Open access peer-reviewed chapter

Surface Plasmonics and Its Applications in Infrared Sensing

By Guiru Gu, Xuejun Lu, Thitikorn Kemsri and Yingjie Zhang

Submitted: September 20th 2016Reviewed: January 9th 2017Published: June 21st 2017

DOI: 10.5772/67410

Downloaded: 900

Abstract

Surface plasmonic waves have been extensively researched due to their strong surface confinement. The strong surface confinement allows high absorption in an infrared (IR) detector with a thin active absorption region. The excitation of surface plasmonic resonance (SPR) depends on the metallic structures and the interface materials. This enables engineering of plasmonic-enhanced IR detector properties (e.g. detection wavelength, polarization and angular dependence) by properly designing the plasmonic structures. This chapter first gives a brief review of the surface plasmonic waves, followed by the description of SPR excitation in a metallic two-dimensional (2D) sub-wavelength hole array (2DSHA) structure. The applications of the 2DSHA SPR in IR detector enhancement are then presented with a discussion of the polarization and angular dependence.

Keywords

  • surface plasmonic waves
  • quantum dot
  • infrared detection

1. Introduction

Surface electromagnetic waves (also called surface plasmon polaritons, SPPs), predicted by Ritchie in 1957 [1], are coherent electron oscillations at a metal surface. The coherent electron oscillations generate electromagnetic (EM) waves that travel along the dielectric/metal interface [2]. Depending on the metal structures, certain resonant modes of the surface electromagnetic waves can be strongly absorbed (or excited) [2]. This is generally referred to as excitation of surface plasmonic resonance (SPR) modes. SPRs propagate along the metal-dielectric interface. The intensity profile exponentially decreases with the distance from metal-dielectric interface, that is, SPs are confined at the metal-dielectric interface. Such surface confinement effects have been used in Raman scattering, that is, surface-enhanced Raman scattering (SERS) [35] and spectroscopy [6, 7] and have been reported with thousands of times sensitivity enhancement [3, 4]. In addition to SERS, the SPR enhancement technology also provides a promising technique to concentrate EM energy on surface area and thus enables high quantum efficiency with a thin active absorption layer in a photodetector. Significant performance enhancements in a quantum dot infrared photodetector (QDIP) have been reported [815].

In addition, the SPR modes can also change EM field distribution and thus offers an effective technique for EM field engineering to achieve specific transmission and/or receiving patterns with polarization and detection spectrum engineering capability [11, 16]. In this chapter, a brief review of the fundamental concepts of SPRs will be first given and followed by the description of their applications in infrared detections. SPR-based EM field engineering will also be presented with the discussion of polarization and receiving angle control.

2. Review of surface plasmonic resonance

2.1. Plasma frequency, complex conductivity and the Drude model

Plasmonic waves are electromagnetic waves. They follow Maxwell equations, which can be expressed as follows in the phasor domain

D= ρ,E1
×E=jωμH,E2
B=0,E3
×H=J+jωεE,E4

where Jis the current density, D,E, are the electric flux density and the electric field, respectively, and B, Hare the magnetic flux density and the magnetic field, respectively, ωis the angular frequency, and εand μare the permittivity and permeability of the material, respectively. The current density Jis related to Eby:

J=σE,E5

where σis the conductivity of the material. Eq. (4) can therefore be expressed as follows:

×H=jωε(1+σjωϵ)E,E6

where εc=ε(1+σjωϵ)is the complex permittivity of the material. The Maxwell’s current continuity equation is as follows:

J=etn,E7

where eis the charge of an electron, e=1.6×1019(C), nis the density of electrons in metal.

