We report intense (~10 mW), ultra-broadband (~150 THz wide), terahertz-to-infrared, Gaussian-wavefront emission from nanopore-structured metallic thin films under femtosecond laser pulse irradiation. The proposed underlying mechanism is thermal radiation. The nanostructures of the metal film are produced by random holes in the substrate. Under pulse-train femtosecond laser irradiation, we found dramatically enhanced optical absorption, with an absorptivity that was equal to as much as 95% of the metallic surface nanostructure, due to both an antireflection mechanism and dissipation of excited surface plasmon polaritons into the metal surface.
© 2015 Optical Society of America
Terahertz (THz) to infrared (IR) signals have attracted considerable interest for applications because it can penetrate many materials and reveal the corresponding spectral signatures [1, 2]. THz-to-IR radiation is widely used in materials science, pharmaceutical compound analysis, biomedical imaging, security, and particle physics research [3–5]. Recent research focuses on the development of suitable materials or structures allowing high conversion efficiency from the exciting laser energy to THz energy. Plasmonics offers the prospect of deep subwavelength confinement of light and associated enhancement of the electromagnetic field strength, which is a potential candidate of THz-to-IR generation mechanism [6–8]. Several research groups used femtosecond laser irradiation of metal surfaces to generate THz radiation, most of them applying thin metal films typically deposited onto dielectric substrates with periodic structures [9–12].
Growth of various metallic nanostructures and understanding their optical properties are very important aspects with regard to various applications ranging from waveguides to surface enhanced Raman scattering (SERS) [13–19]. Studies of SERS found that the Raman yield could be increased by up to 6 orders of magnitude at a roughened silver surface which is due to coupling of the photons with surface plasmon modes via surface roughness . In the present work, we use random metallic surface structures that are smaller than the incident laser wavelength to dramatically enhance the coupling of femtosecond laser light to such metals [21, 22]. In contrast with isolated small metal particles or periodic structures (for which surface plasmon resonances occur at sharp individual frequencies), aggregates of the coalesced nanoparticles cause broadening of resonances into a band of frequencies that is similar to the broadband optical response of random metallic fractals . In addition to providing a coupling mechanism, surface roughness features can also serve to localize and further enhance the surface plasmons .
In this letter, unlike the commonly used periodic structures, we characterize an intense THz-to-IR thermal emission from a random nanostructured metal film under femtosecond laser irradiation. The absorptivity was enhanced to as much as 95% because of surface roughness. A fraction of the absorbed laser energy remained as residual thermal energy, which caused the bulk sample temperature to rise. When in thermal equilibrium, the heated sample emitted intense thermal radiation in the THz-to-IR region.
2. Experimental setup
In our experiments, commercial anodic-aluminum-oxide (AAO) membranes (Whatman, Germany) served as the substrate. The diameter and thickness of an AAO membrane were 25 mm and 60 µm, respectively. The average pore diameter of an AAO membrane was approximately 200 nm. Ruthenium (Ru) was deposited onto an AAO membrane with a nominal thickness of 100 nm. The deposition was performed through DC magnetron sputtering using a JGP600 high-vacuum system with a base pressure of 1.5 × 10−5 Pa. The deposition rate was 0.5 nm/s, and the working pressure was 0.3 Pa. The morphology of metallic surface was observed using a scanning electron microscope (SEM), which is shown in Fig. 1(a). The images indicate that the sample surface has a variety of nanoscale structures, including nanoscale voids and nanoprotrusions.
The frequency spectra of the thermal THz-to-IR emission from the metallic nanostructures were characterized using a Fourier-transform Michelson interferometer, which is illustrated in Fig. 1(b). A Ti:Sapphire amplifier laser was used to generate 2.3 W, 100 fs pulses at a 1 KHz repetition rate with a central wavelength of 800 nm. The metallic nanostructured sample was placed in the beam path with the metallic film surface facing the incident beam. The thermal emission from the other side was collected. The high IR transmittance of the substrate ensured that the strong thermal radiation propagated in the forward direction. The optical beam was normally focused on the sample with a spot diameter of 6 mm. A 0.4-mm-thick, high-resistivity silicon wafer was used as a beam splitter. THz-to-IR radiation was detected using a calibrated Golay cell equipped with a 6-mm-diameter diamond input window (Microtech SN:220712-D). This detector has an approximately flat response over a broad spectral range (0-150 THz). The system was purged using dry nitrogen gas to prevent absorption of the water vapor from the ambient air.
3. Results and discussion
Surface roughness can enhance light absorption via both an antireflection effect that is caused by the gradient in the refractive index at an air/solid interface and by surface plasmon absorption [24, 25]. Following laser pulse train irradiation, a fraction of the absorbed pulse energy is retained in the heat-affected zone, dissipates into the bulk sample because of heat conduction, and remains in the sample as residual thermal energy [26, 27]. The remaining energy causes the bulk sample’s temperature to rise. The heated sample will act as a thermal radiation source when the bulk metal reaches thermal equilibrium. The graph in the top of Fig. 2(a) shows the frequency spectra of the THz-to-IR radiation under three different pump powers, as measured using the setup illustrated in Fig. 1(b). The bandwidth of the radiation was in the THz-to-IR region, extending from 0.1 THz up to 150 THz for a pump power of 2.3 W. As the pump power was increased from 0.05 to 2.3 W, the frequency bandwidth was obviously broadened. The spectral feature at 18 THz is the two-phonon absorption feature of the silicon wafer. The frequency spectrum can be simulated by modified Planck's law of black-body radiation Fig. 2(a).
