Emitted Terahertz Power Research Articles

637 μ W emitted terahertz power from photoconductive antennas based on rhodium doped InGaAs

We investigate photoconductive terahertz (THz) emitters compatible with 1550 nm excitation for THz time-domain spectroscopy (TDS). The emitters are based on rhodium (Rh) doped InGaAs grown by molecular beam epitaxy. InGaAs:Rh exhibits a unique combination of ultrashort trapping time, high electron mobility, and high resistivity. THz emitters made of InGaAs:Rh feature an emitted THz power of 637 μW at 28 mW optical power and 60 kV/cm electrical bias field. In particular for a fiber coupled photoconductive emitter, this is an outstanding result. When these emitters are combined with InGaAs:Rh based receivers in a THz TDS system, 6.5 THz bandwidth and a record peak dynamic range of 111 dB can be achieved for a measurement time of 120 s.

Efficient nearly-circularly-polarized terahertz generation from an air plasma pumped by collinear and circularly polarized two-color laser fields

Efficient circularly polarized terahertz (THz) emission is important for THz imaging technology and basic scientific research. In order to achieve this, we have made a simple, all-passive design to achieve collinear and corotating circularly polarized two-color laser fields. The collinearity allows us to manipulate accurately the relative phase between two-color lasers. For a given relative phase, our experiments show that efficient nearly-circularly-polarized THz emission can be generated from an air plasma pumped by the two-color fields. The emitted THz power is \ensuremath{\sim}2.5 times that pumped by linear-polarized two-color laser fields. Our results also indicate that nearly circularly polarization is stable against different pump power and plasma distance.

ErAs:In(Al)GaAs photoconductor-based time domain system with 4.5 THz single shot bandwidth and emitted terahertz power of 164 µW.

Superlattice structures of In(Al)GaAs with localized ErAs trap centers feature excellent material properties for terahertz (THz) generation and detection. The carrier lifetime of these materials as emitter and receiver has been measured as 1.76 ps and 0.39 ps, respectively. Packaged photoconductors driven by a 1550 nm, 90 fs commercial Toptica "TeraFlash pro" system feature a 4.5 THz single shot bandwidth with more than 60 dB dynamic range. The emitted THz power of the ErAs:In(Al)GaAs emitter versus laser power has been recorded with a pyroelectric detector calibrated by the Physikalisch Technische Bundesanstalt (PTB). The maximum power was 164 µW at a laser power of 42 mW and a bias of 200 V.

Experimental Comparison of UTC- and PIN-Photodiodes for Continuous-Wave Terahertz Generation

We carried out an experimental comparison study of the two most established optoelectronic emitters for continuous-wave (cw) terahertz generation: a uni-traveling-carrier photodiode (UTC-PD) and a pin-photodiode (PIN-PD). Both diodes are commercially available and feature a similar package (fiber-pigtailed housings with a hyper-hemispherical silicon lens). We measured the terahertz output as a function of optical illumination power and bias voltage from 50 GHz up to 1 THz, using a precisely calibrated terahertz power detector. We found that both emitters were comparable in their spectral power under the operating conditions specified by the manufacturers. While the PIN-PD turned out to be more robust against varying operating parameters, the UTC-PD showed no saturation of the emitted terahertz power even for 50 mW optical input power. In addition, we compared the terahertz transmission and infrared (IR) blocking ratio of four different filter materials. These filters are a prerequisite for correct measurements of the absolute terahertz power with thermal detectors.

Silicon Field Effect Transistor as the Nonlinear Detector for Terahertz Autocorellators.

We demonstrate that the rectifying field effect transistor, biased to the subthreshold regime, in a large signal regime exhibits a super-linear response to the incident terahertz (THz) power. This phenomenon can be exploited in a variety of experiments which exploit a nonlinear response, such as nonlinear autocorrelation measurements, for direct assessment of intrinsic response time using a pump-probe configuration or for indirect calibration of the oscillating voltage amplitude, which is delivered to the device. For these purposes, we employ a broadband bow-tie antenna coupled Si CMOS field-effect-transistor-based THz detector (TeraFET) in a nonlinear autocorrelation experiment performed with picoseconds-scale pulsed THz radiation. We have found that, in a wide range of gate bias (above the threshold voltage mV), the detected signal follows linearly to the emitted THz power. For gate bias below the threshold voltage (at 350 mV and below), the detected signal increases in a super-linear manner. A combination of these response regimes allows for performing nonlinear autocorrelation measurements with a single device and avoiding cryogenic cooling.