The current density is related to the speed of the charge vby:

J=en0v,E8

where n0is the average electron density in metal. Combining Eqs. (7) and (8), one gets:

n0v=tn,E9

Taking another partial derivative of t, one gets:

n0tv=2t2n,E10

Eq. (10) can be expressed as follows:

n0mma=2t2n,E11

where mis the mass of an electron. The materm is the force on the electron, which is the electric force, F=eE. Eq. (11) can thus be changed to

en0mE=2t2n,E12

In phasor domain, Eq. (12) can be expressed as follows:

en0mE=ωp2n,E13

where ωpis the plasma frequency. Combining Eqs. (1) and (13), one gets

ωp=e2n0mϵE14

Taking the partial derivative of t for Eq. (8), one gets

tJ=en0mmtv,E15

The mtvterm can be replaced by the electric force,  F=eE. Eq. (15) becomes

tJ=e2n0mE,E16

In phasor domain, Eq. (16) can be rewritten as follows:

jωJ=e2n0mE,E17

Combining Eqs. (5) and (17), one gets:

σ=e2n0jωm,E18

The complex permittivity εcis thus given as follows:

εc=ε(1+σjωϵ)=ε(1e2n0ω2mϵ)=ε(1ωp2ω2)E19

The derivations and equations above are for the ideal lossless free-electron gas model. The loss term can be included in the model by introducing a relaxation rate γ, that is,

εc=ε(1ωp2ω2+jωγ)E20

Eq. (20) is the complex permittivity given by the well-known Drude model [17].

2.2. Plasmonic waves at the dielectric and metal interface

Following Raether in Ref. [2], one can analyse plasmonic waves using a transmagnetic (TM) plane wave. Figure 1 shows the incident of a TM plane EM wave with the H-field in the y-direction. Note that the phasor expressions of the H-fields follow the convention of ikRindicating the wave travels in the Rdirection, whereas ikRindicating the wave travels in the − Rdirection. A negative sign needs to be included in Eq. (6) to use this convention.

Figure 1.

Scheme of TM wave incidence at the dielectric and metal interface.

Under the aforementioned convention, the E-field defined by Eq. (6) can be changed to

E=1jωɛc×HE21

From Eq. (21), the Ex can be expressed as follows:

E1i=x^jk1zjωε1H1iej(k1xx+k1zz)E22
E1r=x^jk1zjωε1H1rej(k1xxk1zz)E23
E2t=x^jk2zjωε2cH2tej(k2xx+k2zz)E24

where k1x, k1zare the x and z components of the propagation constant k1, respectively. Similarly, k2x, k2zare the x and z components of the propagation constant k2, respectively.

k1x2+k1z2=k12=ω2ϵ1μ1E25
k2x2+k2z2=k22=ω2ϵ2cμ2E26

where μ1μ2μ0 are the permeability of the material 1, 2 and vacuum, respectively. The boundary conditions are Hyand Excontinuous at the interface (i.e. z=0). Applying the boundary conditions, one gets the following:

H1iejk1xx+H1rejk1xx=H2tejk2xxE27
k1zε1H1iejk1xxk1zε1H1rejk1xx=k2zε2cH2tejk2xxE28

Since Eq. (27) and Eq. (28) are valid for any x, k1x=k2x.

When there is no reflection, that is, H1r=0, Eqs. (27) and (28) become

H1i=H2tE29
k1zε1H1i=k2zε2cH2tE30

Combining Eqs. (29) and (30), one gets the following:

k2z=ε2ck1zε1E31

Plugging Eq. (31) into Eq. (26), one gets the following:

k2x2+k2z2=ε2c2ε12k1z2+k2x2=ω2ϵ2cμ0E32

Combing Eq. (32) with Eq. (25), one can get k1x as follows:

k1x2=ω2μ0ϵ2cϵ1(ε1+ε2c)E33

Eq. (33) can be expressed using relative permittivity ε1r, and ε2cr.

k1x2=ω2μ0ϵ0ϵ2crϵ1r(ε1r+ε2cr)E34

where ε1r=ε1ε0, and ε2cr=ε2cε0.

