Metal-Cavity Nanolasers - IEEE Xplore

Invited Paper

Metal-Cavity Nanolasers Volume 3, Number 2, April 2011 Shun Lien Chuang Dieter Bimberg

DOI: 10.1109/JPHOT.2011.2138690 1943-0655/$26.00 ©2011 IEEE

IEEE Photonics Journal

Metal-Cavity Nanolasers

Metal-Cavity Nanolasers Shun Lien Chuang 1 and Dieter Bimberg 2 (Invited Paper) 1

Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA 2 Institut fu¨r Festko¨rperphysik, Technische Universita¨t Berlin, 10623 Berlin, Germany DOI: 10.1109/JPHOT.2011.2138690 1943-0655/$26.00 Ó2011 IEEE

Manuscript received February 19, 2011; revised March 24, 2011; accepted March 26, 2011. Date of current version April 26, 2011. The work at the University of Illinois at Urbana-Champaign was supported by the Defense Advanced Research Projects Agency’s NACHOS Program under Grant W911NF-07-1-0314. S. L. C. also thanks the support of the Humboldt Research Award from the A. von Humboldt Foundation. The work at Technische Universita¨t Berlin was supported by Deutsche Forschungsgemeinschaft in the frame of SFB787. Corresponding author: S. L. Chuang (e-mail: [email protected]).

Abstract: Recent progress on nanoscale lasers, especially metal-cavity nanolasers, is highlighted. In spite of the metal loss, metal cavities of subwavelength scales have been used successfully for semiconductor lasers by optical or electrical pumping from low to room temperature. We focus on the demonstration of a substrate-free metal-cavity surfaceemitting microlaser operating continuous wave at room temperature with electrical injection. Index Terms: Semiconductor lasers, nanolasers, nanophotonics.

Nanoscale lasers possess advantages such as low power consumption, an ultrasmall footprint, and ultrafast switching [1]–[3]. Potential applications include biochemical sensing [4], imaging [5], and intrachip and interchip short-distance optical interconnects [1], [2]. Practical nanolasers require electrical injection operation at room temperature in continuous-wave mode. Independent nanolasers can form dense arrays of subwavelength pitch for possible near-field scanning and optical atom traps. The smallest laser based on dielectric cavities requires an optical cavity with a dimension of half a wavelength in all three directions, which is often called the diffraction limit. During the last decade, photonic crystal lasers have been extensively studied as candidates for small lasers. However, to have a large quality factor for laser action, many periods of photonic crystal are required, making the size on the order of several wavelengths. To produce a laser breaking the diffraction limit, one approach is to use the plasmonic effect [6]–[15] formed at the interfaces between the metal and semiconductor. In this case, both the physical and effective volume of the optical cavity can be reduced, although it would be at the expense of modal absorption due to the metal loss. By positioning the active materials such as quantum dots or quantum wells (QWs) at the peak of optical fields with an emission wavelength near the cavity resonance, it is possible to enhance the spontaneous and stimulated emission [12] and reduce the lasing threshold. There has been excellent progress in micro- and nanolasers, especially metallic and plasmonic nanolasers [3]. Plasmonic nanolasers via optical pumping have been reported by [7] using a CdS nanowire as the gain medium on top of a silver surface with a 5-nm insulator gap. Nanoparticles with a gold core and dye-doped silica shell have been used to realize spaser-based nanolasers via optical pumping [8]. Electrical injection of metal-cavity lasers with a dimension that is less than a wavelength in one or two dimensions has been demonstrated in [9]–[11].

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Fig. 1. (a) Schematics of our metal-cavity microlaser. The fabricated device has an active region of 14 GaAs/Al0:2 Ga0:8 As quantum wells. Optical feedback is from the bottom silver and top hybrid/DBR mirrors with the surrounding metal sidewall. Silver encapsulation helps mode confinement and scattering reduction. (b) Current dependent spectra of the device lasing under CW current injection at 300 K. (Inset: scanning electron micrograph of a SiNx -passivated cavity before metallization). (c) Light output power as a function of injection current (L–I) curves at various temperatures (10 C–27 C) and the I–V curve at room temperature (27 C) [22].

