2.1.

Single Interface Surface Plasmon Polaritons

Figure 1 shows that the simplest planar structure sustaining SPPs is that of a single, flat interface between a dielectric, nonabsorbing half space ($y>0$) with positive real dielectric constant ${ϵ}_{\mathrm{r},\mathrm{d}}$ and an adjacent conducting half space ($y<0$) described via a dielectric function ${ϵ}_{\mathrm{r},\mathrm{m}}(\omega )$. This structure is termed the single interface SPP waveguide. The metallic character requires that $\text{Re}\{{ϵ}_{\mathrm{r},\mathrm{m}}(\omega )\}<0({ϵ}_{\mathrm{R}}>0)$. For metals, this condition is fulfilled at frequencies below the bulk plasmon frequency ${\omega }_{\mathrm{p}}$.¹

Fig. 1

Single-interface SPP and waveguide. The relative permittivity of the metal is denoted ${ϵ}_{\mathrm{r},\mathrm{m}}$ and that of the dielectric ${ϵ}_{\mathrm{r},\mathrm{d}}$. The distribution of the main TE field component ($E_y$) of the SPP is sketched in blue on the structure. (Adapted from Ref. 33).

Highly conductive metals and good dielectrics are usually used to implement SPP structures. Au is a relevant and good choice given its chemical stability and low loss among the other metals. Ag and Cu are also good choices, but they are reactive so passivation is required. Other good choices are metals satisfying the condition ${ϵ}_{\mathrm{R}}\gg {ϵ}_{\mathrm{I}}$; nevertheless, many other materials may offer advantages for collecting these hot carriers. In Ref. 34, by considering the expected hot carrier distributions based on the electron density of states for 16 different materials ranging from metals (Au, Ag, Cu, Al, Fe, Pt, Ti, and Y), alloys (Ag-Cu, Al-Ga, Au-Pt, and AlOCu), and nanostructures (Ag and Au nanowires, carbon nanotubes, and graphene), the authors predicted the preferred hot carrier type for collection and their expected performance under different illumination conditions. Proper material selection is then achieved by knowing carrier energy distributions, which are extremely sensitive to the details of the electronic bandstructure. For example, Cu and Au generate hot holes that are much more energetic than electrons, while Ag and Al distribute energies more equitably between electrons and holes.³⁵

Metals are dispersive at optical wavelengths. Away from intraband transitions, the permittivity can be accurately modeled via the Drude equation which captures their dispersive character¹

The single-interface SPP waveguide supports a nonradiative SPP mode (one-bound) that is transverse magnetic (TM) and may propagate at any angle in the plane of the interface. The SPP fields ($E_y,E_z$, and $H_x$) are confined in the $y$ direction, with maxima at the interface, and decaying into both media. The main transverse electric (TE) field component ($E_y$) of the SPP is sketched on the structure of Fig. 1 as the thick curve. The field penetration in the metal ${\delta }_{\mathrm{m}}$ is much smaller than that in the dielectric ${\delta }_{\mathrm{d}}$. The SPP is confined because the metal and dielectric have $\text{Re}\{{ϵ}_{\mathrm{r}}\}$ of opposite signs at the wavelength of interest. The wavenumber of the single-interface SPP is given by:¹

For a dielectric cladding that has no loss ($\text{Im}\{{ϵ}_{\mathrm{r},\mathrm{d}}\}=0$), the above simplifies to the following approximate expressions for the real and imaginary parts of the wavenumber:¹

Fig. 2

Dispersion curve of the SPP along a Ag/SiO2 semi-infinite interface; the light line in ${\mathrm{SiO}}_2$ is plotted as the green dash-dot curve. (Adapted from Ref. 33).

The optical density-of-states increases and the group velocity decreases near the energy asymptote. Bend-back in the SPP dispersion is observed for frequencies above the asymptote ($h\nu >3.4\mathrm{eV}$), linking the bound SPP on the right of the light-line to the radiative SPP on the left. The bound SPP (right of the light line in Fig. 2) cannot be directly excited by incident light—an additional structure is needed in order to increase the in-plane wavenumber of the light to match that of the SPP; a corrugated grating or a prism is commonly used to accomplish this task.¹

2.2.

Internal Photoemission

A conventional Schottky photodiode on n-type Si is sketched in Fig. 3(a),³⁶ and used to describe two detection mechanisms that can be used to detect SPPs. The structure is convenient for the detection of SPPs because the latter may propagate along its metal surfaces, particularly along the metal–Si interface.

Fig. 3

(a) Schottky diode on n-Si illuminated by light of photon energy $h\nu$ (green). Reverse biasing ($V_{\mathrm{b}}<0$) is assumed. Electrons are shown by filled dark blue circles and holes by unfilled ones. (b) Energy band diagram of a Schottky contact on n-Si and the three-step IPE process consisting of $p$, photoexcitation, $t$, transport, $e$, emission. $E_{\mathrm{C}}$ and $E_{\mathrm{V}}$ are the conduction and valence band edges, respectively, $E_{\mathrm{F}}$ is the Fermi level, and ${\mathrm{\Phi }}_{\mathrm{B}}$ is the Schottky barrier height. (Adapted from Ref. 32).