Terahertz excitation spectra of InP single crystals

Investigation of terahertz (THz) pulse generation from semi-insulating and n-type InP crystals surfaces is presented in this letter. In order to determine energy separation between the main and subsidiary conduction band valleys, THz pulse amplitude dependences on the photoexcitation wavelength (in a range of 410–950 nm) were measured. These dependences had a clear maximum at ∼540 nm, from which the inter-valley energy separation in the conduction band of InP as equal to 0.75 eV was determined. Moreover, THz generation mechanisms at laser excited surfaces of InP were investigated by additionally analyzing the azimuthal angle dependences of the emitted THz signal amplitude and power. It has been shown that the main physical mechanism of the surface THz emission in this material is the spatial separation of photoexcited electrons and holes, which can also lead to a symmetry similar to the second order optical nonlinearity. Photocurrent surge in the surface electric field can also contribute to the THz emission from a semi-insulating crystal illuminated by optical pulses with the wavelengths close to the absorption edge.

Electro-optically Induced and Manipulated Terahertz Waves from Fe-doped InGaAs Surfaces

We demonstrate the presence of dual simultaneous nonlinear mechanisms: field-induced optical rectification (FIOR) and field-induced surge current (FISC) for the generation of terahertz (THz) pulses from p-type and n-type Fe:In0.53Ga0.47As surfaces upon excitation with femtosecond laser pulses centered at 800 nm wavelength. Experimental investigations of the dependence of the generated THz waves on the incident angular optical polarization, optical irradiance, and the direction and magnitude of applied electric DC fields give confirming results to the proposed THz generation mechanisms. Applying external DC electric fields in the plane of the incident optical field shows efficient capability in manipulating the direction and phase of the generated THz waves, and controlling the refractive index of Fe:In0.53Ga0.47As material in the THz range, in addition to enhancing the emitted THz power up to two orders of magnitude. The fast and reliable response of Fe:In0.53Ga0.47As to the changes in the direction and magnitude of the optical and electrical fields suggests its use in amplitude and phase modulators, and ultrafast optoelectronic systems.

Generation of continuous wave terahertz frequency radiation from metal-organic chemical vapour deposition grown Fe-doped InGaAs and InGaAsP

We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550 nm, and THz radiation was emitted over a bandwidth of greater than 2.4 THz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAs material found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and material doping level were also studied.

Reliability Investigation of Photoconductive Continuous-Wave Terahertz Emitters

The lifetime of photomixers depends significantly on the operation conditions. High values of bias voltage and/or optical power increase the emitted terahertz power, but considerably decrease the operation lifetime. Interdigitated finger photomixers especially face the problem of thermal destruction due to high current densities. We present an Arrhenius analysis of low-temperature-grown-GaAs photomixers and link the photomixer lifetime to the accessible signal-to-noise ratio (SNR). With a coherent continuous-wave terahertz system designed for 1000 operating hours, we achieve an SNR of 30 dB at 1 THz using an integration time of 500 ms.

Terahertz emission from a femtosecond laser focus in a two-color scheme

We critically revise the theory of terahertz emission from a plasma filament induced in a gas media by one or two focusd femtosecond laser pulses. We distinguish a radiation pressure force (RPF) from a ponderomotive force (PF), discuss conditions for one of these forces to be the dominating contribution to the terahertz emission, and also show that the angular distribution of the emitted power critically depends on which of the two forces dominates in a particular experiment. We show that the experimentally observed periodic dependence of the emitted terahertz power on the gas pressure reveals the dominating role of the RPF over the PF, whereas the angular diagram of the emission allows us to determine the predominant direction of the force. We also emphasize that the terahertz emission originated by a transient photocurrent exhibits a different dependency from the phase difference between the first and the second harmonics of the optic laser field, which generally enables the experimental detection of the prevailing mechanism of the terahertz emission from the plasma filament.

