In this paper, we report the study of the frequency-dependent plasmonic enhancement of a circular disk nano-optical antenna array and the photo-response of the optical antenna enhanced photodetector at different frequencies using a femtosecond (fs) laser frequency comb. A fs-laser frequency comb can provide hundreds of evenly spaced harmonic frequencies and thus allows simultaneous measurement of the plasmonic optical antenna enhancement effect at these harmonic frequencies. This offers a highly efficient frequency-dependent measurement approach compared to the conventional method of modulating of a c.w. laser, which measures the frequency response at each frequency. The impulse response of the circular disk nano-optical antenna array and the electric-field (E-field) distribution profile are simulated under a fs laser illumination. The light intensity spectrum is simulated and verified to have uniform intensities on the harmonic frequencies within the ±5 GHz frequency range. The photocurrent densities in different regions of a GaAs p-i-n photodetector are analyzed together with their frequency dependence at the harmonic frequencies of the fs laser frequency comb with a repetition rate of f 0 = 94.2 MHz. A circular disk nano-optical antenna array enhanced GaAs p-i-n photodetector was fabricated and measured using a fs laser frequency comb with the same repetition rate. The nano-optical antenna can provide ~20 dB enhancement for the harmonic frequencies and extend the detector cut-off frequency from 2.4 GHz to 4.2 GHz.
In this paper, we present the analysis of the evanescent waveguide modes in a 2D subwavelength hole array (2DSHA) plasmonic structure and their relationship with the excited surface plasmonic resonance (SPR). It is found that when the metal film thickness is larger than 0.0625 µm, the excited SPRs have a linear dependence on the evanescent waveguide modes, whereas when the metal film is thin, the linear relationship does not hold, primarily due to the coupling of the SPRs on the asymmetric interfaces of the metal film. The linear dependence is also verified by curve-fitting the Fano transmission profiles of the 2DSHA structures. 2DSHA enhanced quantum dot infrared photodetectors with various metal film thicknesses were fabricated and tested. The measured plasmonic enhancement ratios agree well with the analysis.
Extended shortwave infrared (e-SWIR) photodetectors and imaging focal plane arrays covering the wavelength beyond the conventional In0.53Ga0.47As cutoff wavelength of 1.65 micrometers (µm) can find numerous applications in infrared sensing and imaging. This paper reports voltage-tunable e-SWIR photodetectors based on the conventional gallium antimonide (GaSb) n–i–p and p–i–n homojunctions on GaSb substrates, which offer bias-dependent photodetection band tuning with a simple structure and high material crystal quality due to the perfect lattice matching on the substrates. Detection bands between the cutoff wavelengths of 1.7 µm and 1.9 µm can be tuned with a low reverse bias voltage of <0.1 volts (V). The mechanism of the voltage-dependent band-tuning was analyzed and attributed to the Moss–Burstein effect, which changes the electron and hole filling factors under different reverse bias voltages. This analysis agreed with the experimental data. The Moss–Burstein effect-induced voltage-dependent band-tuning mechanism can provide useful guidance for the designs of e-SWIR photodetectors.
In this paper, we report the analysis of surface current confinement in circular ring optical antennas and its enhancement effect to the photoresponse of quantum dot (QD) longwave infrared (LWIR) photodetectors. Circular ring optical antennas with various inner and outer diameters were simulated to determine the electric near-field (E-field) and surface current distribution. Over 80 times E-field enhancement was obtained when the width of the ring is as narrow as 2 nanometers (nm) where the surface current is strongly confined. In addition, the E-field (|E|) shows a direct linear relationship with the surface current density. The linear relationship is analyzed using an RLC circuit model. The E-field enhancement of the circular ring optical antenna is compared with that of the two-dimensional metallic subwavelength hole arrays structure (2DSHA) and is found to have higher enhancement with a broader spectral band coverage. The circular ring optical antenna enhanced LWIR QD infrared photodetectors were fabricated and measured. The ring optical antenna with stronger surface current confinement shows a higher photocurrent enhancement. The experimental results agree well with the simulation. Broader band photocurrent enhancement than the 2DSHA structure was also verified.
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