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The photochemical reactivity of diphenyldiazomethane 1 and phenyl 1- and 2 adamantyl diazomethanes 2 and 3, respectively, was investigated by transient absorption spectroscopy (TA). Photoelimination of N2 upon UV excitation takes place in the anti-Kasha ultrafast photochemical reaction from the upper excited singlet states to deliver singlet carbenes, which were, in the case of 1 and 2, detected by fs-TA. The reactivity of the carbenes differs with respect to the substituent at the carbene center. The singlet car-1 in a nonpolar solvent delivers the triplet carbene by intersystem crossing (ISC). Singlet car-2 does not undergo ISC but reacts in the intermolecular insertion reactions into C–H bonds. Car-3 has an α-C–H bond next to the carbene center and reacts rapidly in the intramolecular C– H insertion reaction to deliver alkene, precluding its detection by fs-TA. However, the isolation of ketone photoproducts from 3 is highly indicative of triplet car-3’s intermediate formation. The TA spectra from the S1–S3 states of 1–3 were computed using time-dependent density functional theory, while the multiconfigurational perturbation theory to the second order was used for the absorption spectra of the corresponding singlet and triplet carbenes. The modeled and measured spectra are in good agreement, and the computations corroborate the assignments of the key short-lived intermediates.

Ultrafast dynamics of three different diazoalkanes (1, 2, 3) was investigated with femtosecond transient absorption spectroscopy using third harmonic generated UV pump pulses and broadband white light supercontinuum probe pulses. Diazoalkanes are important reagents in organic synthesis that produce carbenes via photoinduced elimination of nitrogen. Such reactions can occur from higher excited states which allows investigation of anti-Kasha photochemistry.[1] Photogenerated carbenes are widely used as short- lived reactive intermediates where the spin state of carbene plays an important role. We observe intersystem crossing from singlet to triplet state of the carbene[2] formed from 1 but not from 2 or 3. Although being different only by substitution of adamantane for phenol, 2 and 3 display different spectral and temporal features from 1. We aim to unravel reaction pathways with combining measurements and computational studies.

By using ultrafast transient absorption, photochemical generation of QM from naphthol and adamantyl-naphthol in a solution at room temperature was tracked in real time with time resolution of 300fs. Surprisingly, although molecules possess the same chromophore (naphthol) the photochemical pathway of QM generation by photodehydration is different. In the case of adamantyl-naphthol, ground state QMs are produced directly via excited state QMs with no other intermediates or species participating in a reaction.

Results of femtosecond pump-probe study of excited-state dynamics of all-trans-β-carotene are presented with focus on experimental details and advanced data analysis schemes such as soft-modeling multivariate curve resolution alternating least-squares analysis (MCR-ALS) and the generalized two-dimensional (2D) correlation spectroscopy data analysis. Conclusion drawn from the analysis indicate that the observed S* state is not the hot S1v’ state but instead a separate singlet state, a different conformational state of a S1 state that could be formed by twisting of the carotenoid backbone.

Thanks to increased availability of ultrashort lasers pulses (ps and fs) there are numerous studies dealing with ultrashort laser interaction with hard dental tissue which could lead to replacement of mechanical drills. In our research we employed fs pulses generated in a Ti:Saphire amplifier with pulse length of 120 fs at 800 nm and with 1 kHz pulse repetition to form cavity in a tooth sample. Simultaneously with formation of cavity we measured the temperature with thermocouple at the place where the dental pulp was and numerically modelled the heat transfer in order to assess safe parameters for future employment of fs laser for hard dental tissue preparation. Compared to previous work, we used lower repetition rates and slower scanning speeds. We used finite element analysis to develop fully three-dimensional heat conduction model with associated boundary conditions. Values of the thermophysical properties are taken from literature. Fourier’s heat conduction equations were used. We modeled the incident heat flux from the laser as a moving spatially distributed volumetric heat source where we used the known value for the optical penetration depth in dentine and Beer-Lambert law to model incident heat flux in the direction normal to the incident top surface. The conditions for safe operating are found and can be checked by altering dimensions, powers, and scanning speeds in our model. This work can contribute to assessment of the use of fs lasers in restorative dentistry

Ultrafast dynamics of phenolic and adamantyl compounds was studied by measuring time- resolved absorption changes using transient absorption spectroscopy technique (TA). Phenolic and adamantyl compounds used were adamantyl-phenol, BODIPY- phenol, naphtol and adamantyl-naphtol. Probe pulse spectrum was supercontinuum generated in calcium- fluoride (CaF2) window pumped by 800 nm or 400 nm. In the case of pumping with 400 nm white light spectrum shifted more towards UV but is inherently more unstable because the second harmonic (400 nm) is less stable than fundamental (800nm) by itself. Pump beam was frequency doubled and prism compressed output of the NOPA. NOPA output gave wavelength variability in the range 500-700 nm which was then compressed by chirped mirrors and frequency doubled in BBO crystal which resulted in wavelength variability in the range 260-300 nm. After prism compressing the pump pulse, durations of sub 20 fs were obtained which roughly defined the resolution of the measurement. Measurements were performed at 1kHz repetition rate with pump pulse energies at around 100 nJ. All of the samples undergo photochemical reaction of quinone methide generation after excitation in UV at around 275 nm. Significance of the product molecules is that quinone methides alkylate and cross-link DNA strands, which underpins the antiproliferative activity of some antitumor antibiotics. BODIPY- phenol is interesting because it does not undergo any photochemical reaction when excited into its S1 state (500 nm) but it does undergo a quinone methide generation reaction when excited into its S3 state (276 nm). At last we wanted to investigate the difference in the dynamics of naphtol and adamantyl-naphtol samples. All of the reactions are reversible, i.e. quinone methides transfer back to its initial molecules after some time so the samples are preserved but required to be flown because the lifetime of quinone methides is comparable to laser’s repetition rate so by flowing the sample we ensure that every pump pulse gets absorbed by unexcited (‘fresh’) part of the sample and we are avoiding multiple excitations. Currently, partial quantum chemical TD-DFT simulations of the photochemical reaction of the adamantylphenol are available as calculations are still in progress. Simulations of the reactions of the other 3 samples are still yet to be done. Goal of the TA measurements of the first sample was to investigate which reaction pathway do molecules take as adamantyl- phenols can undergo a reaction in which products (quinone methides) are generated in its excited state then relaxed to its ground state and transferred back to its initial molecules or initial molecules can take a reaction pathway which includes conical intersection by which products are generated immediately in their ground state in which case TA measurements show one transient state less, etc. Spectral dynamics of the first sample was quantitatively investigated and the results show fast dynamics at around 800 fs which will be assigned to one of the reaction pathways after TD-DFT quantum chemical calculations are finished. Other 3 samples were qualitatively investigated, spectral dynamics and time dynamics is shown and compared. Due to ultrashort pump pulses (very large intensity at the focus) coherent artifacts of the TA method, such as cross-phase modulation and two-photon absorption were present and largely interfered with the global analysis which had to be done to have a complete description of the photochemistry of adamantyl-phenol. Extraction and smoothening of coherent artifacts is currently being done which will greatly help with the global analysis and time dynamics analysis such as fitting the exponential decay at a certain wavelength and obtaining decay rates.

