Mid/far-infrared few-cycle-pulse emission via resonant mixing in semiconductor heterostructures

Abstract

Mid/far-infrared emission from a semiconductor multiple quantum well structure under femtosecond optical pulse excitation is studied. It is shown that resonant nonlinear-wave mixing in the quantum wells can be used for the generation of ultra-short mid/far-infrared pulses with a duration of a few cycles or even a single cycle. Explicit analytical formulas for the mid/far-infrared radiation field and polarization in a simple three-level model of a quantum well are presented and compared with numerical simulations. The power of the mid/far-infrared emission and the down conversion efficiency of the resonant nonlinear-wave mixing are discussed.     

Few-cycle terahertz generation and spectroscopy of nanostructures

We report on new schemes for terahertz (THz) generation. The THz efficiency of photoconducting antennas can be increased by using a cavity effect for the near-infrared pump beam. The cavity is formed by a molecular beam epitaxy grown semiconductor Bragg mirror below the photoconducting layer. The optical confinement is accompanied by an electrical confinement suppressing undesired leakage currents and providing a constant electric field in the active layers. The performance of this cavity-enhanced emitter is further improved by using a mobility optimized low-temperature GaAs layer. This emitter is successfully used in a femtosecond Ti:sapphire laser cavity for highly efficient intracavity THz generation, where the photoconductive layer serves also as a saturable absorber. The broadband THz pulses generated are used for time-resolved spectroscopy of nanostructures. We study the dynamics of intersubband transitions in semiconductor quantum wells. The relaxation of carriers excited by a near-infrared pump pulse is investigated by measuring the THz absorption between the different subbands with our THz pulses. For transition energies below the optical phonon energy we find relatively long relaxation times with a strong dependence on the excited carrier density.     

Broadly tunable femtosecond pulse generation in the near and mid-infrared

High-repetition-rate (80-MHz) femtosecond infrared pulses are generated by difference frequency mixing (DFM) a femtosecond Ti:sapphire laser with a phase-locked synchronized cw mode-locked Nd:YAG picosecond laser. This DFM scheme is of particular interest for generating ultrashort near-IR pulses (~10 fs) because group velocity mismatch with a pump pulse can be ignored. The simplicity and the broad wavelength tunability (from the near IR to the mid-IR) of this scheme is demonstrated. Short (125-fs FWHM) optical pulses in the near IR around 1.5 mum are obtained with noncritical type-I phase-matched LiB(3) O(5). We also used a similar scheme to generate mid-infrared pulses at 3.0 mum with type-II phase-matched KTiOPO(4).     

Femtosecond response of a free-standing LT-GaAs photoconductive switch

We present a novel, free-standing low-temperature GaAs (LT-GaAs) photoconductive switch and demonstrate its femtosecond performance. A 1-microm-thick layer of a single-crystal LT-GaAs was patterned into 5-10-microm-wide and 15-30-microm-long bars, separated from their GaAs substrate and, subsequently, placed across gold coplanar transmission lines deposited on a Si substrate, forming a photoconductive switch. The switch was excited with 110-fs-wide optical pulses, and its photoresponse was measured with an electro-optic sampling system. Using 810-nm optical radiation, we recorded an electrical transient as short as 360 fs (1.25 THz, 3-dB bandwidth) and established that the photo-carrier lifetime in our LT-GaAs was 150 fs. Our free-standing devices exhibited quantum efficiency of the order of approximately 7%, and their photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the excitation power, below a well-defined saturation threshold.     

Femtosecond optical parametric generation of noncollinear phase matching for a biaxial crystal

In an optical parametric generation of a femtosecond pulse for a biaxial crystal, the interaction of three waves can be used as a model of noncollinear phase matching in which the group velocities of the interacting pulses are suitably linked to each other. For satisfaction of group-velocity matching, the tunable parametric generation of femtosecond pulses must use noncollinear phase matching. We consider three conditions of group-velocity matching for femtosecond pulses. Signal and idler pulses can be obtained when the coupled-wave equations, including the group-velocity mismatch and group-velocity dispersion effects, are solved. A Fourier method is an effective method for solving the equations, and from the solution of the equations the relation between duration of output pulses and wavelengths can be obtained. In a comparison of collinear and noncollinear matches, when the latter is group-velocity matched, the duration of its outpulses are smaller, and the outpulses can be continually tuned from the visible to the mid-infrared.     

