High power room-temperature design of terahertz quantum cascade laser based on difference frequency generation

Terahertz (THz) quantum cascade lasers (QCLs) are key elements for high-power terahertz beam generation for integrated applications. In this study, we design a highly nonlinear THz-QCL active region in order to increase the output power of the device especially at lower THz frequencies based on difference frequency generation (DFG) process. It has been shown that the output power increases for a 3.2 THz structure up to 1.2 μW at room temperature in comparison with the reported power of P = 0.3 μW in [1]. The mid-IR wavelengths associated with this laser are λ₁ = 12.12 μm and λ₂ = 13.93 μm, which are mixed in a medium with high second-order nonlinearity. A similar approach has been used to design an active region with THz frequency of 1.8 THz. The output power of this structure reaches to 1 μW at room temperature where the mid-IR wavelengths are λ₁ = 12.05 μm, λ₂ = 12.99 μm.     

Enhancement of terahertz wave difference frequency generation based on a compact walk-off compensated KTP OPO

A compact, walk-off compensated dual-wavelength KTP OPO near the degenerate point of 2.128 μm pumped by a Nd:YAG pulsed laser is employed as the pump for terahertz (THz) source based on difference frequency generation (DFG) in a GaSe crystal. Coherent THz radiation that is continuously tunable in the range of 81-1617 μm (0.186-3.7 THz) is achieved. An enhancement of 76.7% in average for the THz energies at different wavelengths is realized using the walk-off compensated KTP OPO than the common one. Using a 8 mm-long GaSe crystal, the maximum output THz pulse energy is 48.9 nJ with the peak power of 11 W, corresponding to the energy conversion efficiency of 5.4 × 10^− 6 and the photon conversion efficiency of about 0.09%.     

Tunable terahertz generation from one CO2 laser in a GaSe crystal

Coherent terahertz pulses have been generated at a range of 236.3-1104.5 μm (0.27-1.3 THz) by one CO₂ laser with dual-wavelength output based on collinearly phase-matched different frequency generation (DFG) in a GaSe crystal. This source has the advantages of compact and simplicity for tuning. The output power of the THz pulse and phase-matching conditions were investigated. The maximum single pulse energy of 11 nJ was generated at a frequency of 1.23 THz (243.6 μm), corresponding to a peak output power 182 mW.     

Narrow-band terahertz wave generation and detection in one periodically poled lithium niobate crystal

In this article, we present studies on therahertz (THz) wave generation and frequency up-conversion in a periodically poled lithium niobate (PPLN) crystal. A frequency at 1.37 THz was generated as femtosecond pump pulses passed through a PPLN crystal with grating periods of 30 μm. The pump-induced THz wave interacts with the probe wave in the crystal by frequency mixing. The frequency up-converted THz wave is easily detected by a normal photodiode. A new scheme for generation and detection of THz wave in one non-linear crystal was proposed.     

Infrared and far-infrared spectroscopy of CH3OH: TeraHertz laser lines and assignments

We use a ¹³CO₂ laser as optical pumping source to search for new THz laser lines generated from ¹³CH₃OH. Nineteen new THz laser lines (also identified as far-infrared, FIR) ranging from 42.3 μm (7.1 THz) to 717.7 μm (0.42 THz) are reported. They are characterized in wavelength, offset, relative polarization, relative intensity, and optimum working pressure. We have assigned eight laser lines to specific rotational energy levels in the excited state associated with the C-O stretching mode.     

Silicon-on-insulator microcavity light emitting diodes with two Si/SiO2 Bragg reflectors

Light emitting pn-diodes were fabricated on a 5.8 μm thick n-type Si device layer of a silicon-on-insulator (SOI) wafer using standard silicon technology and boron implantation. The thickness of the Si device layer was reduced to 1.3 μm, corresponding to a 4λ-cavity for λ=1150 nm light. Electroluminescence spectra of these low Q-factor microcavities are presented. Addition of Si/SiO₂ Bragg reflectors on the top and bottom of the device (3.5 and 5.5 pairs, respectively) is predicted to lead to spectral emission enhancement by ∼270.     

Use of plasma polymerisation process for fabrication of bio-MEMS for micro-fluidic devices

Using a plasma polymerisation process with optical lithography, wet and dry etching techniques we have fabricated an organic micro-fluidic device (OMDF) on silicon/glass substrate. An asymmetric electrode array used in micro-fluidic device (MFD) with small electrode (4 μm wide) separated from the large electrode (20 μm wide) by 20 μm and 6 μm gaps in both sides respectively. In this study we have found that plasma polymerisation process is not only important for changing the surface chemical and physical properties but also has advantage in bonding of these micro devices at low temperature (∼100 °C) due to low T_g of polymeric material. The fluidic velocity measurement shows a maximum of about 450 μm/s in a 150 μm channel width of organic micro-fluidic devices after plasma surface modification.     

Generation of widely tunable Fourier-transform-limited terahertz pulses using narrowband near-infrared laser radiation

Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δν < 1 MHz, λ ∼ 800 nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10 MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS.     

