Intense Far-infrared Radiation Research Articles

SUNRISE: The rich molecular inventory of high-redshift dusty galaxies revealed by broadband spectral line surveys

Understanding the nature of high-redshift dusty galaxies requires a comprehensive view of their interstellar medium (ISM) and molecular complexity. However, the molecular ISM at high redshifts is commonly studied using only a few species beyond 12C16O, limiting our understanding. In this paper, we present the results of deep 3 mm spectral line surveys using the NOrthern Extended Millimeter Array (NOEMA) targeting two strongly lensed dusty galaxies observed when the Universe was less than 1.8 Gyr old: APM 08279+5255, a quasar at redshift z = 3.911, and NCv1.143 (H-ATLAS J125632.7+233625), a z = 3.565 starburst galaxy. The spectral line surveys cover rest-frame frequencies from about 330 to 550 GHz for both galaxies. We report the detection of 38 and 25 emission lines in APM 08279+5255 and NCv1.143, respectively. These lines originate from 17 species, namely CO, 13CO, C18O, CN, CCH, HCN, HCO+, HNC, CS, C34S, H2O, H3O+, NO, N2H+, CH, c-C3H2, and the vibrationally excited HCN and neutral carbon. The spectra reveal the chemical richness and the complexity of the physical properties of the ISM. By comparing the spectra of the two sources and combining the analysis of the molecular gas excitation, we find that the physical properties and the chemical imprints of the ISM are different: the molecular gas is more excited in APM 08279+5255, which exhibits higher molecular gas temperatures and densities compared to NCv1.143; the molecular abundances in APM 08279+5255 are akin to the values of local active galactic nuclei (AGN), showing boosted relative abundances of the dense gas tracers that might be related to high-temperature chemistry and/or the X-ray-dominated regions, while NCv1.143 more closely resembles local starburst galaxies. The most significant differences between the two sources are found in H2O: the 448 GHz ortho-H2O(423 − 330) line is significantly brighter in APM 08279+5255, which is likely linked to the intense far-infrared radiation from the dust powered by AGN. Our astrochemical model suggests that, at such high column densities, far-ultraviolet radiation is less important in regulating the ISM, while cosmic rays (and/or X-rays and shocks) are the key players in shaping the molecular abundances and the initial conditions of star formation. Both our observed CO isotopologs line ratios and the derived extreme ISM conditions (high gas temperatures, densities, and cosmic-ray ionization rates) suggest the presence of a top-heavy stellar initial mass function. From the ∼330–550 GHz continuum, we also find evidence of nonthermal millimeter flux excess in APM 08279+5255 that might be related to the central supermassive black hole. Such deep spectral line surveys open a new window into the physics and chemistry of the ISM and the radiation field of galaxies in the early Universe.

Cosmic rays, γ-rays, and neutrinos in the starburst nuclei of Arp 220

The cores of Arp 220, the closest ultra-luminous infrared starburst galaxy, provide an opportunity to study interactions of cosmic rays under extreme conditions. In this paper, we model the populations of cosmic rays produced by supernovae in the central molecular zones of both starburst nuclei. We find that ~65 - 100% of cosmic rays are absorbed in these regions due to their huge molecular gas contents, and thus, the nuclei of Arp 220 nearly complete proton calorimeters. As the cosmic ray protons collide with the interstellar medium, they produce secondary electrons that are also contained within the system and radiate synchrotron emission. Using results from chi-squared tests between the model and the observed radio spectral energy distribution, we predict the emergent gamma-ray and high-energy neutrino spectra and find the magnetic field to be at milligauss levels. Because of the extremely intense far-infrared radiation fields, the gamma-ray spectrum steepens significantly at TeV energies due to gamma-gamma absorption.

Nonlinear dynamics in far-infrared driven quantum-well intersubband transitions

We study the effect of many-body interactions on the collective response of confined electrons in doped quantum-well (QW) heterostructures to intense far-infrared radiation. Absorption line shapes are computed both by numerically integrating the equations of motion and by using the appropriately time-averaged equations. For a two-subband double-QW system, optical bistability and period-doubling bifurcations are observed and their parameter range of activity is given. For a three-subband asymmetric triple-QW system driven at $\ensuremath{\omega}\ensuremath{\approx}{E}_{2}\ensuremath{-}{E}_{0},$ Hopf bifurcations occur which generate a strong response at a frequency incommensurate with the drive frequency or any natural frequency of the system.

