共查询到20条相似文献,搜索用时 15 毫秒
1.
Spontaneous (SP) and piezoelectric (PZ) polarization effects on the intraband relaxation time for a wurtzite (WZ) GaN/AlGaN
quantum well (QW) are investigated theoretically as functions of the sheet carrier density and well thickness. The self-consistent
(SC) model with the SP and PZ polarizations shows that linewidths for carrier–carrier and carrier–phonon scatterings are significantly
reduced compared to those for the flat-band (FB) model without SP and PZ polarization. In particular, line-widths for the
e–h and h–e scatterings are reduced by about two orders of magnitude at a sheet carrier density as low as 2×1012 cm-2 compared to the case of the FB model. This is attributed to the decrease of the matrix element due to the spatial separation
between electron and hole wave functions. In the case of the e–e and h–h scatterings, the reduction of linewidths is mainly
attributed to the decrease of the scattering matrix element due to the increase of the inverse screening length. Linewidths
for e–h and h–e scatterings gradually increase with the sheet carrier density since the screening field increases, while linewidths
for the other scatterings are almost independent of the sheet carrier density. The SC model also shows that linewidths for
the carrier–carrier and carrier–phonon scatterings are nearly, constant irrespective of well thickness except for e–h and
h–e scatterings. In the case of e–h and h–e scatterings, linewidths greatly decrease with the well width because of the increase
of the spatial separation of wave functions.
Received in final version: 13 July 2000 / Accepted: 13 July 2000 / Published online: 16 August 2000 相似文献
2.
An optical clock based on an Er3+ fiber femtosecond laser and a two-mode He–Ne/CH4 optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er3+ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH4 laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent
connection between the optical frequency of the He–Ne/CH4 standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock
is measured against a commercial hydrogen maser. The measured relative instability is 1×10−12 at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift
of the He–Ne/CH4 optical standard is dominant. 相似文献
3.
D.?Aumiler T.?Ban N.?Vuji?i? S.?Vdovi? H.?Skenderovi? G.?Pichler 《Applied physics. B, Lasers and optics》2009,97(3):553-560
We introduce a method for determination of the absolute frequencies of comb lines within an optical frequency comb spectrum.
The method utilizes the experimental and theoretical approach of the velocity-selective optical pumping of the atomic ground
state hyperfine levels induced by resonant pulse-train excitation. The information on the laser pulse repetition frequency
and carrier–envelope offset are physically mapped onto the 87Rb ground state hyperfine level population velocity distributions. Theoretical spectra are calculated using an iterative analytic
solution of the optical Bloch equations describing the resonant pulse-train excitation of four-level 87Rb atoms. They are employed to fit the measured spectra and obtain the parameters of the frequency comb, thus providing a
practical algorithm which can be used in real-time measurements. 相似文献
4.
A photonic approach for microwave/millimeter-wave (MMW) frequency measurement is proposed and demonstrated based on stimulated
Brillouin scattering (SBS) of a 20-km standard single mode fiber (SSMF). After the MMW signal is modulated to a laser source
with two sidebands using optical carrier suppression (OCS) modulation, its frequency can be easily measured by monitoring
the SBS-induced amplification with a power meter. Due to the 1-pm resolution of a tunable pump laser source, a frequency measurement
range of 1–40 GHz is demonstrated in our experiment with a frequency resolution of 125 MHz. We believe the frequency measurement
range can be further extended to satisfy photonic radar front-end processing application. 相似文献
5.
Optimization of the glass cylindrical active element (AE) of a solid-state laser is carried out. Dependences of the optical
quality of a laser medium on scale-geometry parameters of the AE with different-intensity optical pumping are obtained. The
possibility of considerable increasing of the laser efficiency by using an aperture diaphragm is shown.
Baltic State Technical University, 1, 1st. Krasnoarmeiskaya St., St.-Petersburg, 198005, Russia. Translated from Zhurnal Prikladnoi
Spektroskopii, Vol. 64, No. 6, pp. 734–737, November–December, 1997. 相似文献
6.