Generation of high average power 1 kHz shaped THz pulses via optical rectification

Generation of near single-cycle THz pulses from lithium niobate with 3.3 μJ energy, 3.3 mW average power, 1.2 THz central frequency and 4 MW peak power was demonstrated by tilting the intensity front of the pump pulses from a 1 kHz Ti:sapphire laser. THz pulse intensity as high as 200 MW/cm² was achieved. The energy conversion efficiency was 7 × 10⁻⁴. The capability of the present scheme to generate high energy shaped THz pulses was also demonstrated by using a sequence of optical pump pulses.     

Growth of thin Fe(0 0 1) films for terahertz emission experiments

The electrical and magnetic properties of thin iron (Fe) films have sparked significant scientific interest. Our interest, however, is in the fundamental interactions between light and matter. We have discovered a novel application for thin Fe films. These films are sources of terahertz (THz) radiation when stimulated by an incident laser pulse. After intense femtosecond pulse excitation by a Ti:sapphire laser, these films emit picosecond, broadband THz frequencies. The terahertz emission provides a direct measure of the induced ultrafast change in magnetization within the Fe film. The THz generation experiments and the growth of appropriate thin Fe films for these experiments are discussed. Several criteria are used to select the substrate and film growth conditions, including that the substrate must permit the epitaxial growth of a continuous, monocrystalline or single crystal film, yet must also be transparent to the emitted THz radiation. An Fe(0 0 1) film grown on the (0 0 1) surface of a magnesium oxide (MgO) substrate makes an ideal sample. The Fe films are grown by physical vapor deposition (PVD) in an ultrahigh vacuum (UHV) system. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) are used to characterize the Fe(0 0 1) films. Two substrate surface preparation methods are investigated. Fe(0 0 1) films grown on MgO(0 0 1) substrates that are used as-received and films grown on MgO(0 0 1) substrates that have been UV/ozone-cleaned ex vacuo and annealed in vacuo produce the same results in the THz generation experiments. Either substrate preparation method permits the growth of samples suitable for the THz emission experiments.     

Phase-matched third-harmonic UV generation using low-order modes in a glass micro-fiber

We observe phase-matched third-harmonic generation at 355 nm in a low-order mode of a sub-micron diameter glass fiber. The third-harmonic signal exhibits a sharp resonance for a fiber diameter d = 0.49 ± 0.02 μm, in excellent agreement with the value d = 0.506 μm predicted by theory. The third-harmonic conversion efficiency is 2 × 10⁻⁶, and is limited by the pump power (1 kW) and effective fiber length (100 μm).     

Temperature phase-matching properties of mixed chalcopyrite AgGa1−xInxS2 crystal

AgGa_1−xIn_xS₂ with x = 0.14 ± 0.01 was found to be 90° phase-matchable for the second harmonic generation (SHG) of CO₂ laser radiation at 10.591 μm at 203 °C. In addition, temperature-tuned 90° phase-matched difference frequency generation (DFG) at 4.02 μm was demonstrated by mixing the idler output of a Nd:YAG third harmonic pumped β-BBO optical parametric oscillator and its fundamental source at 1.0642 μm. The Sellmeier and thermo-optic dispersion formulas that reproduce well these experimental data are presented.     

Dynamic optical arbitrary waveform generation and detection in InP photonic integrated circuits for Tb/s optical communications

This article presents recent results in the development of optical arbitrary waveform generation (OAWG) technologies based on optical frequency combs and indium phosphide devices. A novel spectral-slice dynamic-OAWG approach and waveform shapers with customized spectral multiplexers and modulators, enable continuous generation of high fidelity optical waveforms accessing bandwidths in excess of 1 THz. We show results for two types integrated waveform shapers, a 100 GHz electrically controlled device with 10 channels spaced at 10 GHz and a 1 THz optically controlled device with 100 channels spaced at 10 GHz. Additionally, we include results from a 640 GHz waveform measurement device with 16 channels and 40 GHz spacing.     

Tunable dual-wavelength terahertz wave power splitter

We demonstrate, for the first time, a tunable dual-wavelength terahertz wave power splitter based on the multimode interference effect and self-imaging principle in 1 × 2 × 2 photonic crystal waveguides. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The simulation results demonstrate that the power splitter not only split the input power into output1 and output2 branches with equal power at frequency of 1.09 THz, but also split the power into output3 and output4 branches symmetrically at frequency of 1.20 THz. Furthermore, for the frequency of 1.09 THz, the input terahertz wave power can be split into output1 and output2 branches with an arbitrary ratio by tuning the refractive index of the tuning rods.     

