Can a metal film make nano-sized frequency doublers efficient?
Putting a thin semiconductor waveguide just above a silver film squeezes infrared light into a tiny region and makes frequency doubling far stronger than in the same structure on glass.
Source
Direct visualization of phase-matched efficient second harmonic and broadband sum frequency generation in hybrid plasmonic nanostructures
Study at a glance
- Design
- Other — AlGaInP waveguides and disks released onto Al2O3/Ag or glass, excited with tunable or broadband infrared pulses; far-field SHG/SFG imaging and spectra compared with COMSOL simulations
- N
- No sample size; several waveguide widths and 1 μm disks were measured
- Population
- Lithographically defined AlGaInP nanostructures on silver versus glass substrates
- Outcome
- SHG/SFG conversion efficiency, far-field visualization of phase matching, enhancement over photonic counterparts
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What they did
The authors made AlGaInP semiconductor waveguides and disks about 110 nm thick and placed them on a silver film with a thin alumina layer, or on glass for comparison. They coupled telecom-band infrared light in and imaged the red light produced by second-harmonic generation (doubling) and sum-frequency generation (adding two different frequencies) from above. They compared measured patterns and efficiencies with mode simulations.
What they found
Light leaking from higher-order modes let them watch the doubled signal build up or fade along the waveguide, directly showing where phase matching held; at one wavelength they estimated a coherence length of about 12 μm. Structures on silver produced over 1500 times more signal than the same waveguides on glass, which gave essentially nothing. In 1 μm disks, broadband infrared light gave a sum-frequency efficiency of 14.8% per megawatt, about five times that of single-wavelength doubling, and the doubled power scaled with input power to a measured exponent of 2.1.
The limits
What it doesn't show
The efficiencies are high per unit volume but absolute output powers were only nanowatts or picowatts, and propagation losses in the plasmonic modes limited amplification to a few micrometres. Some phase-matched modes could not be seen in the far-field setup, and a quoted higher efficiency for a shorter waveguide is an extrapolation rather than a measurement. Only one semiconductor-metal combination was tested.
Key terms
- Second-harmonic generation
- A nonlinear process where two photons of the same frequency combine into one photon of twice the frequency.
- Sum-frequency generation
- A nonlinear process where two photons of different frequencies combine into one photon at the sum of their frequencies.
- Phase matching
- The condition that the generated wave stays in step with the driving wave so the signal keeps adding up along the propagation direction.
- Coherence length
- The distance over which generated light stays in phase with the fundamental before it starts cancelling itself.
- Hybrid plasmonic mode
- A guided light mode shared between a dielectric or semiconductor and a nearby metal, confining light well below the diffraction limit.
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Quiz yourself
What does sum-frequency generation do?
Common questions
Why is the metal film needed?
It supports hybrid plasmonic modes that confine light much more tightly than a waveguide on glass can, and its image charges strengthen the field inside the semiconductor.
How could they 'see' phase matching?
Some generated modes leak into free space, so the camera image of the waveguide shows how the signal grows or shrinks along its length.
Why is broadband light better in the disks?
Each infrared wavelength can pair with many others, so many sum-frequency processes add up coherently and raise total efficiency.
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