Two mechanisms for photodetection are sketched in Fig. 3(a): the first is the creation of EHPs in Si due to the absorption of incident radiation of energy $h\nu$ greater than the bandgap energy $E_{\mathrm{g}}$. This mechanism, labeled EHP, involves the three steps of optical absorption and creation of EHPs, separation of EHPs, and transport under reverse bias for collection as the photocurrent. The second is the IPE of hot carriers (electrons in this case) created in the metal due to absorption of incident radiation of energy $h\nu$. This mechanism, labeled IPE, is described via the energy band diagram of Fig. 3(b); IPE is also a three-step process consisting of photoexcitation of hot (energetic) carriers in the metal, transport with scattering of hot carriers toward the metal–Si interface, and the emission of hot carriers over the Schottky barrier into the semiconductor where they are collected as the photocurrent. IPE requires that $h\nu$ be greater than the Schottky barrier energy ${\mathrm{\Phi }}_{\mathrm{B}}$, and is a useful process for ${\mathrm{\Phi }}_{\mathrm{B}}<h\nu <E_{\mathrm{g}}$, i.e., for detection at energies below the semiconductor bandgap.

The internal quantum efficiency ${\eta }_{\mathrm{i}}$ is defined as the number of carriers that contribute to the photocurrent $I_{\mathrm{p}}$ per absorbed photon per second

For detection using IPE, if $h\nu \ll E_{\mathrm{F}}$ and there is no bandgap then it can be assumed that the density of states is approximately constant, so ${\eta }_{\mathrm{i}}$ is given approximately by³⁷

The responsivity of a photodetector is given by the ratio of the photocurrent to the incident optical power and can be expressed in terms of ${\eta }_{\mathrm{i}}$ and ${\eta }_{\mathrm{e}}$ as

IPE can be enhanced in several plasmonic structures as discussed in the following sections. It should be mentioned that the creation of EHPs can also be enhanced by involving SPPs. It is also important to mention that plasmon hot carrier generation results in higher-energy electrons than direct excitation.³⁸

The probability for photon absorption is proportional to the square of the local electric field inside the metal. Planar metal surfaces reflect most of the incident light, and light absorption is not very efficient. The importance of plasmons to enhance light absorption in the near-surface region of a metal and consequently enhance photoemission processes was highlighted a long time ago.³⁹ In metallic nanostructures, light absorption can be further enhanced by exciting localized SP resonances. This produces an antenna effect resulting in light collection from an area that is larger than the physical size of the nanostructure.⁴⁰ The associated light concentration has been verified by, and used in, surface-enhanced processes, including Raman spectroscopy,⁴¹ photochemistry,⁴² and the photoexcitation of nearby quantum emitters.²⁵ Knowledge of the imaginary part of the permittivity of a metal also provides a means for optimizing hot carrier generation. For instance, by tuning a plasmon mode to energies where the imaginary part of the permittivity is large, maximizing the hot carrier production is possible. This tuning can be performed either by exploiting the unique geometrical tunability of localized surface plasmon resonances (LSPRs) of metallic nanoparticles, or by fabricating composite structures consisting of both noble metals⁴³ and metals with strong intraband transitions (such as transition metals).⁴⁴

Figure 4(a) shows photoexcitation following the illumination of a metal nanoparticle with a femtosecond source pulse. Excitation of SPPs guides the flow of light (Poynting vector) toward and into the nanoparticle. Larger nanoparticles and shorter lifetimes result in higher carrier production rates but smaller energies.⁴⁵ By following the trajectories of photoemitted electrons created by intense laser pulses, researchers have also been able to map the evanescent fields around plasmonic nanoparticles. In Fig. 4(b) one can see that electrons are preferentially emitted from regions of high field intensity (hot spots) and subsequently accelerated within the evanescent tail of plasmons resonating on nanoparticles.

Fig. 4

(a) Photoexcitation processes following the illumination of a metal nanoparticle with a femtosecond laser pulse; the excitation of a localized SP redirects the flow of light (Poynting vector) towards and into the nanoparticle. (Adapted from Refs. 31 and 46). (b) Simulation of field enhancement in a plasmonic bowtie nanoantenna and the resulting electron trajectories. (Adapted from Ref. 47).

This enables the steering of photoemitted carriers in directions determined by particle geometry.⁴⁷ Precisely designed plasmonic nanostructures can significantly enhance photon absorption and convert incident light into hot carriers.⁴⁸

3.1.

Grating Detectors

A grating structure can be used as an alternative to a prism to increase the in-plane momentum of the incident light in order to match that of the SPP,¹ providing compactness and manufacturing advantages and high detection sensitivity. In particular, the grating coupling method is convenient to excite SPPs on a detector,^17,20 because the arbitrary wavenumber of the diffracted light can be generated by simply changing the grating pitch. The coupling condition of an SPP at a metal surface containing a grating is described by the following equation:

Figure 5 shows a grating structure forming an Au/Si Schottky-type SPP detector, producing a responsivity of $38\mu \mathrm{A}/\mathrm{W}$ (under no electrical bias).⁶²

Fig. 5

(a) Structure of Au/Si Schottky-type SPP detector with a nanoslit grating. (b) Slit pitch dependencies on SPP intensity and photocurrent for ring-type grating when slit pitch varies in the range of 440 to 1280 nm. (Adapted from Ref. 62).

An efficient nanoslit grating arrangement that affects the phase conditions at both the air/Au interface and the Au/Si interface has been studied. When the slit pitch was nearly equal to an integral multiple of the SPP wavelength at the Au/Si interface, a higher intensity SPP mode was excited at the Au/Si interface outside the slits. Additionally, while the phase matching conditions were satisfied at the Au/Si interface, the antiphase interference of the SPP mode along the air/Au interface outside the slits was also confirmed. Experimentally, similar results were confirmed as an SPP photodetector with a ring-type grating that has a polarization-independent photocurrent.

