Abstract
Black phosphorus possesses several attractive properties for optoelectronics, notably a direct and layer dependent bandgap that varies from the visible to mid-infrared and the ability to transfer the material to nearly arbitrary substrates. A less utilized property of black phosphorus for optoelectronics is the nonlinear photoresponse. The photocarrier lifetime in black phosphorus exhibits a strong nonlinear dependence on the excitation density that is utilized in the present work for optoelectronic mixing. In this scheme, two telecommunications-band lasers are intensity-modulated by a radio frequency (RF) and local oscillator (LO) frequency and focused onto a black phosphorus photoconductive detector. Above the saturation carrier density, the photocurrent is proportional to the square root of the optical power which produces photocurrents at the sum and difference frequencies of the input beams. The bandwidth of the mixing process increases from 10 to 100 MHz for incident powers of 0.01 to 1 mW, respectively. An excess carrier model accurately describes the power dependence of the cutoff frequency and mixing conversion, which are both limited by photocarrier recombination. Optimizing our device geometry to support larger bias fields and decreased carrier transit times could increase the maximum RF/LO frequency beyond a GHz by reducing the excess carrier lifetime. Frequency mixing based on the photocarrier nonlinearity in multilayer black phosphorus demonstrated here can be readily extended to mid-infrared wavelengths as long as 4 µm.
Highlights
Optoelectronic mixing via a local oscillator applied directly to the active material of a nonlinear detector generally allows for the optimization of the detector design for the lower intermediate frequencies (IFs)
The first demonstration of optoelectronic down conversion was based on an AC biased photodiode: in addition to the DC bias, an electrical local oscillator (LO) AC bias scitation.org/journal/app is applied to the photodiode, generating the IF.25,26
This strong nonlinearity, in combination with the versatility of a material that can be used on a wide variety of substrates, makes black phosphorus an extremely attractive material for optoelectronic mixing
Summary
Black phosphorus has generated interest as a versatile material for optoelectronics covering a large spectral range. Similar to graphene and other 2D materials, its layered structure allows for the mechanical exfoliation of atomically thin layers that can be transferred to substrates like SiO2/Si, quartz, or even waveguides. Flakes of different thicknesses feature a direct bandgap between the bulk value of 0.3 eV for thick flakes (more than 10 layers) to around 2 eV for single atomic layers. While the single atomic layers are highly unstable under ambient conditions and will deteriorate without a protective layer in less than a minute, thicker flakes are more robust and can remain viable with a simple capping layer for many months. Because of these favorable properties, black phosphorus has been employed as an active material for both detectors and modulators in the infrared range. Black phosphorus exhibits a photoconductive response that scales nonlinearly with applied optical power. The origin of this nonlinearity is radiative carrier recombination, which results in decreased carrier collection efficiency as the applied optical power is increased. While the single atomic layers are highly unstable under ambient conditions and will deteriorate without a protective layer in less than a minute, thicker flakes are more robust and can remain viable with a simple capping layer for many months.10–12 Because of these favorable properties, black phosphorus has been employed as an active material for both detectors and modulators in the infrared range.. While the bias voltage shows no influence on the device bandwidth, the cutoff frequency is observed to shift from 10 MHz to 100 MHz with increasing optical power We utilize this nonlinearity for the optoelectronic mixing of two independent laser sources that are modulated at frequencies of several hundred MHz and serve as radio frequency (RF) and local oscillator (LO) inputs to the mixer. The bandwidth of our device could potentially be extended beyond a GHz by shortening the black phosphorus channel and applying larger bias fields to sweep the carriers from the device. The carrier nonlinearity utilized for mixing depends on the excited carrier density, indicating that the mixer presented here could be used at wavelengths as long as 4 μm due to the low bandgap of bulk black phosphorus
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