Two types of mirror structures with saturable absorption are under consideration: monolithic mirrors grown from semiconductor materials and mirrors with a dielectric reflector, with quantum well containing semiconductor structures transferred to the dielectric. Both types of mirrors manifest high reflectivity in the NIR range of the spectrum: the table width is about 100 nm for semiconductor reflectors and more than 200 nm for dielectric reflectors. It is shown that a maximum depth of absorption modulation from 1 to 40% is possible. The recovery time of the saturable absorber (2 ps) makes these mirrors significantly fit for using in lasers with a pulse repetition rate of 1 GHz.

A double-modulation technique was developed and tested experimentally for the detection of the kinetics of reflection of subpicosecond pulses of probe near-IR radiation from a sample (or transmission through it) after the action of exciting radiation pulses. A probe signal is detected at a frequency that is equal to the sum of stabilized unequal frequencies of interrupting exciting and probe radiations, thus permitting the elimination of the contribution of scattered exciting radiation from the valid signal. When detecting the semiconductor- sample reflectance kinetics with a time resolution of 0.13 ps, the reflection sensitivity ΔR/R = 5 × 10–6 was reached.

Collision-induced stimulated photon echo generated at transition 0 <-> 1 was analyzed theoretically and investigated experimentally in the gaseous mixture of ytterbium vapour diluted with a large amount of buffer gas xenon in the presence of a weak longitudinal magnetic field. The inter-combination transition of Yb-174 (6s(2)) S-1(0) <-> (6s6p) P-3(1) was used; all experimental parameters were carefully controlled for their correspondence to the broad spectral line conditions. The curve representing the collision-induced stimulated photon echo variations versus a weak magnetic field strength showed very good agreement with the corresponding theoretical curve; this agreement permitted getting the decay rates for Yb-174 level P-3(1) orientation and alignment in collisions with Xe.

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For the first time, the collision induced stimulated photon echo generated at transition S-1(0) -> P-3(1) of Yb-174 (type 0-1) in the mixture of gases Yb + Xe was investigated in the presence of weak longitudinal magnetic field, with experimental parameters corresponding to broad spectral line conditions. Comparison of the experimental echo amplitude versus magnetic field strength dependence with the theoretical curve shows a very good agreement, giving rise to an improved estimate for the difference between alignment and orientation decay rates.

1 Институт физики полупроводников им. А.В. Ржанова СО РАН 2 Новосибирский государственный университет 630090, г. Новосибирск, ул. Пирогова, 2 * E-mail: rubtsova@isp.nsc.ru DOI:10.31868/RFL2018.184 Предложенные в 90-е годы [1] полупроводниковые зеркала с насыщающимся поглощением, известные в англоязычной литературе как SESAM (semiconductor saturable absorbtion mirror), остаются эффективным и сравнительно недорогим способом получения режима пассивной синхронизации мод для лазеров среднего уровня мощности. В разработке таких зеркал, которые должны иметь высокую отражательную способность в области усиления лазера, обеспечивать заданный уровень насыщаемых потерь при минимуме ненасыщаемых потерь и обладать высоком быстродействием, важно развивать методы диагностики. Мы имеем примеры успешного дизайна и изготовления таких зеркал в ИФП СО РАН, а также их использования в фемтосекундных лазерах Yb 3+ :KY(WO4)2 [2] и Yb:KGW ближнего инфракрасного диапазона [3] – совместно с БНТУ, Минск. Эти результаты вдохновляют на поиски путей расширения диапазона доступных плотностей мощности для таких зеркал. Это направление исследований актуально ещё и потому, что зеркала, разработанные для конкретного лазера, оказываются лучше коммерчески доступных. В докладе представлены результаты исследования имеющихся зеркал с насыщающимся поглощением с высоким быстродействием в более широком диапазоне интенсивностей излучения накачки методом накачка-зондирование. Результаты интерпретированы в рамках модели, учитывающей вклады в отражение фото-генерированных экситонов и свободных носителей заряда [4]. Обсуждаются проблемы, связанные с дальнейшим ростом допустимой для нормального функционирования и целостности зеркал плотности сфокусированного излучения и способы их преодоления путем изменения конструкции.

The photon echo formed at the intercombination (6s(2)) S-1(0) <-> (6s6p) P-3(1) (type 0-1) transition of ytterbium atoms Yb-174 by two pulses of resonance radiation was investigated as a function of the time delay between exciting pulses (the photon echo kinetics). For exciting radiation pulses of the same linear polarization in the delay region 70-250 ns, the photon echo kinetics is close to the exponential one both in pure ytterbium and in its mixtures with buffer gases Ar and Xe. The relaxation cross sections were determined for all three cases, taking into account the isotopic composition of xenon and ytterbium. For the first time, the complex nonmonotonous kinetics of the collision-induced photon echo was observed in the same mixtures for the forming pulses of linear mutually orthogonal polarization. The relative efficiency of buffer atoms Yb (other isotopes than Yb-174), Xe and Ar in the collision-induced photon echo formation was obtained.

A new idea based on the collision-induced stimulated photon echo in the presence of weak longitudinal magnetic field is applied to the depolarizing collisions research in a gaseous mixture of ytterbium vapour with xenon. Comparison of experimental data with theoretical prediction for the collision-induced stimulated photon echo in the weak magnetic field shows that the alignment decay rate of state 3P1 in 174Yb is higher than the orientation decay rate.

