Free Carrier Heating Research Articles

Competition between Direct Detection Mechanisms in Planar Bow-Tie Microwave Diodes on the Base of InAlAs/InGaAs/InAlAs Heterostructures.

The application of the unique properties of terahertz radiation is increasingly needed in sensors, especially in those operating at room temperature without an external bias voltage. Bow-tie microwave diodes on the base of InGaAs semiconductor structures meet these requirements. These diodes operate on the basis of free-carrier heating in microwave electric fields, which allows for the use of such sensors in millimeter- and submillimeter-wavelength ranges. However, there still exists some uncertainty concerning the origin of the voltage detected across these diodes. This work provides a more detailed analysis of the detection mechanisms in InAlAs/InGaAs selectively doped bow-tie-shaped semiconductor structures. The influence of the InAs inserts in the InGaAs layer is investigated under various illumination and temperature conditions. A study of the voltage-power characteristics, the voltage sensitivity dependence on frequency in the Ka range, temperature dependence of the detected voltage and its relaxation characteristics lead to the conclusion that a photo-gradient electromotive force arises in bow-tie diodes under simultaneous light illumination and microwave radiation.

Ultrafast excitation of conduction-band electrons by high-intensity ultrashort laser pulses in band-gap solids: Vinogradov equation versus Drude model

Heating of conduction-band electrons is one of the major processes of energy absorption and transfer in high-intensity ultrafast laser–solid interactions. It is frequently simulated by assuming a high rate of electron-particle collisions. We explore the approximation of low-rate electron–phonon collisions based on the Vinogradov equation for the intraband absorption rate by conduction-band electrons performing laser-driven oscillations. Band-structure modification by the ponderomotive energy of the ultrafast oscillations is taken into account. The Vinogradov equation combined with the Keldysh formula for the interband transition rate delivers a highly nonequilibrium energy distribution of the conduction electrons. Reported results suggest a substantial revision of the traditional models of ultrafast free-carrier heating by intense laser pulses.

Boosting the terahertz nonlinearity of graphene by orientation disorder

The conical band structure is the cornerstone of graphene’s ultra-broadband optical conductivity. For practical use of graphene in nonlinear photonics, however, substantial increases of the light–matter interaction strength will be required while preserving the promising features of monolayers, as the interaction of light with a single atomic layer is limited due to the extremely short interaction length and low density of state, particularly for the long-wavelength region. Here, we report that this demand can be fulfilled by random stacking of high-quality large-area monolayer graphene up to a requested number of layers, which leads to the electronic interaction between layers being effectively switched off due to turbostratic disorder. The nonlinear characteristics of randomly stacked multilayer graphene (RSMG), which originates from a thermo-modulational feedback mechanism through ultrafast free-carrier heating and temperature-dependent carrier-phonon collisions, show clear improvements in the terahertz (THz) regime with increasing layer numbers, whereas as-grown multilayer graphene (AGMG) exhibits limited behaviors due to strong interlayer coupling. This controllable nonlinearity enhancement provides an ideal prerequisite for developing efficient graphene-based THz photonic devices.

Terahertz photoconductivity of Pb1−xSnxTe(In)

We have analyzed photoconductivity in Pb1−xSnxTe(In) under the action of ∼100 ns long terahertz laser pulses with the wavelength varying from 90 to 280 μm in the temperature range 4.2–300 K. Strong photoresponse has been observed at all laser wavelengths used. Two types of photoresponse have been detected. Positive persistent photoconductivity, which is observed at T<10 K is due to photoexcitation of impurity states, whereas negative nonpersistent photoresponse prevailing at higher temperatures T∼25 K results from free carrier heating. Specific features of photoconductivity mechanisms are discussed.

Ambient temperature or moderately cooled semiconductor hot electron bolometer for mm and sub-mm regions

AbstractA model of semiconductor hot electron bolometer (SHEB), in which electromagnetic radiation heats only electrons in narrow-gap semiconductor without its lattice slow-response heating, is considered. Free carrier heating changes the generation-recombination processes that are the reason of semiconductor resistance rise. It is estimated, that Hg0.8Cd0.2Te detector noise equivalent power (NEP) for mm and sub-mm radiation wavelength range can reach NEP ∼10−11 W at Δf = 1 Hz signal gain frequency bandwidth. Measurements performed at electromagnetic wave frequencies v = 36, 39, 55, 75 GHz, and at 0.89 and 1.58 THz too, with non-optimized Hg0.8Cd0.2Te antenna-coupled bolometer prototype confirmed the basic concept of SHEB. The experimental sensitivity Sv ∼2 V/W at T = 300 K and the calculated both Johnson-Nyquist and generation-recombination noise values gave estimation of SHEB NEP ∼3.5 × 10−10 W at the band-width Δf = 1 Hz and v = 36 GHz.