Light coupling, guiding, and photodetection can be combined in the same structure. Figure 6 shows a simple metal/semiconductor SPP structure consisting of an SPP waveguide and a detector where the SPP couples from the waveguide to the photodetector.⁴ The air/Au interface is used for guiding SPPs and the SPPs propagate at this interface without large losses. To convert the SPPs into an electrical signal using IPE, the propagating SPPs are coupled to the Au/Si interface by the diffraction structure consisting of a multislit structure and a disk array. A tunable laser was set to the wavelength of 1550 nm, emitting an optical power in the range of 0 to 10 mW. The photocurrents detected at the Au/Si interface are much larger when compared with those detected for the device without the diffraction structure (26 times for the multislit structure and 10 times for the disk array). This kind of device is targeted to be used as a low-loss SPP waveguide and detector in photonic integrated circuit applications and wavelength filters controlled by the slit pitches.

Fig. 6

Scanning electron micrographs (SEMs) of (a) a device used to evaluate the photoresponse measured by the devices with diffraction structures, (b) the multislit structure, (c) the disk array structure, and (d) no diffraction structure in place, (e) photoresponses of the devices with each of the diffraction structures, and (f) polarization angle dependence of the photocurrent detected by the device with the multislit structure. (Adapted from Ref. 4).

Sobhani et al. presented a grating Schottky photodetector based on IPE with the geometry shown in Fig. 7, enabling a three times narrower spectral response than observed for nanoantenna-based devices.²⁷ The responsivity of this device ($\sim 0.6\mathrm{mA}/\mathrm{W}$ at zero bias voltage) is similar to responsivities reported in the literature for SPP detectors.³

Fig. 7

(a) Schematic of a gold grating on an n-type Si with a 2-nm Ti adhesion layer, oriented transverse to the laser polarization. The polarization of the incident beam and its $k$ vector are represented by yellow and green arrows, respectively, (b) SEM image of a gold grating structure with a grating thickness $(T)=200\mathrm{nm}$, interslit distance $(D)=950\mathrm{nm}$, and slit width $(W)=250\mathrm{nm}$. For all structures the array measured $12\times 12{\mu \mathrm{m}}^2$. The scale bar is $1\mu \mathrm{m}$. (c) Responsivities of grating-based photodetectors for three different gold layer thicknesses, $T=93\mathrm{nm}$ (black), 170 nm (grey), and 200 nm (green), showing a strong intensity dependence on grating thickness. (Adapted from Ref. 27).

The internal quantum efficiency of this photodetector is $\sim 0.2\%$, 20 times larger than the previously reported literature value of 0.01% for nanoantenna-based devices.²⁹ Unlike previous devices, this responsivity maintains its narrow full width at half maximum (FWHM) bandwidth of nominally 54 meV, narrower by a factor of more than three compared to previously reported devices.²⁹

Although studies have been carried out on various aspects of nanogratings and their detection applications, achieving a high infrared (subbandgap) responsivity over a broad spectral range on low-cost Si remains a difficult (and worthwhile) challenge. In Ref. 63, Alavirad et al. proposed a plasmonic Schottky photodetector consisting of a Au nanograting on a thin metal patch on $p$-type Si, as shown in Fig. 8, providing detection via the IPE of hot holes generated in the patch along the Schottky contact. This structure produced some of the highest responsivities reported to date in the literature, of about $13\mathrm{mA}/\mathrm{W}$ (with no electrical bias) at ${\lambda }_0\sim 1550\mathrm{nm}$. The RC time constant limited electrical bandwidth of this device is predicted to be up to $\sim 26\mathrm{GHz}$. Gathered into arrays, these small area devices are capable of aggregating bandwidths into the $\mathrm{Tb}/\mathrm{s}$ range. These detectors have many important advantages such as simplicity, compatibility with silicon, and low-cost fabrication.

Fig. 8

(a) SEMs of an uncoated $\mathrm{Au}/p\text{-}\mathrm{Si}$ nanograting photodetector. The pitch of the device is measured as 406 nm (400 nm designed) and the thickness of the Au patch is measured as 29.8 nm (20 nm designed) and its duty cycle of this is 60%. (b) Measured photocurrent response of three grating photodetectors; $V_{\mathrm{B}}=-100\mathrm{mV}$, $\mathrm{\Lambda }=\mathrm{390,400}$, and 410 nm, duty cycle of 62% and patch diameter of $25\mu \mathrm{m}$. (c) Photocurrent generated by grating detectors versus incident power $P_{\text{inc}}$ measured for three different pitches. A linear fit is applied to the data and the slope corresponds to the responsivity of the device. (Adapted from Ref. 63).

3.2.

MIM, MOM, MSM, and MIS Detectors

Other nanostructures investigated as SPP photodetectors include gratings or arrays of metal nanowires on metal–oxide–semiconductor structures (MOS),⁶⁴ metal–oxide–metal (MOM) structures,⁶⁵ or on a commercial photodiode.⁶⁶ In these planar junctions, the excitation of propagating SPPs can enhance efficiency. Brongersma et al. have examined the phenomenon of photocurrent generation by hot electrons in wavelength-scale metal–insulator–metal (MIM) tunneling devices⁶⁷ and enhanced their efficiency by reshaping one of the metal contacts of the $\mathrm{Au}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{Au}$ junction into an array of metal nanowires that support resonating SPPs, as depicted in Fig. 9. The voltage dependence, spectral dependence, nanowire width dependence, and polarization dependence of the photocurrent were examined experimentally.