Figure 2(b) shows the surface temperature under the different pump powers measured using an IR thermal camera follows a Gaussian distribution, which results from the Gaussian shape of the laser beam intensity profile. The right panel in Fig. 2(b) plots the temperature of the cross section, as indicated by the dashed white line in the image. We used the Gaussian distribution of the surface temperature to calculate the total radiation spectrum by integrating the spot result over the irradiated area. It is clear that intense pump power caused high surface temperature, thereby resulting in strong thermal radiation.
In the simulation, the emissivity was assumed to be equal to the absorptivity of the incident laser, and it was assumed that the emissivity was independent of wavelength. Kirchhoff’s law implies that in thermal equilibrium, the emissivity equals the absorptivity. Previous work has indicated that Kirchhoff’s law is still applicable for samples with surface structures that are much smaller than the wavelength of the light with which they are irradiated . Therefore, in this experiment, the emissivity was enhanced because of the surface structure, thereby resulting in efficient thermal emission.
The good agreement between the measured and simulated results demonstrates that our samples indeed behaved like modified blackbodies with constant emissivity. The measured spectra are slightly different from the simulated spectra because of uncertainties in the system response, such as imperfect wave collimation and spectroscopic absorption of residual water vapor from the ambient air, sample substrate and metal film itself.
A plot of laser absorptivity versus incidence angle is presented in the top of Fig. 3. The laser incidence angle was changed from −50° to 50° with a step size of 5°. Normal incidence corresponds to an angle of 0°. The range was limited by the available space in the setup. The laser power was fixed to be 1.5 W. For an s-polarized laser beam, the absorptivity decreased as the incidence angle increased. In contrast, for a p-polarized laser beam, the absorptivity increased with the incidence angle. The results can be easily explained by the Fresnel equation.
To further demonstrate that the absorption of light was enhanced by the surface nanostructures, we compared the absorptivity with that of a flat metal surface. The flat Ru metal surface was fabricated through deposition on a polished alumina plate via magnetron sputtering. The film with a thickness of 100 nm was homogeneous and evenly distributed on the substrate. This comparison reveals that the absorptivity at normal incidence was dramatically enhanced from 40% for the flat surface (dots) to 90% for the nanostructured surface (squares).
The output power of the THz-to-IR radiation emitted from a nanostructured metal film versus the incidence angle was measured using calibrated Golay cell, with the sample and detector spatially fixed at an optimized position. These measurements were similar to the results for the absorptivity, as shown in the bottom of Fig. 3. As the incidence angle was increased, the THz-to-IR power decreased for s-polarized laser but increased for p-polarized laser. This result demonstrates that higher absorptivity resulted in more efficient thermal radiation.
The in-plane angular distribution of the THz-to-IR thermal emission was measured by rotating the Golay cell detector in the horizontal plane around the center of the sample. The angle between the sample-detector direction and the laser incidence direction was recorded. The distance between the sample and detector was fixed to be 17 cm. The power of the incident laser was 1.5 W for all of the measurements. Figure 4 presents the angular distribution measured for incidence angles of 0° and 30°, with the metal surface facing the incident laser beam. The angular distribution for the 30° incidence angle (the blue data points) is similar to the distribution for the 0° incidence angle (the red data points), except that the pattern is rotated by 30°. It was found that the angular distributions exhibited a characteristic pattern of circles tangent to the sample surface, regardless of the incidence angle, which is consistent with Lambert’s cosine law.
Although the results shown in Fig. 4 were measured with the p-polarized laser, an identical angular distribution pattern was also obtained with the s-polarized optical laser, albeit with lower THz-to-IR intensity for an incidence angle of 30°. The patterns were the same for arbitrary polarizations of the incident laser at an incidence angle of 0°, and they exhibited an axially symmetric distribution of the THz-to-IR emission around the laser propagation direction.
To complete the characterization of the thermal radiation, we investigated the spatial distribution of the beam at 7 cm away from the sample. The laser was arbitrarily polarized and normally incident on the nanostructured metallic surface. The measured beam profile was Gaussian, as indicated by the 3D plot shown in Fig. 5. Compared with the deformed THz wave beam profile produced from laser-induced air plasma , this beam profile is more suited for large-scale standoff sensing and imaging applications.
The overall generation efficiency in our study could be optimized further. According to the Stefan–Boltzmann law, the total thermal radiation power depends on the surface temperature to the fourth power. Thus, higher surface temperatures generate more efficient thermal radiation. It has been found that the surface temperature is proportional to , where is the absorptivity of the incident laser and and are the thermal diffusivity and conductivity of the metal film, respectively . The absorptivity can be significantly enhanced by modifying the surface structure. In our case, the surface structure could be easily modified by changing the pore diameter and density, depositing a suitably thick metal film, or annealing after magnetron sputtering. Such surface optimization should maximize the absorptivity. Furthermore, the type of metal could be selected to maximize the temperature.
In conclusion, we presented a metallic nanostructure that was produced by random holes in the substrate. The laser absorptivity was dramatically enhanced because of the surface structure. Strong THz-to-IR radiation was detected. The agreement between the observed and simulated frequency spectrum and the angular distribution of the detected radiation demonstrate that the underlying generation mechanism is thermal radiation. The excellent beam profile of this nanostructure makes it a good candidate for a laser-based mW-level THz-to-IR radiation source.
The authors acknowledge support from the National Natural Science Foundation of China under grant no. 11374007 and the Foundation for the Author of National Excellent Doctoral Dissertation of PR China under grant no. 201237. This work was funded by the National Keystone Basic Research Program (973 Program) under grant no. 2014CB339806-1. It was also supported by the Hong Kong Scholars Program.
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