Ilya Utkin

Optical resolution photoacoustic microscopy using novel high-repetition-rate passively Q-switched microchip and fiber lasers

Optical-resolution photoacoustic microscopy (OR-PAM) is a novel imaging technology for visualizing optically absorbing superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection. Since scanning of the illumination spot is required, OR-PAM imaging speed is limited by both scanning speed and laser pulse repetition rate. Unfortunately, lasers with high repetition rates and suitable pulse durations and energies are not widely available and can be cost-prohibitive and bulky. We are developing compact, passively Q-switched fiber and microchip laser sources for this application. The properties of these lasers are discussed, and pulse repetition rates up to 100 kHz are demonstrated. OR-PAM imaging was conducted using a previously developed photoacoustic probe, which enabled flexible scanning of the focused output of the lasers. Phantom studies demonstrate the ability to image with lateral spatial resolution of 7±2 μm with the microchip laser system and 15±5 μm with the fiber laser system. We believe that the high pulse repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where real-time imaging performance is essential.

Surface-Enhanced Raman Scattering (SERS) Detection of Low Concentrations of Tryptophan Amino Acid in Silver Colloid

The surface-enhanced Raman scattering (SERS) spectrum of L-tryptophan has been studied in the concentration range 1.4 × 10−8 to 5 × 10−4 M. A borohydride-reduced silver colloid was employed as the nanoparticle enhancing agent and different electrolytes have been tested for activation of the colloid. The optimum conditions have been determined for achieving high sensitivity of detection. The experimental procedure developed, which includes the use of a composite electrolyte (a mixture of NaHCO3 and NaCl) for colloid activation, results in very high enhancement of the Raman signal (up to 108). This gives the possibility of studying SERS spectra of L-tryptophan at concentrations as low as 10−8 M, which is several orders of magnitude lower than previously reported in the literature. The observed SERS spectra were very reproducible and were detectable 2 minutes after mixing, reaching maximum strength approximately 15 minutes after mixing. The spectral characteristics were stable over the entire period of observation. We have found that SERS spectra of tryptophan in silver colloid differ in several features at low (below ∼10−5 M) and at high (above ∼10−4 M) concentrations. The most important difference is the absence of the peak near 1000 cm−1 at low concentrations, which is usually assigned to the indole ring breathing mode. The observed spectra allow us to suggest that at low concentrations Trp molecules bind to the surface through the indole ring, which remains flat on the surface. This is in contrast to the previously reported observation of SERS spectra from Trp performed at concentration levels above 10−5 M.

Two-wavelength passively Q-switched ytterbium doped fiber laser

A high power passively Q-switched dual wavelength Yb fiber laser using a Cr4+:YAG saturable absorber has been realized. Two wavelengths centered at 1040 nm and 1070 nm are generated directly from the cladding pumped Yb doped fiber laser. The pulse trains exhibit regions of stability and instability dependent on the pump power. At a pump power of 7.8 W, 1040 nm and 1070 nm pulses are generated alternatively, with pulse durations of 105 ns, pulse-repetition rates of 32 KHz and average pulse energies of 56 microJ and 47 microJ, respectively. A theoretical model is developed to simulate the two-wavelength Q-switched operation, which gives qualitative agreement with the experimental observations.

Passively Q-switched Yb 3+ :NaY(WO 4 ) 2 laser with GaAs saturable absorber

Passively Q-switched Yb(3+):NaY(WO(4))(2) lasers have been demonstrated using a GaAs saturable absorber. Under continuous wave pump mode, significant pulse shortening effects have been observed at high pump powers. At a pump power of 12 W, stable Q-switched output has been obtained with a pulse duration of 5 ns, pulse repetition rate of 83 kHz and a pulse to pulse timing jitter of less than 2%. With pulsed pump mode, much longer pulse duration and reduced pulse stability have been observed. It is proposed that the heating of GaAs may play an important role in the Q-switched operations under CW pump conditions.

High-peak-power subnanosecond passively Q-switched ytterbium-doped fiber laser

A passively Q-switched ytterbium doped fiber laser has been demonstrated with a Cr(4+):yttrium aluminum garnet saturable absorber and distributed stimulated Brillouin scattering. A linearly polarized output with approximately 375 kW peak power and a pulse duration as short as 490 ps have been obtained. A theoretical model is developed to simulate passive Q switching with the stimulated Brillouin scattering, which shows good agreement with the experiment.