In 2010, significant progress on micro- and nanolasers has been made, i.e., subwavelength nanolasers via optical pumping [16]–[19], nanopillar lasers on silicon substrate [20], electrical injection Fabry–Perot metal-cavity lasers at 240 K [21], and substrate-free metal-cavity surfaceemitting microlasers at room temperature [22]–[26]. At the University of California at San Diego, metallo-dielectric subwavelength lasers [16] using an InGaAsP multiple quantum well (QW) active layer disk surrounded by an aluminum/silica bilayer shield as the cavity were made by optical pumping at room temperature. The importance of the optimized thickness of the insulating silica is emphasized to reduce the threshold gain for optical pumping at room temperature. The feedback is provided by a mode cutoff plug-in structure which forbids the propagating mode inside, thus achieving a high reflectivity mirror. At University of California at Berkeley, subwavelength nanopatch lasers using top and bottom metals (gold) to form the nanocavity with InP/InGaAsP/InP materials with a physical volume ð0:01930 Þ were demonstrated at 78 K [17], [18] by optical pumping. Due to their resemblance to patch antennas in microwave technology, the structures emit light from the sidewalls with constructive/destructive interferences in the surface normal direction and are suitable for beam divergence control. Polarization controllability has been demonstrated by tuning the geometry of the nanopatches. Silver nanopan plasmonic lasers [19] with a volume of 0:56ð0 =2nÞ3 have also been demonstrated at 8 K with a subnanometer linewidth by optical pumping. Whispering gallery modes in silver defined cavity were identified in nanopan plasmonic lasers. Nanolasers using InGaAs nanopillars grown on silicon substrate by optical pumping at room temperature have also been reported by UC Berkeley [20]. Until recently, the electrical injection of metal-cavity semiconductor lasers has demonstrated significant progress, such as high-temperature (240 K) continuous-wave (CW) operation using a Fabry–Perot type with emission from the bottom aperture by Arizona State University and Technical University of Eindhoven [21], as well as a CW room temperature surfaceemitting microlaser bonded on silicon with a physical volume of 1230 by the University of Illinois and the Technical University of Berlin [22]–[26]. The size of the laser is now not limited by the diffraction limit. Nanolasers will have a large impact on our technology if they are integrable to current electronic architecture. From an application point of view, nanolasers with integrability to current electronic platforms (i.e., silicon) will lead to advanced photonic integrated circuits. Several nanolasers have shown a promising future for integration either by direct growth of nanopillars (without metal coating) on a silicon substrate [20] or by stacking the devices onto the electronic platform. We demonstrated experimentally a metalcavity surface-emitting microlaser with metal on the top and surrounding sidewall and a bottomdistributed Bragg reflector (DBR), which lases at room temperature under CW operation (see Fig. 1) [22]–[26]. The active region consists of 14 pairs of GaAs/Al0:2 Ga0:8 As QWs. Multiple QWs uniformly distributed in the active region are used to provide enough optical gain without worrying about the longitudinal standing wave (node/peak) effects. A 17.5-pair n-doped quarter-wavelength DBR acts as both the feedback and the electron injector. The integration to silicon was demonstrated by flipchip bonding to a gold coated silicon substrate with the complete removal of the GaAs substrate to