Ultrafast dynamics of adamantyl-phenol was studied by measuring time-resolved absorption changes using transient absorption spectroscopy technique. Adamantyl-phenol molecules dissolved in acetonitrile-water mixture (1:1) were excited by femtosecond laser pulses with the wavelength of 267nm (pump pulse) while their absorption as a function of time was tested with the probe pulses. Probe pulse spectrum was supercontinuum generated in calcium-fluoride (CaF2) crystal pumped by 800nm. Resulting signal was positive, meaning the molecules have negligible fluorescence. After the excitation to first excited electronic state photochemical reaction of photodehydratation takes place and the formation of quinone methide (QM) begins via conical intersection. Quantum-chemical TD-DFT calculations indicate immediate ground state QM formation and the simulated ground state QM spectra is compared to measured one. Obtained results proved the functionality of the used experimental setup which will be used for future research and ultrafast transient absorption measurements of many different samples.

Metodom ultrabrze tranzijentne apsorpcije proučena je ultrabrza dinamika molekule rodamina B (C28H31ClN2O3) te dvolomnost inducirana Kerr efektom u benzenu i otopini vode i acetonitrila (C2H3N). Molekule rodamina B otopljene u acetonitrilu pobuđivane su femtosekundnim laserskim pulsevima valne duljine 267nm (pumpni puls) koji su dobiveni u postavu za stvaranje trećeg harmonika dok je njihova apsorpcija kao funkcija vremena ispitivana probnim pulsevima čiji je spektar superkontinuum generiran u kalcij-fuoridnoj (CaF2) pločici. Mjerenjima je dobiven signal visoke vrijednosti koji se sastojao od pozitivnog i negativnog dijela. Pozitivan dio uzrokovan je apsorpcijom višeg stanja u koje su molekule pobuđene pumpnom zrakom dok je negativan signal uzrokovan stimuliranom emisijom i izbjeljivanjem osnovnog stanja što je zaključeno promatranjem emisijske i apsorpcijske krivulje molekule rodamina B. Mjerenjem dvolomnosti inducirane Kerr efektom proučeno je trajanje te raspored spektralnih komponenti probnog pulsa u vremenu. Zaključeno je da je puls uvelike proširen u vremenu u odnosu na fundamentalni te da posjeduje pozitivan cvrkut (niže frekvencije se nalaze u vodećem krilu pulsa). Dobiveni rezultati pokazuju da je korišteni postav moguće koristiti za buduća proučavanja raznih uzoraka metodom ultrabrze tranzijentne apsorpcije.

Objectives Ultra-short laser pulses have the potential for ablation of various materials without thermal damage and with high precision which makes them very convenient tool in many applications. The main advantages of material structuring by ultra-short pulses are localized energy deposition, well-defined ablation thresholds and limited heat diffusion into material. Precise drilling, sufficient material removal rate (mm3 /s) and low temperature rise in tooth pulp (less than 5 K) are main prerequisite to using femtosecond laser (fs) in cavity preparation. Most of the studies so far dealt with evaluating ablation efficiency in dentin and enamel and very few with heat transfer to tooth tissue. Methods The fs pulses used in this study were generated in a Ti: Saphire amplifier with pulse length of 120 fs at 800 nm and with 1 kHz pulse repetition. The laser light is directed through a microscopic objective onto the tooth sample which were mounted on computer controlled XY translation stage. The tooth samples were prepared by diamond blade saw and thermocouple of small diameter was inserted where the pulp used to be. Simultaneously with rectangular scanning of the sample the temperature was monitored at 10 times per second rate. Results Several rectangular holes in dentin and enamel were drilled using different scanning speeds and laser intensities in order to measure temperature rise and find optimal parameters for practical use of fs laser in restorative dentistry. Temperature inside pulp chamber depended on different laser intensities and ranged between 1, 5 – 3, 7 °C. Conclusions Fully three-dimensional heat conduction model with associated boundary conditions by employing finite element analysis was developed in this study. Fourier’s heat conduction equations were used with thermophysical constants from literature. Ultra-short laser pulses can be safely used for cavity preparation.

International audience; We investigated static and dynamic lattice properties in a quasi-one-dimensional charge-ordered semiconductor (NbSe 4) 3 I by using Raman, femtosecond pump-probe spectroscopy and x-ray diffraction. In addition to a welldocumented pseudo-Jahn-Teller ferrodistortive structural transition at T C = 274 K, where the displacements of Nb ions lead to ferroelectric (FE) in-chain polarization with opposite direction in adjacent chains, all methods suggest an additional lowering of symmetry at T * ≈ 160 K. Although antiferroelectric (AFE) phase is partially formed at T C , our results consistently point to an enhancement of the interchain order at T * , thus leading to AFE order-disorder transition, as supported by the earlier dielectric and structural studies.