Tolerance and tuning properties of the optical parametric processes using periodically poled RbTiOAsO4

The maximal tolerance parameters of poling period and phase-matching, temperature in second harmonic generation (SHG) using periodically poled RbTiOAsO₄(PPRTA) as a function of the fundamental wavelength are investigated theoretically. The tolerance of the poling period ΔΛ of PPRTA is found larger than that of PPLN and PPKTP when the fundamental wavelength is beyond 2 μm, which reaches its maximum ΔΛ_max for PPRTA at a fundamental wavelength of 2.7433 μm. However, the tolerance for the phase-matching temperature ΔT of PPRTA is found smaller than that of PPLN and PPKTP with an exception that PPRTA has a larger tolerance of the temperature or a larger temperature phase-matching bandwidth at fundamental wavelength of 2.2474 μm, where the maximum of ΔT(ΔT_max) is obtained. Furthermore, the tuning characteristics of the optical parametric processes using PPRTA for collinear quasi-phase-matching (CQPM) is analyzed. The combination of temperature tuning and poling period tuning enables a quasi-continuous wavelength tuning range of 1493.2-1593.7 nm for the signal and 3201.8-3699.2 nm for the idler, where poling period of 39 μm, 39.5 μm and 40 μm and a temperature between 20 and 120° have been employed in the corresponding theoretical analysis.     

Silicon donor and Stokes terahertz lasers

Two types of lasers based on hydrogen-like impurity-related transitions in bulk silicon operate at frequencies between 1 and 7 THz (wavelength range of 50-230 μm). These lasers operate under mid-infrared optical pumping of n-doped silicon crystals at low temperatures (<30 K). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized in the first type of terahertz silicon lasers. These lasers have a gain ∼1-3 cm⁻¹ above the laser thresholds (>1 kW cm⁻²) and provide 10 ps-1 μs pulses with a few mW output power on discrete lines. Raman-type Stokes stimulated emission in the range 4.6-5.8 THz has been observed from silicon crystals doped by antimony and phosphorus donors when optically excited by radiation from a tunable infrared free electron laser. The scattering occurs on the 1s(E)→1s(A₁) donor electronic transition accompanied by an emission of the intervalley transverse acoustic g-phonon. The Stokes lasing has a peak power of a few tenths of a mW and a pulse width of a few ns. The Raman optical gain is about 7.4 cm GW⁻¹ and the optical threshold intensity is ∼100 kW cm⁻².     

Theoretical study of the electro-optic effect of aperiodically poled lithium niobate in a Q-switched dual-wavelength laser

The electro-optic effect of aperiodically poled lithium niobate (APLN) has been theoretically investigated and proposed to use as a Q-switch in a simultaneous dual-wavelength laser. Our analysis shows that the polarization planes of the z-polarized (or y-polarized) dual-wavelength beams can be simultaneously rotated by 90° through a well-constructed APLN with an external electric field applied along the y-axis, which enables Q-switch function in a dual-wavelength laser cavity. Using a Nd:YVO₄ laser operating at 1.0643 μm and 1.3419 μm as an example, we present a design method of APLN by using the so-called simulated annealing algorithm. The influence of the domain errors in fabricating an APLN device is also studied. The results show that the device is not susceptible to the fabrication errors.     

Modified grating-based external cavity diode laser for simultaneous dual-wavelengths operation

We have reported a modified V-shaped external cavity, which is constructed around a commercial diode laser operating at a center wavelength of λ=785 nm by adding a new coated glass plate with about 50% reflectivity to the cavity. This allows simultaneous dual-wavelengths operation in the vicinity of Δν_min=0.18 THz to Δν_max=0.22 THz, which can be used as laser source for terahertz generation either for semiconductor devices or nonlinear schemes.     

Multi-band terahertz two-handed metamaterial based on the combined ring and cross pairs

We present the design of a Multiband two-handed metamaterial (MM) with the composite structure combined ring and cross pairs (RP + CP) in the terahertz regime. The dual-band left-handed and single-band right-handed transmission properties of this composite structure were explored by the FDTD method. The MM exhibits left-handed and right-handed transmission pass bands around the frequencies of 0.43 THz, 1.32 THz and 0.85 THz for the terahertz electromagnetic (EM) wave normal incidence, respectively. The surface currents distributions are demonstrated to discuss the physical mechanism of the left-handed properties of the proposed MM. The retrieved equivalent EM parameters and the refraction phenomenon based on a wedge-shaped model verify the left-handed properties. Furthermore, the dual-band left-handed transmission properties of RP + CP structure can be tunable individually by changing the structural parameters. The presented design of terahertz two-handed MM offers a flexibility for investigation of their novel EM properties, and important terahertz device applications.     

Temperature dependent narrow-band terahertz pulse generation in periodically poled crystals via difference frequency generation

Femtosecond optical pulse is used to generate narrow-band terahertz pulses depending on a quasi-phase-matched condition in periodically poled lithium niobate (PPLN) and stoichiometric lithium tantalate (PPSLT) crystals by difference frequency generation. The origin of narrow-band THz generation proved that the two frequency components of the fs pulse contribute to the frequency mixing. By cryogenic cooling, the absorption of THz waves in the crystal is significantly reduced which results in efficient THz generation. Simultaneously generated forward and backward THz pulses were 1.38 and 0.65 THz with as narrow as the bandwidth of 32 GHz in the PPSLT sample. Temperature dependence of the generated THz waveforms had good agreement with the simulation result using one dimensional plane-wave propagation model.