硅太赫兹波导配置及性能分析

针对现有太赫兹辐射源在输出频率可调性及输出功率方面的局限性。本文从非线性介质的Maxwell方程入手对非线性聚合物的光学性质进行理论分析,建立硅波导配置的数学模型。利用硅材料的高折射率,设计了硅太赫兹波导。分析了太赫兹波导模式图,讨论了硅太赫兹波导模式的有效折射率、波导损耗、连续波输出功率与输出频率的关系,实验结果表明:硅太赫兹波导产生的太赫兹波连续可调,输出太赫兹波频率范围宽至0.1THz到15THz,输出功率可达到微瓦级。     

THz generation from plasmonic nanoparticle arrays

We investigate the generation of THz pulses when arrays of silver nanoparticles are irradiated by femtosecond laser pulses, providing the first reproducible experimental evidence in support of recent theoretical predictions of such an effect. We assess our results in the context of a model where photoelectrons are produced by plasmon-mediated multiphoton excitation, and THz radiation is generated via the acceleration of the ejected electrons by ponderomotive forces arising from the inhomogeneous plasmon field. By exploring the dependence of the THz emission on the femtosecond pulse intensity and as a function of metal nanoparticle morphology, and by comparing measurements to numerical modeling, we are able to verify the role of the particle plasmon mode in this process.     

Phase, amplitude, and polarization shaping with a pulse shaper in a Mach-Zehnder interferometer

We present a shaper scheme that fully controls the spectral phase, amplitude, and polarization of femtosecond laser pulses. In particular, it enables independent manipulation over the major axis orientation and the axis ratio of the polarization ellipse. This is accomplished by integrating a 4f-shaper setup in both arms of a Mach-Zehnder interferometer and rotating the polarization by 90 degrees in one of the arms before overlaying the beams. The generated pulses are resolved in a simple and intuitive detection scheme.     

Terahertz Radiation from YBCO Thin Film Log-Periodic Antennas

We study terahertz (THz) radiation properties from YBa₂Cu₃O^7– (YBCO) thin film log-periodic antennas. Three sets of wire grid polarizers are employed to characterize the frequency-dependent polarization of the THz beam excited with a femtosecond laser. The polarized radiation has clear resonance at frequencies corresponding to the antenna structures. InSb hot-electron bolometer is used to estimate the radiation power after being calibrated in the THz beam generation and detection system. The radiation power increases quadratically with increasing bias current and reaches over 20 W in the average at an excitation power of 98 m W.     

Wave packet engineering using a phase-programmable femtosecond optical source

Abstract

A new optical telecommunication method combining time and frequency domain multiplexing is proposed using phase-controlled femtosecond pulses. Each pulse in a pulse train can be used as a data packet with data bits in the frequency domain. We call the new principle ‘wave packet engineering’, which adjusts the amplitude and phase of the wave function in device materials arbitrarily by controlling the spectral phase of femtosecond pulses. The optical phase-to-amplitude converter is demonstrated with organic dye molecules, in which the phase information in the phase-modulated pulses can be demodulated into the luminescence intensity. The luminescence intensity from cyanine dye molecules is observed to be chirp dependent, and is explained quantum mechanically in terms of coherent population transfer. The design principle of the device using semiconductor coupled quantum nanostructures is also discussed in terms of wave packet engineering.     

Submillimeter planar gyrotrons with transverse diffraction output of radiation

In order to increase the integral output power of short-wave gyrotrons, it is suggested to use a planar scheme with the transverse (relative to the direction of electron translation) diffraction output of radiation. An advantage of the planar design in comparison to the traditional cylindrical gyrotron geometry is the possibility to ensure the coherence of radiation at a greater oversized factor by using a diffraction mechanism of mode selection with respect to the transverse coordinate. The results of simulation of the nonlinear dynamics of a planar gyrotron with a polyhelical ribbon electron beam show that it is possible to reach an output power of several hundred kilowatt at frequencies up to 1 THz. An additional advantage of the proposed scheme is the possibility of frequency tuning by changing the distance between plates.     

Far infrared radiation generated during femtosecond laser excitation of a semiconductor surface in magnetic field

The effect of a magnetic field on the generation of electromagnetic radiation pulses in the terahertz frequency range from a semiconductor surface excited by an ultrashort laser pulse is considered within the framework of a hydrodynamic model. The appearance of a photocurrent component in the Hall direction leads to elliptic polarization of the microwave radiation and to a severalfold increase in the generation efficiency. This is consistent with the results of Monte Carlo modeling of a self-consistent field and photogenerated carrier dynamics.     

Evaluation of quantum-cascade lasers as local oscillators for infrared heterodyne spectroscopy

We report experiments evaluating the feasibility of quantum-cascade lasers (QCLs) at mid-infrared wavelengths for use as local oscillators (LOs) in a heterodyne receiver. Performance tests with continuous-wave (cw) lasers around 9.6 and 9.2 microm were carried out investigating optical output power, laser linewidth, and tunability. A direct comparison with a CO2 gas laser LO is presented as well. The achieved system sensitivity in a heterodyne spectrometer of only a factor of 2 above the quantum limit together with the measured linewidth of less than 1.5 MHz shows that QCLs are suitable laser sources for heterodyne spectroscopy with sufficient output power to replace gas lasers as LOs even in high-sensitivity astronomical heterodyne receivers. In addition, our experiments show that the tunability of the lasers can be greatly enhanced by use of an external cavity.     