Razumijevanje procesa relaksacije i prijenosa energije u kompleksnim molekulskim sustavima nezamislivo je bez spektroskopske analize na ultrakratkoj (femtosekundnoj) vremenskoj skali. Najraširenija i najjednostavnija tehnika tranzijentne apsorpcije daje vremensku ovisnost apsorpcije molekule nakon pobuđenja u obliku dvodimenzionalne matrice koja se zatim može analizirati metodom globalne analize gdje se testiraju razni kinetički modeli relaksacije sa ciljem dobivanja najboljeg slaganja sa eksperimentom. Ovakav pristup očigledno se oslanja na određeni model, no postoje i alternativne analitičke metode koje koriste princip multivarijacijske analize i ne zahtijevaju upotrebu modela. Na nekoliko primjera pokazati ću primjenu ovih metoda. All-trans beta karoten važan je pigment koji sudjeluje u procesu fotosinteze a čija eletronska struktura još nije do kraja razrješena. Najveće debate I dalje se vode oko kontroverznog S* tamnog stanja smještenog između dobro poznatog prvog i drugog singletnog pobuđenog stanja beta karotena. Multivarijacijska analiza tranzijentne apsorpcije ukazuje na vjerojatnost postojanja tamnog stanja kao novog elektronskog stanja. Na jednom drugom primjeru kromofore s razgranatom strukturom (heksamer) rezultati globalne analize tranzijentne apsorpcije i mjerenja tranzijentne anizotropije omogućila su detaljan uvid u kompleksni relaksacijski mehanizam ove molekule. Struktura heksamera vrlo je slična strukturi dobro poznatog pigmenta alokocijanina (APC). Inicijalno pobuđena delokalizirana ekscitonska stanja nastala zbog snažnog vezanja dipolnih momenata prijelaza dviju kromofora unutar svake od tri grane heksamera lokaliziraju se u jednoj od grana. Slijedi relaksacija u dugoživuće ekscimersko stanje koje se formira u grani sa najnižom energijom. Utjecaj ekscitonskih i ekscimerskih stanja na dinamiku i mehanizam prijenosa energije u ovom sustavu važan je za primjenu u razvoju novih ekasnijih solarnih ćelija. U zadnjem primjeru razmatrana je relaksacija metalnih nanoklastera sačinjenih od atoma zlata. Nanoklasteri sačinjeni su od dovoljno malog broja atoma (nekoliko desetaka) da njihovi optički spektri pokazuju svojstva molekularnih spektara. U pobuđenom nanoklasteru najčešće dolazi do intramolekularnog prijenosa naboja između liganada zaštitne ovojnice i jezgre koju čine atomi metala. Mjerenjem tranzijentne apsorpcije u otapalima različite polarnosti otkriveno je da polarnost otapala bitno mijenja dinamiku relaksacije odnosno proces prijenosa naboja. Također su uočene oscilacije u kinetičkim prolima tranzijentnih spektara nastale zbog elekron-fonon vezanja. Period ovih oscilacija također je pokazao ovisnost o polarnosti otapala čime su dobivene nove važne informacije o relaksacijskom mehanizmu ovih zanimljivih sustava

Istraživana je nelinearna interakcija atomskih para alkalija i laserskih izvora u različitim uvjetima. Elektromagnetski inducirana transparencija nastaje zbog interakcije izmedu kontinuiranih lasera i atomskih para rubidija u V shemi vezanja stanja. Transparencija je ponajviše rezultat smanjenja populacije osnovnog nivoa zbog saturacije prijelaza vezanog jakim veznim laserom. Usporedbom s teorijskim modelom zaključeno je da, osim saturacije, i stvorena koherencija u atomima takoder doprinosi povećanju transparencije. Istražena je interakcija femtosekundnih pulseva s rezonantnim parama rubidija različitih koncentracija korištenjem tehnike frekventno razlučivog optičkog uzorkovanja (FROG). Efekti prostiranja femtosekundnih laserskih pulseva optički tankim sredstvom mogu se opisati stvaranjem 0-π pulseva kod kojih dolazi do značajnih vremenskih modulacija električnog polja pulsa. Dobiveni rezultati usporedeni su s teorijskim simulacijama prostiranja pulsa rezonantnim medijem u okviru linearne disperzijske teorije, čime su odredene granice primjene FROG tehnike na karakterizaciju kompleksnih pulseva. FROG tehnikom mjereni su i pulsevi nakon rezonantne nelinearne interakcije s gustim parama rubidija. Rezultat takve interakcije je konusna emisija. U okviru aproksimacije adijabatske slijednosti zbog različitog predznaka razlike izmedu centralne valne duljine pulsa i valne duljine najbliže rezonantne linije dolazi do prostornog samofokusiranja odnosno samodefokusiranja. U frekventnoj domeni dolazi do samomodulacije faze pulsa koja takoder ovisi o predznaku razlike valnih duljina pulsa i rezonantne linije. Mjerenja faze pulsa potvrdila su gore navedene teze kroz različito ponašanje spektralne i vremenske faze pulsa ovisno o položaju spektra pulsa u odnosu na rezonanciju. Degenerirano četverovalno miješanje (DFWM) femtosekundnih pulseva u natrijevim parama mjereno je u režimu tranzijentne rešetke u BOXCARS geometriji. Proučavan je vremenski razvoj UV emisije uzrokovane višefotonskom apsorpcijom u mediju kao i vremenski razvoj generiranog DFWM signala. Modulacije intenziteta tranzijenata objašnjavaju se kvantnim udarima zbog koherentnog pobudivanja stanja fine strukture. Period modulacije odreden je razlikom energija nivoa različitog spina. Modulacije eksponencijalno trnu u vremenu odredenom karakterističnim vremenom relaksacija koherencija. This thesis examines nonlinear interaction between the alkali vapor and different laser sources in various conditions. Interaction between cw lasers and rubidium atomic vapor in the V coupling scheme produces electromagnetically induced transparency, where the main contibution to the induced transparency comes from nonlinear saturation effects on the strongly coupled transition. By comparing the theoretical model and experimental results it is shown that part of the induced transparency comes from coherences induced between the uncoupled transitions. Interaction between the femtosecond (fs) pulses and resonant rubidium atomic vapor is investigated using frequency-resolved optical gating (FROG) technique. Propagation of the fs pulses in the optically thin medium results in formation of 0-π pulses which have strong modulation of the electric field amplitude on a time scale of several ps. Since it is impossible to cover the whole time evolution of the 0-π pulse, the FROG reconstruction process can never be accurate enough. Propagation of nearly-resonant pulses through the optically thick rubidium vapor results in conical emission due to the self-focusing of the pulse in time domain and self-phase modulation in frequency domain. Using the adiabatic following approximation, it is shown that the physical processes depend on the sign of detuning of pulse wavelength from the rubidium resonance lines. The pulse electric field phase in both domains, retrieved from the FROG measurements, shows different behavior with respect to the sign of the pulse detuning. Degenerate four-wave mixing (DFWM) of femtosecond pulses in sodium vapor was performed in BOXCARS geometry. When on three-photon atomic resonance, transient grating signal appeared on the frequency of the third harmonic of fundamental frequency at negative delay times with modulations whose period corresponds to the energy difference between the coherently excited upper state fine stucture levels. In positive time delays signal appears at the fundamental frequency and there is no signal at third harmonic frequency. This is explained by destructive interference between the fundamental field and the field at the frequency of third harmonic. The amplitudes of modulations at negative time delays have exponential time dependence determined by the coherence relaxation time.

