光激發半導體產生高功率寬頻THz輻射源之研究

Abstract

本文探討光激發在不同種類的半導體表面產生THz輻射。首先在高功率光源的研究中，我們報導利用紐曲式飽和布拉格反射體當穩定被動鎖模元件並加入連續波放大器在雷射共振腔內可產生高達1.62W的輸出。此飽和布拉格反射體的暫態微分反射率由4%變成-0.6%隨著激發光波長從755nm到760nm，顯示在能隙附近有著很強的非線性效應與吸收。 雷射共振腔的端面改成一般的飽和布拉格反射體在0.88T的強磁場施加下，並以此紐曲式飽和布拉格反射體當被動鎖模元件，可產生將近45nW的THz輻射功率其中心輻射頻率與頻寬約在0.7THz。 同樣情況下以InAs塊材放在共振腔內與入射光成85度的斜角入射情形下，可產生的輻射功率與中心頻率分別約為5nW與0.4THz。 在高功率與寬頻THz輻射源的研究中，我們用三種不同的元件來分析砷離子佈值砷化鎵與半絕緣性砷化鎵的輻射特性。在1mm到0.02mm間隙的光導天線研究中，我們發現已半絕緣性砷化鎵為基版的天線隨偏壓的增加而呈現性增加到最後將近4kV/vm即因電流過大而超過儀器極限，而砷離子佈值砷化鎵卻在1mm與0.5mm的天線中發現飽和的現象，而偏壓已加到8kV/cm且振幅已與半絕緣性砷化鎵時相當或更高。兩者的THz電場強度、幅射中心與頻寬均約為0.15kV/cm、0.5THz 與 1THz，這表示只要將樣品降到適度的低溫下再加大偏壓產生更高功率的輻射是可能的。 在5mm間隙共振偶極天線的研究上，砷離子佈值砷化鎵材料可產生輻射的中心頻率為較半絕緣性砷化鎵時為高(分別為0.9THz與0.7THz，頻寬約為1.2THz)，且當施加更高的偏壓時，輻射振幅已超過半絕緣性砷化鎵。 在5mm與2mm間隙，直徑約300mm與120mm的螺旋天線研究中，間隙為2mm的半導體材料有著中心波長低到0.32THz與頻寬高達0.74THz寬頻輻射可用來作為材料在THz波段的頻譜分析。 利用非線性現象產生THz的輻射中，我們以10對扭曲式多層膜量子阱在1T的強磁場下產生在能隙附近有共振現象的THz輻射，其中心頻率約為0.7THz對應頻寬約0.9THz，亦比中心頻率僅有0.45THz的半絕緣性砷化鎵為高，表示MQW非線性效應主導THz幅射引致中心頻率往高頻移動。

In this thesis, we demonstrate the optical excited semiconductor to generate high power and broadband THz radiation. We report a new type of femtosecond amplification scheme in which a strained saturable bragg reflector (SSBR) with low saturation flurence was used for self-starting mode-locking, and an intracavity CW amplifier system was used to scale the average output power. As the amplifier rod was pumped from 0 W to 15 W, the output power was linearly increased from 1.1 W to 1.62 W. Stable mode locking was achieved at the same time. For the research of the passive mode-locker-SSBR, the magnitude of transient DR/R have large changes from 4% to -0.6% from 755nm to 760nm caused by the large optical nonlinearities of band filling, screening effect, surface field enhancement and free carrier absorption near the band edge. Intra-cavity saturable Bragg Reflector in the magnetic field of 0.88T with self-started mode-locking by Strained Saturable Bragg Reflector can generate THz-radiation with an average power~45 nW. Its center frequency and bandwidth is around 0.7 THz. Bulk InAs was placed in the cavity with 85 degree shallow incident angle for high optical reflectivity. The average power and spectrum of THz-radiation was around 5 nW and peaked around 0.4 THz with respectivity. The advantage of this scheme is the simple emitter structure and applicability to other laser media. In the study of THz emitter, we had demonstrate three types of antennas for the investigate of high power and road band THz emitter. In the mid-aperture photoconductive antenna, voltage dependence THz radiation in S.I. GaAs is linearly increased, however it is saturated in the 1mm and 0.5mm of GaAs:As+ case. Both THz field, center frequency and bandwidth are around 0.15kV/cm, 0.5THz and 1THz. This implied that it could be applied for even higher voltage for higher THz radiation. Saturation behavior is shown in the power dependence THz radiation from S.I. GaAs with gap size of 1mm and 0.5 mm. However, it is more linearly in the case of GaAs:As+. For even higher pumping power, it is possible to generate higher THz radiation amplitude. In dipole antenna, waveform compared between S.I. GaAs and multi implant GaAs:As+ show that the negative peak of GaAs:As+ is higher than the S.I. GaAs which from the fast photocurrent decay time of GaAs:As+. With spectrum blue shifted from 0.7THz to 0.9THz and band width around 1.2THz in GaAs:As+ compared to S.I. GaAs. For the broadband emitter in spiral antenna, the low frequency component is enhanced, however the higher frequency had not shown the increase behavior. In the 2um gap size spiral antenna, it present the lower center frequency of 0.32 THz and larger bandwidth of 0.74 THz shown the applicable in the broad band THz emitter. The fast carrier life time and high break down voltage of GaAs:As+ had shown the candidate for the high power and broad band THz radiation. For the nonlinearity material of the broadband emitter, we report the optically excited THz radiation from ten MBE-grown strained MQWs on an (100)-oriented semi-insulating GaAs substrate in a 1 T magnetic field. Obviously resonant absorption had shown at band edge which perform the nonlinear effect induce THz radiation. Center frequency and bandwidth are around 0.7THz and 0.9THz respectively which higher than the case of S.I. GaAs with center frequency of 0.45THz. In the thin MQW sample, the THz emission is still significant. This indicates the possibility of designing MQWs as efficient THz emitters in the future.