Nanophotonics 2020; 9(12): 3965–3975 Research article Dmitry Yu. Fedyanin*, Alexey V. Krasavin, Aleksey V. Arsenin and Anatoly V. Zayats Lasing at the nanoscale: coherent emission of surface plasmons by an electrically driven nanolaser https://doi.org/10.1515/nanoph-2020-0157 photonics, coherent nanospectroscopy, and single- Received February 28, 2020; accepted June 26, 2020; molecule biosensing. published online July 20, 2020 Keywords: integrated nanolaser; plasmonics; surface Abstract: Plasmonics offers a unique opportunity to break plasmon polariton amplification. the diffraction limit of light and bring photonic devices to the nanoscale. As the most prominent example, an inte- grated nanolaser is a key to truly nanoscale photonic cir- 1 Introduction cuits required for optical communication, sensing applications and high-density data storage. Here, we Nanophotonics is ubiquitous in many applications ranging develop a concept of an electrically driven subwavelength from optical interconnects and sensing to high-density data surface-plasmon-polariton nanolaser, which is based on a storage and information processing [1–6]. Particularly, the novel amplification scheme, with all linear dimensions plasmonic approach offers an opportunity to deal with smaller than the operational free-space wavelength λ and a photonic signals at the nanoscale via coupling to the free- mode volume of under λ3/30. The proposed pumping electron oscillations in a metal. It can provide ultracompact approach is based on a double-heterostructure tunneling components for optical interconnects such as modulators, Schottky barrier diode and gives the possibility to reduce photodetectors, waveguides and incoherent and coherent the physical size of the device and ensure in-plane emis- nanoscale optical sources [7–13]. The implementation of the sion so that the nanolaser output can be naturally coupled latter is a great challenge since a significant amount of the to a plasmonic or nanophotonic waveguide circuitry. With surface plasmon field is concentrated in the metal that re- the high energy efficiency (8% at 300 K and 37% at 150 K), sults in high Joule losses. Nevertheless, the idea of SPASER the output power of up to 100 μW and the ability to operate (from surface plasmon amplification by stimulated emission at room temperature, the proposed surface plasmon radiation) [14, 15] was proposed inspiring further research polariton nanolaser opens up new avenues in diverse for novel truly nanoscale optical sources with efficient application areas, ranging from ultrawideband optical pumping schemes. While incoherent sources of surface communication on a chip to low-power nonlinear plasmon polaritons (SPPs) can be relatively easily inte- grated in a nanophotonic circuitry [1, 16, 17], the situation is more complicated in the case of coherent SPP sources and amplifiers, since to reach the lasing threshold one has to *Corresponding author: Dmitry Yu. Fedyanin, Laboratory of Nanooptics and Plasmonics, Center for Photonics and 2D Materials, compensate not only Joule but also radiation losses, which Moscow Institute of Physics and Technology, 9 Institutsky Lane, can be quite large due to a small mode volume of the plas- Dolgoprudny 141700, Russian Federation, monic cavity [13, 18]. The threshold gain may become very E-mail: [email protected]. https://orcid.org/0000- high, posing a serious challenge for the creation of the 0001-7264-0726 required population inversion. Although full compensation Alexey V. Krasavin and Anatoly V. Zayats: Department of Physics and London Centre for Nanotechnology, King’s College London, Strand, of the SPP losses at the nanoscale has been demonstrated London WC2R 2LS, UK. https://orcid.org/0000-0003-2522-5735 [19–24], it was achieved under optical pumping, which is (A.V. Krasavin). https://orcid.org/0000-0003-0566-4087 hardly compatible with on-chip integration [25–28], the (A.V. Zayats) target area of prospective applications. In this regard, an Aleksey V. Arsenin: Laboratory of Nanooptics and Plasmonics, Center electrical pumping scheme is undeniably more desired and for Photonics and 2D Materials, Moscow Institute of Physics and Technology, 9 Institutsky Lane, Dolgoprudny 141700, Russian must be developed for practical application of plasmonic Federation. https://orcid.org/0000-0002-7506-4389 components [25, 26, 29, 30]. Open Access. © 2020 Dmitry Yu. Fedyanin et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 3966 D.Yu. Fedyanin et al.: Coherent emission of surface plasmons The possibility to implement electrical pumping in dramatic reduction of the physical size of the device, sim- plasmonic devices is strongly restrained by a very limited plifies its design, ensures in-plane emission