It is not a new idea to think of the use of the femtosecond laser in the cooling and trapping process of atoms. However, since the pulse train has a broad frequency spectrum of the equidistant comb lines, it has the capacity to supply the frequencies of comb lines that almost simultaneously may serve in the photoassociation and molecular stabilization processes. Our previous work may serve in pointing to the way of realizing several tasks at once, or in several steps. We shall start with the rubidium vapor which is laser cooled in a usual way (MOT), and then will use appropriately shaped optical frequency comb to perform photoassociation. The details of the proposed experiments will be discussed at the conference.

Istraživana je nelinearna interakcija atomskih para alkalija i laserskih izvora u različitim uvjetima. Elektromagnetski inducirana transparencija nastaje zbog interakcije između kontinuiranih lasera i atomskih para rubidija u V shemi vezanja stanja. Transparencija je ponajviše rezultat smanjenja populacije osnovnog nivoa zbog saturacije prijelaza vezanog jakim veznim laserom. Usporedbom s teorijskim modelom zaključeno je da, osim saturacije, i stvorena koherencija u atomima također doprinosi povećanju transparencije. Istražena je interakcija femtosekundnih pulseva s rezonantnim parama rubidija različitih koncentracija korištenjem tehnike frekventno razlučivog optičkog uzorkovanja (FROG). Efekti prostiranja femtosekundnih laserskih pulseva optički tankim sredstvom mogu se opisati stvaranjem 0-pi pulseva kod kojih dolazi do značajnih vremenskih modulacija električnog polja pulsa. Dobiveni rezultati uspoređeni su s teorijskim simulacijama prostiranja pulsa rezonantnim medijem u okviru linearne disperzijske teorije, čime su određene granice primjene FROG tehnike na karakterizaciju kompleksnih pulseva. FROG tehnikom mjereni su i pulsevi nakon rezonantne nelinearne interakcije s gustim parama rubidija. Rezultat takve interakcije je konusna emisija. U okviru aproksimacije adijabatske slijednosti zbog različitog predznaka razlike izmedu centralne valne duljine pulsa i valne duljine najbliže rezonantne linije dolazi do prostornog samofokusiranja odnosno samodefokusiranja. U frekventnoj domeni dolazi do samomodulacije faze pulsa koja takoder ovisi o predznaku razlike valnih duljina pulsa i rezonantne linije. Mjerenja faze pulsa potvrdila su gore navedene teze kroz različito ponašanje spektralne i vremenske faze pulsa ovisno o položaju spektra pulsa u odnosu na rezonanciju. Degenerirano četverovalno miješanje (DFWM) femtosekundnih pulseva u natrijevim parama mjereno je u režimu tranzijentne rešetke u BOXCARS geometriji. Proučavan je vremenski razvoj UV emisije uzrokovane višefotonskom apsorpcijom u mediju kao i vremenski razvoj generiranog DFWM signala. Modulacije intenziteta tranzijenata objašnjavaju se kvantnim udarima zbog koherentnog pobuđivanja stanja fine strukture. Period modulacije određen je razlikom energija nivoa različitog spina. Modulacije eksponencijalno trnu u vremenu određenom karakterističnim vremenom relaksacija koherencija.

Generiranje trećeg harmonika (TH) upadnog zračenja u nekom mediju je uvjetovano postojanjem optičke supsceptibilnosti trećeg reda. Treći harmonik je pojačan ukoliko postoji rezonantni nivo u sistemu koji odgovara trofotonskoj (TP) pobudi. U slučaju kada se dvije laserske zrake susreću pod kutem različitim od nule do zračenja trećeg harmonika dolazi u dva pravca koja leže između upadnih snopova – zračenje je s kutno ugođenom fazom i pojačano u odnosu na zračenje emitirano u upadnim pravcima. Ovo relativno pojačanje kutno ugođenih zraka prema kolinearnom zračenju može pod određenim uvjetima biti i posljedica interferencije pri trofotonski rezonantnoj pobudi atomskog prijelaza. Kada je TP prijelaz s osnovnog na pobuđeni nivo ujedno i dipolno dozvoljeni (jednofotonski) prijelaz, dolazi do interferncije polja koje je nastalo miješanjem fundamentalnog polja (sum-frequency field) i rezonantnog (jednofotonskog) polja. Destruktivna interferencija dovodi do slabljenja rezonantnog zračenja. U fazno ugođenim pravcima nema polja upadnog zračenja pa tamo niti ne dolazi do destruktivne interferencije. Generiranje TH u uvjetima kutno ugođene faze promatrano je u atomima natrija gdje je fundamentalno fs pulsno zračenje ugođeno na 804 nm što dovodi do TP pobuđenja 6p nivoa i stvaranje trećeg harmonika na 268 nm. Uvedena je i treća zraka na fundamentalnoj frekvenciji u fazno ugođenoj geometriji pomoću koje se može kontrolirati treći harmonik koji je generiran od prve dvije zrake. Ovo otvara mogućnosti za optičko uključivanje i isključivanje (optical switching). Mjerenja su također značajna radi moguće primjene u laserskoj mikroskopiji pomoću trofotonske apsorpcije.