Hot carriers in tunnel diode

We present the first results of experimental study of free carrier heating in a degenerate semiconductor when the carriers were excited by CO2 laser light and microwave radiation of 10 and 35 GHz. Tunnel GaAs p−n diodes were used as objects under investigation. It is shown that the carrier heating reduces the dark tunnel current, while at high enough forward bias the thermo-diffusive current is responsible for the formation of the signal. The magnitude of the electromotive force arising under the microwave irradiation depends linearly on the power and increases with the decrease of the semiconductor lattice temperature. It is nearly independ of the microwave frequency.

Investigation of Degenerate Carrier Heating in Tunneling GaAs p-n Junction

We report on first experimental investigation of photoelectrical properties of tunneling GaAs p-n junction. It was observed that the tunneling current in degenerately doped p-n junction can be significantly changed by infrared laser radiation. Experimental results are interpreted in terms of classical tunneling current model including free-carrier heating under infrared excitation. The hot carrier temperature is evaluated.

Photoelectrical properties of GaAs/AlGaAs p-n heterojunctions under infrared laser illumination

We report on results of an experimental study of photoresponse, induced by pulsed CO2 laser in GaAs/AlGaAs p-n heterojunction and compare with that of GaAs homojunction. We show that the signal originates from free carrier heating in the structure. The dependence of the photocurrent on light intensity was established to be linear in the measured intensity range. Moreover, we demonstrate the possibility to employ this phenomenon in devices such as infrared detector and infrared photocapacitor as well, which successful operation does not require any cryogenic cooling.

Quenching of excitonic quantum-well photoluminescence by intense far-infrared radiation: Free-carrier heating.

Undoped GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As and GaAs/AlAs quantum wells are simultaneously excited by weak visible light and intense far-infrared (FIR) radiation with electric fields polarized parallel to the planes of the quantum wells. The frequency of the FIR radiation ranges from 6 to 119 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with intensities up to 700 kW/${\mathrm{cm}}^{2}$. Peaks in the excitonic photoluminescence (PL) are broadened and quenched by the intense FIR radiation; the PL line shapes are consistent with the FIR radiation heating the carriers without significantly heating the lattice. Despite the excitonic nature of the PL, the power and frequency dependence of the carrier heating is consistent with free-carrier absorption of FIR radiation. Energy and momentum relaxation times for the free carriers are extracted from fits to a Drude model. We suggest that the FIR radiation heats free carriers that are not contributing to luminescence, and that these hot carriers in turn heat the luminescing excitons.

Microwave modulated and thermal modulated photoluminescence studies of PbI2 layered semiconductor

The exciton luminescence of PbI2 layered semiconductor has been examined utilizing microwave modulated photoluminescence (MMPL) and thermal modulated photoluminescence (TMPL) spectroscopy. In the MMPL experiment, the microwave electric field induces a nonequilibrium heating of free carriers. In the TMPL experiment, modulation creates an equilibrium thermal redistribution of carriers among the emitting states. The results revealed that MMPL and TMPL can be applied as high-resolution spectroscopic techniques.

Microwave modulated and thermal modulated photoluminescence studies of 2H-lead iodide

Lead iodide, PbI 2 , is a layered semiconductor. The luminescence processes, of 2H-PbI 2 single crystals, have been examined utilizing microwave modulated photoluminescence (MMPL) and thermal modulated photoluminescence (TMPL) spectroscopies. In the MMPL experiment, the microwave electric field induces a thermal heating of free carriers. The hot carriers effect on luminescence processes is discussed. On the basis of the experimental evidence, we suggest that the hot carriers influence the luminescence processes of excitons by reducing their cross section of formation. The TMPL experiments showed that thermal modulation has created a bolometric effect followed by equilibrium redistribution of carriers among the emitting states

Ultrafast refractive index dynamics in AlGaAs diode laser amplifiers

Ultrafast refractive index dynamics in AlGaAs diode laser amplifiers were studied using 440 fs optical pulses. Refractive index changes consistent with a free carrier heating model proposed to explain earlier studies of gain dynamics in diode lasers are exhibited. We also observe an apparently instantaneous refractive index change that corresponds to an n2⊥ of −5×10−12 cm2/W.