kx=k0Re[ϵ2crϵ1r(ε1r+ε2cr)]+jk0Im[ϵ2crϵ1r(ε1r+ε2cr)]E35

k2zcan be obtained from Eq. (34) as follows:

k2z=k0Re[ϵ2crϵ2cr(ε1r+ε2cr)]+jk0Im[ϵ2crϵ2cr(ε1r+ε2cr)]E36

where Re(.) and Im(.) are taking the real and the imaginary parts. The plasmonic wave propagates in x and z directions with loss described by the imaginary parts of the corresponding directions, that is, kspp=kx. Since ε2cris generally much larger than ε1r, Eq. (36) can be simplified as follows:

kx=k0ϵ1rE37

2.3. Plasmonic wave excitation in a metallic two-dimensional (2D) sub-wavelength hole array (2DSHA) structure

Surface plasmonic waves can be excited by numerous structures. The metallic 2DSHA structure [18] is one of the commonly used plasmonic structures for performance enhancement in infrared photodetectors [8, 1012]. Figure 2 shows a schematic structure with the light incident scheme of the metallic 2DSHA array structure. The 2DSHA structure is a square lattice with the period of p, and the diameter of the hole is d. The periodical structure provides a grating vector of Λ=2πp. Surface plasmonic wave can be excited when the grating vector of Λmatches the kspp, that is, kspp=2πp.

Figure 2.

Schematic structure with the light incident scheme of the metallic 2DSHA array structure. The 2DSHA structure is square lattice with a period of p, and the diameter of the hole is d.

Figure 3 shows the cross section of the light incidence on the 2DSHA structure. The excited plasmonic waves and the scattered waves by the 2DSHA structure are indicated in the figure.

Figure 3.

Schematic of the cross-section of the light incidence on the 2DSHA structure. The 2DSHA structure generates plasmonic waves and scattering waves.

From E-M field boundary conditions, the following relations hold:

H1s=H2tE38
H1s+=H2t+E39
k1zε1=k2zε2cE40

The excitation efficiency ηcan be defined by the overlap integral of the incident filed with the plasmonic waves, that is,

η=H(x,z)ei(ksppx+k2zz)|z=0dxE41

where H(x,z)is the magnetic field of the incident wave modified by the 2DSHA, that is, the periodical 2DSHA structure will introduce periodical variation in H(x,z)as well.

2.4. Near-field E-field distribution in the 2DSHA structure

The near-field E-field component can be simulated as well. Figure 4 shows the top (x-y) and cross-section (x-z) views of the Excomponent at different wavelengths. The 2DSHA structure is square lattice with a period of 2.3 µm, and the diameter of the hole is 1.15 µm. The simulation is performed using CST Microwave studio®. The incident light is linearly polarized surface normal plane wave with E-field magnitude of 1 V/m. The colour scale bar represents the magnitude of the Ex.

Figure 4.

Top and cross-section views of the Ex component at different wavelengths simulated using CST Microwave studio®. The incident light is a linearly polarized surface normal plane wave with E-field magnitude of 1 V/m.

Figure 5 shows the top (x-y) and cross-section (x-z) views of the Ezcomponent at different wavelengths. From Figures 4 and 5, both Ex and Ez strongly depend on the excitation wavelengths. Ez is high between 7.6 and 8.5 µm, whereas Ex is strong between 7.6 and 8.0 µm. Ez is primarily at the edge of the holes, whereas Ex is inside the holes. The period of the 2DSHA p= 2.3µm, kspp= 2πp.

Figure 5.

Top and cross-section views of the Ez component at different wavelengths simulated using CST Microwave studio®. The incident light and the simulation conditions are the same.

From Eq. (35), ksppis

kspp=2πλ0Re[ϵ2crϵ1r(ε1r+ε2cr)]+j2πλ0Im[ϵ2crϵ1r(ε1r+ε2cr)]E42

where λ0 is the free space wavelength, εm and εd are the relative permittivity of the metal and GaAs, respectively. The dielectric constant of GaAs in the long-wave infrared region is 10.98 [19]. The dielectric constant of Au is calculated to be −1896 + i684 at 7.6 µm [1]. The calculated plasmonic resonant wavelength is λsp = 7.6 µm. At the resonant wavelength (i.e. λ=7.6 µm), the E-fields are strongly confined near the surface. Such strong surface confinement of plasmonic waves enables high absorption using a thin absorption layer and thus leads to significant enhancement in quantum dot infrared photodetectors (QDIPs) [8, 1013, 15]. In the following sections, we will first give a brief introduction of QDIPs, followed by the description of surface plasmonic enhancement in QDIPs.