U ovom radu istražujemo 5S-5D dvofotonske prijelaze u atomima rubidija (85Rb i 87Rb) koji nastaju kao posljedica interakcije femtosekundnog frekventnog češlja s atomskim nivoima oba rubidijeva izotopa pri sobnoj temperaturi. Naseljenost 5D pobuđenog stanja određujemo pomoću fluorescentnog signala na 420 nm, koji nastaje kao rezultat relaksacije s 5D na 5P atomsko stanje, od kud atomi spontanom emisijom prelaze u 5S osnovno stanje. Također, ispitivali smo ovisnost dvofotonskog fluorescentnog signala uslijed interakcije frekventnog češlja s perturbiranim energetskim nivoima rubidijevih atoma pod utjecajem vanskog magnetskog polja. Uočili smo da za različite jakosti vanjskog magnetskog polja koje vode do poželjnog cijepanja energetskih atomskih nivoa dolazi do pojačanja dvofotonskog fluorescentnog signala. Za očekivati je da fluorescentni signal bude znatno pojačan kada je atomska Larmorova frekvencija 5S1/2 osnovnog stanja višekratnik frekvencije repeticije femtosekundnog lasera (frep=80.53 MHz). Račun pokazuje da se Zeemanova koherencija prvog reda dobije za 114.8 G (za 87Rb, Fg=1) te za 115.3 G (za 87Rb, Fg=2), što naši eksperimenti i potvrđuju.

Površina pulsa je u koherentnoj optici direktno povezana sa populacijom atoma koja ostaje u pobuđenom stanju nakon prolaska pulsa kroz atomsku paru. Puls površine nula (0-pi puls) je puls koji nakon prolaska kroz paru ostavlja atomsku populaciju u nepromijenjenom stanju. U našem istraživanju koristili smo vlak 0-pi pulseva u svrhu proučavanja procesa koherentne akumulacije populacije. Propagacijom femtosekundnog pulsa male snage kroz rezonantnu rubidijevu paru generiran je 0-pi puls. Ovakvo rezonantno oblikovanje pulsa mijenja vremenski oblik električnog polja, a jačina promjene ovisi prvenstveno o koncentraciji Rb atoma u pari. Generirani puls je karakteriziran eksperimentalno koristeći FROG (Frequency Resolved Optical Gating) metodu i teorijski u okviru linearne disperzijske aproksimacije. Pobuđivanjem Rb atoma vlakom pulseva za koji vrijedi da je vrijeme između dva pulsa kraće od vremena relaksacije pobuđenog atomskog stanja dolazi do efekata akumulacije populacije i koherencije. Pokazali smo da ovi akumulacijski efekti jako ovise o obliku električnog polja 0-pi pulsa unutar pulsnog vlaka. Za jako oblikovane pulseve efekti akumulacije potpuno iščezavaju. Eksperimentalni rezultati dobiveni upotrebom MDFCS (Modified Frequency Comb Spectroscopy) metode potvrđeni su odgovarajućim teorijskim modelom.

Kombinacija laserskih dioda u konfiguraciji sa vanjskim rezonatorom i poluvodičkog optičkog pojačala (engl. tapered amplifier) obično se naziva MOPA (master-oscillator power amplifier) sistemom. Takav sistem naročito je primjenjiv u situacijama gdje se zahtjeva laserski izvor velike snage i dobro definiranih optičkih svojstava (uzak spektar kontinuirano promjenjiv u što većem rasponu valnih duljina). Eksperimenti fotoasocijacije ultrahladnih atoma u ultrahladnih molekula tipičan su primjer. Kako se na Institutu za fiziku upravo dovršava magneto-optička stupica kao preduvjet za eksperimente sa ultrahladnim atomima i molekulama, izrada MOPA sistema nametnula se kao logična nadogradnja eksperimenta. Osim u području laserskog hlađenja, ovakvi sistemi su korisni i u proučavanju kvantne interferencije stomskih sistema: elektomagnetski inducirane transparencije. Ti efekti već su bili predmetom istraživanja grupe , ali je upravo nedostatak dovoljno snažnog, tzv. pumpnog lasera, umanjio kvalitetu dobivenih rezultata.

