Quantification of Free-Carrier Absorption in Silicon Nanocrystals with an Optical Microcavity

Geballe Laboratory of Advanced Materials, Stanford University, Stanford, California 94305, USA.
Nano Letters (Impact Factor: 13.59). 11/2008; 8(11):3787-93. DOI: 10.1021/nl8021016
Source: PubMed


We present a highly sensitive and accurate microcavity-based technique to quantify the free-carrier absorption (FCA) cross-section of semiconductor quantum-dot ensembles. The procedure is based on measuring the pump-intensity-dependent broadening of the whispering gallery modes (WGMs) of microdisk resonators. We have applied this technique to determine the FCA cross-section of Si nanocrystals (Si-ncs) in the visible-near-infrared wavelength range. Our procedure accounts for the size distribution effects by including the measured wavelength dependence of the excitation cross-section and the decay rate of photoexcited carriers in the analysis. By monitoring the WGM widths at various wavelengths in the 700-900 nm wavelength range, we found that the FCA cross-section follows an approximately quadratic wavelength dependence. The magnitude of the FCA cross-section of Si nanocrystals was determined to be a factor of 7 higher than that in bulk Si. For this reason, these findings have important implications for the design of Si-based lasers and all-optical switching devices in which FCA plays a critical role.

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    • "Versatile passive and functional devices including couplers, multiplexers and demultiplexers, resonators, filters, and modulators have been emerged to meet the demands of the next-generation photonic integrated circuits. To fabricate a Si-based modulator with high modulation depth, several kinds of Si-based micro-or nano-structures, including rib/slot waveguide [1] [2] [3] [4] [5], nano-wire [6] [7] [8] and ring-resonator [9] were developed with mature fabrication technology in current Si industry. Recently, Si quantum dot (Si-QD) has been studied in several applications of silicon photonics, such as light emitting diodes (LEDs), distributed Bragg reflectors, waveguide amplifiers, modulator and switch, etc. [10] [11] [12] [13] [14] [15] [16] [17] [18]. "
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    ABSTRACT: The Si quantum dot doped SiO x rib waveguide based free-carrier absorption modulator with enhanced all-optical modulation depth is demonstrated by integrating with a micro-ring waveguide resonator. The micro-ring waveguide resonator with a Q-factor of 6x10 3 induces a throughput transfer function in wavelength domain, such a transmittance notch can be blue-shifted by varying the excited free-carrier density of Si-QD. When injecting the continuous-wave probe at central wavelength of the transmittance notch, the probe signal can be inversely modulated by optically pumping the micro-ring waveguide resonator to blue-shift the notch away from its original wavelength. By optimizing the pump wavelengths, the largest free-carrier absorption (FCA) loss and highest free-carrier density can be enhanced to 2.9 cm-1 and 7.83×10 16 cm-3 , respectively. With the excited free-carrier density of ~1.3×10 16 cm-3 inside the micro-ring waveguide, the maximal wavelength of the transmittance notch can be blue-shifted by 0.033 nm. The optical pumping also induces the broadened linewidth of transmittance notch from 0.25 to 0.27 nm. With the integrated micro-ring waveguide resonator, the all-optical modulation depth can be further enhanced from 52.5% to 63.5% by shifting the notched transmission spectrum of the micro-ring waveguide with the excited free-carrier density of Si-QD at probe wavelength of 1563.5 nm. Index Terms—Si quantum dots, free-carrier absorption, ring waveguide resonator, data inverter.
    Full-text · Article · Jun 2015 · IEEE Journal of Selected Topics in Quantum Electronics
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    • "As expected, the distribution of NCs has smaller radii than than the NCs of Ref. [4], as the NC emission has been pushed to shorter wavelengths. By integrating the distribution for all radii, we find a total nanocrystal density of N N C = 7 × 10 18 cm −3 , which is comparable to the densities obtained in other works [2] [4]. Figure 8: (a) The Si-NC density distrubution of this sample as a function of Si-NC radius. "
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    ABSTRACT: One dimensional nanobeam photonic crystal cavities are fabricated in silicon dioxide with silicon nanocrystals. Quality factors of over 9 x 10^3 are found in experiment, matching theoretical predictions, with mode volumes of 1.5(lambda/n)^3 . Photoluminescence from the cavity modes is observed in the visible wavelength range 600-820 nm. Studies of the lossy characteristics of the cavities are conducted at varying temperatures and pump powers. Free carrier absorption effects are found to be significant at pump powers as low as a few hundred nanowatts.
    Full-text · Article · Mar 2010 · Physical review. B, Condensed matter
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