Deterministic generation of singlet state of N atoms in coupled cavities via adiabatic passage of a dark state

In this paper, we propose an efficient scheme to generate the N-atom singlet state via adiabatic passage of a dark state in cavity quantum electrodynamics (QED) system. Appropriate Rabi frequencies of the classical fields are selected to realize the present scheme. We discuss the influence of decoherence induced by cavity decay and atomic spontaneous emission by numerical calculation. The result shows that the scheme is insensitive to atomic spontaneous emission as the atomic excited state is hardly populated in adiabatic evolution.     

Near-room-temperature mid-infrared quantum well photodetector

We demonstrate InGaAs mid-infrared quantum well infrared photodetectors (MIR PV-QWIPs) that enable cost-effective mature GaAs-based detection and imaging technologies, with exceptional material uniformity, reproducibility, and yield, over a large area, with high spectral selectivity, innate polarization sensitivity, radiation hardness, high detectivity, and high speed operation at TEC temperatures without bias.     

Prelase stabilization of the polarization state and frequency of a Q-switched, diode-pumped, Nd:YAG laser

We present an experimental investigation of the energy statistics of the linear polarization components of pulses from a Nd:YAG laser that is repetitively Q-switched with an acousto-optic modulator. Varying the modulator-induced diffraction losses leads to changes in the pulse polarization state and the energy statistics of the polarization components. For conventional Q-switching there is no laser oscillation during the low-Q intervals, and we find that the orthogonal components of the pulses can display large relative energy fluctuations even though the total pulse energy is quite stable. In the prelase mode, a weak continuous-wave background seeds the Q-switched pulses and results in the emission of highly linearly polarized, single-longitudinal-mode pulses with small relative energy fluctuations.     

Integrated planar terahertz resonators for femtomolar sensitivity label-free detection of DNA hybridization

A promising label-free approach for the analysis of genetic material by means of detecting the hybridization of polynucleotides with electromagnetic waves at terahertz (THz) frequencies is presented. Using an integrated waveguide approach, incorporating resonant THz structures as sample carriers and transducers for the analysis of the DNA molecules, we achieve a sensitivity down to femtomolar levels. The approach is demonstrated with time-domain ultrafast techniques based on femtosecond laser pulses for generating and electro-optically detecting broadband THz signals, although the principle can certainly be transferred to other THz technologies.     

Effect of intensity dependent higher-order dispersion on femtosecond pulse propagation in quantum well waveguides

We have studied theoretically the dispersion of ultrafast coherent pulses in GaAs/AlGaAs quantum well waveguide structures as a function of optical intensity. Semiconductor Bloch equations are used to obtain the polarization induced in the medium due to an incident Gaussian electromagnetic beam. The partial differential equation describing the pulse propagation in the presence of group velocity dispersion is used to analyze the role of higher-order dispersion on femtosecond pulse propagation in the waveguide. Due consideration has been given to the intensity dependent optical susceptibility of the medium. The results of the numerical analysis manifest significant influence of higher-order dispersion on femtosecond pulse propagation over short waveguide distance.     

Investigation of polarization state of terahertz radiation from compact laser-induced plasma in air

A comprehensive theoretical study is presented on the polarization state of THz wave radiation from a two-colour laser-induced plasma, and its dependency on optical pulse polarizations and phase difference. We extend the so-called photocurrent model to calculate three-dimensional distribution of photocurrent from which the polarization state of the THz radiation is calculated. It is shown that the calculation results from the photocurrent model, is in agreement with the previously reported experiments. Moreover, we show the possibility of creating any desired polarization state (i.e. elliptical, circular or linear) for the THz radiation through such generation mechanism by adjusting proper parameters.     

Nondestructive defect identification with terahertz time-of-flight tomography

We demonstrate the application of terahertz (THz) time-of-flight tomographic imaging to identify the distribution of defects in foam materials. Based on THz time-domain spectroscopy technology, THz imaging probes targets with picosecond pulses of broad-band radiation in the frequency range from 100 GHz to 3 THz. The reflected THz wave from the target is measured using electrooptic sampling, which provides two-dimensional images with phase and amplitude information, as well as the spectroscopic properties of the object. The depth information is recorded in the THz time-domain waveform. Several reconstruction models are developed for tomographic imaging of defects inside foam. Foam insulation of space shuttle fuel tanks, with prebuilt defects, are investigated with THz tomographic imaging. Most prebuilt defects are pinpointed and models used to identify different kinds of defects are discussed.