P. Campbell 《Hyperfine Interactions》2006,171(1-3):143-148
The use of optical pumping in an RF linear Paul trap is shown to permit efficient collinear laser spectroscopy in systems
where atomic ground state resonance lines are ill-suited to the fast beam technique. The case of yttrium, where laser spectroscopy
has been used to study 86–90,92–102Y and isomeric states of 87–90,93,96,97,98Y, is highlighted. 相似文献
7.
8.
The results of a nonlinear-absorption research in a film of a-Si: H are reported. The absorption is induced by a picosecond laser pulse with quantum energy only slightly exceeding the band gap width of the material. Picosecond pulses obtained by optical methane and hydrogen pumping and stimulated Raman scattering were used in experiments for resonance excitation of the sample. The total absorption is shown to be the sum of the free carrier absorption and absorption by holes trapped on local levels in the “tail” of the conduction band. 相似文献
9.
A comparative analysis of the spectral-luminescent characteristics and generation parameters of a number of 3-imidazopyridylcoumarin
derivatives has been carried out under nano- and microsecond excitation by coherent and incoherent light. New coumarin dyes
are offered that feature the ability to lase under different types of pumping in the spectral range 525–580 nm; high lasing
efficiency reaching 20% and 1% under laser and lamp pumping, respectively; and high photochemical stability (3–5 times higher
than that of rhodamine 6 G) under conditions of powerful nonmonochromatic pumping. 相似文献
10.
L.E. Batay A.A. Demidovich A.N. Kuzmin A.N. Titov M. Mond S. Kück 《Applied physics. B, Lasers and optics》2002,75(4-5):457-461
A new laser medium – Yb,Tm:KY(WO4)2 – for diode pumped solid state laser applications operating around 1.9 to 2.0 μm has been investigated and the main laser
characteristics are presented. Diode pumping at 981 nm and around 805 nm was realised. For 981-nm pumping, the excitation
occurs into Yb3+ ions followed by an energy transfer to Tm3+ions. A slope efficiency of 19% was realised. For pumping around 805 nm, the excitation occurs directly into the Tm3+ ions. Here a maximum slope efficiency of 52%, an optical efficiency of 40%, and output powers of more than 1 W were realised.
Using a birefringent quartz plate as an intracavity tuning element, the tunability of the Yb,Tm:KY(WO4)2 laser in the spectral range of 1.85–2.0 μm has been demonstrated. The possibility of laser operation in a microchip cavity
configuration for this material has also been shown.
Received: 12 March 2002 / Revised version: 20 May 2002 / Published online: 25 September 2002
RID="*"
ID="*"Corresponding author. Fax: +49-531/592-4116, E-mail: stefan.kueck@ptb.de 相似文献
11.
Margarita A. Deneva Pepa I. Uzunova Marin N. Nenchev 《Optical and Quantum Electronics》2007,39(3):193-212
We report (theory, experimental check) an improved approach for generation of a tunable, subnanosecond pulse (0.1–0.4 ns),
based on a single pulsation (“spike”) separation from the transient oscillations in a dye laser with active mirror (AMIR).
A pumping by 20–50 ns pulses from Q-switched Nd:YAG laser is considered. The separation is in original, two-spectral selective
channels cavity, where the forced by AMIR quenched generation at one of the wavelength stops initially started spiking generation
at the other wavelength after the first spike development. The AMIR quickly starts the quenching generation at a precisely
controlled moment and with necessary intensity thus assuring the desired separation. An advantage is a high reproducibility
of the separation for high (~250%) pump power fluctuations combined with tuning in large range (~20 nm). To obtain such an
operation we form ~1 ns leading front pump pulse by electrooptical temporal cutting of the input pump pulse and use an optical
delay line. This increases also a few times the power in the separated spike (to be ~100 kW). Our approach widens the combinations
of lasers for effective applications of spike separation technique (dye lasers excited by Q-switched solid-state or Cu-vapor
lasers). 相似文献
12.
