Abstract
We demonstrate the generation of nanosecond mid-infrared pulses via fast modulation of thermal emissivity enabled by the absorption of visible pump pulses in unpatterned silicon and gallium arsenide. The free-carrier dynamics in these materials result in nanosecond-scale modulation of thermal emissivity, which leads to nanosecond pulsed thermal emission. To our knowledge, the nanosecond thermal-emissivity modulation in this work is three orders of magnitude faster than what has been previously demonstrated. We also indirectly observed subnanosecond thermal pulses from hot carriers in semiconductors. The experiments are well described by our multiphysics model. Our method of converting visible pulses into the mid infrared using modulated emissivity obeys different scaling laws and can have significant wavelength tunability compared to approaches based on conventional nonlinearities.
Highlights
Short optical pulses have applications that range from telecommunication to ultrafast science to materials processing
In this work, mid-infrared pulses are generated by rapidly modulating thermal emission from heated semiconductors using a visible pulsed laser (λ = 515 nm) (Fig. 1a)
Undoped semiconductors are usually poor thermal emitters at photon energies below their band gap (e.g., silicon (Si) or gallium arsenide (GaAs) in the mid infrared21) but can become highly emissive via the presence of free carriers that can be generated via absorption of an above-gap optical pump pulse
Summary
Short optical pulses have applications that range from telecommunication to ultrafast science to materials processing. Existing technologies have significant limitations; for example, mode locking of quantum cascade lasers is challenging and has resulted in sources with low power and limited tunability[1,2], while down conversion of near-infrared pulses using nonlinear optics requires complex and expensive instrumentation[3,4]. We explore an approach for generating short pulses in the mid infrared based on fast optically driven modulation of thermal emission. The modulation of thermal emission can be realized via dynamic changes in either of these two parameters. Even faster temperature changes can be realized by decoupling the electronic and lattice temperatures: electrons can be driven far out of thermal equilibrium with phonons for a very short amount of time when pumped by a laser pulse[7]. Observations of hotelectron thermal emission have been reported in graphene[8] and in metals[9], not in semiconductors
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