3. QDIPs in mid-wave infrared (MWIR) and long-wave infrared (MWIR/LWIR) detection

QDIPs are based on intersubband transitions in self-assembled InAs quantum dots (QDs). The simplified band diagram and the schematic structures of a QD infrared photodetector (QDIP) are shown in Figure 6(a) and 6(b), respectively. The s, p, d, f, represent the energy levels of a QD with the wetting layers (WL). A typical QDIP consists of vertically-stacked InAs quantum dots layers with GaAs capping layers. The electrons are excited by the normal incident light and subsequently collected through the top electrode and generate photocurrent. This is a unipolar photodetector, where only conduction band is involved in the photodetection and photocurrent generation process.

Figure 6.

Schematic structure a unipolar QDIP with the intersubband transition: (a) simplified band diagram and (b) schematic of the QDIP. The s, p, d and f, represent the energy levels of a QD.

The QDIP technology offers a promising technology in MWIR and LWIR photodetection due to the advantages provided by the three-dimensional (3D) quantum confinement of carriers—including intrinsic sensitivity to normal incident radiation [13], high photoconductive (PC) gain, high quantum efficiency [15] and photoresponsivity [16]. The normal incidence detection capability greatly simplifies the fabrication complexity for a large format (1K × 1K) FPA. The high photoconductive (PC) gain and high photoresponsivity provide a promising way for MWIR and LWIR sensing and detection.

Figure 7 shows the photodetection spectrum of a QDIP. It covers a broadband IR spectrum from 3 to 9 µm. The insert of Figure 7 shows an atomic force microscopic (AFM) picture of the QDs by self-assembled epi-growth. The growth gives high-density QDs with uniform sizes. The InAs/GaAs QD material is a mature material system with low substrate cost, high material quality and large wafer growth capability. It can offer low-cost MWIR and LWIR photodetectors and focal plane arrays (FPAs) with simplified fabrication processes and high yield. Detailed QDIP performance, such as photoconductive gains, noise, photoresponsivity and photodetectivity can be found in literature [2027].

Figure 7.

Photodetection spectrum of a QDIP. It shows a broad IR detection spectrum from 3 to 9 µm. The insert shows an AFM image of the QDs by self-assembled epi-growth.

4. 2DSHA plasmonic-enhanced QDIPs

4.1. Backside configured 2DSHA-enhanced QDIPs

Surface confinement of plasmonic waves enables high absorption using a thin absorption layer and thus leads to significant enhancement in QDIPs [8, 1013, 15]. We have developed a backside-configured plasmonic enhancement technology by fabricating the 2DSHA plasmonic structures on top of a QDIP and illuminating the QDIP from the opposite side of the plasmonic structure (i. e. backside illumination) [12]. Figure 8(a) and 8(b) show the schematic cross-section structures of the backside-configured plasmonic QDIP and the reference QDIP without the plasmonic structure, respectively. Figure 8(c) and 8(d) show scanning electron microscopic (SEM) images of the plasmonic structures on a QDIP and a close-up view of the plasmonic structure, respectively.

Figure 8.

(a) Schematic cross-section structure of the backside-configured 2DSHA plasmonic structure on the QDIP. The IR incident light illuminates from the substrate side; (b) ref-QDIP without the plasmonic structure; (c) SEM image of the 2DSHA plasmonic structure on the QDIP and (d) close-up view of the plasmonic structure (© IOP Publishing. Reproduced with permission from Ref. [12]. All rights reserved).

Figure 9 shows the measured photocurrent spectra of the backside-configured plasmonic QDIP compared with the top-side configured plasmonic QDIP and the reference QDIP. The backside-configured plasmonic QDIP clearly shows higher plasmonic enhancement than those of the top-side configured plasmonic QDIP and the reference QDIP.

Figure 9.

Measured photocurrent spectra of the backside-configured plasmonic QDIP compared with the top-side configured plasmonic QDIP and the reference QDIP. The backside-configured plasmonic QDIP clearly shows higher plasmonic enhancement.