Fig. 9

(a) Schematic illustration of a typical device featuring a series of MIM crossbar junctions formed between a wide bottom electrode and a set of nanoscale top electrodes. The directions of the electric field for TE and TM illumination are as depicted. (b) SEM image of a device with a 470-nm-wide top electrode. The inset shows a cross-sectional image of the device, composed of an oxide layer sandwiched by top and bottom metal layers. (c) Photocurrent image of the device taken at an illumination wavelength of 400 nm with TM polarization overlaid on the SEM image of the device. The red color corresponds to 7 pA and blue indicates a close to zero photocurrent. (Adapted from Ref. 67).

Figure 9(c) shows an image photocurrent taken from a device with a 470-nm-wide stripe overlaid on its SEM image (no bias voltage). It is clearly observed that photocurrent is generated only in the regions where the top electrodes overlay the wide bottom electrode to form a series of nanoscale MIM junctions. Finally, they measured the spectral dependence of the responsivity of a detector with 750-nm-wide stripes upon illumination with TM polarized light with the bias voltages in the range from $-0.4$ to $+0.4\mathrm{V}$ and obtained a maximum value of $\sim 0.55\mathrm{mA}/\mathrm{W}$ in the visible range. In modern optical communication systems, high efficiency (realized from some parameters such as size, responsivity, dark current, and photocurrent) photodetectors at the communication window with the wavelength of 1550 nm are strongly desired. Wu et al. designed an MIM IPE-based photodetector exhibiting an unbiased responsivity of $0.1\mathrm{mA}/\mathrm{W}$ and an ultranarrow response band ($\mathrm{FWHM}=4.66\mathrm{meV}$), which promises to be a candidate as the compact photodetector operating in communication band.⁶⁸

Casalino et al. presented a Si on insulator (SOI) waveguide metal–semiconductor–metal (MSM) photodetector, based on IPE and working at a wavelength of 1550 nm.⁶⁹ Taking advantage of the MSM structure, their device is able to increase responsivity by lowering the barrier. At the same time, a very low dark current is achieved both by depositing a very small active metal area and by realizing a device with asymmetric electrodes.⁷⁰ Figure 10(a) shows the SOI structure obtained by depositing the absorbing Cu metal strictly around the output vertical facet of an SOI rib waveguide.

Fig. 10

(a) Schematic of the proposed photodetector integrated with SOI waveguide and (b) responsivity versus reverse voltage applied at 1550 nm. (Adapted from Ref. 69).

This active metal is in contact with a large Al pad deposited on the buried oxide layer of the SOI structure, and therefore, does not contribute to the dark current or the junction capacitance. A second large Al electrode was deposited on the Si substrate to collect the generated photocurrent. In Fig. 10(b), the responsivity of MSM photodetector versus bias voltage measured at 1550 nm is shown. It reaches a maximum value of about $4.5\mathrm{mA}/\mathrm{W}$ at $-21\mathrm{V}$.

Another alternative for conventional metal–semiconductor Schottky junction is a choice of metal–insulator–semiconductor (MIS) heterojunction which significantly reduces dark current, resulting in increased device performance.⁷¹

3.3.

Nanoparticles Detectors

In recent years, plasmonic energy conversion has been proposed as a promising alternative to conventional electron–hole separation in semiconductor structures. This emerging method is based on the generation of hot carriers in plasmonic devices through the decay of SPs. LSPRs occur in well-designed nanostructures in which confined free electrons oscillate with the same frequency as the incident light and eventually enter resonance, giving rise to enhanced, highly localized electromagnetic fields. Therefore, such nanostructures have been investigated as very efficient light-trapping elements that can be integrated into photovoltaic cells to considerably increase the quantum efficiency of conventional schemes.²⁶ Many studies have considered plasmonic hot-electron generation with applications to both photovoltaic and catalytic devices.

Many of these studies considered Au or Ag nanoparticles in contact with ${\mathrm{TiO}}_2$,^72–75 as shown in Fig. 11. The Au-nanoislands (Au-NIs), annealed at 800°C, tightly contacted the ${\mathrm{TiO}}_2$ surface and appeared to be incorporated into the interfacial structure atomic layers. Electron energy loss spectroscopy (EELS) measurements were obtained to characterize the interfacial structure of the $\mathrm{Au}\text{-}\mathrm{NIs}/{\mathrm{TiO}}_2$. In EELS, the corresponding core edge [the titanium (Ti) ${\mathrm{L}}_{23}$ edge] in the spectrum provides information about the electronic nanostructure and symmetry coordination of the excited atom. Measurements show extinction spectra of $\mathrm{Au}\text{-}\mathrm{NIs}/{\mathrm{TiO}}_2$ prepared under 800°C annealing. Although the extinction values at the peak wavelength are different, all the $\mathrm{Au}\text{-}\mathrm{NIs}/{\mathrm{TiO}}_2$ show plasmon peak at the wavelength range from 600 to 630 nm.

Fig. 11

(a) High-resolution transmission electron microscopy images of the cross-section at the interface of the Au-NIs and the ${\mathrm{TiO}}_2$ annealed at 800°C. (b) The incident photocurrent efficiency action spectrum measured with the $\mathrm{Au}\text{-}\mathrm{NIs}/{\mathrm{TiO}}_2$ photoelectrode (red) and the extinction spectrum of the $\mathrm{Au}\text{-}\mathrm{NIs}/{\mathrm{TiO}}_2$ photoelectrode (black). The applied potential was 0.3 V versus a saturated calomel electrode. (Adapted from Ref. 73).

