Surface plasmons in metal hole arrays have been studied extensively in the context of extraordinary optical transmission, but so far these arrays have not been studied as resonators for surface plasmon lasing at optical frequencies. We experimentally study a metal hole array with a semiconductor (InGaAs) gain layer placed in close (20 nm) proximity of the metal hole array. As a function of increasing pump power, we observe an intense and spectrally narrow peak, with a clear threshold. This laser emission is donut shaped and radially polarized. Three experimental observations support that the system shows surface plasmon lasing. First, the full wavelength dispersion of the observed resonances can be understood by using a single surface plasmon mode of the system. Second, the polarization of these resonances is as expected for surface plasmons. Third, the magnitude of the avoided crossing, which results from mode coupling at the holes, has a similar magnitude as found in simulations using surface plasmons.
A metal film perforated by a regular array of subwavelength holes shows unexpectedly large transmission at particular wavelengths, a phenomenon known as the extraordinary optical transmission (EOT) of metal hole arrays. EOT was first attributed to surface plasmon polaritons, stimulating a renewed interest in plasmonics and metallic surfaces with subwavelength features. Experiments soon revealed that the field diffracted at a hole or slit is not a surface plasmon polariton mode alone. Further theoretical analysis predicted that the extra contribution, from quasi-cylindrical waves, also affects EOT. Here we report the experimental demonstration of the relative importance of surface plasmon polaritons and quasi-cylindrical waves in EOT by considering hole arrays of different hole densities. From the measured transmission spectra, we determine microscopic scattering parameters which allow us to show that quasi-cylindrical waves affect EOT only for high densities, when the hole spacing is roughly one wavelength. Apart from providing a deeper understanding of EOT, the determination of microscopic scattering parameters from the measurement of macroscopic optical properties paves the way to novel design strategies.
We study the effect of frequency detuning on light focused through turbid media. By shaping the wavefront of the incident beam light is focused through an opaque scattering layer. When detuning the laser we observe a gradual decrease of the focus intensity, while the position, size,and shape of the focus remain the same within experimental accuracy. The frequency dependence of the focus intensity follows a measured speckle correlation function. We support our experimental findings with calculations based on transport theory. Our results imply wavefront shaping methods can be generalized to allow focusing of optical pulses in turbid media.
In the past decade, metal hole arrays have been studied intensively in the context of extraordinary optical transmission (EOT). Recently it was shown that surface plasmons on optically pumped hole arrays can show laser action. So far, however, it is not demonstrated that the optical transmission of these arrays can also be increased using gain. In this Letter, we present a dramatic increase of the EOT via loss compensation of surface plasmons, accompanied by spectral narrowing of the resonance. These experiments allow us to quantify the modal gain experienced by the surface plasmon. Interestingly, the transmission minimum of the Fano-resonance becomes smaller.
The optical intensity transmitted through a random pattern of subwavelength holes in a metal film exhibits a speckle pattern. We study the variation of this speckle pattern as a function of wavelength. We find that the resulting speckle correlation function (SCF) separates into a wavelength-dependent part and a wavelength-independent background. The wavelength dependence is caused by surface plasmons excited at one hole and coupled out at another hole, while the constant background originates from light transmitted directly through the holes. By analyzing the SCF for a set of samples of varying hole density, we find the propagation length of the surface plasmons and the scattering losses induced by the holes.
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