Subwavelength Nanopatch Cavities for Semiconductor Plasmon Lasers

In the past few years much progress has been made in tightly confining light in high quality factor optical microcavities and in defects in 1D and 2D photonic crystals [1], [2], [3]. Modal volumes close to the diffraction limit of (λ/2n) 3 (where λ is the mode wavelength and n is the refractive index seen by the mode) have been achieved in some of these structures [1], [2], [3]. Optically and electrically pumped photonic crystal defect lasers with modal volumes few times the diffraction limit have also been demonstrated [4], [5], [6]. Feedback structures, such as Bragg reflectors and photonic crystals, needed to achieve such small modal volumes made the overall size of the laser structure several times larger than the wavelength since at least a few periods of the feedback structure were required for adequate mode confinement [1], [2], [3], [4], [5], [6]. Plasmonic structures for photonic applications have been extensively studied in the last few years [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The large wavevector values of plasmon-polaritons near the surface plasmon resonance frequency have been used to achieve subwavelength device dimensions. Surface plasmon confined optical modes in waveguides have exhibited modal loss values ranging from 0.3 dB/µm to 30 dB/µm in the visible-to-near-IR wavelength range [14], [15]. While large wavevector values, and small device sizes (compared to the free-space wavelength), are possible for frequencies near the surface plasmon resonance frequency (which corresponds to wavelengths in the 0.4-0.6 µm range for most important metals, such as Silver and Gold), the losses are also higher at frequencies close to the surface plasmon resonance frequency [9], [10]. In the near-IR 1.0-1.6 µm wavelength range, although the losses are smaller, the wavevector values of plasmon-polaritons are also smaller [9], [10]. Plasmon propagation in waveguides coupled to gain media has also been studied theoretically and values between 500 cm -1 and 5000 cm -1 for the material gain required for lossless propagation have been reported [13], [19]. An advantage of operating at frequencies much smaller than the surface plasmon resonance frequency is that the plasmon fields are not strongly confined near the surface of the metal and can therefore have significant overlap with an external gain medium. For realizing lasing in surface plasmon confined nanoscale optical cavities, the gain needs to not only compensate for intrinsic cavity losses (including surface plasmon losses) but also for losses due to external radiation. In addition, the output radiation patterns need to be well behaved for practical applications. In the mid-IR and far-IR wavelength range, where surface plasmon losses are considerably smaller compared to those at visible and near-IR wavelengths, surface plasmon mode confinement in dual-metal waveguides has been used to achieve lasing in quantum cascade devices [20], [21], [22]. However, these laser structures did not have subwavelength sizes in all three dimensions.

In this paper, we propose and analyze a family of nanoscale optical cavities for electrically-pumped surfaceemitting near-IR semiconductor lasers -semiconductor nanopatch lasers (or SNLs) -that have dimensions in the few hundred nanometer range and cavity volumes (not just modal volumes) approaching (λ/2n) 3 [23]. Surface plasmons are used not only for mode confinement but also for output beam shaping to realize single-lobe farfield output radiation patterns with narrow beam waists. The lasers discussed here are in many ways nanoscale optical versions of micropatch antennas (or microstrip patch antennas) that are commonly used at microwave/RF frequencies [24]. We show that in the near-IR wavelength region (1.0-1.6 µm) cavity losses in nanopatch lasers (including surface plasmon losses) can be compensated by gain from conventional III-V materials because surface plasmons can themselves be used to provide large overlap of the cavity mode with the gain region. The material threshold gain values needed to achieve lasing can be smaller than 700 cm -1 . Despite surface plasmon losses, the external radiation efficiencies are between 10%-30%. Compared to all-dielectric microcavity semiconductor lasers reported in the literature [4], [5], [6], [7], the proposed nanopatch lasers have much smaller cavity vol

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