In further recent developments, multiple combinations of materials have been proposed to enable hot carrier detectors and catalytic devices. Some examples include multilayer assemblies of Pt nanoparticles on ${\mathrm{TiO}}_2$ thin films,⁷⁶ Ag-decorated ${\mathrm{TiO}}_2$ nanotube arrays⁷⁷ as shown in Fig. 12, ZnO nanorods decorated with Au nanoparticles,⁷⁸ and hierarchical Au-ZnO flower-rod heterostructures⁷⁹ as shown in Fig. 13.

Fig. 12

SEM of Ag nanoparticles in ${\mathrm{TiO}}_2$ nanotubes. (Adapted from Ref. 77).

Fig. 13

SEM of Ag on ZnO flower-rod heterostructures. (Adapted from Ref. 79).

Referring to Fig. 12, Au nanoparticles are well dispersed on the surface of ${\mathrm{TiO}}_2$ nanotubes of a metallic state. The SPP resonance (SPR) effect of Ag nanoparticles extends the visible light response and consequently enhances the absorption capacity of ${\mathrm{TiO}}_2$. They can also efficiently restrain the recombination of photo-generated EHPs in ${\mathrm{TiO}}_2$ nanotubes. Experimental results prove that $\mathrm{Ag}/{\mathrm{TiO}}_2$ nanotube devices have better photocatalytic performance than pure ${\mathrm{TiO}}_2$ nanotubes and their corresponding hydrogen evolution rate is 3.3 times as that of the latter.

The Au/ZnO samples in Fig. 13 are made through a thermal decomposition method. Au nanoparticles were loaded on oriented hierarchical ZnO flower-rod heterostructures. Measurements reveal that there is an electronic interaction between the Au nanoparticles and ZnO flower-rods, and more oxygen molecules are adsorbed on the surface of Au/ZnO heterostructures. They show a strong absorbance in the visible region due to SPR and enhance the separation of EHPs, resulting in the improvement of photocatalytic activity under visible light radiation.

Small metal nanoparticles exhibit resonant responses under optical excitation, dependent on the structure’s shape, size, and composition, the dielectric environment in which they find themselves, and the wavelength of illumination.²³ Nazirzadeh et al. presented a broadband near-infrared (NIR) Si Schottky photodetector consisting of Au nanoislands (randomly sized) formed on an n-type Si substrate, fabricated by rapid thermal annealing of a thin Au film.⁵⁶ Their device structure is shown in Fig. 14. Its potential monolithic integration with electronics makes it promising for ultralow-cost NIR imaging in large volume applications such as automotive and security, in addition to telecommunications. The measured photoresponse of the devices extend up to ${\lambda }_0=2000\mathrm{nm}$ and the responsivities at 1.3 and $1.55\mu \mathrm{m}$ are 2 and $0.6\mathrm{mA}/\mathrm{W}$, respectively.

Fig. 14

(a) Depiction of broad-band NIR Si Schottky photodetector. Au nanoislands are formed on n-type Si by rapid thermal annealing of a thin Au film. Si substrate is used as the bottom contact and Au nanoislands together with the AZO capping layer constitute the top contact, (b) SEM image of the samples annealed at 450°C. (Adapted from Ref. 56).

Several works have provided insight into the localization of hot electron generation in plasmonic nanostructures. Dombi et al. demonstrated the ultrafast generation of hot electrons from tailored metallic nanoparticles and realized the role of plasmonic field enhancement in this process by comparing resonant and off-resonant particles, as well as different nanorod geometries. These nanoparticles offer the possibility of tailoring nanoscale electric fields with high-level control over the plasmonic resonance, which can be overlapped with the spectrum of the exciting laser pulse to achieve field enhancements of the incident radiation by factors of up to several hundred.⁴⁷ Kazuma et al.⁸⁰ showed that sites in Ag nanorods on ${\mathrm{TiO}}_2$ exposed to higher electromagnetic fields generate more hot electrons. After SPP excitation and hot electron generation, oxidized ${\mathrm{Ag}}^{+}\mathrm{ions}$ diffuse into the water layer, adsorb on the ${\mathrm{TiO}}_2$ substrate, and eventually recombine with electrons from ${\mathrm{TiO}}_2$, leading to the formation of satellite redeposited Ag islands, as shown in Fig. 15 (top).

Fig. 15

Oxidation of Ag nanorods and redeposition of small Ag nanoparticles takes place as a result of plasmon-induced charge separation. After LSPR excitation and hot-electron generation, the oxidized ${\mathrm{Ag}}^{+}\mathrm{ions}$ diffuse in the water layer adsorbed on the ${\mathrm{TiO}}_2$ substrate and eventually recombine with electrons from ${\mathrm{TiO}}_2$, forming satellite redeposited islands. The AFM images show Ag nanorods with various ARs that have undergone excitation of multipole plasmon modes from $m=1$ to 4 after irradiation with 800 nm. The rods experience changes in length depending on the incident wavelength and mode excited. The horizontal scale bars are 50 nm in length. (Adapted from Ref. 80).

This effect was observed in nanorods with different aspect ratios (ARs) that sustained different multipole plasmon modes ($m$ is 1, 2, 3, 4), as shown in the atomic force microscopy (AFM) images in the bottom of Fig. 15. Islands appeared in the regions where the SPP electromagnetic field was more intense, confirming the hypothesis that the charge generation process is induced and/or enhanced by the intense fields on the nanostructures.