Figure 10(a) shows the simulated E-field intensities of the top and backside-configured plasmonic structures, respectively. The backside-configured plasmonic structure can induce stronger E-field at the interface and therefore can provide larger plasmonic enhancement [12].

Figure 10.

Simulated E-field intensities: (a) top-configured plasmonic structure and (b) backside-configured plasmonic structure. The backside-configured plasmonic structure can induce stronger E-field at the interface and therefore can provide larger plasmonic enhancement (© IOP Publishing. Reproduced with permission from Ref. [12]. All rights reserved).

4.2. Wavelength-tuning and multispectral enhancement

From Eq. (42) and kspp= 2πp, one gets:

λsp=pm2+n2Re[ϵ2crϵ1r(ε1r+ε2cr)]E43

where (m, n) are the orders of the grating vectors. Therefore, the plasmonic resonance wavelength can be tuned by changing the period of the 2DSHA plasmonic structure. The plasmon resonant peaks λsp of different (m, n) orders for various 2DSHA periods p are shown in Figure 11 that exhibits the linear dependence of the resonant peak wavelength λsp with the period p of the 2DSHA for different (m, n) orders. As predicted by Eq. (43), good linearity is obtained for both the (1, 0)th or (0, 1)th and the (1, 1)th order plasmonic resonant peaks.

Figure 11.

(a) Plasmonic resonant peaks λsp of different (i, j) orders for various 2DSHA periods; (b) linear dependence of the resonant peak wavelength λsp with the period of different (m, n) orders.

We define the photocurrent enhancement ratio R as follows:

R=IplasmonicIrefE44

where Iplasmonic is the photocurrent of the plasmonic QDIP and Iref is the photocurrent of the reference QDIP without the 2DSHA structures.

Figure 12 shows the photocurrent enhancement ratio spectra for all the plasmonic QDIPs with different periods from 1.4 to 2.0 µm. The periods of the 2DSHA plasmonic structures are marked on the curves. The insert shows a microscopic picture of a QDIP with a 2DSHA plasmonic structure. The IR incidence is from the backside of the QDIPs, that is, backside-configured 2DSHA plasmonic structures.

Figure 12.

Photocurrent enhancement ratio spectra for plasmonic QDIPs with different periods varying from 1.4 to 2.0 µm. The insert is a microscopic picture of a QDIP with a 2DSHA plasmonic structure (Reproduced from Ref. [28] with the permission of AIP Publishing).

As shown in Figure 12, the enhancement wavelengths can be tuned by varying the periods of the 2DSHA plasmonic structures. This allows one to achieve multispectral QDIPs without fabricating an individual filter on each photodetector [28].

4.3. Polarization dependence and polarimetric detection

From Eq. (43), the plasmonic resonant wavelength depends on the period p. By changing the plasmonic lattice from a square lattice to a rectangular lattice with different periods in the x and y directions, one can engineer the resonant wavelengths at different polarizations.

Figure 13(a) shows an SEM picture of a rectangular lattice 2DSHA plasmonic structure with the periods of 2.6 and 3.0 µm in the x-direction and the y-direction, respectively. Figure 13(b)(d) shows the transmission spectra of the un-polarized, 90° polarized (i.e. polarization in y-direction), and 0° polarized (i.e. polarization in x-direction), respectively. The resonant wavelengths correspond to the orders in the x- and y- directions are marked in the figures. The un-polarized light shows transmission in both (0, 1) and (1, 0) orders. However, the 0° polarized light only shows transmission at the (1, 0) order resonant wavelength and the 90° polarized light only shows transmission at the (0, 1) order.

Figure 13.

(a) SEM picture of rectangular lattice 2DSHA structure. The periods in x- and y- directions are 2.6 and 3.0 µm, respectively. (b), (c) and (d) transmission spectra at of the un-polarized, 90° polarized (i.e. polarization in y-direction), and 0° polarized (i.e. polarization in x-direction), respectively. The polarized light only shows transmission at their corresponding resonant wavelengths (© IOP Publishing. Reproduced with permission from Ref. [11]. All rights reserved).