The peak-power-density stability and beam-wander precision of a probe laser are important factors affecting the inspection
results in precise thin-film optical measurements. These factors are also key to evaluating a probe laser for in-line long-time
operation of precise thin-film optical measurements. The peak-power density and beam wander of liner helium–neon (He–Ne) lasers,
random He–Ne lasers, and diode lasers as functions of time are investigated experimentally using a beam profiler. It is found
that the linear polarized He–Ne laser is considered to be a promising candidate for a probe laser employed in precise thin-film
optical measurements due to better peak-power-density stability and beam-wander precision. Both the peak-power-density stability
and beam-wander precision of He–Ne lasers are usually better than that of diode lasers, but an adequate warm-up of He–Ne laser
for 30 min is required before thin-film optical measurements are made. After 12 h operation, the linear polarized He–Ne laser
is suitable for precise thin-film optical measurements because both the peak-power-density stability and the beam-wander precision
reach the minimum level. A cost-effective system composed of two linear polarized He–Ne lasers for long-term operation is
proposed. This system can operate for around 0.5–1.2 years in precise thin-film optical measurements under the normal operating
life of a He–Ne laser by switching the probe laser every 18 h. 相似文献
13.
We predict that nonlinear ultrafast electron photoemission by strong optical fields and, potentially, other nonlinear optical
responses of metal nanostructures significantly depend on the absolute (carrier–envelope) phase of excitation pulses. Strong
enhancement of the local optical fields produces these responses at excitation intensities lower by order(s) of magnitude
than for known systems. Prospective applications include control of ultrafast electron emission and electron injection into
nanosystems. A wider class of prospective applications is the determination of the absolute phase of pulses emitted by lasers
and atoms, molecules, and condensed matter at relatively low intensities.
PACS 78.67.-n; 78.47.+p; 79.60.Jv; 73.20.Mf 相似文献
14.
P. G. Eliseev 《Opto-Electronics Review》2008,16(3):199-207
The anti-Stokes luminescence is a mechanism of the optical refrigeration in semiconductor light sources. The heavily doped
semiconductors are considered as a material for the laser cooling. The limitation of this mechanism appears to be connected
with a transition from the non-degenerate to degenerate occupation. This transition occurs at higher pumping rate (along with
the transition to the optical gain and lasing) and at lower temperature. Thus, the limit for the laser cooling can be indicated.
The minimal obtainable temperature is about 60–120 K depending on the doping level. The laser cooling of a semiconductor is
impeded by the difficulty of extracting the spontaneous emission from a radiating body that is characterized by large angle
of the total internal reflection. 相似文献
15.
G. Bismut B. Pasquiou D. Ciampini B. Laburthe-Tolra E. Maréchal L. Vernac O. Gorceix 《Applied physics. B, Lasers and optics》2011,102(1):1-9
We report on a strategy to maximize the number of chromium atoms transferred from a magneto-optical trap into an optical trap
through accumulation in metastable states via strong optical pumping. We analyse how the number of atoms in a chromium Bose–Einstein
condensate can be raised by a proper handling of the metastable state populations. Four laser diodes have been implemented
to address the four levels that are populated during the MOT phase. The individual importance of each state is specified.
To stabilize two of our laser diode, we have developed a simple ultrastable passive reference cavity whose long term stability
is better than 1 MHz. 相似文献
16.
K. Suchanek M. Suchanek A. Nikiel T. Pałasz M. Abboud A. Sinatra P.-J. Nacher G. Tastevin Z. Olejniczak T. Dohnalik 《The European physical journal. Special topics》2007,144(1):67-74
An accurate optical method to measure the nuclear
polarization of 3He atoms in the 11S ground state is
described. The absorption of a weak, probe laser beam is used to
measure the relative populations of two hyperfine sublevels of the
23S metastable state that are not addressed by the pumping
laser beam. Since a common spin temperature between the ground and
metastable states is established by metastable exchange
collisions, the nuclear polarization can be derived from these
absorption measurements. The method is highly sensitive, robust,
and can be used to monitor the dynamics of optical pumping and
relaxation processes without interfering with them. It was
successfully implemented and tested in the 0.45–2.0 T magnetic
field range at the 3He gas pressure up to 67 mbar. 相似文献
17.