We shall discuss the direct observation of the velocity-selective optical pumping of the Rb ground state hyperfine levels induced by femtosecond pulse-train excitation. A modified direct frequency comb spectroscopy based on the fixed frequency comb and a weak cw scanning probe laser was developed and the experimental results are compared with theoretical evaluations in the context of the density matrix formalism. A mode-locked femtosecond laser is used to physically map the laser frequency comb into the velocity comb of the excited Rb atoms at room temperature. Upon resonant excitation by discrete optical frequencies the velocity distribution of the excited Rb atoms shows comblike structure. Simultaneously, velocity-selective population transfer occurs between ground-state hyperfine levels. This direct observation of the velocity-selective optical pumping of the ground state hyperfine levels induced by the femtosecond (fs) laser oscillator was also tested experimentally and theoretically in the case Cs D2 (852 nm) and Cs D1 (894 nm) cesium line. The frequency comb excitation changes the usual Doppler absorption profile into a specific periodic, comblike structure. Coherent control of atomic interactions with photons from the pulse-train caused accumulation effects of atomic coherences and populations. We further investigated the coherence accumulation effects in Rb atoms excited by a train of femtosecond pulses. The effect of the pulse train excitation on the cw probe laser transmission was investigated in dependence on the probe laser power. Results show that accumulation of population and coherence can be effectively reduced and eventually destroyed by increasing the cw laser intensity. A strong cw laser can therefore serve as a switch from the pulse-train to pulse-by-pulse type of interaction of Rb atoms with the fs laser. Our extended model well describes these findings. We introduced a new and simple method for determination of the absolute frequencies of comb lines within the optical frequency comb spectrum. A combined experimental and theoretical approach is presented which utilizes the effect of velocity selective optical pumping of hyperfine lines induced by the resonant frequency comb excitation. The laser pulse repetition frequency and carrier envelope offset are physically mapped onto the 87Rb ground state hyperfine level population velocity distributions. An iterative analytic solution to the optical Bloch equations describing the resonant pulse-train excitation of 4-level 87Rb atoms is found. Theoretical results are used to fit the observed mapping and obtain the parameters of the frequency comb, thus providing a practical algorithm which can be used in real-time measurements. Finally we added appropriate atomic Rb filter cell at different temperatures, which modifies amplitudes and phases of the frequency comb lines of the femtosecond laser. Therefore, the direct observation of the velocity selective optical pumping of the 87, 85Rb ground state 5 2S1/2 hyperfine levels induced by the modified femtosecond (fs) pulse train will be changed. A weak cw scanning probe laser will show amplitude changes of the modified comb lines when transmitted through Rb filter cell. Phase changes of the comb lines will be visible in the time domain behavior of the cw probe laser. We hope to apply atomic filter experiments in the selective pumping of the ultracold rubidium cloud in the photoassociation experiments.

We investigate the time dynamics of a four-level rubidium atomic system excited by a train of 0π pulses. Resonant 0π pulse shaping is achieved by propagation of weak femtosecond (fs) pulses through rubidium vapor and characterized using frequency resolved optical gating and within the linear dispersion theory. The excitation of the Rb atoms by 0π pulse train is studied experimentally by modified direct frequency comb spectroscopy and theoretically within the density-matrix approach. We show that accumulation of populations and coherences, as typical pulse train excitation effects, strongly depend upon the strength of the resonant 0π pulse shaping. Stronger pulse shaping reduces the coherent accumulation effects, eventually leading to the disappearance of the velocity selective optical pumping observed in the frequency domain.

We investigated the resonance D2 line of rubidium vapour by ECDL. The observed Doppler-broadend profiles consist of four lines, two of them resulting from 85Rb absorption and the other two from 87Rb. We measured line shape dependence on the magnetic field strength. Measured experimental results show good agreement with applied theoretical model.

In the present experiment the 5S-5D two-photon transitions in 85Rb and 87Rb atoms as a result of the interaction of the femtosecond frequency comb with atomic levels of both rubidium isotopes are investigated. The main problem in studying of two-photon transitions is in maximization of the excitation efficiency of the desired state. There are two general cases: those transition with an intermediate resonance and those in which the pulse spectrum is far detuned from an intermediate resonance. In order to investigate the dependence of the two-photon fluorescence signal as a result of interaction of the frequency comb with perturbed energy-level pattern an external magnetic field was applied.

In this work we report the CE from the dense rubidium vapor excited by the output from the mode-locked, 100 fs laser oscillator. The cone emission was observed only when the laser beam propagated through the high-density vapor (N>5x1016 cm− 3) and for the smaller densities no CE was observed. The laser central wavelength was changed in the wide interval, from the far blue wing of the D2 line to the far red wing of the D1 line. The observed CE in the far blue wing of the D2 line is interpreted as a consequence of the self-focusing and self-phase modulation according to the adiabatic following limit. The emission in the red wing of the D2 line shows defocusing of the propagating beam. In contrast to the CE reported so far, the bright central spot was not observed in the far blue wing. In the near blue wing the central spot appears due to the two- photon resonance at 778 nm. The temporal shape of the CE, measured by cross-correlation in the SHG crystal shows pulse breaking and extreme lengthening.

Induced transparency on a blue probe light in a Doppler broadened V-type mismatched system using a low power near infrared coupling diode laser was experimentally observed. Extensive theoretical model was used which utilizes the three-level V-type scheme. Appropriate modifications were made to take into account the hyperfine splitting of the ground 5 2S1=2 and excited 5 2P3=2 and 6 2P3=2 states. Calculated probe absorption line profiles reproduce with great accuracy measured absorption profiles. The influence of the 5 2P3/2 – 6 2P3/2 unlinked states coherence on the induced transparency was discussed in the context of the used theoretical model.

An experimental technique with enhanced sensitivity for the rubidium atom velocity comb observation will be presented. Rubidium atom velocity comb, observed as modulation of probe cw laser transmission, is a result of the velocity selective optical pumping induced by the resonant frequency comb excitation of Rb atoms. Resonant excitation of the rubidium atoms by discrete frequency comb optical spectrum results in the comb-like velocity distribution of the excited state hyperfine level populations and velocity-selective population transfer between the Rb ground state hyperfine levels. Decrease of the effective relaxation time of the Rb system is observed in the strong probe case, leading to the gradual change from the frequency comb excitation to the pulse by pulse excitation. We expand the theoretical model presented in our previous papers to directly calculate the probe laser absorption and show preliminary results which support the experimental data.

We studied the emission from dense rubidium vapor under 100-fs pulsed laser excitation in the 730–820 nm range. Self-focusing along with conical emission (CE) was observed for the excitation in the blue wing of D2 atomic resonance. This effect was earlier observed for fs-pulsed excitation in Cs2 molecules. For the far blue-wing excitation, the CE consists of a bright ring without central spot. The magnitude of the cone angle as a function of detuning from the resonance obeys ~∆-1.5 dependence (∆ being frequency detuning from the resonance) which is in accordance with the theory of self-focusing. The evidence of self-focusing along with the spectral broadening leads to the conclusion that the self-phase modulation generates observed phenomena. Self-defocusing occured for the red-wing excitation. When the laser wavelength is tuned at two-photon allowed transition at 778 nm, 5s1/2 → 5d3/2, the output pattern shrinks into a small spot whose wavelength is around 762 nm. This can be explained by two-photon resonantly enhanced FWM.

