Letter pubs.acs.org/NanoLett
Enhanced Detection of Broadband Incoherent Light with Nanoridge Plasmonics Jeong-Hyeon Kim†,‡ and Jong-Souk Yeo*,†,‡ †
School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University, Incheon 406-840, Republic of Korea S Supporting Information *
ABSTRACT: Emerging photonic integrated circuit technologies require integrative functionality at ultrahigh speed and dimensional compatibility with ultrasmall electronics. Plasmonics offers a promise of addressing these challenges with novel nanophotonic approaches for on-chip information processing or sensing applications. Short communication range and strong light-matter interaction enabled by on-chip plasmonics allow us to extend beyond a conventional approach of integrating coherent and narrowband light source. Such hybrid electronic and photonic interconnection desires a on-chip photodetector that is highly responsive to broadband incoherent light, yet provides elegant design for nanoscale integration. Here we demonstrate an ultracompact broadband photodetection with greatly enhanced photoresponsivity using plasmonic nanoridge geometry. The nanoridge photodetector confines a wide spectrum of electromagnetic energy in a nanostructure through the excitation of multiple plasmons, which thus enables the detection of weak and broadband light. With nanoscale design, material, and dimensional compatibility for the integration, the nanoridge photodetector opens up a new possibility of highly sensitive on-chip photodetection for future integrated circuits and sensing applications. KEYWORDS: Plasmonics, nanoridge, integrated photonics, broadband photodetection, responsivity
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Plasmon-induced hot electrons can either relax through electron−electron scattering and electron−phonon collisions26−30 or be captured as a photocurrent by passing over a potential barrier between metal and adjacent semiconductor.8,10,24,25,31 The former process produces phonon-limited mobility of electrons reducing the amount of current upon illumination, thus resulting in a negative photocurrent,30,32 while the latter approach generates hot electron-induced photocurrent even at zero bias voltage. The required characteristics for on-chip photodetectors can differ from the photodetectors used in typical optical communications. Unlike long-range communications, a chipscale communication in integrated on-chip photonics may not necessarily require light sources with spatial and temporal coherency, narrow bandwidths, and high-output power. Instead, more integration-friendly, power-efficient, and cost-effective nanoscale incoherent light source operating in a broad spectrum such as nano-LEDs33 may provide more feasible path toward future integrated on-chip optical interconnection. Such photonic integrated systems will require an on-chip photodetector that is highly responsive to incoherent broadband light and easier to integrate with electronic counterparts. However, the integrative functionality of sensitive photodetection for incoherent broadband light is yet to be demonstrated.
hotonic integrated circuits and on-chip optical interconnects have emerged as alternatives to conventional electronics with their advantages such as design simplification and architectural and physical benefits.1,2 Major requirements to enable photonic integrated circuits include dimensional compatibility and integrative functionality with electronic components. Among the essential enablers for integrated photonics, on-chip photodetectors have been extensively researched with various materials and structures such as graphene,3−6 quantum-dots,7 nanoantennas,8,9 and nanogratings.10 A recent approach adopted in on-chip optical integration for subwavelength confinement is coupling light with coherent electron oscillations called plasmons.11−14 Plasmon resonances at metallic interfaces or nanostructures such as nanorods,8 Cshaped nanoapertures,15 and dipole or bow-tie nanoantennas16,17 greatly enhance optical near field as a result of strong light−matter interaction, squeezing light into nanoscale overcoming diffraction limit.18 The resonant modes are coupled within a few tens of femtoseconds,19 which enables ultrafast optical components along with size-compatibility for silicon electronics. Fast plasmonic photodetectors such as a waveguideintegrated metal-semiconductor−metal photodetector20 show feasible paths toward photonic integrated circuits. Plasmonics thus provides unique prospects for on-chip photodetection in hybrid electronic and photonic circuits 21 and further applications such as optical sensing.22,23 Plasmonic photodetectors primarily use photoexcited electrons called hot electrons for generating photocurrent.8,10,24,25 © XXXX American Chemical Society
Received: November 12, 2014 Revised: February 28, 2015
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Figure 1. Plasmonic nanoridge photodetector. (a) Illustration of the experimental setup and device geometry. The highlighted region in the illustration indicates the pathway of light actively interacting with nanoridges whose power is defined as the effective power, Peff. Bottom figure shows the three-dimensional atomic force microscopy image of the gold NRAs. (b) Polarization dependence of LSPR. The upper image shows scanning electron microscopy (SEM) image of NRAs indicating the polarization directions and the corresponding angles in the polar plot. The lower polar plot shows polarization-dependent LSPR, which is characterized by scattering intensity. The scattering intensity is calculated as the integral of nanoridge’s scattering spectra over its plasmon resonance band (500−600 nm). Scale bar, 1 μm.
