Rocking bidirectional lasers

We study the emission properties of a class A bidirectional laser under the action of an amplitude modulated injected signal, i.e. a rocked bidirectional laser. We derive two coupled autonomous amplitude equations valid close to the emission threshold and study their solutions. The most relevant result is that while in the absence of rocking the laser can only emit in either of the two unidirectional solutions, under suitable rocking conditions cw bidirectional emission appears and, moreover, it coexist bistably with unidirectional emission.     

Insight into the thermionic emission regimes under gold film thermal relaxation excited by a femtosecond pulse

We theoretically investigated different thermal relaxation participating in the ultrafast thermionic emission processes on gold film surface with a femtosecond pulse excitation. The thermionic emission regimes under the two temperature relaxation and the thermal diffusion relaxation were demonstrated. The simulations showed that the thermionic emission properties can be defined in the regime under two temperature relaxation by reducing the laser fluence, or widening the pulse duration or increasing the laser wavelength. It was also found that there exists a transition between the two distinct thermionic emission regimes under peculiar laser parameters of laser fluence, pulse duration and laser wavelength. The results were explained as significant intervene of laser irradiation parameters into gold film thermal relaxation processes.     

Novel design concepts of widely tunable germanium terahertz lasers

We explore the full parameter space for laser operation and compile guidelines for building improved Ge lasers, especially for continuous wave applications. We present laser emission from p-type Ge lasers with small volumes at high repetition rates and with inter-contact distances as low as 750 μm. The emission is analyzed as a function of the current–voltage characteristic. The Poisson equation is solved to determine the electric field distribution and two new laser designs are presented.     

Coherent subterahertz radiation from femtosecond infrared filaments in air

We report on what is believed to be the first observation of coherent subterahertz (sub-THz) emission from a 1-m string in the atmosphere. The sub-THz pulse emitted by the filamentary structure from an intense IR femtosecond laser pulse is detected perpendicularly to the laser propagation axis by use of two heterodyne detectors at 94+/-1 and 118+/-1GHz . We describe the characteristics of this emission and show evidence of constructive interference between two separate strings.     

CW dye laser emission in the range 7540–8880 Å

We report cw dye laser emission covering the range 7540–8880 Å with two cyanine dyes pumped by the red lines of a krypton laser.     

Defect mode emission of a dye doped cholesteric polymer network

We have observed enhanced fluorescence and laser emission due to a photonic defect mode in a dye doped cholesteric polymer network. The defect is caused by a phase jump of the cholesteric helix at the interface of two stacked layers of a cholesteric polymer film. Fluorescence spectra show an additional resonant mode inside the photonic stop band. Pulsed excitation gives rise to laser emission of the defect mode, with an exceptionally low lasing threshold. The defect mode emission has a circular polarization whose sense of rotation is opposite to that of the cholesteric helix.     

双色高频激光作用下原子低阶次谐波的理论研究

通过数值求解含时薛定谔方程,研究了高频双色激光脉冲与原子相互作用产生的光辐射.研究表明,光辐射谱中既有基频光的谐波,又可观测到谐波能量附近的多个频率的光辐射产生,且辐射的峰值强度随着入射激光强度的提高呈指数增强,相邻辐射频率差值为入射的两束激光脉冲频率差.     

Terahertz radiation source in air based on bifilamentation of femtosecond laser pulses

A new terahertz (THz) source in air based on the bifilamentation of femtosecond laser pulses is reported. This THz radiation is 1 order of magnitude more intense than the transition-Cherenkov THz emission from femtosecond laser filaments reported recently and shows different angular and polarization properties. We attribute it to the emission from a bimodal transmission line created by two plasma filaments.     

Two color oscillations of TE-CO2 laser and applications to two photon process of optically pumped FIR laser

We have developed the resonance system which is able to operate a TE-CO₂ discharge tube in two color emission source. The two color source is excellent in pumping efficiency, spacial overlap and time synchronization of the two emission CO₂ lines.We indicate that the competitive operation of the two emissions is well explained by a simple model.When a TE-CO₂ laser operates at the total pressure of below half atmospheric pressure, we make clear in this work that the operation of a TE-CO₂ laser forms depression like hole burning, with regards to the J of the upper laser level by J-J collisional relaxation during the laser emission.As an example of the two-photon source, two-photon pumping of NH₃ and FIR emissions are given in this paper.     