Naši nedavno objavljeni radovi opisuju rezonantno pobuđenje atoma rubidija diskretnim spektrom frekventnog češlja što dovodi do češljolike raspodjele brzina populacija pobuđenih hiperfinih nivoa i brzinski-selektivnog prebacivanja populacije između hiperfinih nivoa osnovnog stanja rubidija. Opažene modulacije apsorpcijskog profila probnog lasera posljedica su brzinski-selektivnog optičkog pumpanja induciranog pobuđenjem frekventnim češljem. Korištenjem fazno-osjetljivog pojačala eliminira se Dopplerova pozadina iz signala čime se povećava osjetljivost mjerenja te se opažaju samo modulacije u transmisiji probnog lasera. Struktura i intenzitet opaženih oscilacija jedinstveni su kod svake od četiri apsorpcijske linije i zrcale strukturu hiperfinih nivoa kao i vrijednosti odgovaraju¢ih dipolnih momenata prijelaza. Opažena promjena interakcije iz VSOP-a u režimu slabe probe do brzinski neovisnog optičkog pumpanja u režimu jake probe upućuju na zaključak da se priroda atomskog pobuđenja mijenja od pobuđenja nizom pulseva prema pobuđenju s pojedinačnim pulsom.

The frequency spectrum of the pulse train consists of a series of fringes separated by the pulse repetition rate. The fringes are regular in frequency space if the pulses in the pulse train have a defined phase relation relative to each other. In the systems where the atomic coherence relaxation time is longer than the pulse repetition period the atoms interact with the spectrum of the pulse train, and not with the spectrum of a single pulse. Modelocked, phase-stabilized femtosecond (fs) lasers with high repetition rates produce stabilized wide-bandwidth optical frequency combs. In our recent papers we presented the observation of the velocity selective population transfer between the Rb ground state hyperfine levels induced by fs pulse train excitation. We developed a modified direct frequency comb spectroscopy (DFCS) which uses a fixed frequency comb for the 85, 87Rb 5 2S1/2 → 5 2P1/2, 3/2 excitation (mode-locked Ti:sapphire laser with pulse duration of 100 fs and pulse repetition of 80 MHz) and a weak cw scanning probe (ECDL at 780 nm) for ground levels population monitoring. The Rb(5 2P1/2, 3/2) excited atomic levels have the relaxation many times greater than the fs laser repetition period. In the time domain this leads to population and coherence accumulation effects. This corresponds to the interaction of the Rb atoms with the fs frequency comb in the frequency domain. As a result, velocity selective excited state hyperfine level populations are obtained, i.e. the mapping of the frequency comb to the atomic velocity comb. Simultaneously, velocity selective optical pumping of the ground hyperfine levels is achieved. We measured the 85, 87Rb(5 2S1/2) hyperfine levels population by monitoring the 5 2S1/2→5 2P3/2 probe laser absorption. Modulations in the 5 2S1/2→5 2P3/2 hyperfine absorption line profiles are observed as a direct consequence of the velocity selective optical pumping induced by the frequency comb excitation. The 85, 87Rb 5 2S1/2 → 5 2P1/2, 3/2 fs pulse train excitation of a Doppler broadened rubidium vapor was investigated theoretically in the context of the density matrix formalism and the results are compared with the experiment. The agreement between theory and experiment is excellent. The dependence of the observed modulations upon the wavelength and power of the fs laser and external magnetic field was investigated experimentally. The expansion of the present investigation to the cesium case was performed. We foresee an application of the results of this work in the field of spectroscopy of ultracold atoms and the atomic beam experiments. In the systems where Doppler broadening is negligible, by varying the comb optical frequency spectrum it is possible to directly manipulate the fractional populations of hyperfine ground state levels. In addition, we demonstrated that the frequency comb is suitable for velocity selective optical pumping which opens the way to a completely new scheme of laser cooling.

Konusna emisija (CE)u gustim metalnim parama već godinama privlači pažnju kao značajan fenomen iz područja nelinearne optike, a prepoznaje se kao difuzni prsten (jedan ili više njih) koji nastaje oko središnje točke, koja odgovara laserskom zračenju. Pobudim gustih rubidijevih para intenzivnim laserskim pulsevima u trajanu od 100 fs i u rasponu valnih duljina od 730 do 820 nm, opažaju se zanimljivi uzorci koji se djelomično pripisuju konusnoj emisiji. Pri pobudi u plavom krilu D2 atomske linije može se opaziti proces samofokusiranja, zajedno s procesom CE. Činjenica da je prisutno samofokusiranje i spektralno širenje vode do zaključka da je modulacija vlastite faze (SPM) osnovni proces koji je odgovoran za nastajanje opaženog fenomena. Samodefokusiranje u gustim rubidijevim parama se opaža kada je lasersko zračenje ugođeno u crveno krilo rubidijeve D2 atomske linije. Taj efekt se vidi kao razmazana velika pjega u dalekom polju gledanja.

We present results of our recent experiments where cesium vapor was illuminated by the femtosecond laser frequency comb centered at either D2 line at 852 nm or D1 line at 894 nm. This action changed the usual Doppler profile into very peculiar profile with periodic structure that can be observed by cw laser scanning probe. The periodic structure reflected the frequency spectrum of the pulse train consisting of a series of fringes separated by the pulse repetition rate. In cesium system the atomic coherence relaxation time is longer than the laser pulse repetition period. Cs atoms interact with the spectrum of the pulse train, and not with the spectrum of a single pulse. This opens up a possibility for the high resolution spectroscopy, where the observed linewidths are much less than the Fourier transform limit of the individual pulse in the train. In our recent papers we presented the observation of the velocity selective population transfer between the Rb ground state hyperfine levels induced by fs pulse train excitation. We developed a modified direct frequency comb spectroscopy (DFCS) which uses a fixed frequency comb for the 85, 87Rb 52S1/2 - 52P1/2, 3/2 excitation (mode-locked Ti:sapphire laser with pulse duration of 100 fs and pulse repetition of 80 MHz) and a weak cw scanning probe (ECDL at 780 nm) for ground levels population monitoring. The Rb(52P1/2, 3/2) excited atomic levels have the relaxation times greater than the fs laser repetition period. We shall present velocity selective optical pumping of the ground hyperfine levels in cesium atoms achieved by femtosecond laser oscillator. Modulations in the hyperfine absorption line profiles are observed as a direct consequence of the velocity selective optical pumping induced by the frequency comb excitation. In addition, we show the results of the use of a hyperfine Cs filter, which selectively absorbs frequency comb lines over the Doppler profile.