Experiment and Numerical Modeling of High-Power Passively Q-Switched Ytterbium-Doped Double-Clad Fiber Lasers

Two high-power and high-repetition-rate passively Q-switched Yb-doped double-clad fiber lasers have been demonstrated with small (5.4 ¿m) and medium (10 ¿m) core diameters using Cr4+ :YAG as an external saturable absorber. At a pump power of 13.8 W, 7.8 W single-mode outputs have been obtained with a pulse repetition rate of 120 kHz, pulse energy of 65 ¿J , and pulse duration of 116 ns. A theoretical model is developed to predict the laser spectrum and numerically simulate the output characteristics versus pump power. The focused beam in Cr4+:YAG is assumed to have a Gaussian profile and is analyzed using traveling wave rate equations. The effect of amplified spontaneous emission is also investigated in the simulation, which shows reasonable agreement with experimental observations.

Two-wavelength ytterbium-doped fiber laser with sustained relaxation oscillation

Dual-wavelength radiation around 1044 nm and 1070 nm has been generated directly from a cladding pumped Yb-doped fiber laser. Outputs of the two wavelengths exhibit sustained relaxation oscillation with anticorrelated dynamics. Modeling of the transient built up of the free-running laser shows the two-wavelength behavior can be attributed to the existence of two gain peaks of Yb-doped fiber under intermediate pump conditions.

High sensitivity surface enhanced Raman scattering detection of organic molecules and amino acids

Surface enhanced Raman scattering is a promising technique for high sensitivity analytical applications. However, there are issues which have to be addressed in order to make SERS a reliable technique such as the optimization of conditions for any given analyte, understanding the kinetic processes of binding of the target molecules to the nanostructures and understanding the evolution and coagulation of the nanostructures, in the case of colloidal solutions. The background electrolyte is a very important factor in SERS experiments. Here we report a detailed study of the influence of the addition of different types of electrolytes on the amplitude and kinetics of the SERS signal in silver colloids. Different amino acids and organic dyes were used as test molecules in the concentration range of 10-8 to 10-4 M. We found that a new proposed electrolyte containing HCO3, CO3, Cl and SO4 ions provides very high enhancement of Raman signal in organic molecules we studied. The advantages of the composite electrolyte are especially noticeable at low concentration of tryptophan where we observed 108 enhancement of Raman signal, approximately 300 times larger than for the case of commonly used electrolyte sodium chloride.

High Repetition Rate Passively Q-switched Fiber and Microchip Lasers for Optical Resolution Photoacoustic Imaging

Optical-resolution photoacoustic microscopy is a novel imaging technology for visualizing optically-absorbing superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection. Since scanning of the illumination spot is required, the imaging speed is limited by the scanning speed and the laser pulse repetition rate. Unfortunately, lasers with high-repetition rate and suitable pulse durations and energies are difficult to find. We are developing compact laser sources for this application. Passively Q-switched fiber and microchip lasers with pulse repetition rates up to 300 kHz are demonstrated. Using a diode-pumped microchip laser fiber-coupled to a large mode-area Yb-doped fiber amplifier we obtained 60μJ 1-ns pulses at the frequency-doubled 532-nm wavelength. The pulse-repetition rate was determined by the power of the microchip laser pump source at 808nm and may exceed 10 kHz. Additionally, a passively Q-switched fiber laser utilizing a Yb-doped double-cladding fiber and an external saturable absorber has shown to produce 250ns pulses at repetition rates of 100-300 KHz. A photoacoustic probe enabling flexible scanning of the focused output of these lasers consisted of a 45-degree glass prism in an optical index-matching fluid. Photoacoustic signals exiting the sample are deflected by the prism to an ultrasound transducer. Phantom studies with a 7.5-micron carbon fiber demonstrate the ability to image with optical rather than acoustic resolution. We believe that the high pulse-repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where realtime imaging performance is essential.

Surface-enhanced Raman scattering detection of amino acids and peptides in a microfluidic device

Here we report the results of investigations of Surface Enhanced Raman Scattering (SERS) from amino acids and peptides. In order to obtain optimum signals a standard microfluidic chip has been modified with the help of laser micromachining technique to increase scattering light collection efficiency. We have studied the SERS signals from the following amino acids: tryptophan (Trp), phenylalanine (Phe) and glycine (Gly) and peptides Trp-Trp and Gly-Gly-Gly. The optimum conditions for observing the spectrum from these amino acids and peptides have determined. In our studies the highest enhancement observed is from the amino acid Trp. Large signal enhancements were observed and the lowest detectable concentration of Trp was estimated to 4·10 -9 M.