Charge separation of dense two-dimensional electron-hole gases: mechanism for exciton ring pattern formation

We report on new experiments and theory that unambiguously resolve the recent puzzling observation of large diameter exciton emission halos around a laser excitation spot in two dimensional systems. We find a novel separation of plasmas of opposite charge with emission from the sharp circular boundary between these two regions. This charge separation allows for cooling of initially hot optically generated carriers as they dwell in the charge reservoirs for very long times.     

Episodic synchronization via dynamic injection

We report the occurrence of spontaneous synchronizing events between two semiconductor lasers, when the emission of a frequency- and intensity-chaotic driving laser is unidirectionally coupled into a second stable response laser. The driving laser is driven chaotic by delayed optical feedback, the response laser is a device-identical solitary laser. We demonstrate the onset of an episodic synchronization regime when the two lasers are spectrally detuned with respect to each other. By a joint experimental and modeling analysis we can attribute the onset and the duration of the episodes to properties of spectral overlap of both lasers. This effect can even give rise to seemingly anticorrelated intensity behavior. We expect episodic synchronization to be a generic scenario for the loss of synchronization of chaotic oscillators exhibiting frequency cycles.     

Modulation of a saturable absorber Q-switched laser by a train of picosecond pulses

In this experiment, the transmission of the saturable absorber of a mode-locked laser is modulated optically by a train of ultra-short pulses emitted by another laser, with some power reflected inside the cavity to initiate the emission. The laser threshold is only attained during the opening time of the modulator, and stimulated emission is prevented at any other time. Once pulses are formed, the photon density is sufficiently high for auto-modulating the dye.Q-switching of the laser is only realised if the difference between the time of a round-trip in the cavity and the period of modulation is less than ±17 psec. The emission is a series of pulses of 5.2 nsec period lasting for 60 nsec. The total energy in all pulses is 60 mJ for a neodymium doped glass rod of 8 mm diameter.We describe a method for measuring the noise of the series of pulses using a saturated photo-cell. We find a ratio of 100 between the energy of one pulse and the energy emitted between two pulses.     

High resolution emission spectra of laser-excited molecular iodine as the excitation frequency is tuned through and away from resonance

We have studied the ΔJ = 2 rotationally shifted emission lines in the region of the strong absorption of molecular iodine which occurs within the 5145 Å argon ion laser line. We used an etalon tuned, single frequency argon ion laser with a linewidth of 20 MHz to excite the iodine emission and recorded the spectra of the rotational lines with Fabry-Perot spectrometers having resolutions up to 70 MHz (0.0023 cm^-1). To overcome Doppler linewidth limitations we took spectra of the emission at small angles to the exciting beam and found the lines to have widths less than our instrumental resolution and frequencies which accurately tuned with the incident laser frequency. We recorded the emission lines for laser frequencies in the absorption line center and out into the absorption wing. Our spectra show that the intensity of the emission lines follows the absorption line profile while the frequency of the emission lines is determined by the laser frequency; the intensity is maximum at the absorption line center, falling by 10⁴ as the laser frequency is moved off the line center, while the line position maintains a constant frequency shift from the laser frequency.     

Chaotic two-mode lasing

We consider a homogeneously broadened, two-mode laser under conditions of rapid relaxation of off-diagonal density-matrix elements. Treating as an example the case of two transverse modes, we show that a spatially inhomogeneous pumping rate in such a laser can give rise to chaotic emission.     

Diode laser frequency-noise suppression by >50dB by use of electro-optic parametric master oscillators

We demonstrate a novel frequency-stabilization scheme for laser diodes that is capable of linewidth reduction by more than 5 orders of magnitude. In this master-slave scheme the diode laser emission is frequency-offset phase locked to the emission of an electro-optic parametric oscillator (EOPO), and the laser diode is simultaneously used as the EOPO pump source. As a result the initial frequency fluctuations of the pump are reduced to the intrinsic noise level of the EOPO, which can be extremely small. We demonstrate that subhertz linewidths of the beat notes of the signals of two of these systems can be readily achieved if acoustic perturbations are suppressed.     

