摘要:

The emission and detection efficiencies of photoconducting THz transmitters and receivers are found to be sensitive to the polarization of the optical gating pulse. Signal amplitudes from GaAs coplanar stripline transmitters and silicon-on-sapphire dipole receivers vary by factors of up to 4 and 3, respectively, with rotation of the exciting pulse polarization. In both cases, maximum signal is obtained when the polarization of the normally incident light is perpendicular to the edge of the metal electrodes. This polarization sensitivity, which appears to arise from differences in the spatial distribution of photoexcited carriers in the semiconductor, needs to be considered when optimizing the signal-to-noise ratio in coherent THz spectroscopy. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121278

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Second-harmonic generation (SHG) has been applied to study the influence of defects in short-period(SimGen)pstrained-layer superlattices. With a misfit stacking fault defect density of∼1010 cm−2shown by transmission electron microscopy, it is found that the SH intensity increases by about one order of magnitude from that of the defect-free samples. We propose that the inhomogeneous strain field around the fault planes in the superlattice layers is responsible for this abrupt increase of SHG. The expected symmetry and the magnitude of the nonlinear susceptibility from these stacking fault defects are shown to be in agreement with the experimental observations. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121279

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this letter we report the measurement of the field enhancement at the tip of a scanning tunneling microscope, by means of the detection of the optical rectification current. A field enhancement factor between 1000 and 2000 is obtained for highly oriented pyrolytic graphite and between 300 and 600 for gold. Field enhancement factors found are strongly dependent on the particular tip used. The magnitude of the emitted light at the field enhanced region, calculated from the measured optical voltage, could be easily detected by a simple photodiode. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121280

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Liquid crystal alignment layers are prepared using a noncontact method based on laser ablation. Nonpolarized light from a KrF excimer laser at 248 nm is exposed through a phase mask to etch gratings of period 1.1 &mgr;m onto polyimide alignment layers. Twisted nematic cells were prepared using one rubbed and one grating aligned surface, and azimuthal anchoring energies were found from measurements of the twist angles as a function of grating depth. The measured anchoring energies agree with those predicted from the minimization of elastic strain energy when the liquid crystal directors at the surface are aligned parallel to the groove of the grating. This suggests that topographical rather than epitaxial alignment is achieved. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121281

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We introduce and demonstrate a novel operating mode in near-field optical microscopy. The tip is used to simultaneously optically probe the sample and induce a highly localized strain in the area under study by pushing the tip into the sample. From knowledge of total tip-sample compression and tip geometry, we estimate the magnitude of stress, and show that localized uniaxial-like stresses in excess of 10 kbar can be achieved. We apply this method to a sample of InAlAs self-assembled quantum dots. A blueshift of quantum dot emission lines consistent with estimates of the strain is observed, as well as a quenching of the photoluminescence with strain. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121282

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We report the measurements of nonlinearity effects on analog transmission of long wavelength vertical-cavity surface-emitting lasers. The experiments involved measuring single-mode fiber-optic transmissions of microwave signals (100–1000 MHz) at 1.54 &mgr;m to characterize the dynamic range, intermodulation distortion and frequency response. Spur free dynamic ranges up to 69 dB were demonstrated. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121283

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a low-energy photonic pulse. This phenomenon is interpreted in terms of the excitonic energy diagram of the crystal, which presents a four-level pathway for the photoexcitation and emission, similar to the one observed in classical inorganic-based laser materials. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121284

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The slope efficiency and threshold current density of 1.3 &mgr;m AlGaInAs/InP lasers with AlInAs–AlGaInAs multiquantum barrier (MQB) are experimentally studied and compared with the conventional step-index separate confinement heterostructure (SCH) laser. With the MQBs at the guiding layers, the characteristic temperature can be improved as much as 10 K as compared with the conventional SCH laser. This is attributed to the suppression of electron and hole leakage currents. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121285

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Lasers and light-emitting diodes with multistage, type I InAsSb/InAsP quantum well active regions are reported. These ten stage, cascaded devices were grown by metalorganic chemical vapor deposition. The broadband light-emitting diodes produced high average powers,>2 mW(∼80 K,3.7 &mgr;m) and>0.1 mW(∼300 K,4.3 &mgr;m). A 3.8–3.9 &mgr;m laser structure operated up toT=180 K.At 80 K, peak power>100 mWand a slope efficiency of 48&percent; (4.8&percent; per stage) were observed in our gain guided lasers. The slope efficiency was strongly dependent on cavity length, and analysis of efficiency data suggests an internal differential quantum efficiency>1and a loss coefficient⩾100 cm−1.©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121286

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A model for the formation of holographic surface relief gratings in azobenzene functionalized polymers is presented. Forces leading to migration of polymer chains upon exposure to light in the absorption band of the azo chromophore are attributed to dipoles interacting with the gradient of the electric field present in the polymer material. Efficienttrans–ciscycling in the azobenzenes allows cooperative movement of the chromophores under the influence of gradient forces. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121287

出版商:AIP    

年代:1998

数据来源: AIP