and increases choice of low-loss plasmonic metals. Gold, silver and the operating temperature to above 300 K. Contrary to the copper form rectifying Schottky contacts to direct- electrically pumped metal coated nanopillar lasers [26, 33, bandgap semiconductors [31], while low-resistance 35, 41], which radiate photons into free space in a dipole-like ohmic contacts are needed for efficient electron and fashion, the proposed SPP source is intrinsically integrable hole injection and creation of the population inversion in with nanophotonic circuitry. Unlike nanolasers based on the gain medium [25, 32]. Therefore, one has to use ohmic localized optical modes, which are difficult to pump elec- contacts made of highly absorptive materials (such as ti- trically [42], the subwavelength size of the proposed elec- tanium, chromium, palladium, and different alloys [25, trical SPP source is achieved with a propagating plasmonic 26, 33–35]), place it at a large distance from the active mode, and the device can be referred as a SPPASER (surface region of the structure and rely on the refractive index plasmon polariton amplification by stimulated emission of contrast between semiconductor materials to localize the radiation) or an SPP spaser [43]. The proposed approach optical mode and prevent its overlap with the ohmic enables the SPP to be efficiently out-coupled to a bus plas- contact [33, 35]. On the other hand, many proposed monic or photonic waveguide without losing energy nanolaser designs based on low-absorptive structures through the emission into the surrounding space, which is (see, for example, Refs. [36–38]) can hardly be pumped particularly important in large-scale integrated nano- electrically due to the high-resistance of metal- photonic circuits, where performance and scalability are semiconductor contacts, which does not allow to inject limited by the crosstalk noise. asufficient number of electrons and holes into the gain medium for the creation of the population inversion and full compensation of the high optical losses of the lasing 2 Results and discussion mode. The high-resistance contact limitation greatly complicates the design of electrically pumped nanolasers 2.1 Implementation of electrical pumping and increases their physical size compared to the optically pumped counterparts. To overcome this problem, it was The proposed amplification scheme based on the Au/n+- + proposed to use the minority carrier injection properties of InAs0.4P0.6/In0.72Ga0.28As/p -Al0.29In0.71As double-hetero- Schottky contacts and inject electrons into the gain structure tunneling Schottky barrier diode is shown in semiconductor medium directly from the plasmonic Figure 1 (see Supplementary Information Section 1 for de- metal, avoiding the commonly used intermediate semi- tails). It gives the possibility to use the low-loss SPP sup- conductor electron injection layer [27, 30]. However, there porting interface between gold and the 100-nm-thick is only one binary III–Vsemiconductormaterial(InAs) electron injection n+-InAsP layer as a low-resistance that forms Schottky contacts to plasmonic metals with a tunneling contact to the n+-InAsP electron injection layer barrier height greater than the bandgap energy of the with nearly ideal ohmic characteristics. Therefore, the semiconductor [31, 39], which is required for the creation material gain in InGaAs is provided in the near-field of the population inversion [27, 30]. This limits the oper- proximity to the gold contact, exactly where the plasmonic ation wavelength to around 3 µm and the operation tem- modes are localized, which gives a crucial advantage over perature to below 150 K [40]. In addition, the inability of traditional electrical pumpings schemes of plasmonic and the Schottky barrier to block the majority carriers under photonic nanolasers based on highly absorptive ohmic forward bias leads to a quite high leakage current even in contacts [29, 33, 35, 41, 44]. The n-doped InGaAs layer the presence of a single heterostructure [30], which sandwiched between the p+-AlInAs and n+-InAsP layers greatly limits the energy efficiency of the amplification with bandgap energies exceeding that of InGaAs acts as an scheme. active region of the semiconductor structure. Under high Here, we propose an efficient electrically driven single- forward bias, electrons and holes are injected into InGaAs mode deep-subwavelength coherent optical source from the InAsP and AlInAs sides, respectively, while high featuring a unique combination of strong nanophotonic potential barriers for electrons at the InGaAs/AlInAs het- guiding and a novel amplification scheme based on a erojunction and for holes at the InAsP/InGaAs hetero- double-heterostructure tunneling Schottky