Comparison of photoexcited p-InAs THz radiation source with conventional thermal radiation sources

P -type InAs excited by ultrashort optical pulses has been shown to be a strong emitter of terahertz radiation. In a direct comparison between a p-InAs emitter and conventional thermal radiation sources, we demonstrate that under typical excitation conditions p-InAs produces more radiation below 1.2 THz than a globar. By treating the globar as a blackbody emitter we calibrate a silicon bolometer which is used to determine the power of the p-InAs emitter. The emitted terahertz power was found to be 98±10 nW in this experiment.

Strong terahertz emission from (100) p-type InAs

Terahertz emission has been observed from (100) Zn-acceptor-doped InAs under illumination by fs pulses of near-infrared radiation. Turning the crystal about the surface normal produces two maxima per rotation, whether the angle of incidence is 45° or 75°, in contrast to (111) p-InAs, where three maxima per rotation have been reported. The emitted terahertz power has a quadratic variation with the pump power and decreases with increasing temperature in the range 20–300K. This behavior is consistent with a photocurrent surge being the dominant terahertz generating mechanism at low excitation fluences. The p-type InAs generates about two orders of magnitude more power than the standard unbiased terahertz emitter, 1mm thick ZnTe.

Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches

A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photoexcitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage, pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration (<40 fs) laser pulses the energy distribution of the Gaussian pulses constrains the emitted power, while for long (>40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented. The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results. We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes.

Terahertz radiation from narrow-gap semiconductors photoexcited by femtosecond laser pulses

Large increase of the emitted terahertz power has been observed for p-InAs samples with the p-doping levels of ≈10 16–10 17 cm −3. This increase was explained by a large surface depletion layer and an electric-field-induced optical rectification effect in the layer. Terahertz fields radiated by the samples of all three investigated Cd x Hg 1 − x Te layers was of the same order of magnitude. No azimuthal angle dependence of the radiated signal was detected, what evidences that linear current surge effect is dominating over nonlinear optical rectification. Azimuthal angle and magnetic fields emission witness that it is caused by linear photo-Dember type processes.

Terahertz emission from p-lnAs due to the instantaneous polarization

Terahertz radiation from differently doped n- and p-type InAs crystal surfaces was investigated by time-resolved measurement. Large increase of the emitted terahertz power has been observed for p-InAs samples with the p-doping levels of approximately 1016–1017cm−3. This increase was explained by a large surface depletion layer and an electric-field-induced optical rectification effect in this layer.

TEMPERATURE DEPENDENCE OF TERAHERTZ EMISSION FROM SILICON DEVICES DOPED WITH BORON

In this paper, we report on electrically pumped terahertz emitters based on silicon doped with boron acceptors. At cryogenic temperatures, three narrow spectral emission lines attributed to radiative transitions from p-like excited hydrogenic states to the s-like Γ8 ground state associated with the boron dopants were observed centered around 8 THz. The spectral emission line center frequencies were in remarkable agreement with values reported from absorption measurements and theoretical calculations. The total time-resolved terahertz emission power was found to be up to 31 μW per device facet. We have solved the rate equations describing the populations in the hydrogenic dopant states involved in the emission mechanism and derived expressions for the current pumping and temperature dependence of the emitted terahertz power, yielding excellent agreement with the experimental data. These results suggest that silicon-based terahertz emitters may be fabricated without epitaxial quantum wells. The observed temperature dependence suggests that electric field assisted thermal escape of carriers from upper hydrogenic states may be responsible for lower output powers at higher temperatures.

Inversionless amplification of coherent terahertz radiation

We experimentally demonstrate inversionless amplification of the emission from electronic transitions in a solid state system inside a cavity. A semiconductor-based terahertz (THz) emitter is exposed to a coherent, phasematched THz background field during the emission process leading to an increase in the overall emitted THz power of more than one order of magnitude. The experimental concept and results are discussed within a three-level model and an extended density matrix approach.