Prof. Xiang Zhang's Laboratory

at UC Berkeley

Site Updated:
10/24/2009

Plasmon Laser

Objective

Demonstration of light amplification of stimulated emission by radiation (lasing action) utilizing surface plasmon polaritons (SPP) at sub-wavelength scales. 

Background

Since the first demonstration of the laser in the middle of the last century laser technology has made tremendous advances towards higher power, faster and smaller light sources. However, the diffraction limit of light imposed a fundamental boundary of how small a laser could be made. This physical constrain seemed to be beatable when Bergmann et. al. proposed a laser devices utilizing localized SPPs. While SPPs offer optical confinement, they come with a trade-off, that is, they introduce ohmic losses to the plasmon mode decreasing the modes propagation length. Thus, the requirements on the laser gain medium and the cavity mirror quality were high. While former Xlab student M. Ambati [1] showed that SPP losses can be reduced via a gain (Er+) material, it was the hybrid-plasmon mode concept, [2] developed by Xlab researchers R. Oulton and V. Sorger that made the realization of a plasmon laser with deep sub-wavelength optical confinement possible. (See the plasmon waveguide page) In brief, a high dielectric gain material (e.g. a semiconductor nanowire) separated from an metal interface by a nanometer thin oxide layer forms an optical capacitor based on polarization charges. This design allows for mode area confinement up to l2/400 significant modal overlap with the gain material to provide optical amplification.    

Results

Xlab researches realized a plasmon laser by utilizing the hybrid plasmon mode concept by placing a Cadmium Sulfide nanowire bridged by a 5 nm thin Magnesium Fluoride layer atop of a Silver film oxide and exciting (pumping) the device optically (Fig. 1a). The sub-wavelength optical confinement for such a laser can clearly be seen from the electric field distribution (Fig. 1b).
Upon increasing the pump intensity, we observe the onset of amplified spontaneous emission peaks. These correspond to the longitudinal cavity modes that form when propagation losses are compensated by gain allowing plasmonic modes to resonate between the reflective nanowire end-facets (Fig. 2a). Increasing the pump even further produces sharp (FWHM < 0.5 nm) lasing peaks. The plasmon laser light output power as a function of pump intensity yields the characteristic super-linear curve, well known for lasers devices (Fig. 2b). The lasing light exciting at the end facets of the nanowire and can be clearly seen by this microscope image (Fig. 2c). The internal processes of this laser (exciton generation and annihilation, see [3] for details) provide for a 10% efficient laser, which is strong enough to in order to observe the plasmon laser signal with the naked eye in room-light (Fig. 2d).

Proving the sub-wavelength confinement of plasmon laser can be done in multiple ways (refer to Ref. 3 and SOM for details): monitoring the characteristic polarization of the hybrid SPP mode or utilizing gain-clamping upon reaching laser threshold, which we checked via the frequency-pulling effect of lasers a feature of strong gain dispersion and mode confinement. Another method is by measuring the emission rate, which is inverse proportional to the optical confinement (Purcell Effect). We observed an increase of the emission rate of the plasmon laser up to 6 times compare to emission into free space. This increased rate directly relates to the mode confinement via the so called “Purcell factor”, a measure of the emission rate normalized by the rate into free space, which is inverse proportional to the optical confinement (Figure 3a). Lastly, the power output vs. pump intensity shows a signature of threshold-less lasing. That is, with increasing mode confinement (i.e. decreasing laser diameter, d) the spontaneous emission factor, b, increases showing a nearly ‘flat’ curve for smallest lasers.

This first demonstration of a semiconductor plasmon laser with sub-wavelength confinement is an important milestone towards true nano-scale photonic circuits, single molecule detection and other applications like optical computing and non-linear optics with low light intensities.      
     


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Figure 1 | The deep sub-wavelength plasmonic laser. a The plasmonic laser consists of a Cadmium Sulphide semiconductor nanowire atop a Silver substrate, separated by a nanometre scale MgF2 layer. This structure supports a new hybrid-plasmon mode [2] whose mode size can be 100 times smaller than a diffraction limited spot. Inset shows a scanning electron microscope image of a typical plasmonic laser. b Electric field distribution and direction |E(x,y)| of a hybrid plasmonic mode at a wavelength l = 489 nm corresponding to the CdS I2 exciton line.



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Figure 2 | Laser oscillation and threshold characteristics of plasmonic and photonic lasers. a Fabry-Perot laser oscillation of a plasmonic laser. b Shows the non-linear response of the output power to the peak pump intensity. c Microscope image of a plasmon laser in action with the scattered light output is from the end-facets. d The lasing single as observed with the naked eye in room light emphasizing the high efficiency of these devices. Inset, zoom-in showing the lasing output and the objective lens collecting the laser emission. 

   


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Figure 3 | Signatures of plasmonic and photonic lasers. a Purcell factors determined from the emission rate measurements showing the clear increase with increasing field confinement (reduction in MgF2 gap, h). An increase of up to 6 times was measured for the smallest laser. b The dependence of measured output power over pump intensity highlights clear differences in the physics underlying the plasmonic (h = 5 nm) and photonic lasers. Photonic lasers exhibit a clear transition between spontaneous emission and laser operation characterized by a clear ‘kink’, corresponding to the laser threshold. The large value of bobserved in the plasmonic lasers is associated with so-called threshold-less operation and attributed to the strong mode confinement.
    • References from Xlab

      [1] Ambati, M., Nam, S. H., Ulin-Avila, E., Genov, D. A., Bartal, G. & Zhang, X. Observation of Stimulated Emission of Surface Plasmon Polaritons. Nano Lett. 8, 3998–4001 (2008). view pdf

      [2] Oulton, R. F., Sorger, V. J., Genov, D. A., Pile, D. F. P. & Zhang, X. A hybrid plasmonic waveguide for sub-wavelength confinement and long-range propagation. Nature Photonics 2, 495-500 (2008). view pdf

      [3] Oulton, R. F., Sorger, V. J., Zentgraf, T., Ma, R.-M., Gladden, C., Dai, L., Bartal, G. Zhang, X. Plasmon lasers at deep subwavelength scale. Nature, October (2009). view pdf

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