Nonlinear terahertz properties of n-type quantum-well heterostructures

In this letter, we study the effect of many body interactions on the collective response of confined electrons in doped quantum-well (QW) heterostructures to intense far-infrared radiation. Absorption lineshapes are computed both by numerically integrating the equations of motion and by using the appropriately time-averaged equations. For a two-subband double-QW system optical bistability is observed and its parameter range is given. For an asymmetric triple-QW structure driven at /spl omega//spl ap/E/sub 2/-E/sub 0/ Hopf bifurcations occur which generate a strong response at a frequency incommensurate with the drive frequency or any natural frequency of the system.

Subharmonic generation in a driven asymmetric quantum well

A realistic model of a doped quantum well heterostructure exhibits a subharmonic signal in the presence of intense far-infrared radiation. Nonlinearity enters the model through the effective potential due to the density of the electrons in the well. Driven at twice the resonant frequency of a ``bare'' two-level system the electron states undergo a subcritical pitchfork bifurcation. The signature of this nonlinear bifurcation is an emitted subharmonic radiation at half of the frequency of the drive. This nonperturbative response is found by applying the technique of averaging from nonlinear dynamical systems theory to the density matrix dynamical equations, for a many-body two-level system of electrons confined in the asymmetric square well, of Galdrikian and Birnir [Phys. Rev. Lett. 29, 3308 (1996)]. The averaged equations determine the parameter ranges of the subharmonic response and they turn out to be well within the capabilities of present quantum well technology and occur at moderate values of field strength of the drive. Potential technological applications of subharmonic generation include frequency conversion and the production of ``squeezed'' states of THz radiation.

Near-infrared sideband generation induced by intense far-infrared radiation in GaAs quantum wells

GaAs quantum wells are simultaneously illuminated with near-infrared (NIR) radiation at frequency ωnir and intense far-infrared (FIR) radiation from a free-electron laser at ωfir. Magnetic fields up to 9 T are applied. Strong and narrow sidebands are observed at ωsideband=ωnir±2ωfir. The intensity of the sidebands is enhanced when either ωsideband or ωnir is near the onset of NIR absorption in the quantum well, or when ωfir is near the free-electron cyclotron frequency. We attribute these sidebands to four-wave mixing of NIR and FIR photons whose energies differ by more than a factor of 100.

Period doubling and strange attractors in quantum wells.

A realistic model of a doped quantum well heterostructure exhibits a subharmonic signal in the presence of intense far-infrared radiation. Nonlinearity enters the model through the effective potential due to the density of electrons in the well. Dephasing and energy relaxation are introduced through a simple density matrix approach. The resultant dynamics show the period-doubling routes to chaos, in the form of strange attractors. All material parameters used are well within the present capabilities of present quantum well technology, and the driving field amplitudes and frequencies are easily obtainable with a free electron laser.

Nonlinear multiphoton resonances in quantum wells

The periodically driven particle in a box serves as a starting point for the qualitative analysis of nonintegrable systems, and as a simple model of semiconductor quantum wells exposed to intense far-infrared radiation. Classically, the phase space contains chaotic low-momentum trajectories, bounded by a sharp transition to regular motion for high momenta. The quantized system contains invariant states which localize upon classical invariant surfaces. We model the time evolution of the ground state (in the classical system, an ensemble of low-momentum trajectories) under various driving amplitudes and frequencies. Whereas the dynamics of the classical system are governed by the presence of KAM tori, the quantized system also displays sharp, nonperturbative resonances at particular driving parameters. These resonances correspond to the mixing of Floquet states at avoided crossings in the quasienergy spectrum. This form of transport should survive when many-body effects, dissipation, and the continuum are taken into account.

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.

Photoluminescence as a probe of the interaction of intense far-infrared radiation with semiconductor quantum structures

At the UCSB free-electron laser we have realized a set-up to detect the photoluminescence from semiconductor quantum structures while they are irradiated with intense far-infrared radiation. The effect of the radiation on both quantum wells and quantum-well wires has been identified with non-resonant carrier heating. The confined carriers in these structures have energy-level spacings in the far-infrared region of the spectrum. The tunability of the free-electron laser therefore allows the study of resonant effects as well.