G. Baum C. D. Caldwell W. Schröder 《Applied Physics A: Materials Science & Processing》1980,21(2):121-126
A lithium-6 atomic beam is spin-polarized by means of optical pumping with a single-mode dye laser operating on the resonance
transition. Simultaneous pumping of both hyperfine substates is achieved by frequency-splitting the laser light with an acousto-optic
modulator. A polarization dependent signal, obtained by probing the optical activity of the beam with linearly polarized light,
is utilized in a microprocessor-controlled laser stabilization scheme. The polarization is analyzed with a sextupole magnet
and its overall value is 0.70 for an intensity of 1×1014 atms s−1. By reversing the sense of circular polarization of the pumping light the atomic beam polarization is easily reversed in
direction. 相似文献
18.
K.W. Adu Q. Xiong H.R. Gutierrez G. Chen P.C. Eklund 《Applied Physics A: Materials Science & Processing》2006,85(3):287-297
Raman scattering is shown to be an effective probe of optical and surface optical phonons in highly crystalline semiconducting nanowires (SNWs). We show that the confinement model of Richter et al. well describes the nanowire diameter dependence of the asymmetric broadening of the one-phonon band in Si nanowires observed at ∼520 cm-1. We also show that the use of high laser flux (∼0.1 mW/μm2) leads to a second mechanism that can asymmetrically broaden the 520 cm-1 Raman band. This broadening has nothing to do with confinement, and can qualitatively be understood in terms of inhomogeneous laser heating. A model is presented that supports this explanation. The production of SNWs via the vapor–liquid–solid growth mechanism leads, in many cases, to an instability in the nanowire diameter or cross-sectional area. In the second part of this review, we show that this instability activates the surface optical (SO) phonon Raman scattering. Examples of this phenomenon are shown for GaP and ZnS nanowires. The former and latter have, respectively, cylindrical and rectangular cross sections. We show that the cross-sectional shape of the nanowire is important for a quantitative analysis of these SO modes. PACS 78.67.-n; 78.67.Lt; 78.30.-j; 78.30.Fs; 72.10.Di 相似文献
19.
V. Dierolf A.B. Kutsenko C. Sandmann F. Tallian W. von der Osten 《Applied physics. B, Lasers and optics》1999,68(5):767-775
Detailed excited state absorption measurements under pumping at 980 nm and 1.5 μm together with conventional absorption and
emission spectroscopy is employed to investigate optical transitions of Er3+ in Ti:LiNbO3 channel-waveguides. The experimental data were evaluated using the Judd–Ofelt method giving parameters close to those in
the bulk. The good agreement between theoretical prediction and measurements allows us to calculate cross section, lifetimes,
and branching ratios. Based on these results we developed and tested a model which is able to predict the conditions for which
laser operation at 550 nm and 2.7 μm will be possible under 980 nm pumping.
Received: 9 December 1998 / Revised version: 8 January 1999 / Published online: 24 March 1999 相似文献
20.
J.L. Pan J.E. McManis M. Gupta M.P. Young J.M. Woodall 《Applied Physics A: Materials Science & Processing》2008,90(1):105-112
Materials exhibiting strong optical emission also exhibit strong absorption at the same wavelengths because both emission
and absorption are governed by the same optical dipole and density-of-states. Laser action requires a carrier injection large
enough for emission to exceed absorption at laser wavelengths. Thus, strong self-absorption at luminescent wavelengths raises
the operating current of LEDs, lasers, and optical amplifiers. Here we demonstrate that, contrary to conventional expectations,
materials designed with novel deep centers achieve surprisingly large optical emission while, simultaneously, the inverse
process of optical absorption remains very small. A striking consequence is that materials designed with our novel deep centers
achieve transparency at a carrier injection which is four-orders-of-magnitude lower than in all technologically important
semiconductors. Simultaneously, and surprisingly, our novel deep centers in GaAs achieve an optical gain, Einstein B coefficient,
and radiative efficiency significantly larger than in direct-band-gap materials at 1.3–1.5 μm. We engineered this dramatic
reduction of the injection to achieve transparency while retaining strong optical emission in our novel material by making
use of a Franck–Condon shift of absorption away from luminescent wavelengths.
PACS 71.55.Eq; 71.55.-i; 78.67.-n; 81.10.-h; 85.60.Jb 相似文献