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May 1, 2015 - Breakthroughs in Photonics 2014: Advances in Plasmonic Nanolasers. Volume 7, Number 3, June 2015. Ankun Yang. Teri W. Odom.
Invited Paper

Breakthroughs in Photonics 2014: Advances in Plasmonic Nanolasers Volume 7, Number 3, June 2015 Ankun Yang Teri W. Odom

DOI: 10.1109/JPHOT.2015.2413773 1943-0655 Ó 2015 IEEE

IEEE Photonics Journal

Breakthroughs in Photonics 2014

Breakthroughs in Photonics 2014: Advances in Plasmonic Nanolasers Ankun Yang1 and Teri W. Odom1,2 (Invited Paper) 1

Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208 USA 2 Department of Chemistry, Northwestern University, Evanston, IL 60208 USA DOI: 10.1109/JPHOT.2015.2413773 1943-0655 Ó 2015 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received February 20, 2015; revised March 13, 2015; accepted March 13, 2015. Date of publication March 18, 2015; date of current version May 1, 2015. Corresponding author: T. W. Odom (e-mail: [email protected]).

Abstract: Plasmonic nanolasers are promising as nanoscale coherent sources of optical fields because they support ultrasmall sizes and show ultrafast dynamics. Major advances over the last several years have focused on nanocavity design, materials improvements in quality of both the plasmonic materials and gain media, and roomtemperature operation. In 2014, the most significant results focused on tunable nanolasing emission and strategies to time-resolve the dynamics. Index Terms: Nanolasers, plasmonic lasers, light sources, surface plasmons, optical cavities, nanophotonics.

1. Introduction Lasers are widely used in optics and biology [1]–[3] and have driven research applications to low thresholds, fast speeds, and small sizes [4]–[6]. In nanophotonics, coherent light sources with sizes below that of the diffraction limit are promising for integrating miniaturized photonic and electronic elements and increasing the speed of optical communication. One strategy to confine light into a subwavelength optical mode is to exploit surface plasmons (SPs), collective oscillations of free electrons at a metal-dielectric interface [7]. However, SPs excited by photons exhibit short lifetimes because of energy dissipation in the metal. To overcome disadvantages while maintaining small sizes and localized optical fields, Bergman and Stockman proposed a surface plasmon amplification by stimulated emission of radiation (spaser) in 2003 [8]. The spaser concept, or more generally plasmonic nanolasers, relies on the building up of strong coherent fields in a spatial footprint much smaller than the emission wavelength and has been expanded by many theoretical proposals [9]–[11] and experimental demonstrations [12]–[18]. Compared to conventional lasers, plasmonic nanolasers can support unusual nanocavity architectures resulting in amplification of electromagnetic (EM) fields that are spatially localized at the nanoscale and that show ultrafast emission with fs-time scales [8], [11]–[13], [17]. Furthermore, enabled by the tightly focused, enhanced optical fields supported by SPs, plasmonic nanolasers also offer the ability to amplify weak physical and chemical light-matter interactions on the nanoscale.

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Breakthroughs in Photonics 2014

Fig. 1. Plasmonic nanolaser architectures. (a) Semiconductor nanowire sitting on a metal film separated by a dielectric spacer (adapted by permission from Macmillan Ltd.: Nature [12], copyright 2009). (b) Gold core coated with dye-doped silica shell (adapted by permission from Macmillan Ltd.: Nature [13], copyright 2009). (c) Gold nanoparticle arrays surrounded by dye molecules (adapted by permission from Macmillan Ltd.: Nature Nanotechnology [17], copyright 2013).

