利用氧化矽與氮化矽中摻雜矽量子點進行全光放大/調變/波長轉換研究

Abstract

本研究中，我們成功利用矽量子點鑲嵌於氧化矽/氮化矽光波導結構達成全光光放大/調變/波長轉換之目的。 在第一部分，我們利用有限位勢法模擬了矽量子點能隙與耦合放光效率隨矽量子點尺寸大小的變化。並且透過光激螢光光譜與時析螢光光譜分析法，區分出各尺寸大小之矽量子點鑲嵌於氧化矽薄膜內的放光機制，其中包含了中性氧缺陷(455 nm)、E’d缺陷(520 nm)以及矽量子點內因量子侷限效應之放光。透過外部注入一調變訊號光至矽量子點光波導放大器內(SiO1.24:Si-QD/SiO1.42:Si-QD)，我們觀察到在波長785 nm以及650 nm的情況下，其小訊號增益分別為14.7 dB/6.5 dB。藉由將功率相依之增益係數導入光波導放大器增益之模擬，我們可以成功模擬出矽量子點光波導放大器內增益飽和的現象，並估算出矽量子點之最大增益以及飽和功率。 在第二部分，由於矽量子點在紅外光波長具有極強的自由載子吸收特性，因此我們進行全光自由載子吸收調變器的研究。我們發現當量子點尺寸由4.3 nm縮小至1.7 nm時，其自由載子吸收面積會由2.8*10-17 cm2 縮減至8*10-18 cm2，雖然自由載子吸收面積會隨著矽量子點尺寸縮減而劣化，但是我們發現自由載子活期將由~10us 增快至480 ns，這是由於在動量空間中，量子侷限效應使得電子電洞波函數之耦合率會隨量子點尺寸縮小而增加。接著，我們成功利用矽量子點建構出全光自由載子吸收調變器，並且呈現了全光反向歸零訊號格式的調變，也發現當尺寸由4.3 nm縮減至1.7 nm，其最佳的調變位元率可由100kbit/s增快至2Mbit/s。 在最後一部分的研究中，我們探討了矽量子點鑲嵌於氮化矽薄膜內之非線性光學特性以及其應用。透過Z-scan量測法，我們得知當薄膜內富矽含量由16.3%增加至23.4%時，富矽氮化矽於800 nm波長下之非線性折射係數由5.7*10-13增大至 9.2*10-12 cm2/W。透過矽量子點內極強的量子侷限效應，摻雜矽量子點之氮化矽薄膜內的非線性折射率可被增強一至兩個數量級(相較於結晶矽與標準氮化矽)。隨後，我們利用矽量子點鑲嵌於氮化矽薄膜製備光波導結構，並且首次觀測到鑲嵌矽量子點的氮化矽內非線性四波混頻現象。在長度為8 mm的氮化矽波導內，其最佳波長轉換效率、3-dB轉換頻寬分別為-46 dB、18 nm；透過色散模擬，我們發現實驗觀測的轉換效率以及轉換頻寬已十分接近數值模擬結果。最後，我們利用了鑲嵌矽量子點的氮化矽薄膜製備環形共振腔結構，透過非線性克爾效應與波長設定，在環形共振腔內可分別達成全光正向及反向訊號的快速調變。在激發光尖峰功率為3 W的情況下，當提高薄膜內富矽含量由16.3% (R 0.5)增加至23.4% (R 0.9)時，我們觀測到克爾效應導致的非線性折射率增量由2*10-5 增加至 1.6*10-4。這意味著與非線性折射相關的克爾係數隨著富矽含量增加由1.4*10-14 變大至1.6*10-13 cm2/W。最終，我們成功利用了矽量子點鑲嵌於氮化矽環形共振腔結構展示了全光非歸零訊號之正向與反向格式轉換器，其調變位元率至少可達12 Gbit/s。 關鍵字：矽量子點、光波導、量子侷限效應、增強自發輻射、自由載子吸收、四波混頻、非線性克爾效應。

All-optical amplification, modulation, and wavelength conversion have been demonstrated by using the Si-QD doped in SiOx/SiNx waveguide. In the first part, the bandgap energy and radiative recombination rate of the Si-QD are theoretically simulated by the finite potential well approximation. The luminescent mechanisms, including NOV, E'd defect and Si-QDs related spontaneous emission, in SiOx:Si-QD are clearly distinguished by combining the PL with TRPL analysis. The small-signal gain of the SiOx:Si-QD waveguide is demonstrated by injecting the modulated probe signal into the optically pumped Si-QDs. The small-signal gain of 14.7 dB/6.5 dB at 785-/650-nm are measured in the SiO1.24:Si-QD/SiO1.42:Si-QD waveguide amplifiers. By considering the power-dependent gain coefficient in the Si-QD based waveguide amplifier, the peak power gain and saturation power in the Si-QD based waveguide amplifiers are determined. In the second part, the free-carrier absorption in the Si-QD is utilized to demonstrate the all-optical FCA modulator. The FCA cross-section at 1550 nm is reduced from 2.8*10-17 cm2 to 8*10-18 cm2 with shrinking Si-QD size from 4.3 nm to 1.7 nm. Although the FCA cross-section is degraded when shrinking the Si-QD size, the free-carrier relaxation lifetime in SiOx:Si-QD waveguide is shortened from ~10 us to 0.48 us due to the increased momentum overlapping factor of electron-hole wave-functions in Si-QDs. The achievable bit rate of the inverted data modulation with RZ-OOK data format at 1550 nm has been significantly increased from 100 kbit/s to 2 Mbit/s by shrinking Si-QD size from 4.3 nm to 1.7 nm. In the rest part, the nonlinearity of the Si-QD doped in SiNx matrix and corresponding applications are discussed. Based on the Z-scan measurement, the nonlinear refractive index at 800 nm for Si-rich SiNx film is increased from 5.7*10-13 to 9.2*10-12 cm2/W when increasing the excessive Si concentration from 16.3% to 23.4%. Such optical nonlinearity enhancement in the Si-rich SiNx film can be attributed to the strong quantum confinement effect in the Si-QD doped in the SiNx matrix. Then, the FWM has been firstly demonstrated by using the SiNx:Si-QD channel waveguide. The maximum conversion efficiency of -46 dB with the 3-dB bandwidth of 18 nm has been measured in the 8-mm long Si-rich SiNx channel waveguide. Furthermore, the all-optical data modulation in the SiNx based ring resonator has been achieved by using the nonlinear Kerr effect of the Si-QD. The refractive index change induced by the nonlinear Kerr effect is increased from 2*10-5 to 1.6*10-4 for SiNx R0.5 to R0.9 ring resonator under the pump pulse excitation with peak power of 3W. It indicates that the nonlinear refractive index at ~1550 nm are increased from 1.4*10-14 to 1.6*10-13 cm2/W by increasing the excessive Si concentration in SiNx films from 16.3% (R0.5) to 23.4% (R0.9). Finally, the 12 Gbit/s all-optical data conversion and inversion with NRZ-OOK data format has been firstly demonstrated by nonlinear Kerr effect in the SiNx based ring resonator.