Quang Duc Pham

Three-dimensional positioning of optically trapped nanoparticles

We firstly demonstrate the three-dimensional (3D) measurement of a nanometer-sized sphere held in optical tweezers in water using an in-line digital holographic microscope with a green light emitting diode. Suppressing the movement with optical tweezers enabled us to detect the three-dimensional position of a polystyrene sphere with a diameter of 200 nm. The positioning resolutions of the microscope were 3.2 nm in the transverse direction and 3.4 nm in the axial direction, from the standard deviation of measurements of the 200 nm sphere fixed on glass. Changes in the Brownian motion in response to a change in the trapping laser power were measured. We also demonstrated that this holographic measurement is an effective method for determining the threshold power of the optical trapping.

Three-dimensional subpixel estimation in holographic position measurement of an optically trapped nanoparticle

We propose three-dimensional (3D) subpixel estimation in the position measurement of a nanoparticle held in optical tweezers in water by using an in-line, low-coherence digital holographic microscope. The 3D subpixel estimation was performed with the addition of axial subpixel estimation to the lateral subpixel estimation introduced in our previous work [Appl. Opt.50, H183 (2011)]. The axial subpixel estimation allowed the step length in the diffraction calculation of a hologram to be increased to ~20 nm while keeping the axial resolution of ~3 nm. This drastically decreased the computation time of the diffraction calculation to less than 10% of the two-dimensional subpixel estimation.

Selectable-wavelength low-coherence digital holography with chromatic phase shifter

We propose a new digital holography method using an ultra-broadband light source and a chromatic phase-shifter. The chromatic phase-shifter gives different frequency shifts for respective spectral frequencies so that the spectrum of the light reflected from the object can be measured to reveal the spectral property of the object, and arbitrary selection of signals in the temporal frequency domain enables single- and multi-wavelength measurements with wide dynamic range. A theoretical analysis, computer simulations, and optical experiments were performed to verify the advantages of the proposed method.

Optical frequency comb interference profilometry using compressive sensing

We describe a new optical system using an ultra-stable mode-locked frequency comb femtosecond laser and compressive sensing to measure an objects surface profile. The ultra-stable frequency comb laser was used to precisely measure an object with a large depth, over a wide dynamic range. The compressive sensing technique was able to obtain the spatial information of the object with two single-pixel fast photo-receivers, with no mechanical scanning and fewer measurements than the number of sampling points. An optical experiment was performed to verify the advantages of the proposed method.

Digital holographic microscope with low-frequency attenuation filter for position measurement of a nanoparticle.

We propose a new method for three-dimensional (3D) position measurement of nanoparticles using an in-line digital holographic microscope. The method improves the signal-to-noise ratio of the amplitude of the interference fringes to achieve higher accuracy in the position measurement by increasing weak scattered light from a nanoparticle relative to the reference light by using a low spatial frequency attenuation filter. We demonstrated the improvements of signal-to-noise ratio of the optical system and contrast of the interference fringes, allowing the 3D positions of nanoparticles to be determined more precisely.

Enhanced intensity variation for multiple-plane phase retrieval using a spatial light modulator as a convenient tunable diffuser

In the multiple-plane phase retrieval method, a tedious-to-fabricate phase diffuser plate is used to increase the axial intensity variation for a nonstagnating iterative reconstruction of a smooth object wavefront. Here we show that a spatial light modulator (SLM) can be used as an easily controllable diffuser for phase retrieval. The polarization modulation at the SLM facilitates independent formation of orthogonally polarized scattered and specularly reflected beams. Through an analyzer, the polarization states are filtered enabling beam interference, thereby efficiently encoding the phase information in the axially diverse speckle intensity measurements. The technique is described using wave propagation and Jones calculus, and demonstrated experimentally on technical and biological samples.

Optical frequency comb profilometry with a compressive sensing–based single-pixel camera composed of digital micromirror devices and a two-frequency method for meter-order depth measurements

Abstract. We demonstrated optical-frequency-comb profilometry with a compressive sensing–based single-pixel camera composed of digital micromirror devices and a photodetector for measurement of large-depth objects. Use of two harmonic frequencies allowed us to measure an object with a meter-order depth without any 2π-phase ambiguity. Profilometry with 10×10 pixels using the 12th and 13th harmonics of the fundamental frequency of 76 MHz gave a noise level of ∼10  μm in terms of standard deviation. Linking the measurement results obtained using one and two frequencies gave an axial dynamic range of ∼3.9×105.

Interference imaging profilometry using optical frequency comb and compressive sensing

We describe a new optical system using an ultra-stable mode-locked frequency comb femtosecond laser and the compressive sensing to measure an object surface profile. The highly-stabilized frequency comb laser is used to measure an object with a large depth very precisely without any 2π phase ambiguity. The compressive sensing is able to perform profilometry with two single pixel fast photo-receivers and few measurements. We demonstrate high dynamic range profilometry for a large depth object.

Combining phase images measured in the radio frequency and the optical frequency ranges

Phase images measured in the radio frequency (RF) and optical frequency (OF) ranges, whose difference was about 4×105, were combined on the basis of a pattern matching method. RF phase imaging was implemented with an optical frequency-comb femtosecond laser and a single-pixel camera to measure a meter-order depth object with micrometer-order accuracy. OF phase imaging was implemented with an optical interferometer using a low-coherence femtosecond laser pulse to measure the profile with nanometer-order accuracy and high spatial resolution. Combining the images obtained from both phase measurement systems enabled profilometry of a large depth object with high lateral and axial resolutions.

Frequency comb-based depth imaging assisted by a low-coherence optical interferometer

We proposed a system composed of a frequency comb interferometer and an optical interferometer of an optical frequency comb mode-locked femtosecond laser to measure the profile of an object. The profile of the object was fast measured by the frequency comb interferometer by means of a single pixel camera. Because of the larger size of the sampling point of the masks integrated in the single pixel camera and the long wavelength of the radio frequency, the profile of the object obtained in this step has low spatial resolution. Since then the measurement was carried out by the optical interferometer using Michelson’s setup. The very accurate and high lateral and axial resolution object’s profile can be partly observed by using low-coherence interference chromatic phase shifting technique. Resultantly, the combination of the frequency comb interferometer and the optical interferometer by matching the relative phases allowed measuring the object’s profile with meter order depth and nano-scale resolution. Employing the proposed system, the whole object or parts of the object with very high resolution can be determined and the measurement time was reduced, dramatically.