2. Design of Plasmonic Nanolasers Plasmonic nanolasers consist of noble metal nanostructures that support SP modes as optical feedback and gain materials that have spectral (emission) and spatial (position) overlap with the SP mode. In this nanodevice, SPs are first excited and then amplified by resonant energy transfer from the gain medium. This type of design is important because 1) the gain can compensate for intrinsic losses in the metal nanocavities, and 2) the EM fields of the metal nanostructures can enhance both spontaneous and stimulated emission of the surrounding gain. Two types of plasmonic nanolasers were reported based on different nanocavity designs five years ago [12], [13]. The first design relied on propagating surface plasmon polaritons (SPPs) in a nanowire-on-metal film platform [see Fig. 1(a)] [12], where SPPs were confined to the thin dielectric layer between the semiconducting (CdS) nanowire and the plasmonic (Ag) film. The nanowire served two key functions, where 1) the end facets supported a microscale Fabry– Pérot cavity, and 2) the CdS nanowire functioned as the gain material. This nanowire-on-film platform has been widely exploited in numerous studies [15], [19]–[22]. The second design was based on localized surface plasmons (LSPs) from Au nanoparticles (NPs) [see Fig. 1(b)], where the Au NP core acted as a nanoscopic feedback cavity and the organic dye-doped silica shell provided the optical gain [13]. This simple architecture is the closest to the predicted spaser [9] but has seen limited follow-up reports. Directionality is not an inherent characteristic of plasmonic nanolasers [9], [23] but can be obtained through structural engineering [10], [17]. Zheludev proposed that arranging plasmonic resonators into a 2-D array could produce a lasing spaser, where the in-plane oscillations of the trapped-mode currents could result in lasing emission normal to the plane of the array [10]. Experimentally, a strongly coupled plasmonic NP array [see Fig. 1(c)] was reported to exhibit surface-normal lasing emission at room temperature [17]. Lasing action was achieved via amplification of lattice plasmons produced from the diffractive coupling of LSPs from individual noble metal (Au or Ag) NPs [17], [24]. Other nanolasers based on 2-D arrays of different plasmonic resonator shapes have included 3-D bowties [16] and circular nanoholes [18], [25]. Similar to photonic cavities, metal nanocavities of plasmonic nanolasers can be characterized by a Purcell factor F [26] associated with Fermi's Golden rule [27]. F quantifies the enhancement of light-matter interaction and is proportional to the quality factor of the cavity and inversely proportional to the mode volume. Because plasmonic nanocavities are leaky, the use of an effective mode volume has been suggested so that the calculated solution converges over all

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space instead of just around the resonator size [28]–[31]. Using the conventional definition, plasmonic nanolasers can access F values from 6 up to 200 [12], [17] by taking advantage of ultra-small mode volumes. Another important parameter in assessing cavity effects is the spontaneous emission factor  that characterizes how much emission couples into the SP mode. The threshold of a laser is defined as total cavity loss divided by  [32], [33]. In general, plasmonic nanolasers have high ; therefore, lasing emission can be achieved with a reasonable threshold despite high cavity losses. Coaxial nanocavities are an architecture that can maximize cavity quantum electrodynamics and modify the spontaneous emission coupling into the lasing mode. With an experimentally determined  ¼ 0:95, a thresholdless nanolaser was realized at 4.5 K [4].

3. Challenges and Recent Advances in Plasmonic Nanolasers Plasmonic nanolasers have no cutoff for device size but can have compromised performance because of losses from the metal. Recent advances in nanolasers focused on suppressing loss and amplifying plasmons with gain through new designs and materials. Intrinsic ohmic losses from the non-radiative dephasing of electrons via scattering with electrons, phonons, and impurities were mitigated by confining the EM fields to the dielectric layer [12], [19], [20]. Scattering and radiation losses from roughened metal surfaces and cavity boundaries were reduced by using atomically flat metal films and arrays of plasmonic cavities [15], [17], [19], [34]. Typical gain materials to compensate losses in plasmonic nanolasers mostly rely on inorganic semiconductors [12], [14] and organic dye molecules [13], [17]. Semiconductors are robust with gain on the order of 10,000 cm1 ; however, they exhibit high non-radiative recombination rates, which reduces optical gain at room temperature. Therefore, the first demonstration of plasmonic nanolasers based on semiconductor gain operated at low temperature [12]. By careful design of the nanocavities to suppress radiative losses [14], [21] and enhanced exciton-SP energy transfer [20], room-temperature lasing operation could be achieved in nanowire-on-film systems. In contrast, organic dye molecules have high quantum yields and can readily compensate metal loss and achieve lasing at room temperature [13], [17]. Dye molecules can be incorporated in silica shells or in a polymer matrix surrounding the plasmonic NPs; however, a disadvantage is photo-induced bleaching of fluorescence that can compromise their long-term stability under high pump intensities. In 2014, two classes of advances were reported: 1) tuning of lasing emission wavelength and 2) resolving the dynamics of plasmonic nanolasers. Although different reports have demonstrated different wavelengths, a systematic approach to manipulate lasing emission over a broad spectral range has been lacking [35]. Progress in tailoring the gain material as well as the quality of the plasmonic structure has enabled broadband tunability. Lasing emission across the visible spectrum was reported in a nanorod-on-film system consisting of a semiconducting InGaN@GaN core-shell nanorod, an atomic layer deposition (ALD) deposited Al2O3 layer, and an epitaxial Ag film [see Fig. 2(a)] [19]. By changing the In composition in the InGaN core, the photoluminescence of the nanorod emitter was tuned from blue to red across the visible spectrum upon excitation by a diode laser (405 nm). Due to the interplay between the dispersion of the Ag and the nanorod emitter, the group index (ratio of vacuum velocity of light to group velocity of cavity mode) near the InGaN emission was strongly enhanced, resulting in “autotuning” of the plasmonic mode to match the emission of the gain media. Although reduced SPP scattering losses from epitaxial Ag films [15] and ALD deposited Al2O3 dielectrics enabled operation under continuous wave (CW) excitation, relatively low temperatures (7–100 K) were still required. A plasmonic nanolaser based on a similar nanorod-on-film architecture achieved room-temperature operation under CW excitation by combining 10 pairs of InGaN/GaN quantum-wells in the nanorod [21]. The morphology of the semiconducting nanorod, in particular the quality of the surfaces and the facets, played a key role in suppressing non-radiative surface recombination and radiative losses. In addition, to address the challenge of high intrinsic losses of conventional plasmonic materials such as Ag in the ultraviolet spectral range, Al with low losses at short