Spectral characteristics of a Yb-doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic laser

We report the experimental observation of random wavelength emission and intensity-dependent central-wavelength shift in a diode-pumped Yb³⁺-doped Y₂O₃ ceramic laser. We show experimentally that, like conventional lasers, the emission of the laser has fixed well-defined transverse modes; however, its instantaneous emission wavelengths change randomly with time. The central wavelength of the laser emission also shifts with the intracavity light intensity. A model was developed to describe the spectral behavior of Yb³⁺-doped lasers. We show that the observed random wavelength emission and central lasing wavelength shift of the laser could be well explained based on the strong reabsorption of light in the gain medium. PACS 42.55.Rz; 42.60.Mi; 42.55.Xi     

Reaching the resolved tunnel regime for a femtosecond oscillator driven field emission electron source

We report on electron emission from tungsten tips with the help of 800 nm-8 fs laser pulses from a Ti:sapphire laser oscillator. We have measured autocorrelation traces of the exciting laser pulse in the photoelectron current, which allows a measurement of the non-linearity as a function of DC voltage applied to the tip. These data are well described by a numerical integration of the one-dimensional Schrödinger equation. The simulation shows us that electron emission resolves the electric field structure of the driving laser pulse, a regime which is currently an extremely fruitful area of research with atoms in the gas phase. For an 8 fs laser pulse with the correct carrier-envelope phase setting the emission duration is as short as 800 nm.     

Transformation of optical properties of crystal media (KGW, YVO4) exposed to quasi-continuous laser radiation in the range of the transmission band of the medium

We present data from an investigation of the properties of the blue (475 nm) and green (520–560 nm) emissions that arise from KGW and YVO₄ crystals upon excitation into the transmission band of the medium by continuous infrared laser radiation the power density of which is in the kilowatt range. As distinct from standard luminescence, the two emissions are narrowband, have a structure, and show a nonlinear character. In the case of blue emission, it is noted that, simultaneously with laser radiation, the crystal experiences a significant action of Raman generation. We demonstrate the excitation of the blue emission by the laser radiation alone and present its dynamic characteristics, showing that its intrinsic time differs from the corresponding times of the Raman and laser generations. Our results show that the green emission has an up-conversion nature with the several photons excitation of residual low-concentration erbium ions. We report that the band intensity ratio of the upconversion luminescence of erbium sharply changes with increasing excitation intensity. The nonlinear interaction that is developed in KGW is accompanied by transformation of the transmission. It is concluded that the investigated processes develop in undoped KGW and YVO₄ crystals as a result of a prolonged exposure of the medium to the laser radiation, which leads to a change in its properties.     

Kinetics of thermalized luminescence of a single quantum dot at room temperature

We have developed a theory of transient secondary emission of a single quantum dot from the lowest energy states of electron-hole pairs. We consider a process in which laser pulses excite a certain highenergy state of electron-hole pairs of a quantum dot at room temperature, with the electronic subsystem then relaxing to low-energy states and photons being emitted. Therefore, the investigated secondary emission process is thermalized luminescence. For definiteness, the developed model takes into account two states of electron-hole pairs that contribute to the luminescence. We have analyzed the dependence of the secondary emission signal on the energy gap between these states, the value of which is determined by the quantum dot size. In terms of the Pauli master kinetic equation, an analytical expression for the time dependent signal of the thermalized luminescence has been obtained. We show that, as the spectral width of the exciting laser pulse tends to zero, this expression yields the signal of stationary luminescence.     

A diode-laser-pumped tunable Ho: YLF laser in the 2 µm region

We report the cw laser emission of Ho: YLF in an astigmatically compensated resonator under pumping by a laser diode array. We measured the laser efficiency at different operating temperatures of the crystal and the non-saturated gain coefficient at the peak wavelength of laser emission. The laser tunability extends from 2.040 µm to 2.075 µm.