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allow its surface emission. The physical size after substrate removal is only 2.0 m in diameter and 2.5 m in total thickness (a volume of 12 30 ), including the overall p i(QWs)-n(DBR) regions. Flipchip bonding with metal allows the integration of our metal-cavity lasers to various substrates, including silicon in our devices. Metal serves as a multifunction medium for reflector, contact, and heatsink. The round-trip resonance phase condition is satisfied by choosing the active layer thickness to match the boundary conditions at both top metal and bottom DBR for the metalconfined fundamental optical mode. Also, with a broadband reflector using metal, the detuning of the cavity mode with the gain peak can thus be reduced, compared with standard vertical-cavity surface emitting lasers [27]. Our devices were mounted on a thermoelectrically cooled copper heat sink for measurements at 300 K under CW operation. Thermal management has been largely improved as a result of efficient heat removal from the surrounding metal and the substrate-free configuration with bonding. We have measured the light output power as a function of the injection current at temperatures from 10 to 27 C, showing temperature-stable operation with a characteristic temperature of 425 K [22], [26]. The light output power is up to 7.5 W at 4.5 mA. We have also measured the laser linewidth and obtained a value of 0.67 A˚ (full-width at halfmaximum) at a bias of 2.8 mA. This is probably the narrowest measured laser linewidth among metal-cavity lasers with electrical injection, which are typically hard to measure due to their low power. A kink at 3.2 mA bias current shows polarization switching behavior, which is confirmed by measuring the polarization resolved L–I curves and emission spectra at various bias currents [22]. We have also developed a rigorous theoretical model, which takes into account the plasmonic dispersion in a nanocavity [12]–[14] and pointed out the importance of using the energy (instead of power) confinement factor [13]. Our theoretical formulation and the resultant rate equations have been applied to study nanolasers such as a nanobowtie laser [12] and a metal-cavity edge-emitting laser [13], [14] for the prediction of lasing threshold and light output power versus injection current (L–I curve). To compare our theory with experimental data [22]–[26], we first calculate the band structure of the GaAs/AlGaAs QW lasers and the optical gain spectrum as a function of increasing carrier density. We also compared the amplified spontaneous emission spectra in the metal cavity with the measured asymmetrical electroluminescence spectra [see Fig. 1(b)] at various injection currents below threshold and obtained good agreement. The band edge of the QW spontaneous emission spectrum and the cavity resonance spectrum creates an asymmetrical lineshape. Our model result of the quality factor Q of 556 of the cold cavity is close to the measured value of 580 at low injection current. We then model the measured light output power as a function of the injection current based on our rate equations and show our theory agrees well with the experimental data shown in Fig. 1(c). We found that at a very small bias current below 0.5 mA, there is no light emission until the spontaneous emission peak wavelength merges with the cavity resonance wavelength. Above 0.5 mA, the spontaneous emission starts to amplify significantly with increasing gain as the current increases. When the optical gain reaches threshold at 1.75 mA, the laser action starts to occur. We have further reduced the size of our metal-cavity surface-emitting lasers by either shrinking the diameter or reducing the number of DBR pairs to only five or even zero, while maintaining a reasonable quality factor for laser action. The results will be reported in the near future [28]. To recap, we have demonstrated experimentally a room-temperature metal-cavity surfaceemitting microlaser [22]–[26] and developed a rigorous model for nanolasers with further reduction in size [13], [14]. Our theory explains the observed asymmetrical optical emission spectrum below threshold and the light output versus injection current (L–I curve). Nanolasers pose intriguing challenges for researchers in photonics, both intellectually and technologically. Due to their compactness in size and substrate-free and/or silicon compatibility, they are promising elements to bridge the gap between nanophotonics and silicon electronics. They have potential applications for ultrahigh density photonic integrated circuits with ultralow power consumption and footprint and ultrafast switching speed. The ultrahigh modulation bandwidth of nanolasers has yet to be demonstrated experimentally [29]–[31]. Further research is necessary to reduce the metal losses in the cavity and to overcome the technological challenges of nanofabrication of nanoscale semiconductor lasers with electrical injection.


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In summary, we have reviewed progress in nanolasers in 2010 with a more extensive discussion of our own contributions to metal-cavity nanolasers. We note that our brief review may be incomplete; nevertheless, we hope it will provide a stimulus for further research on nanolasers.

Acknowledgment The authors would like to thank C. Y. Lu, S. W. Chang, and A. Matsudaira at the University of Illinois at Urbana-Champaign and T. D. Germann and U. W. Pohl, at the Technical University of Berlin for their contributions.

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