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Breakthroughs in Photonics 2014

Fig. 2. Recent advance in plasmonic nanolasers. (a) All-color plasmonic nanolaser (adapted with permission from [19], copyright 2014, American Chemical Society) and (b) ultrafast plasmonic nanowire laser (adapted by permission from Macmillan Ltd.: Nature Physics [22], copyright 2014).

wavelengths (200–400 nm) was used to construct a room temperature ultraviolet plasmonic nanolaser using a GaN–SiO2-Al hybrid structure [20]. The second major advance was experimental demonstration of the ultrafast characteristics of plasmonic nanolasers. Plasmonic nanolasers are expected to operate in the sub-ps regime [11], but measuring the dynamics of plasmonic nanolaser pulses is challenging for electrical detection techniques as well as time-resolved nonlinear spectroscopies. Using a two-pump technique, pulses from hybrid plasmonic zinc oxide (ZnO) nanowire lasers as short as 800 fs were demonstrated [see Fig. 2(b)] [22]. Here, two energetically identical pump pulses with a variable time delay were used to excite the ZnO nanowire laser. The first pump pulse was intense enough to induce lasing emission, and the weaker second pump pulse could not trigger lasing on its own. The strong pump pulse created population inversion and generated an output laser pulse that partially depleted the excited state population. Depending on the delay, the second pump pulse added to the inversion using the residual excited state population to facilitate lasing. The termination of the first lasing pulse was determined by the time when maximum output pulse intensity was achieved, which was 1.9 ps. Taking into account the time needed to start the first lasing pulse (1.1 ps), the pulse width of the plasmonic nanolaser was determined to be less than 1 ps. This work established the characteristic response times of plasmonic nanolasers and verified the ultrafast dynamics predicted in theory [11].

4. Conclusion Plasmonic nanolasers have made considerable progress since their conception in 2003 and had many breakthroughs in 2014, especially regarding lasing wavelength tunability and determination of the modulation speeds [19], [22]. Creative applications of nanolasers were also reported, such as the use of such devices to detect explosives by monitoring the changes of the

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lasing emission [36]. Moving forward, top-down nanofabrication techniques are needed to produce nanowire-on-film plasmonic nanolasers because the bottom-up semiconductor nanowire growth techniques and post-growth assembly on separately prepared dielectric and plasmonic layers are not amenable for large-scale production [12], [14], [15]. Innovative approaches to integrate the nanoscale coherent light sources with photonic components for on-chip applications are also important [35]. In addition, other challenges exist in areas that have only begun to be addressed, such as far-field beam directionality [17], [18] and electrical pumping [37]. In particular, electrical pumping needs metal contacts that will not perturb the optical modes. Also, whether practical threshold currents can be achieved is under debate because very high current densities (104–106 A/cm2) are required to 1) balance the Purcell-effect induced fast spontaneous emission rate and achieve population inversion and 2) compensate high losses at the plasmon frequency [38], [39]. Such challenges create opportunities for new materials, creative designs, and sophisticated nanofabrication techniques to push forward prospects of the nanoscale light sources. With such advances, the quest for even smaller and faster plasmonic nanolasers will lead to unprecedented ultra-compact devices with ultra-fast operation speeds.

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