3.4.

Waveguide Detectors

Schottky photodetectors based on IPE, integrated with a thin metal stripe waveguide in symmetric⁸¹ and asymmetric⁵⁰ claddings have also been proposed, operating with long range and single-interface SPPs, respectively. Figure 16(a) shows a sketch of an asymmetric SPP waveguide photodetector consisting of a thin narrow metal stripe on Si with air on top.⁵⁰ Structures were fabricated as Au or Al stripes on n-Si forming Schottky contacts thereon.⁵⁰ The $s{a}_{\mathrm{b}}^0$ mode,⁸² localized to the metal–Si interface, was excited at wavelengths below the bandgap of Si via butt-coupling to a tapered polarization-maintaining single mode fiber (PM-SMF). This mode propagates along the metal–Si interface with strong absorption, creating hot carriers in the metal along the Schottky contact, some of which cross over the Schottky barrier and are collected as the photocurrent. Figure 16(b) presents the spectral response in Fowler form of a Au stripe of width $2.5\mu \mathrm{m}$ and thickness 135 nm on n-Si, for a reverse bias of 0.2 V and an incident optical power of 2 mW.⁵⁰ The intercept with the abscissa yields the cut-off photon energy, $\sim 0.765\mathrm{eV}$, corresponding to a cut-off wavelength of ${\lambda }_0\sim 1620\mathrm{nm}$. Responsivities up to 1 mA/W were reported with this photodetector schematic. Figure 16(c) shows a measured photocurrent map of a Au on p-Si detector, generated by scanning a tapered PM-SMF over the end facet [following Fig. 16(a)] using a piezoelectric nanopositioner.⁶ The Au stripe width and thickness in this case were $1.5\mu \mathrm{m}$ and 40 nm, the reverse bias was 0.1 V, the incident optical power was 1 mW, and the excitation wavelength was 1310 nm. The photocurrent was largest when the PM-SMF was perfectly aligned with the stripe maximizing the overlap between the incident beam and the $s{a}_{\mathrm{b}}^0$ mode of the waveguide.

Fig. 16

(a) Sketch of an SPP waveguide detector consisting of a metal stripe forming a Schottky contact on Si, excited in the $s{a}_{\mathrm{b}}^0$ mode⁸² via butt-coupling to a tapered polarization-maintaining single mode fiber (PM-SMF). (Adapted from Ref. 50). (b) Spectral response of a Au on n-Si detector as sketched in (a), plotted in Fowler form. The Au stripe width and thickness were $2.5\mu \mathrm{m}$ and 135 nm, the reverse bias was 200 mV and the incident optical power was 2 mW. (Adapted from Ref. 50). (c) Photocurrent map ($\mu \mathrm{A}$) of a Au on p-Si detector as sketched in (a), generated by scanning a tapered PM-SMF over the end facet using a piezoelectric nanopositioner. The Au stripe width and thickness were $1.5\mu \mathrm{m}$ and 40 nm (sketched as the black horizontal line), the reverse bias was 100 mV, the incident optical power was 1 mW, and the excitation wavelength was 1310 nm. (Adapted from Ref. 6).

Other studies involving asymmetric metal stripe Schottky detectors include operation under a strong reverse bias approaching breakdown where a $10\times$ increase in responsivity relative to low bias was reported,⁸³ and a performance comparison of such detectors for two metals on p-Si and n-Si, two SPP modes of operation, and several stripe geometries.⁸⁴

3.5.

Optical Antenna Detectors

The electric near-fields produced by nanoantennas can be of much greater intensity than the incident electric fields,¹⁰ therefore, the photodetection volume can shrink, potentially leading to faster devices with compelling signal to noise characteristics.⁸⁵ Mousavi et al. investigated an SPP photodetector concept based on the enhancement of electrical near-field in low-defect, low-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ detection volumes located in the gaps of an array of metal nanodipole antennas.²⁴ Significant enhancement in responsivity in the presence of nanodipoles at telecom wavelengths is predicted ($\sim 100\mathrm{mA}/\mathrm{W}$ at 1550 nm). The 3-dB electrical bandwidth of the device is estimated based on its RC rise time and the hole transit time through the detection volume for the cases of conventional and ballistic transport in InGaAs and is found to range from $\sim 0.7$ to 4 THz. Knight et al. reported an active optical monopole array which can be considered as a highly compact, wavelength-specific, and polarization-specific light detector.²⁹

Desiatov et al. proposed a nanoscale broadband Si plasmonic Schottky photodetector with high responsivity and improved signal-to-noise ratio operating in the subbandgap regime.⁵⁵ Figures 17(a) and 17(b) show SEM images of a typical fabricated device.⁵⁵ The apex of pyramid was found to be $\sim 50\mathrm{nm}$. Due to the large cross-section of the pyramid, light is collected from a large area which corresponds to its base, concentrated toward the nanoapex of the pyramid, absorbed in the metal, and generates hot electrons. At the apex of the pyramid, the electric field intensity is enhanced by a factor of $\sim 30$ compared with the electromagnetic intensity in the base of the silicon pyramid. Theoretical investigation proves that most of the hot electrons are generated in the metallic part of the pyramid close to the pyramid’s apex. Figure 16(c) shows the measured $I\text{-}V$ characteristic of a typical device for different wavelengths of illumination at a constant incident optical power. The responsivity of the device is found to be 5, 12, and $30\mathrm{mA}/\mathrm{W}$ for incident optical wavelengths of 1550, 1300, and 1064 nm, respectively.