to off-resonant wavelength. Mercury−xenon lamp with a monochromator was used to provide the infrared incoherent light. The light couples out of the fiber to a much wider region than the entire area of the NRAs. Therefore, the effective power of the light falling on the NRAs is calculated to range from 0.4− 520 nW, about 4−8 orders of magnitude smaller than that of a typical laser source. The calculation of the effective power is discussed in detail in section 1 and Figure S2 of the Supporting Information. The gold NRAs interact with incident light through the excitation of localized surface plasmon resonances (LSPR). Unlike metallic wedges guiding surface plasmon polaritons (SPPs) along their edges,39−41 NRAs do not support SPPs propagating along the edges of nanoridges or propagating from the tip toward the substrate due to their small cross-sectional dimensions.39 Our nanoridge geometry is analogous to the tip portion of tapered plasmonic waveguides42 where nanofocusing of optical energy leads to the LSPR. The term “nanoridge” thus represents a unique nanoscale geometry where the electrons can flow along the ridge under the bias while they are simultaneously affected by the excitation of LSPR upon illumination. A strong polarization dependency of LSPR arises from the geometrical anisotropy30,43,44 of NRAs (Figure 1b). The upper image of Figure 1, panel b indicates the direction of polarization for an incident light with respect to the arrays. The lower image of Figure 1, panel b shows the polar plot for the scattering intensity of the NRAs indicating resonant characteristics from the radiative decay of plasmons.45,46 As the polarization of light is directed closer to the perpendicular axis of NRAs, a stronger localization of electromagnetic energy occurs due to the excitation of LSPR; the scattering intensity as a direct indicator of LSPR is maximized for the light polarized normal to the arrays.
Here we irradiate gold nanoridge arrays (NRAs) fabricated using focused ion beam (FIB) with a far field incoherent light from a fiber-coupled halogen lamp and then examine their electrical responses under the broadband visible illumination. The anisotropic cross-section of the nanoridge allows multiple excitations of localized plasmons and simultaneous absorptions for a wide range of wavelengths, which result in a photodetection of broadband light and greatly enhanced photoresponsivities. Thanks to the state-of-the-art nanofabrication technology along with the simple device structure, the plasmonic nanoridge photodetector paves a way for future on-chip photodetection highly responsive to a nanoscale light source.34−37 Gold NRAs as a key element of the photodetector were fabricated by sequential top-down processes including photolithography, lift-off, and ion beam milling on glass substrates. The array structure was adopted for intensifying electrical transduction of optical signals. Spacing between nanoridges was controlled to be about 1 μm, which is far enough to eliminate near field coupling between nanoridges and thus sufficient to demonstrate the behavior of an isolated nanoridge.38 The width and length of a single nanoridge was controlled to be 100−200 nm and a few hundreds of micrometers, respectively. The total size of the single nanoridge is as small as 30 μm2 and can be further reduced by decreasing its length. The electrical response of NRAs upon illumination was measured in a probe station with a bias voltage applied at the end of the NRAs (Figure 1a). A fiber-coupled (a bundle of multimode optical fibers, 2.1 mm-diameter) halogen lamp was used as a broadband incoherent light source; Figure S1 of the Supporting Information shows the spectrum of the halogen lamp, which mainly spans from 400−700 nm. The power at the fiber end ranges from 0.2−3.5 mW by controlling the power of the halogen source. Infrared light source coupled to the same fiber was additionally used to investigate the response of NRAs B
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Figure 2. Negative photoconductance of NRAs. (a) Polarization effect on photoresponse. Polarization-dependent LSPR of NRAs (Figure 1b) results in polarization-dependent photoresponse. Light exposure with parallel polarization (red line) induces no LSPR and thus shows no photoresponse and shows the same current level as no exposure (black line). Perpendicular polarization of light, on the other hand, gives rise to a negative photoconductance (blue line). (b) Current response to illuminating condition. Definitions of Ion, Ioff, and Iph are indicated. (c) Schematic illustration showing the origin of negative photoconductance. An electron instead of multiple electrons is plotted for simplicity. In the absence of light, electrons are driven to move along one direction due to biasing (the green arrow). In the presence of light, electrons oscillate due to the external electric fields in light (the red arrow) while they move along the longitudinal direction. The relaxation process of their coherent oscillations includes scattering (electron−electron and electron−phonon), which reduces the mean free path of the electrons and thus the conductivity of NRAs.
