of wavelength and Pt is almost invisible in THz region. Mie scattering theory suggests dipolar scattering is
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
K.J. Chau, G.D. Dice, A.Y. Elezzabi, Phys. Rev. Lett. 94 (2005) 173904. A. Alu, N. Engheta, Phys. Rev. E 72 (2005) 016623. A.J. Haes, S.L. Zou, G.C. Schatz, R.P. Van Duyne, J. Phys. Chem. B 108 (2004) 109. M.G. Silveirinha, A. Alu, N. Engheta, Phys. Rev. B 78 (2008) 075107. M.A. Seo, H.R. Park, S.M. Koo, D.J. Park, J.H. Kang, O.K. Suwal, et al., Nat. Photonics 3 (2009) 152. X.L. Xu, P. Parkinson, K.C. Chuang, M.B. Johnston, R.J. Nicholas, L.M. Herz, Phys. Rev. B 82 (2010) 085441. C.H. Yu, N. Caiulo, C.C.H. Lo, K. Tam, S.C. Tsang, Adv. Mater. 18 (2006) 2312. X. Xu, K. Chuang, R.J. Nicholas, M.B. Johnston, L.M. Herz, J. Phys. Chem. C 113 (2009) 18106. X. Xu, L. Song, Y. Shi, Y. Yang, S. Xie, W. Li, Chem. Phys. Lett. 410 (2005) 298. A.D. Rakic, A.B. Djurisic, J.M. Elazar, M.L. Majewski, Appl. Opt. 37 (1998) 5271. P. Bharadwaj, B. Deutsch, L. Novotny, Adv. Opt. Photonics 1 (2009) 438–483. C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light By Small Particles, Wiley, New York (1983), pp. 82–129. P. Taneja, P. Ayyub, R. Chandra, Phys. Rev. B 65 (2002) 245412. U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. D. Dye, J. Ketterson, G. Crabtree, J. Low Temp. Phys. 30 (1978) 813.