Fig. 17

SEM images of a plasmonic enhanced Si pyramids, (a) formation of the nanoapex in the silicon pyramid. (b) SEM image of a final fabricated device, (c) $I\text{-}V$ measurements of the pyramid Schottky device at constant optical power for three different wavelengths and dark measurement. The inset shows the photocurrent versus optical power data and linear fit for 1550 nm wavelength. (Adapted from Ref. 55).

In addition to the ability in tunability of the operating wavelength and bandwidth in SPP photodetectors, there are many interesting plasmonic structures which enable a straightforward tailoring of the polarization dependence. In the past decade, SPRs have been employed in creating metamaterials offering interesting optical properties such as strong chirality⁸⁶ which is some orders of magnitude higher than that of chiral molecules. Distinguishing between the two polarizations of circularly polarized light (CPL) is an inherent issue and a big problem with conventional photodetectors due to the fact that conventional semiconductors lack intrinsic chirality. Li et al. proposed a nanoantenna CPL photodetector in which the ability to distinguish left-handed circularly polarized (LCP) light and right-handed circularly polarized (RCP) light are obtained from the engineered chirality in metamaterial plasmonic nanostructures.⁸⁷ Figure 18(a) shows a periodic array of chiral metamaterials consisting of a unit cell of a Ag nanoantenna on top of a PMMA spacer and an optically thick layer of Ag as the backplane forming a Schottky interface between Si and Ag nanowires. Generated hot electrons can then be captured and the high circular dichroism directly leads to enhanced discrimination between LCP and RCP in the CPL photodetectors. Figures 18(b) and 18(c) show two fabricated (left-hand) LH and (right-hand) RH chiral metamaterials arrays and their theoretical and experimental responses. Responsivities are obtained by illuminating the metamaterials with a CP laser source and measuring photocurrent as a function of the laser handedness and wavelength. The peak value for responsivity is $\sim 2.2\mathrm{mA}/\mathrm{W}$ corresponding to a quantum efficiency of $\sim 0.2\%$ which is two times that of chiral organic semiconductor transistors reported in Ref. 88.

Fig. 18

(a) Schematic of the CPL detector consisting of a chiral metamaterial integrated with a semiconductor that serves as a hot electron acceptor. The Ohmic contact on Si is formed by soldering indium. The circuit is formed by wire bonding to the silver bus bar and indium, (b, c) photoresponsivity spectra of experimentally measured (dots) and theoretically calculated (solid curve) under LCP (blue) and RCP (red) illumination for (b) LH and (c) RH metamaterials. (Adapted from Ref. 87).

One can combine this CPL photodetector with existing linearly polarized IPE photodetectors and make a hot electron polarimeter with the ability to determine the Stokes parameters or the states of polarization of an arbitrarily polarized beam

3.6.

Theoretical Investigations and Proposals

A complete review of the theory of IPE in Schottky junctions has been reported, and the Vickers model enriched by Casalino et al.⁸⁹ The conventional Fowler equation used in the description of IPE was modified. They propose an analytical formulation to calculate the metal thickness that maximizes the efficiency of the devices. An important point addressed by this work is to determine if IPE-based Si devices have the potential to compare with photodetectors based on III-V compounds (e.g., GaAs and GaAsIn) and play a key role in telecommunications.

In general, plasmonic nanostructures are deposited on top of the semiconductor, which is convenient from the fabrication point of view, but this causes only a small percentage of hot electrons to be excited with a wavevector permitting transport across the Schottky barrier. Embedding the nanostructure into the semiconductor introduces vertical Schottky interfaces, which permit emission of ballistic electrons over the Schottky barriers into the semiconductor, increasing the internal quantum efficiency. Scales and Berini presented phenomenological models for the internal quantum efficiency of Schottky barrier photodetectors suitable for the detection of optical radiation below the bandgap energy of the semiconductor based on IPE from the metal into the semiconductor substrate.³⁷ They studied both single and two Schottky barrier configurations. The double-barrier case provides higher enhancement due to emission over two barriers. The models proposed are based on assessing the emission probability of hot carriers as a function of their energy, taking into account multiple reflections within the metal film and energy losses due to internal scattering. Following this model, a Schottky contact detector comprising a symmetric metal stripe buried in p-type Si, capable of detecting SPPs at wavelengths below the bandgap of Si, is described in Ref. 81. End-fire coupled responsivities of about $0.1\mathrm{A}/\mathrm{W}$ and minimum detectable powers of about $-20\mathrm{dBm}$ are predicted at room temperature. They also comprehensively studied the internal quantum efficiency, responsivity, and sensitivity of symmetric surface-plasmon waveguide Schottky detectors, consisting of a thin metal stripe buried in Si. Responsivities of 0.1 to $0.21\mathrm{A}/\mathrm{W}$ and receiver sensitivities of $-24$ and $-18\mathrm{dBm}$ are predicted for 1.5 and 10 GHz electrical bandwidths ($\sim 2.5\mathrm{Gbit}/\mathrm{s}$ and $>10\mathrm{Gbit}/\mathrm{s}$), respectively.