Laser-induced modifications and the mechanism of intrinsic damage in wide gap optical materials

A brief outline of our previous investigations into the mechanism of intrinsic bulk laser induced damage and our main conclusions serve as the introduction to exciting new results obtained by Shen et al. [Phys. Rev. Lett. (June 5. 1989)]: who measured the free electron absorption of 1064 nm photons photoacoustically in NaCl and SiO 2. The electrons were generated with a 266 nm pump pulse by two- or three-photon transitions from the valence band. For a given pump energy which is chosen sufficiently low to avoid heating of the interaction volume by itself, the dependence of the photoacoustic signal and temperature rise were observed as a function of the energy of the 1064 nm pulse and found to be in agreement with the theory of free carrier heating by Epifanov et al. Temperatures approaching the melting point were produced in the materials under prebreakdown conditions. This work provides direct experimental proof that free electron absorption is the primary mechanism for heating wide-gap optical materials by intense laser pulses in the visible and near infrared, confirming the indirect measurements reported earlier by us. No evidence of electron impact ionization and ensuing avalanche formation is found up to the intrinsic damage threshold, throwing doubt onto the commonly held belief that it is responsible for free electron generation and eventual breakdown at 1064 nm.

Laser heating of free electrons in wide-gap optical materials at 1064 nm.

Free-electron absorption of 1064-nm photons is measured photoacoustically in NaCl and ${\mathrm{SiO}}_{2}$. The electrons are generated with a 266-nm pump pulse by two- or three-photon transitions. For a given pump pulse energy, the photoacoustic signal and the rise in lattice temperature are observed as a function of the energy of the 1064-nm pulse and found to be in agreement with the theory of free-carrier heating. Temperatures approaching the melting point are produced in these materials under prebreakdown conditions. No evidence of electron-impact ionization and ensuing avalanche formation is found up to the intrinsic damage threshold.

Investigation of intrinsic optical damage in potassium bromide at 532 nm.

We report here the details of an investigation of intrinsic single-pulse optical damage in KBr at 532 nm [Phys. Rev. Lett. 59, 1605 (1987)]. The technique employed is based on the self-trapped-exciton recombination luminescence, and the temperature dependence of the luminous efficiency is utilized to measure the lattice-temperature rise resulting from the interaction of KBr with intense laser pulses at 532 nm. The mechanism of the laser-solid interaction is shown to be four-photon free-carrier generation and subsequent free-carrier heating with small contributions from laser-generated defect formation, absorption, and relaxation as well as direct recombination of the charge carriers. Single-pulse damage occurs at a temperature very close to the melting point of the material with no indication of electron-avalanche impact ionization.

Recombination-induced heating of free carriers in a semiconductor.

Recombination of externally excited (injected) charge carriers in a semiconductor can strongly change the temperature of the remaining carriers. The temperature is increased if the recombination rate decreases with energy and/or degenerate statistics applies. The normalized heating rate $\frac{[\frac{d({k}_{B}T)}{\mathrm{dt}}]}{{k}_{B}T\ensuremath{\cong}{\ensuremath{\tau}}^{\ensuremath{-}1}}$ (the recombination rate) will thus drastically reduce energy relaxation (cooling) of hot carriers at low temperatures. Cooling may occur if the recombination rate increases with energy.

Modulation of fundamental absorption by free carrier heating in degenerate n-PbTe

An increase of the fundamental interband absorption due to applied electric fields up to 150 V cm −1 has been found in highly doped n-PbTe at 80 K. The results can be explained by an electron temperature model. An energy relaxation time of 1.8 × 10 −12 sec is obtained from the electron temperature vs field strenght relation.

The contribution to self-focusing of laser beams from free-carrier heating effects in InSb in the presence of a static magnetic field

In this paper we have studied the self-focusing of a right-handed circularly polarized laser beam in indium antimonide in the presence of a static magnetic field when the nonlinearity in the dielectric tensor arises because of heating and subsequent mass modulation and redistribution of electrons. The paraxial and WKB approximations have been employed to obtain an analytical solution of the wave equation. It is seen that in n-InSb self-focusing of all frequency waves, off the cyclotron resonance, is possible, whereas in compensated InSb only higher frequencies (ω≳ωc) are focused. In the range ω≳ωc the effect of the magnetic field is to enchance the focusing of the beam.