We have fabricated two-dimensional periodic arrays of titanium nitride (TiN) nanoparticles from epitaxial thin films. The thin films of TiN, deposited on sapphire and single crystalline magnesium oxide substrates by a pulsed laser deposition, are metallic and show reasonably small optical loss in the visible and near infrared regions. The thin films prepared were structured to the arrays of nanoparticles with the pitch of 400 nm by the combination of nanoimprint lithography and reactive ion etching. Optical transmission indicates that the arrays support the collective plasmonic modes, where the localized surface plasmon polaritons in TiN nanoparticles are radiatively coupled through diffraction. Numerical simulation visualizes the intense fields accumulated both in the nanoparticles and in between the particles, confirming that the collective mode originates from the simultaneous excitation of localized surface plasmon polaritons and diffraction. This study experimentally verified that the processing of TiN thin films with the nanoimprint lithography and reactive ion etching is a powerful and versatile way of preparing plasmonic nanostructures.

Conventionally used plasmonic materials generally have low thermal stability, low chemical durability (except gold), and are incompatible with complementary metal–oxide semiconductor processes. However, titanium nitride (TiN), an emerging plasmonic material, possesses gold-like optical properties, but displays relatively large ohmic losses. We fabricated a periodic array of TiN nanoparticles to effectively reduce these losses by coupling the localized surface plasmon resonance with light diffraction. The height of the nanoparticle and the periodicity of the array were designed to match the excitation conditions of both the localized surface plasmon resonance and light diffraction. As a result, the array supported a plasmonic–photonic hybrid mode in the visible region. For the loss mitigation effect to be assessed, photoluminescence (PL) from the light emitting layer on the array was measured. The PL intensity was larger than that from the same layer on a TiN thin film, demonstrating reduced loss. The angular and spectral profiles of the PL could be controlled by the hybrid mode. Our results thus pave the way toward plasmonic devices that can be fabricated using traditional complementary metal–oxide semiconductor processes.

A plasmonic array, consisting of metallic nanocylinders periodically arranged with a pitch comparable to the optical wavelength, is a system in which both the localized surface plasmon polaritons (SPPs) and diffraction in the plane of the array are simultaneously excitable. When combined with a phosphor film, the array acts as a photoluminescence (PL) director and enhancer. Since the array can modify both excitation and emission processes, the overall modification mechanism is generally complex and difficult to understand. Here, we examined the mechanism by simplifying the discussion using an emitter with a high quantum yield, large Stokes shift, and long PL lifetime. Directional PL enhancement as large as five-fold occurred, which is mainly caused by outcoupling, i.e., the PL trapped in the emitter film by total internal reflection is extracted into free space through the SPPs and diffraction. The present scheme is robust and applicable to arbitrary emitters, and it is useful for designing compact and efficient directional illumination devices.

White light-emitting diodes (LEDs), light sources that combine blue LEDs and yellow phosphors, are equipped with bulky optics such as lenses, mirrors, and/or reflectors to shape the light into the required directions. The presence of bulky optics causes optical loss and limits the design. Here, a periodic array of metallic nanocylinders, which exhibits a high scattering efficiency owing to the excitation of localized surface plasmon resonance, is proposed as an alternative means of achieving a directional output without the limitations of bulky optics. A prototype of a directional light emitter is fabricated consisting of an Al nanocylinder array on a yellow phosphor plate and a blue laser. The array shapes the yellow luminescence into the forward direction and generates directional quasi-white light (correlated color temperature of 4900 K). The intensity enhancement reaches a factor of five in the forward direction and is further improved up to a factor of seven by the deposition of a multilayer dichroic mirror on the back side of the phosphor plate, resulting in conversion efficiencies as high as 90 lm/W. Our results pave the way toward the development of efficient and compact directional white-light-source devices without any bulky optics.

Noble metals, particularly gold, have been conventionally used for their suitable optical properties in the field of plasmonics. However, gold has a relatively low melting temperature, especially when nanosized, and the abundance of gold in the earth’s crust is low. These material-related limitations hinder the exploration of the use of plasmonics in several application areas. Transition metal nitrides are promising material alternatives because of their high mechanical and thermal stabilities, in addition to their acceptable plasmonic properties in the visible spectral region. Zirconium nitride (ZrN) is one such promising alternative owing to a higher carrier density than that of titanium nitride (TiN), which has been the most studied complementary material to gold. In this study, we have fabricated periodic arrays of ZrN nanoparticles and found that the ZrN array enhances the photoluminescence from an organic dye on the array; the photoluminescence intensity is increased by as much as 9.7× in the visible region. This result experimentally verifies that ZrN is useful as an alternative material to gold, to further develop plasmonics, and mitigate the conventional material-related limitations.