Knight et al. determined that embedded nanowires under normal incidence can produce a $25\times$ greater efficiency than comparable planar Schottky devices.⁹⁰ This clearly suggests that three-dimensional (3-D) Schottky barriers can be a key design feature for increasing the efficiency of plasmon-based photodetection. A polarization-insensitive and high-efficiency plasmonic subbandgap Si Schottky photodetector operating at telecom wavelengths via IPE is proposed in Ref. 91. The proposed structure, shown in Fig. 19(a), consists of a Si ridge waveguide, a Au Schottky electrode (of thickness $h_{\mathrm{Au}}$) covering both the top and sidewalls of the Si ridge, an aluminum (Al) Ohmic electrode on top of the Si slab, and a thin SU8 insulating film (of thickness $h_{\mathrm{SU}8}$) separating the two electrodes. The hot carrier emission probability $P(\mathrm{E})$ is a very important factor in the estimation of the internal quantum efficiency. Depending on the configuration of the structure, the emission cone may consist of one of the subfigures shown in Fig. 19(b). For the proposed detector, $P(\mathrm{E})$ for Au-Si round interfaces follows the same expression as that for flat interfaces [See Fig. 19(b) top-right side one]. By comparison with a diode with only a top Au contact, the polarization-dependent responsivity is significantly minimized in this photodetector and higher responsivities for both TE and TM polarizations are achieved over a very broad wavelength range of 1.2 to $1.5\mu \mathrm{m}$.

Fig. 19

(a) 3-D schematic diagram of the proposed Si photodetector, (b) emission cones with angle, $\mathrm{\Omega }$: (top-left) a momentum-space illustration; (top-right) real-space illustrations for round Au-Si interfaces, and (bottoms) an asymmetric corner denoted by the red dashed rectangles when (left) $\mathrm{\Omega }\ge \pi /4$ and (right) $\mathrm{\Omega }<\pi /4$, (c) responsivity, $R$, for both TE (solid lines) and TM (dotted lines) polarizations, and (d) PDD of responsivity, for photodetectors with $h_{\mathrm{Au}}=\mathrm{10,20}$, 30, 50, 70, and 90 nm. The SU8 film thickness is $h_{\mathrm{SU}8}=60\mathrm{nm}$. (Adapted from Ref. 91).

Figures 19(c) and 19(d) show the effects of $h_{\mathrm{Au}}$ on the responsivity ($R$) and polarization-dependent deviation (PDD) of the device. Specifically, changing $h_{\mathrm{Au}}$ changes the total absorption and the responsivity of the detector and also the polarization dependence of the responsivity. When $h_{\mathrm{Au}}\ge 50\mathrm{nm}$, the absorption spectrum no longer changes very much for both TE and TM polarizations, because the optical field is tightly confined to the Au-Si interfaces and it can be sufficiently absorbed with a 50-nm-thick Au film. However, PDD is very large over the whole wavelength range for $h_{\mathrm{Au}}\ge 50\mathrm{nm}$. As $h_{\mathrm{Au}}$ decreases, PDD decreases first and then quickly increases. At $h_{\mathrm{Au}}=10\mathrm{nm}$, the internal quantum efficiency is the highest in each part of the Au film among all the cases.

Therefore, the responsivity spectrum rises to the highest level, especially in the short wavelength range, even though the absorption spectrum lies in the same range as the other cases with larger $h_{\mathrm{Au}}$. In this case, the internal quantum efficiency dominates the responsivity spectrum.

4.1.

Gratings

Early investigations were based on grating-coupled detector structures. Corrugated grating couplers were used initially to create integrated and compact structures which are of high interest today. Table 1 briefly reviews the structures and characteristics of the devices explained in this paper.

Table 1

Characteristics of some grating-based photodetectors.

4.2.

MIM, MOM, MSM, and MIS

Referring to the point that in modern optical communication systems, size, responsivity, dark current, and photocurrent of a detector are very important, these kinds of detector structures are popular for high-speed operation because the contact architecture lowers the capacitance and carrier transit times. It is promising to design the contacts in such a detector simultaneously as a nanowire grating to couple incident light to SPPs thereon, such that they overlap strongly with the detection region. Table 2 briefly reviews the structures and characteristics of the devices explained in this paper.

Table 2

Characteristics of some MIM and MSM photodetectors.

4.3.

Nanoparticles

As light strikes metal nanoparticles, SPP resonances can be excited thereon, high near-field enhancement factors can be generated, and strong absorption or scattering leading to light trapping can occur. These effects are useful to increase the absorptance of detectors, particularly those based on a thin absorption layer, or at wavelengths where the absorption is low. These effects are promising to improve the performance of thin-film Si or organic solar cells with application to photovoltaic and catalytic devices. Table 3 briefly reviews the structures and characteristics of the devices explained in this paper.

Table 3

Characteristics of some nanoparticles-based photodetectors.

4.4.

Waveguides

SPP waveguides can be formed into SPP photodetectors by replacing a dielectric region with a detection medium. Such detectors are attractive because they can be integrated with other plasmonic or photonic waveguides, they can increase the absorptance, shrink dimensions, enhance the detection process by exploiting enhanced or localized guided fields, and provide broad electrical and optical bandwidths. Table 4 briefly reviews the structures and characteristics of the devices explained in this paper.

Table 4

Characteristics of some waveguide-based photodetectors.

4.5.

Nanoantennas

Optical nanoantennas are passive structures that capture part of an incident optical wave to produce oscillating near-fields at defined locations along its geometry. Combining this function with photodetection leads to integrated nanoscale photodetectors having compelling characteristics, as the antennas allow the detection volume to shrink, leading to fast devices with high signal-to-noise characteristics, and to polarization-resolved spectroscopic nanodetectors. Such properties have attracted much interest lately, due to the implications they have both in fundamental research and in technological applications. Table 5 briefly reviews the structures and characteristics of the devices explained in this paper.

Table 5

Characteristics of some nanoantennas-based photodetectors.