of the photocurrent yields a negative value. This translates in our study that the NRAs become less conductive in the presence of light. The decrease in conductivity under illumination, called negative photoconductance, was previously observed in plasmon-supporting geometries such as a thin metal film32 and graphene nanostructures.30 The negative photoconductance of NRAs can be differentiated from the plasmonically enhanced photon drag effect responsible for the photoinduced electrical signals,49−51 which is dominated by propagating SPPs. As mentioned earlier in the paper, the unique geometry of the nanoridge supports LSPRs across the transverse direction. However, the nanoridges are uniform along the longitudinal direction, and our measurement geometry provides mostly normal incidence of weak and incoherent light on NRAs (Figure 1a), thus SPPs associated with photon drag effect49−51 can hardly be generated on the surface of NRAs. On the basis of the discussions so far, the observed phenomena of negative photoconductance can be understood in terms of a momentum transfer from incident photons to collective oscillations of free electrons, that is, plasmons in a way that disturb the flow of biased electrons under the applied field. Plasmons in nanostructures excited by an external electromagnetic radiation relax their energies in two ways; one is a radiative decay through re-emission of photons45,46,52,53 and the other is a nonradiative decay through electron scattering and electron−phonon coupling.26−30 On the basis of the increased scattering under the nonradiative plasmon decay, the mean free path of electrons in the nanoridge is decreased upon illumination. It is important to note that the nonradiative
The electrical conductivity of NRAs is changed under illumination as a result of the plasmonic interaction between visible light from the halogen lamp and electrons in gold. Figure 2, panel a shows continuous measurements of current at three different conditions: without light, with light polarized parallel to the arrays, and with light polarized perpendicular to the arrays. While the NRAs do not show response to the parallel polarization, the perpendicularly polarized light clearly indicates a decrease of current level. This polarization anisotropy of the photoresponse suggests that the electrical change is assisted by LSPR, not just by radiation induced temperature change and subsequent increase in resistance, known conventionally as a bolometric effect. The fact that the negative photoresponse is originated from plasmonic resonance can also be supported by an experiment with off-resonant excitation. We have confirmed that the gold NRAs do not show photoresponse to the incoherent infrared light (800 and 1550 nm) generated by a Hg(Xe) lamp, as those wavelengths are outside of the resonance in visible range for the given geometry of the gold NRAs. This result also clearly distinguishes the plasmonics-based mechanism of negative photoconductance from the conventional bolometric effect. The current response of NRAs to illuminating condition is shown in Figure 2, panel b. The expected response time of this nanoplasmonic device is less than a few picoseconds as reported elsewhere19,46,47 and discussed in detail in section 2 of the Supporting Information. A photocurrent Iph is defined as the difference between Ion and Ioff as indicated in Figure 2, panel b, where Ion and Ioff are currents measured with light on and off, respectively. As discussed in previous studies,30,48 the definition C
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Figure 3. Radiant power dependence of photocurrent and calculated responsivity and SNR. (a) Photocurrents of two NRAs of different widths as a function of irradiance. Saturation of the photocurrent indicates the dissipation of plasmon-induced heat through the substrate. (b) Calculated responsivity of NRAs in linear regime. Error bars represent standard deviations based on the distribution of the photocurrents. (c) Calculated signalto-noise ratio (SNR) of the NRAs (185 nm width). The definitions of signal and noise are shown in Figure S4 of the Supporting Information.
