Far-infrared Laser Research Articles

Terahertz quartz enhanced photo-acoustic sensor

A quartz enhanced photo-acoustic sensor employing a single-mode quantum cascade laser emitting at 3.93 Terahertz (THz) is reported. A custom tuning fork with a 1 mm spatial separation between the prongs allows the focusing of the THz laser beam between them, while preventing the prongs illumination. A methanol transition with line-strength of 4.28 × 10−21 cm has been selected as target spectroscopic line. At a laser optical power of ∼ 40 μW, we reach a sensitivity of 7 parts per million in 4s integration time, corresponding to a 1σ normalized noise-equivalent absorption of 2 × 10−10 cm−1W/Hz½.

The Dynamics of Rubidium Atoms in THz Laser Fields

By using B-spline basis set combined with model potential, energy levels and wave functions of rubidium atoms are obtained. Using a time-dependent multi-level approach, we study the dependence of the population of rubidium atoms excited from a low lying state to a target state on the parameters of a single or two broadband terahertz laser pulses. The population redistribution between the states n = 23 and n = 24 due to the interaction with terahertz pulses is analyzed in detail. Population oscillation in the initial and final states as a function of the time delay between two half cycle pulses is shown, and the oscillation period is the same as the Kepler period of the selected states. The calculation results agree well with the experiment and can be explained by the semiclassical picture.

Stable single-mode operation of surface-emitting terahertz lasers with graded photonic heterostructure resonators

Graded photonic heterostructures (GPH) can be regarded as energy wells for photons. We show that judicious engineering of such photonic wells, obtained by tailoring the grading and the slit width of the GPH resonator, allows one to ensure spectrally single-mode emission on the fundamental symmetric mode in the whole lasing dynamical range of terahertz quantum cascade lasers. Furthermore, the radiative character of the symmetric mode leads to single-mode emission with mW output power in continuous-wave operation, as well as to single-lobed far-field beam patterns. A careful combination of theoretical analysis and experimental observations reveals that the results stem from interplay between mode competition and spatial hole burning effects.

Improved density measurement by FIR laser interferometer on EAST tokamak

A three-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer is in operation since 2010 to measure the line averaged electron density on experimental advanced superconducting tokamak (EAST). The HCN laser signal is improved by means of a new schottky barrier diode (SBD) detector. The improved measurement and latest experiment results of the three-channel FIR laser interferometer on EAST tokamak are reported.

Monolithically integrated two-dimensional arrays of surface-emitting photonic-crystal terahertz lasers

We demonstrate two-dimensional arrays of surface-emitting THz frequency photonic-crystal quantum cascade lasers covering a broad spectral range. The arrays are made of 16 unit elements, each of them spectrally single mode and featuring a narrow divergence emission pattern. The array emission frequencies cover a range of ≈0.2 THz around an emission frequency of ≈2.75 THz. Large size and small size devices have been developed. The former devices operate up to a maximum temperature of ≈90K with 2 to 3 mW output power in continuous wave regime.

Effect of Terahertz Laser Irradiation on Erythrocyte Aggregation in Healthy Blood

The study is devoted to a topical field of research – the interaction of terahertz radiation with biological structures. This paper examines the impact of the Novosibirsk terahertz free electron laser on the aggregation and morphology of red blood cells of prepared blood samples in vitro. The aim of this study was to determine the safe action of the laser radiation on red blood cells and the identification of specific effects that arise in the erythrocytes under the action of terahertz radiation and lead to non-destructive disaggregation of individual cells of erythrocytes. The methodology of sample preparation of the objects, obtaining and analyzing micrographs based on the classification of red blood cells and particles model colloidal system of the area into classes corresponding to the individual and aggregate particles, was developed. Found that 5sec exposure of 200 mkl sample of red blood cells by the terahertz radiation with wavelength of 130–146 mm and an average power density of 8–10 W/cm2 does not lead to visible changes in cell morphology and the number of aggregated particles. The exposure of more than 25 seconds can lead to lysis of erythrocytes. When exposed for 10–15 sec the average number of erythrocytes in the aggregates is reduced by 10–15 % in some cases – up to 80 %

Dissection of the frustules of the diatom Synedra acus under the action of picosecond impulses of submillimeter laser irradiation

Diatom algae realize highly intriguing processes of biosynthesis of siliceous structures in living cells under moderate conditions. Investigation of diatom physiology is complicated by frustule (siliceous exoskeleton). Frustules consist of valves and girdle bands which are adhered to each other by means of organic substances. Removal of the frustule from the lipid membrane of diatom cells would open new possibilities for study of silicon metabolism in diatoms. We found that submillimeter laser irradiation produced by a free-electron laser causes splitting of diatom frustules without destruction of cell content. This finding opens the way to direct study of diatom cell membrane and to isolation of cell organelles, including silica deposition vesicles. We suppose that the dissection action of the submillimeter irradiation results from unusual ultrasonic waves produced by the short (30-100ps) but high-power (1MW) terahertz laser impulses at 5.6MHz frequency.

