1. Introduction

There is an ongoing interest to control the emission and absorption of patterned surfaces using plasmon and surface polariton resonances to increase the performance of the photovoltaic devices [1–3]. Furthermore, plasmon resonances of small metallic particles deposited/patterned on the surfaces are most commonly utilized to enhance light absorption and scattering [1]. The plasmon resonances are controlled by the dimensions of the metallic nanoparticles, with small/large aspect ratio nanowires having resonances in the visible/mid-infrared region [2].

For surfaces supporting surface phonon polaritons or multilayer structures the thermal radiation becomes coherent and strongly polarized [4–8]. The same increased coherence and pronounced polarization of thermal radiation are also observed from sub-wavelength dimensions nanoheaters with an antenna-like angular radiation pattern [9,10]. Specifically, these investigations showed that the coherence and polarization of the thermal radiation is more significant for narrow nanoheaters and vanishes for wider nanoheaters. The increased polarization can be explained by charge confinement and thus correlated charge fluctuations along the long axis of the nanoheater [11]. Development of sub-wavelength coherent infrared radiation sources are actively explored for near-field and total reflection microscopy where a control of the polarization of the radiation source is highly desired.

In this article we investigate the change in direction of the emitted thermal radiation from metallic nanoheaters as function of lateral dimensions and temperature. It is demonstrated that the direction of the emitted thermal radiation can be rotated from parallel to perpendicular by changing the width/temperature of the nanoheater. Furthermore, we show that this polarization rotation is originated by thermally excited plasmon resonances of the nanoheater detected as peaks in the emission spectrum of the nanoheater.

2. Experiment

Metallic nanoheaters were fabricated by e-beam lithography with four contact pads for accurate resistance measurement and temperature control of the nanoheaters (Fig. 1(a) ) [10]. The nanoheaters are scaled structures with widths ranging from 500 nm up to 2000 nm and lengths being ten times larger than the widths. For clarity, we will denominate the nanoheaters based on their widths while the experiments refer to the scaled structures. By passing an electrical current through the outer electrodes, the nanoheaters are resistively heated up to temperatures of 700° C and the thermal radiation is projected onto an InSb detector (sensitive in the 2 to 5 µm spectral region)(Fig. 1(a)). Before the radiation is captured by the detector, it passes through a rotating infrared polarizer. Typical polarization traces acquired for 500 nm and 1000 nm wide wires are shown in Fig. 1(b) as detected by the InSb detector over the 2-5 µm spectrum. For the 500 nm wide nanoheater the main polarization of the emitted thermal radiation is parallel while for the 1000 nm wide nanoheater the thermal radiation is perpendicular to the nanoheater long axis [11].

 

Fig. 1 (a) Experimental setup for measuring the polarized thermal radiation from individual nanoheaters using a rotating polarizer inserted in front of the InSb detector. Polarization traces (b) acquired for a 500 nm and 1000 nm wide nanoheater where the polarization changes from parallel to a perpendicular orientation.

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The polarized thermal radiation as function of the polarizer rotation angle θ can be represented as a sum of an unpolarized and polarized radiation signal, i.e. $I\left(\theta \right)=I_u+I_p{\sin }^2\left(\theta \right)$, where I_u is the unpolarized background and I_p is the amplitude of the polarized signal. The ratio of polarized and unpolarized signal is defined as the extinction ratio [12], $\epsilon =I_p/I_u$. In Fig. 2 (a) , the extinction ratio is plotted for nanoheaters with widths ranging from 500 nm up to 2000 nm for constant temperature (T = 400° C). In agreement with earlier observations [10], the extinction ratio is very large for narrow nanoheaters (Fig. 2(a)) and the emission is parallel to the long axis of nanoheater. The extinction ratio drops drastically for widths between 600 nm up to 900 nm and a change in orientation occurs. Upon increasing the width above 900 nm the extinction ratio increases while the emission becomes perpendicular to the long axis of nanoheater. Although the effect is small, Fig. 1(b) and Fig. 2(a) clearly show that the main polarization direction rotates from parallel to perpendicular as the width of the nanoheater is increased.

 

Fig. 2 Change of the extinction ratio (a) as function of nanoheater width with polarization rotating for width~700nm and simulations of the degree of polarization for an infinite cylinder (b).