Figure 3, panel a also shows the dependence of photocurrent on the width of nanoridge. Since the radiative decay is linearly proportional to the dimension of a nanostructure,55,56 the plasmons in narrower nanoridges are more likely to relax their energies through the nonradiative decay than those in the wider nanoridges. Thus, the narrower nanoridges demonstrate larger photocurrents. We calculated the responsivity of the nanoridge photodetector as a function of irradiance. The responsivity is defined as a photocurrent per incident radiant power.30 Figure 3, panel b shows the calculated responsivities of the two NRAs from the linear regime shown in Figure 3, panel a. The photocurrents were measured to calculate the responsivity for a given bias voltage. The NRAs with 120 nm width exhibit responsivities as large as −16.3 AW−1 (Figure 3b, the red circles) where the photocurrent was measured as −170 nA with variation of 3%, and the irradiance was 1.95 mW cm−2. Compared to the analogous work previously done with graphene nanoribbon arrays of Freitag et al.,30 the photoresponsivity of the NRAs is enhanced as much as six orders of magnitude even with lower bias voltages. The enhancement is mainly due to the decrease in the input radiant power, which suggests that the nanoridge photodetector is highly responsive to a feeble light. The responsivity can be further increased by applying a higher bias voltage based on the linear bias dependence of the photocurrent (Figure S3, Supporting Information). Figure 3, panel c shows the signal-to-noise ratio (SNR) of the 185 nm NRAs in dB scale. A signal amplitude corresponds to the magnitude of photocurrent. A noise amplitude is defined as a standard deviation of current calculated from 500 successive measurements. Their graphical definitions are presented in Figure S4 of the Supporting Information. Since the noise amplitudes remain in the same order (10−9 A) with increasing input radiant power, the SNR increases rapidly with respect to irradiance especially in the low power region (inset). The photocurrent (i.e., the signal) is a function of the width of nanoridge, bias voltage, and irradiance, whereas the noise of the system is independent of these factors. Thus, the SNR of NRAs can be increased by optimizing the width of nanoridge, applying a higher bias voltage, or increasing the irradiance. To gain an insight into the origin of the high photoresponsivity, we investigated the plasmon resonance bands of
decay of plasmons consequently increases the temperature of electrons and phonons in a noble metal.25 Figure 2, panel c schematically illustrates the difference in electrons’ mean free path in a nanowire between on and off irradiation and describes the virtual journey of the electrons interacting with the phonons and the other electrons. In the absence of light (left), the mean free path of electrons is bounded by the diameter of the nanowire.54 The excitation of electron density oscillations, that is, plasmons, on the other hand, changes the path of the electrons when illuminated (right). Moreover, phonons are coupled during the nonradiative decay of plasmons, which results in the increase of phonon-temperature and scattering with electrons. The right of Figure 2, panel c illustrates only lateral oscillations to the path of the electrons upon illumination to show a contrast in the movement of electrons. In actual situations, though not shown in the illustration for simplicity, the electrons should intricately change their paths through elastic and inelastic collisions with the electrons, phonons, and boundaries. The decrease in the mean free path of electrons upon illumination reduces the electrical conductivity of the nanoridge and the electrical current.54 This consequentially gives rise to the negative photoconductance. Since heat is generated through various scattering processes upon the nonradiative decay of plasmons, the heat transfer among thermal baths (electron, phonon, and matrix)28,29 and through the substrate is an important process to determine the amount of photocurrent. In the range of voltage we have applied, the photocurrent linearly increases with the bias voltage so the effect of a resistive heating on the photocurrent can be ignored (Figure S3, Supporting Information). Figure 3, panel a shows a radiant power dependence of the photocurrents for two NRAs with different widths and indicates a saturating behavior of the photocurrents at higher incident power. As the radiant power increases, the driving force of plasmons becomes larger, which leads to the increase of heat generated upon the decay of plasmons. Thus, the nonlinear behavior of the photocurrents in high power range indicates that the heat dissipation through the substrate becomes dominant above threshold. The photocurrents start to deviate above the power intensity of 370 μW cm−2 for the NRAs used in the experiment. D
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Figure 4. Broadband characteristic of NRAs. (a) Schematics of multiple plasmon excitations in the nanoridge and the optical setup for collecting scattered light from the nanoridge. (b) Dark field microscopy images showing the scattered light from NRAs. The focal point moves from top (number 1) to the bottom (number 10) of the nanoridge. (c) Scattering spectra of NRAs with different focal points. Each plasmon resonance peak in a visible region is extracted and normalized for comparison. Inset shows the outline of nanoridge measured using atomic force microscopy and corresponding focal points (rough estimation).
