Plasmonic infrared emitters based on nanoamorphous carbon
The middle-infrared (mid-IR) spectral range (3 to 15 μm) is of critical importance for thermal imaging, sensing of gases and aerosols, spectroscopy of chemical and biological agents, and environmental monitoring.
Unfortunately, very few radiation sources exist in this range with sufficient power and they are mostly in the development phase. Plasmonic thermal emitters (PTEs) based on periodically structured metallic films provide tunable, narrow-band radiation, much narrower than from a black-body at the same temperature, constituting a simple yet efficient alternative to costly and complex light sources in the mid-IR, such as quantum cascade lasers.
The metallic films used in PTE’s are prone to oxidization and structural damage at high temperatures. Therefore, metallic PTEs cannot be operated above 350°C, limiting the emitted wavelengths to the long part of the mid-IR (4-10μm) and reducing the achievable output power. In addition, metallic films exhibit very high internal stresses that prevent the fabrication of free-standing membranes, resulting in slow switching (heating/cooling) and power efficiency due to the large thermal mass of the substrates typically used to support the metal films.
We have pioneered the development of a highly conductive diamond-like material called nanoamorphous carbon (NAC) that overcomes these limitations, and the development of PTE’s made from NAC with significantly improved thermal stability compared to their metallic counterparts. NAC is extremely stable both mechanically and thermally, and can be doped with metals at very high concentrations (up to 50 at.%) to tune its electrical and optical properties. We have improved the conductivity and mid-IR reflectivity of NAC by doping it with titanium nitride (TiN) and fabricated PTE’s from this composite, which can be operated at up to 600°C in air without any degradation in performance. Diffraction from a fabricated structure is illustrated (above left), showing the high quality of the PTE. The resulting emitted mid-IR radiation has a bandwidth as small as 0.5μm, can be tuned to the desired wavelength by changing the period of the surface pattern and shows an in-band emissivity exceeding that of the non-patterned films, approaching the black-body limit.
When heated, the patterned conducting film supports surface-plasmon polariton modes and emits narrow thermal radiation with a peak wavelength given with the following formula:
Here, λ is the plasmonic wavelength emission, a is the lattice period, i and j are two indices giving the order of the plasmon, and ε1 and ε2 are the dielectric permittivities of the two materials forming the interface, in this case the air/TiN:NAC or TiN:NAC/SiO2. Note that SiO2 is used as a dielectric coupling layer between patterned and unpatterned layers of TiN:NAC. The first two plasmons correspond to the same wavelength (i=0, j=1 and i=1, j=0) and these are the dominant modes in terms of emitted power.
The thermal emission spectrum of the devices can be controlled by changing the lattice spacing a, as shown in the figure below. The devices were heated by bringing them in contact with a miniature hot plate and their emission spectrum was collected using an FTIR equipped with an external emission port. For the samples that were periodically patterned, the thermal emission is contained within a narrow bandwidth of Δλ=0.5 μm to 1.5 μm. The emission spectrum at the same temperature for a sample with identical coatings, but without the periodic lattice of holes is a typical grey-body spectrum with a very broad bandwidth and no pronounced plasmonic emission peak.
The micropatterned devices have in-band emissivities well exceeding that of the non-patterned grey-body. In short, the plasmonic coupling results in both spectral narrowing and an increase of the in-band (useful) emitted radiation at the desired wavelength. Such an emitter can prove very useful as the emission spectrum can be set to a desired wavelength, and unlike black-body radiation the plasmonic peak of emission does not shift in wavelength with the changing temperature. The devices are stable at temperatures up to 600°C for hours of operation in air, without any protective coatings. Free-standing membranes made of NAC are mechanically and thermally stable and are currently being used for the commercial broad-band thermal emitters that can be switched with frequencies up to 100 HZ, unprecedented for thermally driven light sources.
References
- S. Tay, A. Kropachev, I. E. Araci, T. Skotheim, R. A. Norwood, and N. Peyghambarian, "Plasmonic thermal IR emitters based on nanoamorphous carbon," Appl. Phys. Lett. 94, 07113 (2009).