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The thermal radiation emitted by metallic nanoheaters is quite different from Planck’s blackbody radiation [2,11] as plasmon resonances associated with size effects change the emission/scattering. The thermal radiation absorbed/emitted by nanoheaters with absorption cross section ${\sigma }_{abs}$, at temperature T, and in the interval of angular frequency $d\omega$ is given [13] by $dI={\sigma }_{abs}(\hslash {\omega }^3)d\omega /(2{\pi }^3{c}^2(\exp (\hslash \omega /k_{B}T)-1))$. For large aspect ratio structures, nanoheaters can be approximated as infinite cylinders in which case the absorption cross section can be analytically estimated. The absorption cross section for an infinite long cylinder with radius r, in vacuum is [13]: ${\sigma }_{abs}=\genfrac{}{}{0.1ex}{}{2}{k_0}{\displaystyle \colorbox[rgb]{}{$\sum _{m=0}^{\infty }\left[-\mathrm{Re}(a_m)+{\left|a_m\right|}^2\right]$}},$where $k_0=2\pi /{\lambda }_0$ is the wave vector for light in vacuum and $a_m$ are the scattering coefficients. For incident radiation parallel to the cylinder axes, the scattering coefficient is:

$a_m=\genfrac{}{}{0.1ex}{}{k_0{J}_m^{\prime }(k_{0}r)J_m(kr)-k{J}_m(k_{0}r)J_m^{\prime }(kr)}{k_0{H}_m^{(1)}{}^{\prime }(k_{0}r)J_m(kr)-k{H}_m^{(1)}(k_{0}r)J_m^{\prime }(kr)},$ where k is the wave vector inside the cylinder ($k=k_0\sqrt{\epsilon }$ and ε is the dielectric constant of metallic wire [14], i.e. platinum in our case) and J_m and $H_m^{(1)}$are the Bessel function and Henkel function of the first kind, respectively. In this case, the radiation is absorbed along the long axis of the cylinder ($I_{\left|\right|}$).

For incident radiation perpendicular to the cylinder axis, the scattering coefficient is: $a_m=\genfrac{}{}{0.1ex}{}{k{J}_m^{\prime }(k_{0}r)J_m(kr)-k_0{J}_m(k_{0}r)J_m^{\prime }(kr)}{k{H}_m^{(1)}{}^{\prime }(k_{0}r)J_m(kr)-k_0{H}_m^{(1)}(k_{0}r)J_m^{\prime }(kr)},$ and the radiation is absorbed along the perpendicular direction ($I_{\perp }$). The degree of polarization [13] is defined as: $P=\left(I_{\left|\right|}-I_{\perp }\right)/\left(I_{\left|\right|}+I_{\perp }\right)$ and its magnitude $\left|P\right|$ changes from parallel to perpendicular direction at kr~1.5 (Fig. 2(b)) where $k=2\pi /\lambda$ is the wave vector inside the nanoheater and the optical constants of Pt in the studied spectral window were considered [14]. The data in Fig. 2(b) shows that both a change in cylinder radius (r) and the emission wavelength (λ) can change the main polarization. Although the nanoheaters investigated here have a rectangular shape, finite length, and are patterned on insulating substrates (SiO₂) (thus the dielectric constant of the surrounding is different from vacuum), the simple scattering theory for an infinite cylinder still offers a qualitative prediction of the polarization change.

To further investigate the origin for the polarization rotation, spectra of the thermal emission from individual heated nanoheaters were measured using a Fast Fourier Transform Infrared Spectrometer using an InSb detector sensitive in the 2-5 µm regions. The thermal radiation from the nanoheater was normalized by the spectrum of a calibrated blackbody light source to take into account the sensitivity of the optical setup and detector at different wavelengths [11]. Attempts were made to acquire the spectra using a wider spectral band (HgCdTe detectors sensitive in the 3-12 µm regime) but the signal from nanoheaters was too weak to be detected. Emission spectra from an 800 nm wide nanoheater were acquired along two orthogonal polarizations directions: parallel and perpendicular to the nanoheater long axis (Fig. 3 ). The spectra show two different responses for parallel and perpendicular emission. For emission along the axis of nanoheater only the increasing tail of the blackbody radiation is detected with increased signal magnitude at increasing temperatures. However, the perpendicular direction of the spectrum shows a resonance peak in the detection window. To avoid as a first order approximation the plasmon resonance shift due to geometrical variations we chose to characterize fixed aspect ratio nanoheaters where the position of the plasmon resonances should be fixed and determined by the ratio of the length and the width of the nanoheater [13,15,16]. We note that for the nanoheater temperature range studied the blackbody radiation peak as predicted by Wien’s law would be outside of our detection window. From previous studies of thermal radiation from individual nanoheaters these peaks were identified as surface plasmon modes [11] running perpendicular to the nanoheater axis, i.e. λ /2 resonances in antenna terms [17,18].