NRAs, that is, absorbed optical energies. The optical setup of dark field microscopy is shown in Figure 4, panel a; while obliquely incident rays pass through the sample, the scattered light or re-emitted photons as a radiative decay of plasmons is detected by the objective lens located below the device. We observed a unique phenomenon from the optical response of NRAs: rainbow scattering depending on the position of the focal point of light. When the light passes through the top of the NRAs (Figure 4a left), they mainly scatter the light of shorter wavelength. As the focal point moves downward to the bottom of the NRAs (Figure 4a right), they scatter the light of longer wavelength. This is due to an increase of the dimension within the nanoridge’s cross-section; larger dimension leads to the lowering of the resonant energy or the increase of the resonant wavelength. The images of scattered light are shown in Figure 4, panel b, which shows the rainbow colors scattered by the identical NRAs as the focal point moves from the top (number 1) to the bottom (number 10). Corresponding spectra of the dark field images are shown in Figure 4, panel c with the numbers indicated. The resonance peak in a visible regime is extracted and normalized to clearly indicate the shifts in the resonant wavelength. Figure S5 of the Supporting Information shows the original scattering spectra. Inset of Figure 4, panel c shows the nanoridge’s cross-sectional shape measured by atomic force microscopy and the approximate focal points numbered from 1−10. We can identify two trends in the spectra: first, the resonance peak shifts to longer wavelengths as the focal point moves downward; second, the line width becomes narrower as the
focus moves closer to the substrate. The dispersion of resonance peaks spanning the visible range indicates a simultaneous broadband absorption of light by the excitations of multiple plasmons in the nanoridge, which can lead to the enhanced photoresponsivity. The change in the line width of the resonance peaks indicates different damping rate44 in a single nanoridge, where corresponding plasmons are simultaneously excited and coupled one another to form an ensemble. To obtain further insight into the broadband characteristics of the nanoridge geometry, we simulated the electromagnetic fields around the NRAs on the glass substrate by solving partial differential equations in a two-dimensional domain. We assumed an infinite number of infinitely long NRAs and a normal incidence of light for the two-dimensional calculations. The outline of the nanoridge measured by the atomic force microscopy was used in the simulation for obtaining exact results. The optical constants of materials as a function of frequency were referenced from the literature.57 Figure 5 shows cross-sectional plots of norm distributions in electric fields near the nanoridge on the glass substrate at three different wavelengths within a visible range: 420, 505, and 661 nm. The black surface arrows indicate the magnitudes and directions of the electric fields. At the excitation wavelength of 420 nm (Figure 5a), the top section of the nanoridge strongly generates electric fields, which in turn couples with the bottom part to generate weak higher order modes. This coupling leads to a complex multipole field distribution different from a dipolar field distribution. As the excitation wavelength gets longer (Figure 5b,c), the part generating strong E
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, intensity spectrum of broadband light source, calculation of effective power, bias voltage dependence of photocurrent, definition of signal and noise, and original scattering spectra of NRAs. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone:+82 (32) 749 5838. Fax:+82 (32) 818 5801. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. H. Kwon and S. Kwak for helpful discussion. This research was supported by the MSIP (Ministry of Science, ICT, and Future Planning), Korea, under the IT Consilience Creative Program (NIPA-2014-H0201-14-1002) supervised by the NIPA (National IT Industry Promotion Agency).
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REFERENCES
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Figure 5. Numerical calculations for the broadband characteristic. Simulation results showing electric field distributions around the nanoridge of three different representatives in a visible region ((a) 420, (b) 505, and (c) 661 nm). Infinite number of infinitely long NRAs is assumed for two-dimensional calculation. Light of wavelength λ is assumed to be normally incident. The rainbow colors and the black surface arrow indicate the magnitude and direction of electric field, respectively. The position of strong electric fields moves downward with increasing wavelength of incident light. Scale bar, 100 nm.
electric fields due to plasmon resonance moves toward the bottom of the nanoridge. This trend from the modeling agrees well with the spectral results showing broadband characteristics. In summary, we have demonstrated a highly sensitive photodetection of incoherent broadband light with the plasmonic NRAs. We observed the plasmonically driven electrical change of NRAs under illumination; the decay of coherent excitations through electrons’ inelastic scattering and electron−phonon collisions results in the decrease of current. The geometrical anisotropy of the nanoridge’s cross-section enables broadband absorption via multiple excitations of plasmon resonances, which enhances the photocurrents, thus the responsivities. At even lower bias voltage, the nanoridge photodetectors have shown unexpectedly large photoresponsivity with 2−6 orders of magnitude greater in value than the responsivities of previous photodetectors;4,30 we have demonstrated a capability for the detection of weak incoherent light down to 0.4 nW. By taking advantage of the state-of-the-art lithographic technologies, the plasmonic nanoridge photodetector can be easily integrated with high density electronic counterparts owing to its extremely simple and nanoscale geometry. Therefore, the nanoridge photodetector provides novel and more practical method of on-chip photodetection, which meets crucial requirements including incoherent broadband detection, high photoresponsivity, low power consumption, simple fabrication, and integration capability. F
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