Predstavljeni su najnoviji rezultati grupe vezani uz rezonantno pobuđenje atoma rubidija diskretnim spektrom frekventnog češlja što dovodi do češljolike raspodjele brzina populacija pobuđenih hiperfinih nivoa i brzinski-selektivnog prebacivanja populacije između hiperfinih nivoa osnovnog stanja rubidija. Opažene modulacije apsorpcijskog profila probnog lasera posljedica su brzinski-selektivnog optičkog pumpanja induciranog pobuđenjem frekventnim češljem. Eksperimentalni rezultati dobiveni upotrebom MDFCS (Modified Frequency Comb Spectroscopy) metode potvrđeni su odgovarajućim teorijskim modelom.

Induced transparency on a blue 420 nm probe light in a Doppler broadened mismatched V-type system using a low power near infrared (780 nm) coupling laser was experimentally observed. Theoretical modeling was carried out utilizing standard density matrix analysis of three-level V scheme with appropriate modifications to take into account the hyperfine splittings of the 5 2S1/2 ground and 5 2P3/2, 6 2P3/2 excited states. Calculated probe absorption line profiles reproduce with great accuracy measured absorption profiles.

RbCs is the heaviest heteronuclear alkali molecule which is of considerable interest for several applications. It is marked by large spin-orbit interaction stemming from both rubidium and cesium atoms. It also has appreciable permanent electric dipole moment and thus could be used in novel methods of molecular cooling. In addition to all these interesting facts, it can be formed on the surface of the nano-size helium cluster, which as a cryostat helps in forcing the molecule into the lowest vibrational state of lowest triplet or singlet electronic states. It is in principle possible to predict some of the RbCs absorption bands that can be detected in future laser excitation experiments. Laser induced fluorescence from cold RbCs can be also discussed. However, necessary transition dipole moments are not known and additional spectroscopic measurements are needed. We carried out a series of absorption measurements in very dense vapor of Rb-Cs mixture. At temperatures around 650 K we clearly observed a new band peaking at about 563 nm. Since it appeared as a single peak we deduced its triplet origin. However, at slightly higher temperatures (~700 K) another diffuse band extending from 570 till 600 nm appeared, also most probably from RbCs molecules. Resonance lines of Rb and Cs did not reveal any satellite bands as it is well known in homonuclear molecules. Theoretical treatment is very complex, but we hope that absorption measurements at high densities will help in deciding about most prominent transitions in RbCs molecule. We also plan laser induced fluorescence experiments, which will contribute to the knowledge about the structure of heteronuclear alkali molecules. This knowledge could be then applied to spectroscopy of RbCs on the helium clusters.

Recently heteronuclear molecules have been detected at ultra cold conditions and some very interesting laser spectroscopy has been carried out for those molecules under conditions of magneto-optical traps. Heavy alkali heteronuclear molecules became very attractive recently and their observation on helium clusters is desirable, since triplet states are most prominent features in nano-cryostat conditions. Potential curves for heteronuclear molecules are becoming better known in both cases with and without spin-orbit interaction taken into account. Singlet transitions are better known than triplet transitions in usual laser spectroscopy. However, at ultra low temperatures triplet states become more accessible and thus analysis reveal the structure of the potential curves for triplet manifold. Optical transitions within triplet states, sometimes called diffuse bands, may be used for two-photon resonance ionization scheme. We shall discuss absorption spectra at high density alkali vapor mixture at thermal conditions, in the case of excitations from the He droplets, and under ultra cold conditions. Recent observations may contribute to the final interpretation of heteronuclear diffuse bands in the broad spectral region.

We investigated EIT in rubidium system using the ECDL lasers in mismatched wavelength V-type scheme where the probe beam operated at 420nm and the coupling wavelength was locked at the 85Rb 5S1/2(F=2) - 5P3/2 transition. EIT was observed by monitoring the Rb 6P3/2 laser induced fluorescence (LIF). The dependence upon the power of the coupling and probe laser beams of the EIT signal was determined. In addition, temperature, external magnetic field strength and laser beams polarization dependencies were also examined.

Obavljena mjerenja dala su dobar uvid u mogućnosti pojedinih spektroskopskih metoda za određivanje promjena oblika linija pod utjecajem tlaka plemenitog plina. Pokazala se nadmoćnost laserske apsorpcijske spektroskopije koja omogućava mjerenja profila linija velikom točnošću bez korištenja monokromatora, dakle i bez prisutnosti instrumentalnog profila u izmjerenim spektrima što omogućava mjerenja u području valjanosti udarne aproksimacije širenja linija. Najveći nedostatak te metode je teškoća nalaženja odgovarajućih laserskih dioda za mjerenje u području valnih duljina od interesa za pojedino istraživanje. Sam rad sa laserskim diodama zahtjeva puno strpljenja zbog velike nestabilnosti dioda što jako utječe na reproducibilnost mjerenja. Učinjena simulacija spektra smjese Na+K+He pokazalo mogućnosti teorijske simulacije u proučavanju svojstava smeđih patuljaka na temelju njihovih spektara. Za daljnji razvoj ove metode potrebno je uspostaviti usku suradnju sa grupama astrofizičara koji se bave problematikom smeđih patuljaka.