Ultrafast terahertz-field-induced dynamics of superconducting bulk and quasi-1D samples

Within a density-matrix formalism based on the Bardeen–Cooper–Schrieffer (BCS) model and the Bogoliubov–de Gennes equations we provide a description of the dynamics of the non-equilibrium superconducting pairing induced by a terahertz (THz) laser pulse in bulk and quasi-one-dimensional (1D) samples of conventional (BCS-type) superconductors. A cross-over from an adiabatic to a non-adiabatic regime takes place for short and intense THz pulses. In the non-adiabatic regime, the order parameter performs a damped oscillation. We discuss how the parameters of the THz pulse influence the amplitude and the mean value of the oscillation in bulk samples. It is demonstrated that for high intensities the non-adiabatic regime can be reached even for pulses longer than the oscillation period. For the 1D samples we find that the oscillation may attenuate with a different power law. This is analysed by comparing the THz-induced dynamics with the dynamics induced by a sudden switching of the pairing strength, which exhibits essentially the same behaviour. The numerical calculations show that the exponent of the power law depends critically on the density of states in the Debye window and therefore changes in an oscillatory way with the confinement strength. Irregularities in the decay of the oscillation are predicted when the 1D quantum wire is cut short to an elongated zero-dimensional quantum dot structure.

Hyperfine spectrum measurement of an optically pumped far-infrared laser with a Michelson interferometer

In this letter, we present a Michelson interferometer for the hyperfine spectrum measurement of an optically pumped far-infrared laser with a highest frequency resolution of 3–5 GHz. CH3OH gas with a purity of 99.9%, is pumped by the CO2 9P36 and 9R10 laser lines to generate terahertz lasers with frequencies of 2.52 and 3.11 THz, respectively. Moreover, except for the center frequency, which is in good agreement with theoretical work, some additional frequencies on both sides of the center frequency are obtained at a frequency interval of 0.15 THz. Meanwhile, the mechanism behind the observed experimental results is also investigated.

Rashba spin–orbit interaction effect on multiphoton optical transitions in a quantum dot

AbstractWe demonstrate in this work the effect of Rashba spin–orbit interaction (SOI) on multiphoton optical transitions of a quantum dot (QD) in the presence of a terahertz (THz) laser field and external static magnetic field. This combination is solved by accurate nonperturbative Floquet theory. Investigations are made for the optical response of intraband transitions between the various states of the conduction band with spin flipping. It is found that spin‐flip transitions and corresponding multiphoton transitions are strongly affected by Rashba spin orbit coupling and THz laser fields. This aspect will enhance the ongoing efforts for making ultrasensitive optospintronics devices.

Design and implementation of a 150 GHz single-channel millimeter wave interferometer on Joint TEXT tokamak

A simple, single-channel millimeter-wave interferometer system has been designed, fabricated, and installed on the J-TEXT tokamak. For the plasma density anticipated on J-TEXT, a solid-state source operating at 150 GHz has been chosen to minimize errors due to both vibration along the beam path and refraction in the plasma. The new aspect of the interferometer design is to use a subharmonic mixer for detection with a frequency doubled 150 GHz source. It employs a single source which is bias-tuned and modulated with a sawtooth wave form up to 100 kHz in order to generate the intermediate frequency. The 12.5 GHz voltage-controlled oscillator is multiplied to 75 GHz before a final doubler raises it to 150 GHz. A portion of the 75 GHZ power is used for the local oscillator (LO) and is directly connected to the LO input of the subharmonic mixer. The phase is evaluated by a digital phase comparator using a software-based algorithm. Detection noise limits the minimum resolvable phase change with the interferometer to ±0.05 fringe, which corresponds to an averaged electron density change along the chord of ±1.1 × 10(17) m(-2). The maximum measurable electron density is expected to be ∼9 × 10(19) m(-3). A comparison of preliminary results from the millimeter wave interferometer with that from the far-infrared hydrogen cyanide laser (wavelength of 337 μm) interferometer shows good agreement during the pulse flat-top period. The millimeter wave interferometer system will be used as a part of the density feedback control system in the future.

Control of Landau-Zener transition in NaI predissociation with chirped femtosecond laser pulses

We explore the control of the Landau-Zener (LZ) transition in NaI predissociation with low-intensity linearly and quadratically chirped femtosecond far-infrared laser pulses by solving the coupled time-dependent Schrödinger equations. We find that the LZ transition between the excited state and the ground state can be enhanced or be suppressed using a well-delayed control pulse with a proper linear or quadratic chirp. With instantaneously controlling the energy gap, the resonant population inversion induced by Rabi oscillation can facilitate or prohibit the molecular population transfer in the avoided-crossing region. This work provides a deep understanding of coherent control of LZ transition in molecular reaction with chirped femtosecond laser pulses.