The reflectivity kinetics of saturable absorbers based on semiconductor quantum wells was investigated with differential reflectivity Delta R/R = 10-5 accuracy by the pump probe technique for energy fluxes (0,27-5.40).1014 photons cm 2 using a femtosecond Yb3+:KY(WO4)(2) laser, Three kinds of saturable absorbers were investigated: absorbers based on multiple quantum wells with nano -structured barriers grown over a mirror; the same absorber grown over the substrate; absorbers on multiple quantum wells separated by thick barriers grown over the substrate, The substrate contribution gives a stationary background and a two -photon absorption peak for small probe delays, which complicates the kinetics of the samples grown over the substrate. For absorbers grown over the mirror two exponential decays with a short relaxation time tau(1)= 0.2-0.4 ps and with a long relaxation time tau(2) = 7.8 ps were obtained, The last relaxation time is remarkably shorter than in quantum wells with thick barriers.

The kinetics of mirrors with saturable absorption is investigated by a pump-probe singlefrequency technique in reflection of femtosecond pulse radiation with the central wavelength of 1040 nm. The double modulation method with probe radiation detection at the summary frequency allows suppressing the scattered pump radiation contribution and reaching the reflection change sensitivity at a level of 10−5. The kinetics of recovery of the linear reflectivity of the mirror including resonant quantum wells with nanostructured barriers is studied for the surface density of photons in pump pulses of (0.3–5.8) · 1014 cm−2. The time of electron-hole recombination (7.8 ps) is found to be appreciably shorter than in samples with quantum wells separated by thick barriers; the time of ionization of excitons localized in quantum wells (0.2–0.4 ps) increases with the pump intensity.

It was shown experimentally in ytterbium vapour at the transition (6s(2)) S-1(0) (6s6p) P-3(1) (type 0 <-> 1), that the weak longitudinal magnetic field destroys the collision induced two-pulsed photon echo generated in a gas mixture with heavy atomic buffer; in agreement with theoretical prediction.

Our research into photon echo (PE) and stimulated photon echo (SPE) in ytterbium vapour at the transition 0 ↔ 1 is summarized. We present the polarization properties of PE and SPE in pure gas, anisotropy of collision relaxation, collision-induced PE and collision-induced SPE.

Experimental results were obtained at the inter-combination transition (6s(2)) S-1(0) <-> (6s6p) P-3(1) (type 0 <-> 1) of Yb-174 vapour for several combinations of linear and circular polarizations of three exciting pulses. A peculiar case appears when the first pulse has linear polarization crossed with the polarizations of the second and third exciting pulses: the stimulated photon echo (SPE) does not appear in pure ytterbium; dilution of ytterbium by Xe implies a collision induced SPE with non-monotonic amplitude dependence on buffer pressure, and with polarization along the first exciting pulse. For circular polarizations, when the first exciting pulse has polarization opposite to the circular polarizations of the second and third pulses, the SPE does not appear either in pure ytterbium vapour or in its mixture with a heavy atomic buffer. SPE polarization dependence on the third exciting pulse area was demonstrated. This result may be of interest for optical data processing. All the results agree with theoretical predictions.

Collision-induced photon echo generated in ytterbium vapour at the transition (6s2) 1S0 ↔(6s6p) 3P1 (transition of the type 0 ↔ 1) was investigated in the presence of the longitudinal magnetic field strength between zero and 5.6 G. For the weak magnetic fields, not over 0.27 G, the collision-induced echo signal decreases with magnetic field increase. Further increase of the magnetic field strength reveals oscillatory behavior of the echo signal. Experimental results are in qualitative agreement with theoretical predictions.

The polarization of the stimulated photon echo (SPE) generated in a gas at the 0↔1 transition was analysed for the first time for all combinations of three linearly polarized resonant radiation exciting pulses. Experimental results were obtained for the SPE generated at the inter-combination transition (6s2) 1S0↔(6s6p) 3P1 of 174Yb (type 0↔1). In the case where one of the exciting pulses had linear polarization crossed with that of the other two exciting pulses, the SPE polarization proved to be linear and parallel to the crossed pulse polarization. When the second and third exciting pulses had linear polarizations crossed with that of the first one, no SPE in pure ytterbium appeared. For high dilution of ytterbium with the Xe atomic buffer, a collision-induced SPE appeared and showed a non-monotonic amplitude dependence on the buffer pressure and a polarization coinciding with the polarization of the first pulse.

For the first time, stimulated photon echoes (SPE) generated in a gas at the transition 0↔1 by circularly polarized resonant radiation pulses were analysed both theoretically and experimentally. Experiments were performed for the inter-combination transition (6s2) 1S0↔ (6s6p) 3P1 of 174Yb (type 0↔1). The SPE generated by three radiation pulses of identical circular polarizations has the same circular polarization. When the second or the third exciting radiation pulse has a circular polarization opposite to that of the other two pulses, the SPE polarization coincides with that of the oppositely polarized pulse. Finally, when the first exciting radiation pulse has a circular polarization opposite to the circular polarizations of the second pulse and third pulse, the SPE does not appear either in pure ytterbium vapour or in its mixture with a heavy atomic buffer.

Situations when a set of resonant radiation pulses can not create photon echo (PE) or stimulated photon echo (SPE) at relatively simple optically allowed transition in a gas are investigated. Some cases when dilution by atomic buffer can imply PE or SPE generation are analyzed. Discovered experimentally mechanisms, amplitudes, and polarization properties of these collision induced PE and collision induced SPE are in agreement with theoretical predictions. Dynamic Stark suppression of photon echo is also studied.