Far-infrared second-harmonic generation in GaAs/AlxGa1-xAs heterostructures: Perturbative and nonperturbative response.

We report measurements of far-infrared (FIR) harmonic generation from GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heter- ostructures. The samples studied were a modulation-doped ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/GaAs heterojunction and a sample with ten modulation-doped half-parabolic quantum wells. The samples were driven with intense far-infrared radiation from a molecular gas laser at 29.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the University of California--Santa Barbara free-electron laser at 51.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The FIR radiation was polarized parallel to the growth direction. Second harmonics of the FIR were detected from both the semi-insulating GaAs substrate and from the confined electrons. For the heterojunction sample, the second-harmonic power generated by the electrons depended quadratically on fundamental power at low power, as expected from time-dependent perturbation theory. However, this dependence became subquadratic at higher powers, indicating a nonperturbative response. At high FIR powers, electrons were also ionized from the heterojunction and half-parabolic wells. For the heterojunction at f=29.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the perturbative regime, the surface second-order susceptibility was computed to be ${\mathrm{\ensuremath{\chi}}}_{\mathit{S}}^{(2)}$=1.0\ifmmode\pm\else\textpm\fi{}0.75\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{esu}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{3}$. This value agrees, within experimental error, with a simple model of the heterojunction as a triangular quantum well. The second-order polarizability of a conduction electron in the heterojunction is nine orders of magnitude larger than that of a valence electron in pure GaAs.

Nonperturbative resonances in periodically driven quantum wells.

Energy absorption characteristics are computed for a classical and a quantum model of an infinite square well, as a function of driving amplitude and frequency. Nonperturbative resonances are observed corresponding to the replacement of states localised in phase space by more extended states. Their presence is predicted by avoided crossings in the quasienergy spectrum of the Floquet operator. The conditions under which these resonances occur can be realised in experiments on GaAs/Al x Ga 1-x As quantum wells in intense far-infrared radiation

Photoluminescence from AlxGa1-xAs/GaAs quantum wells quenched by intense far-infrared radiation.

We present an experimental study of the effects of intense far-infrared (FIR) radiation on the excitonic photoluminescence (PL) from ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs quantum wells. The FIR electric field was polarized parallel to the plane of undoped, 100-\AA{}-wide ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/GaAs wells. Electron-hole pairs, created by relatively weak visible pulses, were excited by FIR pulses with intensities of up to 70 kW/${\mathrm{cm}}^{2}$ at frequencies of 29.5 and 43.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (3.7 and 5.4 meV). Both quenching and broadening of free-exciton PL peaks were observed for rms FIR field strengths above a threshold of order 100 V/cm.

Generation of widely tunable intense far-infrared radiation pulses by stimulated Raman transitions in methylfluoride gas

We report on the generation of widely tunable intense far-infrared radiation pulses by stimulated Raman transitions in methylfluoride gas. Using a tunable high-pressure CO2 laser we achieved, by P-branch tuning of stimulated Raman transitions in 12CH3F and 13CH3F gases, tunable generation of radiation in a series of intervals in the spectral range from 37 to 72 cm−1 covering 20% of this range. Possibilities of further extension of the tuning regions are also discussed.

Saturation of Impurity Cyclotron Resonance and Effect of 337 µm Radiation on Carrier Numbers and Mobilities in n-InSb

Effect of intense far-infrared radiation on magneto-absorption, photoconduction and photo-Hall effect on n -InSb containing 2.5×10 13 cm -3 electrons were investigated at liquid helium temperatures by using an HCN laser. Saturation of impurity cyclotron resonance (ICR) and enhancement of conduction-electron cyclotron resonance (CCR) with the radiation intensity were found. This can be explained by the effect of relaxation processes where the electrons excited to impurity state (010) drop to Landau state N =0 and accumulate there. The relaxation time for the ICR saturation was obtained as 7×10 -9 sec at 1.7 K. The photoconductivity and the photo-Hall effect were analysed on the basis of the two-band conduction model. The result demonstrates that with an increase of the radiation intensity the number of conduction electrons increases consistently with the proposed relaxation processes, and in addition the impurity-band mobility also increases.