High efficiency and high power continuous-wave semiconductor terahertz lasers at ∼3.1 THz

Continuous-wave operation of semiconductor terahertz quantum cascade lasers at ∼3.1THz based on a bound-to-continuum transition design is described. The material physics and device performance is analyzed in detail. With good injection efficiency in the upper single isolated radiative state and efficient extraction from the lower radiative state, the maximum operating temperature is in excess of 90K in pulsed and 70K in continuous-wave mode. The peak power of 112.5mW was obtained at 10K in pulsed mode, indicating ∼36 photons per injected electron for 120 periods of active region. In continuous-wave operation, collected power of 62mW and 22 photons per electron was also achieved at 10K.

An optically pumped GaN/AlGaN quantum well intersubband terahertz laser

We propose an optically pumped nonpolar GaN/AlGaN quantum well (QW) active region design for terahertz (THz) lasing in the wavelength range of 30 μm ∼ 40 μm and operating at room temperature. The fast longitudinal optical (LO) phonon scattering in GaN/AlGaN QWs is used to depopulate the lower laser state, and more importantly, the large LO phonon energy is utilized to reduce the thermal population of the lasing states at high temperatures. The influences of temperature and pump intensity on gain and electron densities are investigated. Based on our simulations, we predict that with a sufficiently high pump intensity, a room temperature operated THz laser using a nonpolar GaN/AlGaN structure is realizable.

Specificity and Heterogeneity of Terahertz Radiation Effect on Gene Expression in Mouse Mesenchymal Stem Cells

We report that terahertz (THz) irradiation of mouse mesenchymal stem cells (mMSCs) with a single-frequency (SF) 2.52 THz laser or pulsed broadband (centered at 10 THz) source results in irradiation specific heterogenic changes in gene expression. The THz effect depends on irradiation parameters such as the duration and type of THz source, and on the degree of stem cell differentiation. Our microarray survey and RT-PCR experiments demonstrate that prolonged broadband THz irradiation drives mMSCs toward differentiation, while 2-hour irradiation (regardless of THz sources) affects genes transcriptionally active in pluripotent stem cells. The strictly controlled experimental environment indicates minimal temperature changes and the absence of any discernable response to heat shock and cellular stress genes imply a non-thermal response. Computer simulations of the core promoters of two pluripotency markers reveal association between gene upregulation and propensity for DNA breathing. We propose that THz radiation has potential for non-contact control of cellular gene expression.

Graphene Terahertz Lasers: Injection versus Optical Pumping

ABSTRACTIn this paper we demonstrate that graphene is one of the best materials for new types of terahertz lasers as optical and/or injection pumping of graphene can exhibit negative-dynamic conductivity in the terahertz spectral range. We analyze the formation of nonequilibrium states in optically pumped graphene layers and in forward-biased graphene structures with lateral p-i-n junctions and consider the conditions of population inversion and lasing. The latter provides a significant advantage of the injection pumping in realization of graphene terahertz lasers. We benchmark graphene as a prospective material for injection-type terahertz lasers.

Polarization Control of Optically Pumped Terahertz Lasers

ABSTRACTTwo-photon pumping of excited exciton states in semiconductor quantum wells is a tool for realization of ultra-compact terahertz (THz) lasers based on stimulated optical transition between excited 2p and ground 1s exciton state. We show that the probability of two-photon absorption by a 2p-exciton is strongly dependent on the polarization of both photons. Variation of the threshold power for THz lasing by a factor of 5 is predicted by switching from linear to circular pumping. We calculate the polarization dependence of the THz emission and identify photon polarization configurations for achieving maximum THz photon generation quantum efficiency.

Terahertz-Wave Generation Using Graphene: Toward New Types of Terahertz Lasers

This paper reviews recent advances in terahertz-wave generation in graphene toward the creation of new types of terahertz lasers. First, fundamental basis of the optoelectronic properties of graphene is introduced. Second, nonequilibrium carrier relaxation or recombination dynamics in optically or electrically pumped graphene is described to introduce a possibility of negative dynamic conductivity in a wide terahertz range. Third, recent theoretical advances toward the creation of current-injection graphene terahertz lasers are described. Fourth, unique terahertz dynamics of the 2-D plasmons in graphene are described. Finally, the advantages of graphene materials and devices for terahertz-wave generation are summarized.

Silicon optics for focusing of terahertz laser radiation in a given two-dimensional domain

Binary silicon-based diffractive optical element (DOE) - Gaussian-to-Square focuser (diameter of aperture is 30 mm) for the terahertz spectral range has been designed and characterized using terahertz radiation of the Novosibirsk Free Electron Laser (NovoFEL) at the wavelength of 141 mm. The preliminary experiments have demonstrated feasibility of application of binary silicon DOE for focusing of terahertz radiation into pre-given focal domain.

Terahertz zone plates with integrated laser‐ablated bandpass filters

Properties of zone plates with integrated bandpass filters were simulated numerically and demonstrated experimentally for 0.76 THz frequency. Finite-difference time-domain simulations were used to predict properties of a conventional zone plate and laser-ablated zone plate with resonant filter areas. Zone plates were produced employing laser direct writing and characterised by Fourier spectroscopy and an optically pumped terahertz laser.