 

Fig. 3 Parallel and perpendicular thermal radiation spectra from an 800 nm wide nanoheater as function of temperature. A λ/2 resonance peak is detected in the perpendicular orientation spectra for the thermal radiation.

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The increasing temperature of the nanoheaters should broaden and shift the spectral peaks associated with plasmon resonances to smaller wavelength and the relative weight of longitudinal and transversal emission in the detection window should determine the polarization. According to the calculations for infinite cylinder, the polarization should change back to longitudinal modes and then again to transversal modes (higher order plasmon modes being excited) at higher wavelength (radiation temperature) as higher longitudinal and transversal modes interchange [15,16]. We note that the relative weight of the radiation emitted along the two orthogonal directions may change in different spectral windows [18,19].

A clear rotation of the polarized thermal radiation emitted by a 850 nm wide nanoheater is observed as the temperatures of the nanoheater is increased. Polarization traces similar to Fig. 1(b) are acquired for temperatures between 360° C and 640° C and combined in a two dimensional surface plot with axis defined by the rotation angle and temperatures (Fig. 4(a) ). At temperatures below 550° C, thermal emission along the nanoheater axis dominates, while in-between 550° C to 600° C, the polarization starts rotating from a parallel to a perpendicular orientation. At even higher temperature the thermal radiation emission will be perpendicular to the nanoheater long axis. Around 580° C both the parallel and perpendicular emission are comparable in magnitude and the thermal radiation becomes almost unpolarized. The rotation is controlled by the thermally activated transverse surface plasmon modes that will enhance the thermal emission along the perpendicular direction to the nanoheater long axis. We note that in the regime where the polarization of the thermal radiation rotates, the polarized component of the total thermal radiation is small compared to the unpolarized background.

 

Fig. 4 Surface plot (a) of the polarization traces for an 850 nm wide nanoheater as function of temperature. At lower temperatures parallel polarization is dominant and rotates towards a perpendicular direction at higher temperatures. Change in the extinction ratio for the 850 nm nanoheater as function of temperature (b).

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The extinction ratio for the 850 nm wide nanoheater varies as function of nanoheater temperature (Fig. 4(b)) in a very similar way to the extinction ratio as function of the nanoheater width (Fig. 2(a)). These two complimentary data sets demonstrate that the polarization of the thermal radiation can be rotated by either changing the dimensions of the nanoheater or its temperature and is consistent with Fig. 2(b) where a change in radius of the infinite cylinder or emission wavelength (determined by the temperature of the nanoheater) changes the polarization of the emitted thermal radiation.

The change of the polarization direction has been observed for nanoheater patterned on oxide thickness ranging from 90 nm to 110 nm and nanoheater width ranging from 750 nm to 900 nm. For these nanowire widths we observed changes in polarization both from a parallel to perpendicular and also from perpendicular to parallel orientation of the emitted thermal radiation. Once the nanowires width is above 900 nm the radiation direction will have the same orientation up to the 700° C (the highest temperature studied). We note that the polarization component of the thermal radiation is very small for nanoheaters with widths for which the polarization can be rotated. For such nanoheaters, both parallel and perpendicular components of the thermal radiation are comparable in magnitude and the radiation is almost unpolarized. The temperature where transition occurs is dependent on oxide thickness and also on the spectral detection window.

3. Conclusions

Plasmon resonances of metallic nanowires/waveguides are investigated as a possible route for optical information transmission and processing. Recent studies demonstrated the propagation of surface plasmons polaritons over a micron scale distances [20,21] combined with significant nonlinear optical phenomena [22]. Nanoheaters with polarization tunable by temperature will allow the development of local infrared plasmonic radiation sources for local characterization of molecules and quantum structures. Furthermore the nanogap formed in nanoheaters due to electromigrations is actively pursued for single molecule characterization [19].

In conclusion we investigated the polarization of the thermal emission from individual metallic nanoheaters. For very narrow nanoheaters the emission of the thermal radiation is more dominant along the long axis of the nanoheater and rotates to a perpendicular direction as the width is increased. The relative weight of the thermal radiation can be shifted from parallel to perpendicular orientation by increasing the nanoheater temperature. These metallic nanoheaters antennas could be employed as polarized infrared light sources or radiation absorber with enhanced emission/absorption tailored by the nanoheater temperature and plasmon resonances.