Figure 14 shows a microscopic picture of a QDIP with the rectangular lattice 2DSHA structure. The insert shows an SEM picture of the rectangular lattice 2DSHA structure.

Figure 14.

Microscopic picture of a QDIP with the rectangular lattice 2DSHA structure. Insert: SEM picture of the rectangular lattice 2DSHA structure (© IOP Publishing. Reproduced with permission from Ref. [11]. All rights reserved).

Figure 15(a) and (b) shows the measured detection spectra of the plasmonic QDIP at 90° polarization and 0° polarization, respectively. The photo-response of a reference QDIP without the plasmonic structure is also shown in the figures for comparison. The plasmonic QDIP shows different detection wavelengths at different polarizations.

Figure 15.

Measured detection spectra of the plasmonic QDIP at different polarizations: (a) 90° polarization and (b) 0° polarization. Different polarizations show different detection wavelengths (© IOP Publishing. Reproduced with permission from Ref. [11]. All rights reserved).

4.4. Angular dependence of the 2DSHA plasmonic enhancement

Figure 16 shows the schematic view of the top-illuminated 2DSHA plasmonic QDIP with an incident angle θ.

Figure 16.

Schematic view of the top-illuminated 2DSHA plasmonic QDIP with an incident angle θ.

From Eq. (41), the coupling efficiency ηto the plasmonic waves depends on the overlap integral of H(x,z)with the plasmonic wave ei(ksppx+k2zz). At the incident angle θ, the H(x,z)not only periodically modulated by the 2DSHA structure, but also added additional phase term ei(k0sinθx), that is,

H(x,z)e±iΛxeik0sinθxE45

From Eq. (45), the coupling condition can be expressed as follows:

k0sinθ±Λ=ksppE46

Taking the + sign, one can write Eq. (46) in terms of plasmonic resonant wavelength λ0as follows:

λ0={Re[ϵ2crϵ1r(ε1r+ε2cr)]sinθ} ΛE47

Eq. (47) shows that the enhancement wavelength shifts to shorter wavelengths as the incident angle increases.

Figure 17 shows the measured photocurrent of the 2DSHA plasmonic QDIP at different incident angles of 0°, 30°, 45°, and 60°compared with the reference QDIP at the same incident angles. Blue shift of the plasmonic resonant wavelengths λ0is experimentally observed as the incident angle increases.

Figure 17.

Measured photocurrent of the 2DSHA plasmonic QDIP at different incident angles of 0°, 30°, 45° and 60° compared with the reference QDIP at the same incident angles. The plasmonic resonant wavelength λ0 shift to shorter wavelengths as the incident angle increases.

To quantitatively analyse the relationship between the plasmonic wavelength shift and the incident angle, we plot the plasmonic resonant wavelengths λ0versus sinθ. Figure 18 shows the λ0versus sinθ. A linear relation is obtained, which agrees well with Eq. (47). The interception and the slope are 7.6 and −1.4 µm, respectively. The interception 7.6 µm matches the value Re[ϵ2crϵ1r(ε1r+ε2cr)]Λin Eq. (47). However, the slope of −1.4 µm is smaller than Λ=2.3 µm as predicted in Eq. (47). This may be due to the plasmonic scattering-induced phase distribution in H(x,z)in addition to the ei(k0sinθx)term.

Figure 18.

λ0 versus sinθ plot. λ0 varies linearly with sinθ. The interception and the slope are 7.6 and −1.4 µm, respectively.

5. Conclusion

In conclusion, 2DSHA plasmonic structures can effectively induce plasmonic resonant waves. The resonant wavelength depends on the period of the plasmonic structure. The surface confinement effect allows one to improve the performance of QDIPs. The plasmonic enhancement also strongly depends on the polarization and the incident angle. The polarization and incident angle dependence allow polarimetric and polarization engineering in QDIP designs.

© 2017 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Guiru Gu, Xuejun Lu, Thitikorn Kemsri and Yingjie Zhang (June 21st 2017). Surface Plasmonics and Its Applications in Infrared Sensing, Nanoplasmonics - Fundamentals and Applications, Gregory Barbillon, IntechOpen, DOI: 10.5772/67410. Available from:

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